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Progress report on the AEgIS experiment (2017)

The AEgIS/AD-6 collaboration

1. Introduction and Overview

The primary scientific goal of AEgIS is the direct measurement of the Earth‘s localgravitational acceleration g on H. In a first phase of the experiment, a gravity measurementrelying on the observation of the vertical displacement (using a high-resolution position sensitivedetector) of the shadow image produced by an H beam, formed by its passage though a moiredeflectometer, the classical counterpart of a matter wave interferometer, will be attempted. Thepulsed formation of cold antihydrogen atoms, and the production of a horizontally travelingantihydrogen beam are key steps towards this goal.

A second goal of the experiment is to carry out spectroscopic measurements on theantihydrogen atoms in flight; specifically, we intend to use the advantages that a pulsed coldbeam of antihydrogen offers to carry out (inter alia) a measurement of the HFS of antihydrogen.

The essential steps leading to the production of a pulsed cold beam of H and thus both themeasurement of g as well as a precise determination of the HFS of antihydrogen with AEgIS arethe following:

• Production of positrons (e+) through a 22Na source and accumulator;

• Capture and accumulation of p from the AD in a cylindrical Penning trap;

• Cooling of the p to sub-K temperatures

• Production of positronium (Ps) by bombardment of a cryogenic nanoporous material withan intense e+ pulse;

• Excitation of the Ps to a Rydberg state;

• Pulsed formation of H by resonant charge exchange between Rydberg Ps and cold p;

• Pulsed formation of an H beam by Stark acceleration with inhomogeneous electric fields;

• Determination of g in a two-grating moire deflectometer coupled with a position-sensitivedetector.

• Measurement of the HFS of antihydrogen in a microwave flipping cavity coupled with ananalyzing multipolar magnet.

The proposal submitted to the SPSC, and which was approved in December 2008 [1], is basedon achieving these steps, summarized in Fig. 1.

The H formation reaction proceeds according to the equation

Ps∗ + p→ H∗ + e− (1)

where the star denotes a highly excited Rydberg state. The low temperature requirement onthe antiprotons comes from the requirement that the antihydrogen atoms that will be formed

Ps

laser excitation

antiproton trap

positronium converter

e +

Ps

Ps*Ps*

H*

H*H beamH*

accelerating electric �eld

Figure 1. Proposed method for the production of a pulsed beam of cold H atoms.

should have a velocity that is low compared to the (directed) velocity of several 100 m/s thatthey will achieve after acceleration. The charge exchange cross-section is very large for RydbergPositronium [2]: if the Ps-antiproton relative velocity is below 105 m/s its value is about 10−10cm2 for nP s = 17. For relative velocity v lower than about 3104 m/s it increases as 1/v2. Theclassical calculation shows a scaling law as the fourth power of nP s. Taking into account thecorresponding kinetic energy, as well as a smaller contribution due to converted internal energy,H is created at velocities of 25 ∼ 80 m s−1 for antiprotons temperatures of O(0.1K).

In 2016, transfer of antiprotons into the antihydrogen region and formation of positroniumin the same region (via direct injection of ∼ keV positrons into a cryogenic converter sitedabout 13 mm away from the region in which antiprotons are held inside the 1 T magnet) hadbeen implemented. Nevertheless, several key points still remained to be achieved, among themcooling of the ballistically transferred antiprotons to cryogenic temperatures; laser-excitation ofthe formed Ps; implementation of the full antihydrogen production sequence; and commissioningwith antiprotons of the antihydrogen detector.

In the following, we will summarize the advances achieved since the last report in January2017, give an overview of the antiproton run in 2017, and detail our plans and requests for 2018and the following years.

2. Results obtained with positrons in 2017

In 2016, we had just implemented a direct injection technique of positrons from the accumulatordirectly onto the production target; in 2017, this technique was commissioned completely, andthe focus turned on forming and laser-exciting positronium inside the 1T magnet by the sametwo-step laser excitation, from ground state to n = 3 (λ = 205 nm), and then to the Rydbergband (λ ∼1670 nm), as had previously been implemented in the external magnetic-field freeenvironment.

2.1. Laser setup in the 1T Ps production regionA crucial element in laser-exciting Ps in view of forming Rydberg positronium in the immediatevicinity of trapped antiprotons is ensuring an optimal spatial and temporal overlap between anyformed Ps and arriving UV and IR laser pulses.

In 2017, the OPG-OPA (Optical Parametric Generation-Optical Parametric Amplification)laser system was used for Rydberg positronium production using a two-photon scheme fromground state to n=3 with a UV (Ultra Violet) 205.045nm pulse and from n=3 to n=15 ∼17with a MIR (mid-InfraRed) 1695-1710nm pulse. This laser was used for the first time inside theantihydrogen production trap region, which required the following upgrades compared to ourprevious studies conducted in a dedicated apparatus:

Figure 2. Spatial alignment of the laser spots in the interaction area. From left to right: pictureof the positron converter target holder (left), the UV (center) and Rydberg second harmonic(right) spots imaged on a piece of MACOR in the 1T interaction region. The converter targetis held inside the 45 degrees-inclined metallic part of the target holder. Its two ends are markedby two reference fibers (green circles). The fibers can be connected outside of the vacuum eitherto a source of light that can be imaged on a CCD camera looking at the interaction area fromoutside or to a photomultiplier read out on a fast oscilloscope to collect either UV laser light orgamma rays from positron/positronium annihilation. A piece of MACOR has been inserted inthe curved open space visible on the left most picture. The two right most pictures are typicalviews of the beams as diffused by the piece of MACOR and imaged with a CCD camera fromoutside of the vacuum.

• The geometry of the laser transfer line from outside vacuum to the interaction area beingnarrow and constrained, we had to propagate the UV and MIR laser pulses on the exactsame optical path (co-propagating geometry). In order to recombine the two colors, wedeveloped an ad-hoc cost efficient beam combiner based on an equilateral dispersion prism.

• We developed and implemented a laser alignment procedure so that the two beams intersectthe positronium cloud in the right place (laser-positronium alignment). The interaction areawas imaged from outside of the vacuum with a CCD camera equipped with a 30mm lensobjective collecting the laser light coming back from the 1T Ps production region, back-diffused from a piece of MACOR (see Fig. 2) placed immediately behind the interactionarea. The MACOR was thus used as a screen to image the position of the laser beam belowthe positron converter target. MACOR was chosen for its ability to scatter light when beingsubmitted to the UV 205 nm and the second harmonic of the 1700nm Rydberg wavelength.

• The synchronization of the laser pulses to the more general timing of the antihydrogenproduction sequence requires reaching a precision of about 1 ns. We referenced the arrival

of the laser pulses in the interaction area to the positron prompt annihilation peak. Forthat purpose, we developed two sets of diagnostics based on the use of a photomultiplierand/or a photodiode connected to the same fast oscilloscope. One scheme was based oncollecting the gamma rays from the positron annihilation with the outside scintillators andthe laser light with a fast photodiode placed outside vacuum to collect the laser light. Thenwe estimated all the delays introduced by the propagation of the light through the lasertransfer line, the length of the different coaxial cables and the time delay in the responseof the gamma rays detector. We used our knowledge of those delays to adjust the timingbetween the implantation of the positrons in the converter target and the laser pulses witha FPGA (Field-Programmable Gate Array) and two delay Generators (one for the laserthe other one for the positron system). To double check the timing, we later on madeuse of one of the scintillating fibers installed in the target holder (see Fig. 2, highlightedwith green circles) connected to a photomultiplier as a unique time diagnostic for boththe laser pulse and the positron annihilation gamma rays. This allowed us to validate thetiming we chose with nanosecond precision. We optimized the timing based on the SSPALSsignal observed with and without laser excitation (see the positronium excitation section).Subsequently, the signal of the photomultiplier looking at the fiber was used as a diagnosticfor the presence of laser light in the interaction area (see Fig. 3).

• A large effort was made towards remote monitoring and control of the laser performance.Some key examples are the installation of shutters activated remotely to allow or prevent thedifferent laser pulses to be shined on the interaction area and the development of softwareto monitor the UV wavelength coupled with a retro action feedback loop to keep it constantover long periods of time.

Figure 3. Temporal synchronization of the UV laser pulse and the positron prompt annihilationon the positron converter target. Screenshot of the oscilloscope used to acquire the signalfrom the photomultiplier looking at the gamma rays produced by annihilation of positrons andpositronium atoms (light blue curve, channel 2) and at the photomultiplier connected to thefiber collecting UV light inside vacuum in the interaction area (green curve, channel 4).

2.2. Ps formation and laser-excitation in the 1T magnetic field

Experiments on positronium (Ps) laser excitation to Rydberg states in the 1 T magneticfield have been performed during the Summer/Fall 2017. Positrons (e+) extracted from theaccumulator with an energy of 0.3 keV have been accelerated up to 4.6 keV by setting a positive

potential on the kicker tube in the transfer line. Positrons have then been implanted in anefficient e+/Ps converter [3, 4] placed in the 1 T region and kept at a temperature of around10 K. Ps formed in the converter and emitted into the vacuum has been excited to Rydbergstates (Ps*) via a two-step laser procedure previously demonstrated in a dedicated chamber forPs experiments [5]. An UV laser shot (wavelength 205 nm) is used to perform the 13S → 33Ptransition and a synchronous IR laser pulse (wavelength ∼ 1695 nm) for the 33P → Rydbergexcitation.

Ps formation in the 1 T magnetic field has been monitored using the single-shot positronannihilation lifetime spectroscopy (SSPALS) technique. The time distribution of γ-raysgenerated by positrons and positronium annihilations has been detected by plastic scintillatorsplaced around the 1 T vessel (70 cm of distance from the target and 3% of solid angle covered)coupled to a fast photomultiplier tube (PMT, Thorn-EMI 9954). To enhance resolution at thelongest decay times, the signal from the PMT was split and sent to two channels of a 2.5 GHzoscilloscope with high (100 mV/division) and low (1 V/division) gain. Joined data from the twochannels give the final SSPALS spectrum. In Fig.4, the SSPALS spectrum (average of 400 singleshots) measured after positron implantation in the e+/Ps converter is reported (blue curve).The spectrum shows a sharp peak corresponding to the fast 2 gamma annihilations of implantede+ followed by a long tail. The SSPALS spectrum is compared to the detector response (blackcurve in Fig. 4) estimated by measuring the signal generated by high energy cosmic rays in atime window of 15 µs. The detector response reported in Fig.4 is the sum of 700 single cosmicevents. The detector response shows a sharp peak followed by a long tail ascribable to thephosphorescence of the plastic. The presence of an after-pulse introduced by the PMT (around4.5 µs after the prompt peak) is also distinguishable.

Above 1 µs the shape of the tail of the SSPALS spectrum is identical (after normalization ofthe amplitude) to that of the detector response. On the contrary, below 1 µs (see inset in Fig.4),the SSPALS spectrum shows an excess of counts with respect to the detector response due tothe presence of delayed annihilations generated by Ps formation and decay. The geometry of theregion around the e+/Ps converter is shown in Fig.4. Ps emitted into vacuum can self-annihilatein flight with an average lifetime 142 ns if it does not reach any obstacle during its path. Onthe contrary, its lifetime is shortened if Ps reaches one of the surfaces of the objects aroundthe converter. In this process the positron of the Ps atom annihilates with an electron of themedium with opposite spin (pick-off annihilation). Both these annihilation channels contributeto the observed excess of delayed annihilations.

When Ps is excited to Rydberg states its lifetime is increased to several microseconds fromthe 142 ns of the ground state. On resonance for Rydberg Ps formation, a larger fraction of Pscan reach the surface of the obstacles around the converter [5]. In this case, the SSPALS spectrain presence of laser ON is expected to show an excess of delayed annihilations with respect tothe spectra with laser OFF. The comparison between SSPALS spectra with laser on and off isshown in Fig.5. The spectra have been divided by the exponential decay of Ps in free flight invacuum (e−(t/142ns)) to better appreciate the small differences between the two curves.

In order to study the presence and the distribution of the excesses of the delayed annihilationsgiven by Rydberg excitation, one can use the parameter S defined as S=(foff -fon)/foff , wherefoff and fon are the areas below the SSPALS spectra in a given time window with laser off andon, respectively. The S parameter as a function of the time elapsed from the prompt peak isshown in Fig.5. The S parameter has been calculated in time windows of 150 ns width, and isevaluated as a function of time by shifting this window in steps of 25 ns.

The appearance of a relative excess of annihilations in the presence of the UV and IR lasers(resonant frequencies) between 350 and 750 ns is a fingerprint of Ps excitation to long-livedRydberg states.

Figure 4. (Left) SSPALS spectrum measured with laser off (blue) compared to the detectorresponse measured with cosmic rays (black). The spectrum of cosmic rays has been renormalizedto overlap the SSPALS curve for time longer than around 1000 ns up to 15 µs. The differentshape between the SSPALS spectrum and the detector response due to Ps formation anddecay is detailed in the inset. The SSPALS spectrum is the average of 400 single shots. Thedetector response is the sum of 700 single spectra. (Right) Sketch of the geometry around thepositron/positronium converter. The antiproton trap where antihydrogen atoms will be formedis also indicated.

Figure 5. (Left) SSPALS spectra divided by e−(t/142ns) with laser off (blue curve) and laser on(red curve). The SSPALS spectra are the average of 400 single shots.(Right) S=(foff -fon)/foff vs. time after e+ injection into the Ps formation target as extractedfrom the SSPALS spectra (with laser on and off) as shown in Fig. 4. Statistical uncertaintiesare indicated. The continuous lines are Monte Carlo simulations of the S parameter taking intoaccount the full geometry of our converter region for three values of the average velocity of thePs* of 7× 104, 5× 104 and 4× 104 m/s, respectively.

2.3. Ps velocity measurement in the 1T magnetic field

In order to estimate the velocity of the produced Ps*, Monte Carlo simulations of the S parameterhave been performed with the full geometry of our converter region. Since the geometry of thee+/Ps converter, of the antihydrogen formation region and of the surrounding structures isextremely complicated, and because this strongly affects the temporal evolution of the observedannihilation rate (and thus the S parameter), the Monte Carlo simulation has been performeddirectly on the 3D CAD model of the system (consisting actually of a mesh formed by about250000 triangles). Simplified geometric models unfortunately gave quite inaccurate results inthis particular geometry. In each simulation, 1 million events were considered: 75 % of the Pswere promptly annihilated at time zero within the production target, while for the other 25 %,the Ps were propagated before annihilation (notice that this percentage is smaller than the valueof 40-45 % measured in similar targets Fig. 4 because of magnetic quenching in the 1 T field).

The lifetime of the propagating Ps was set either to its standard value of 142 ns (for groundstate o-Ps) or to 10 µs (for Rydberg Ps). In both cases cases, Ps annihilation via pick-offhas been considered to take place just after the collision with any obstacle encountered byPs and Ps*. The Ps were assumed to be emitted isotropically, within an angle of 60 degreesfrom the perpendicular to the surface, from an elliptic area on the converter surface, with aGaussian 2D distribution with RMS =0.28 mm along the vertical direction and RMS=0.13mm along the horizontal one, centred at a distance of 13 mm from the trap axis (theseparameters were estimated experimentally in usual working conditions); after emission the Pswere propagated along straight lines with a velocity with a Gaussian-distributed modulus withdifferent mean values and different RMS (this input comes from previous observations done inthe same experiment performed in other environments [5]; simulations with a standard Maxwell-Boltzmann velocity distribution are on-going).

As the pulse of positrons is not implanted in the production target instantaneously, weassumed a FWHM of about 15 ns (estimated experimentally) for their temporal distribution,and the same FWHM is assumed for Ps emission, but with a delay of 15 ns. Thewhole temporal annihilation signal was finally convoluted with the response of the detector(Scintillator+Photomultiplier) that was experimentally measured for single gamma rays and forcosmic rays (see example reported in Fig. 4).

In order to calculate the S-parameter difference between laser-on and laser-off simulatedtrials, the effect of the laser has been simulated for the moment in a somewhat simplifiedmanner, which should however provide realistic results for the Ps velocities germane to ourexperiment, although we plan to improve it soon by means of a much more accurate model. Forlaser-on trials, simulations were performed by adding an efficiency (now set to 80%) of Rydbergexcitation within a fixed angle (namely 5 degrees) between the direction of the Ps track andthe vertical symmetry plane (perpendicular to the laser beam direction) to imitate the Dopplereffect. On the other hand, for laser-off trials, simulations were obviously performed without anyRydberg excitation.

In Fig. 5 (right), the experimental curve is compared to three different Monte Carlosimulations of the S parameter assuming an average velocity of the Ps* of 7 × 104, 5 × 104

and 4× 104 m/s, respectively. In these simulations, the Ps* speed has been assumed to have aGaussian distribution with a sigma of 20 %. The experimental data are in reasonable agreementwith excitation of Ps with a most probable velocity of the order of 4× 104 m/s, correspondingto a temperature of Ps* of around 100 K, well below the point where the charge exchange crosssection for formation of Rydberg antihydrogen decreases rapidly.

3. Results obtained with antiprotons in 2017

The primary goal of the run period of 2017 (antiprotons were available from May to earlyDecember, during a total of 24 weeks) was to continue to develop and commission the differentsteps required in forming antihydrogen, and on which work was started in 2014. Of primeimportance here are plasma manipulation techniques that compress clouds of antiprotons,positrons and electrons; techniques that allow cooling clouds of antiprotons and increasing thecloud density; and techniques that allow understanding the behaviour over time of these dense,cold clouds of antiprotons and positrons.

In parallel to these, a series of parasitic measurements were carried out in a dedicated simpledevice (optimised for the selection of low-energy antiprotons) mounted on the second beam lineinto the AEgIS zone, using antiproton pulses from the AD that would otherwise not have beentrapped (some antiproton and electron manipulation sequences that are crucial in achieving theconditions for antihydrogen formation exceed the AD cycle time of ∼ 100 s).

3.1. OverviewThe region of the AEgIS apparatus where the antihydrogen should be formed through the chargeexchange process is a Malmberg-Penning trap made of 15 electrodes with 5 mm radius (see fig.6) located in a 1 Tesla magnetic field and mounted below the target used for the Ps formation.We call this trap Htrap and we refer to the electrodes with the names H1

trap, H2trap....H

15trap.

There are a number of experimental constraints that make working with plasmas confinedin this trap particularly challenging. The strongest one is the need to have a semi-transparentregion on top of the trap to let the Rydberg Ps enter inside. We have built the electrodes witha grid with about 80% transparency. The presence of this grid introduces a radial asymmetryof the trap electric field that it is known to induce a radial transport of the plasma and, undergeneral conditions, to limit the plasma trapping time. In addition, another constraint is theneed of small trap radius to reduce as much as possible the distance between the Ps productiontarget and the antiproton cloud and so to limit the losses of Ps due to geometrical reasons. Wehave built the trap with 5 mm inner radius and this value requires higher mechanical accuracy inthe construction, mounting and alignment of the electrodes than that needed for the rest of thelarger radius AEgIS traps. But the most important consequence of the small radius trap is theneed to have a trapped plasma with very small radial extent and this is particularly demandingfrom the experimental point of view because the antiprotons and electrons plasma should betransferred here from a region more than one meter away and in higher (5 T) magnetic field.

The work that we have performed during the past years and have finalized during the 2017run was focused on transporting antiprotons in this trap, cooling and storing them for a timelong enough to allow the antihydrogen formation. After a development phase in the first periodof the 2017 run, we routinely had, stacking 2 AD shots (with nominal intensity 3 ·107 particles),3.5 − 4 · 105 antiprotons with energy below 100 meV ready for antihydrogen formation. Thenumber of antiprotons exceeds the value foreseen in the original AEgIS proposal by about afactor of 4.

The antiprotons are first trapped, cooled and compressed in the traps in the 5 T region, thentransported in the Htrap in the 1 T region and then re-cooled.

3.2. Antiprotons trapping and radial compressionThe entire trap structure used to confine, manipulate and cool antiprotons, positrons andelectrons in AEgIS is a multi-ring structure approx. 2.5 m long mounted in a region withtwo different homogeneous magnetic fields (5 T and 1 T) and a transition region in the middle.

Antiproton trapping is performed in the 5 T region using a trap of about 35 cm length, 1.5cm radius with 9 KV applied to the end electrodes. We have optimized the degrader thickness

1

trapH

2

trapH8

trapH

Figure 6. The trap confining antiprotons during the antihydrogen formation process. Theinner radius is 5 mm. The target converting e+ into Ps is mounted above the trap centered onthe electrode H8

trap.

traversed by the 5. MeV antiprotons provided by AD to achieve a peak antiprotons catchingefficiency of 1.25%, thus giving 3.7 · 105 trapped antiprotons from one AD bunch of 3. · 107

particles.Antiproton cooling is achieved through collisions with a pre-loaded electron plasma with some

108 electrons and density in the range of some 108e−/cm3.We can easily cool up to more than 90% of the trapped antiprotons by properly tailoring the

electron plasma. A considerable fraction of antiprotons are trapped at large radii and they canbe cooled if the electron plasma is large enough. However a very crucial manipulation necessaryto efficiently transport p into the antihydrogen formation region is the radial compression ofthe mixed antiproton and electron plasma. The compression of the plasma with initial radiuscomparable with the trap size does not behave satisfactorilly and we have thus chosen to workwith an electron plasma that only cools a fraction of 50-60 % of the trapped antiprotons butwith an initial radial size that that allows a very efficient radial compression.

The radial compression of the trapped antiprotons is necessary to transport them from the5T to 1T trap region (that induces an increase of the plasma size of

√(5) = 2.2 ) and to achieve

a small radius cloud in the 1 T region as anticipated above.Work on the mixed plasma compression started during previous runs. We have finalized

the procedure and submitted a paper on this subject [6] Compression is achieved by applyingradiofrequency voltages to radial sectors of some trap electrodes (Rotating Wall). We observedthat the compression dynamics for a pure electron plasma behave the same way as that of a mixedantiproton and electron plasma. We have identified one key experimental condition necessaryto perform the mixed plasma compression in which the antiproton density distribution followsthat of the electrons: it is of paramount importance to minimize the tails of the electron densitydistribution. Such electron density tails are remnants of the rotating wall compression and inmany cases can remain unnoticed. We have set up a multistep procedure in which we sequentiallyapply different frequencies of RW signal and we progressively reduce the number of electrons.A ten-fold antiproton radius compression has been achieved, with a typical antiproton radius ofonly 0.17 mm in the 5 T trap.

We also observed centrifugal separation between the electron and the antiproton plasma and

Figure 7. Radial density profile of e−and p after two cases of RW compression. Dotted linesshow unsuccessful compression of the electron plasma tail while the full lines show the highdensity p compression accompanied by the successful compression of the tails of the electronplasma.

we establish that it is important to work with densities of electrons such that this centrifugalseparation is avoided: if not, only the electron plasma get compressed while the antiprotons stayat large radii.

As an example we show in fig.7 how the compression of the electron tails influences thecompression of antiprotons. The plot shows a quantity proportional to the radial profilenz(r) =

∫n(r, z)dz as measured by a MCP coupled to a Phosphor screen and read by a CMOS

camera mounted in the 1T region at the end of the trap system. Particles extracted from thetrap follow the magnetic field lines; with this MCP, we can easily detect electrons or antiprotonsreleased from traps located both in the 5T or in the 1 T regions.

3.3. Antiproton transfer from the 5 T region to the 1 T regionDuring the year 2016 we started to set up a new antiproton transfer procedure called ”ballistictransfer” which is an alternative to the ”adiabatic transfer” tested previously. The AEgIS trapsystem includes many electrodes and it allows to move particles from the 5 T to the 1 T regionby slowly reshaping the trap voltages and shifting the trap in one direction along the z axis,always keeping the trapping condition. This is the ”adiabatic transfer” in which antiprotonsand electrons are transferred together. The presence of electrons helps to compensate for someheating that unavoidably happens during the trap movement and allows to place the two speciesin the final trap where the final cooling can take place. However, in practical conditions, theefficiency of this procedure was limited by the residual misalignement between the 5 T and 1T magnet structure (that induces a diocotron mode with instabilities or increase of the radialsize of the plasma) and by the observation of radial expansions. The ballistic procedure is there-catching in-flight of an antiproton cloud launched from the 5 T trap toward the 1 T trapwhere antihydrogen should be formed. One of the big advantages of the ballistic transfer is thepossibility to apply a controlled shift of the radial position of the antiproton cloud by settingstatic voltages (ranging from -180 to +180 V) to the 4 radial sectors of one cylindrical electrodemounted in transition region between the two magnets. The resulting velocity acquired by theantiprotons in the ExB direction is sufficient, during their transit time below this electrode, toshift their position by some mm and then to compensate for the misalignement of the twomagnets and trap structures. This procedure allows to place the antiproton cloud in theantihydrogen formation trap with the radial extent obtained in the 5T trap with the onlyunavoidable expansion due to the decrease of the magnetic field.

The best value of the shifting electric field changed from the year 2016 to the year 2017because the vacuum chamber was opened after the 2016 run and we did not recover exactlythe same alignment of the previous year. The very important point is that the ExB procedureallows to compensate for these changes and correct again the residual misalignement.

We analyze the images of the antiprotons acquired with the MCP+Phosphor to obtain thecenter of the plasma trapped in the antihydrogen formation trap and the center of the antiprotoncloud dumped from the 5T trap toward the MCP without re-catching. As a reference, if wedo not apply any correction with the shifting electrode, the difference between the two centerswas 2.16 mm in the year 2017. With the voltages applied to the shifting electrode, we are ableto center the antiprotons in the final trap with an accuracy of few tens of microns. Figure8 shows examples of antiproton plasmas dumped from the antihydrogen formation trap afterballistic transfer, re-catching and storing for 1 ms in the case of non-perfect centering with theshift electrodes, as well as for correct centering. In the case of non-perfect centering the plasma,injected off axis, starts a diocotron motion and it forms a ring. The size of the plasma cloudis closely related to the thickness of the rings. The ring then slowly fills up, ultimately leadingto a large cloud. The same figure also shows the centered plasma whose size, after 1ms storagetime in the Htrap, is identical to the one directly imaged from the 5 T trap.

We launch and re-catch antiprotons using custom developed syncronized (up to ns) pulseswith programmable amplitude, length and delay with respect to the trigger. These pulses areelectrically summed to the trap bias and they are used to launch the antiprotons (”opening”the trap) and to re-catch them (”closing” the arrival trap). In order to minimize the energyand time spread of the p we first carefully shape the launch trap before letting the antiprotonsfly along the 1.2 m distance between the two traps with the maximum energy allowed by thepresent hardware (of the order of 100 eV). Then we typically re-catch antiprotons in a trapshaped as a Malmberg trap with the entrance electrode pulsed as explained before. The trap isbiased in order to slow down the incoming antiprotons. The remaining energy and time spreadset the requirements about the trap length and closing time and determine the re-catchingefficiency. We also optimised a similar re-catching procedure using a shorter section of thefinal trap because this choice better matches the procedures with electrons (see the followingparagraph). We typically get a re-catching efficiency of about 90% (both in the long or shortertrap), corresponding to ' 2 · 105 antiprotons from an AD shot with 3 · 107 antiprotons. Thelosses happen around the catching time as shown in figure 9 and not during the flight. Theenergy of the re-trapped antiprotons is close to 10 eV.

3.4. Antiproton cooling in 1 TDuring the ballistic transfer the electrons, due to their higher speed compared with that ofantiprotons, are lost. Cooling of antiprotons in the 1 T trap then requires to load electronsthere. The already mentioned shift between the 1T and 5 T region makes it difficult to directload electrons in the 1 T trap by using the electron gun located in the fringe field of the 5 Teslaregion: the resulting plasma has a quite large radius.

We thus developed a procedure in which electrons are first loaded and compressed in the 5 Ttrap, adiabatically transferred in the first region in the 1 T magnet called ”Big trap” (becauseof its larger radius of 2.3 cm) and recompressed there with a Rotating Wall before finally beingballistically launched toward the 1 T antihydrogen formation trap. This procedure is particularlyconvenient because the storage time of electrons in the Big trap with RW applied is extremelylong (we did not see losses after many hundreds of seconds) so that it is possible to preload theseelectrons at the beginning of the entire antiproton manipulation cycle and leave them ”parked”in the Big trap during all the time necessary to trap, cool and compress antiprotons.

The non-standard design of the antiihydrogen production trap induces particular features inthe dynamics of the trapped plasma. We stored electrons in traps having different lengths and

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Figure 8. Images of an antiproton plasma launched from the 5 T to the Htrap, stored there for1 ms and then dumped on the MCP. The top left image is obtained without any ExB correctionduring the transfer while the top right refers to an imperfect correction. The bottom left plotis obtained with the ExB correction voltages optimized during the 2016 run while finally thebottom right figure refers to the best setting of this year (2017). 43 pix=1mm.

centered in different positions along this trap and we observed a radial transport depending onthe region, length of the trap and plasma density with a non trivial scaling law. As an example,we show the number of electrons and some selected radial profiles for an electron plasma withabout 3 · 107 electrons trapped in the region between the the last electrodes of the structurebefore the antihydrogen formation trap and H4

trap. We see the plasma expansion with almost nolosses of particles until about 300-400 seconds, when losses start to be non-negligible. Longertraps or traps located in different positions show qualitatively similar behavior, typically with afaster time scale.

The first idea to cool antiprotons after the ballistic transfer was to use a procedure similar tothe one adopted in the 5 T magnet for the initial antiproton cooling: this consists in creating an

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Figure 9. Time distribution of the annihilation of antiprotons as seen by scintillatorssurrounding the 1 T region. The first red mark around t=274 sec shows the time when the pulseused to re-catch in-flight antiprotons is applied. The second red mark indicates the beginningof the ramp of the voltages used to dump and count the antiprotons. The peak corresponds toabout 2 ·105 antiprotons. The figure shows that some losses (very few, some % of the final peak)of antiprotons occur around and after the re-catching time. No electrons are loaded in the 1 Ttrap.

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Figure 10. Radial profile of electrons (N= 3107) stored in a Malmberg-like trap with theelectrodes OnAx28 and H4

trap as endcap at 3 different times. We see a substantial expansionover 300 sec.

inner trap for electrons using selected electrodes of the Htrap that provide long enough storagetimes for an electron-only plasma ( e.g using the electrodes between H1

trap and H4trap), preload

here the electrons (using the electron reservoir in the Big trap mentioned before) and thenballistically catch the antiprotons into a longer trap. However we encountered an additionaldifficulty related to the non-standard dynamics of the Htrap trap: we observed that the stabilityof the trapped antiproton cloud is negatively influenced by the presence of the electric field thatcreates the inner electron trap regardless of the fact whether electrons are there or not. Weobserved significant antiproton losses related to the presence and depth of the inner wall thatdiscouraged the use of this strategy. The solution was to catch antiprotons and electrons in thesame trap, without creating an inner potential well.

The final procedure that we have set up consists in first ballistically catching the antiprotonsin the trap shown with the red line in figure 11 and then, again ballistically, adding the electronspreviously ”parked” in the Big trap. Electrons (about 1.5 − 2 · 107 ) are caught by rapidlyopening the trap for a time short enough that antiprotons do not exit (40 ns) by pulsing the

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Figure 11. Potential on the axis (no space charge) of the trap Htrap region used to store theantiprotons that were ballistically trapped. Electrons are added after the antiprotons by fastpulsing the entrance electrode centered at about 80.5 cm in the figure.

entrance electrode: during this time the voltages are the ones shown as a blue line in the figure11.

We observed as expected cooling of the antiprotons with ' 107 electrons in the 1 T region infew seconds or few tens of seconds. After this time we reduce the number of electrons to limitthe expansion rate of the mixed plasma and also to allow an estimation of the temperature.

We measured the axial temperature of the antiprotons by using a procedure quite similarto the one developed in [8] and [7]. After the initial cooling phase we first dump (applyingfast short pulses to one electrode) a large fraction of the electrons but not all of them: thereduction of the number of electrons is necessary because their space charge potential influencesthe temperature measurement but, at the same time, some electrons still must be kept togetherwith the antiprotons to further cool them after the electron kickout: antiprotons are in factheated by the pulses and by the change of the space charge potential in the trap. We keep inthe trap a number of electrons of the order of few 105, so low that we cannot directly countthem on the Faraday Cup. We let the antiprotons cool with the residual electrons for a timeof the order of 10 sec and then we adiabatically move electrons and antiprotons into a differenttrap region, in the direction of positive z, that provides a better overlap with the RydbergPositronium during the antihydrogen formation experiments. Then we wait for a time of theorder of several tens, up to 100 sec to allow reaching the equilibrium temperature. We measurethe temperature by slowly ramping one of the endcap electrodes downwards, and counting thenumber of annihilations of escaping p detected by the surrounding scintillator as a function ofthe trap depth. The first escaping particles are the ones trapped near the axis of the trap and,in ideal conditions if the distribution of the axial velocity is of a Maxwellian type, the followingrelation holds

dN ∝ exp−

(W0+qΦp0

)

kBT1√

(W0 + qΦp0)kBT

dW0 (2)

where W0 is the trap depth on axis due to the applied voltages (the so called vacuum trap) andΦp0 is the potential on axis due to the space charge. The temperature is typically obtained, in a

first approximation, under the assumption that the number of extracted particles is so low thatthe changes of the space charge potential are negligible and by neglecting the variation of the

term√

(W0 + qΦp0)kBT . Under this simplified hypothesis the first few particles escaping from

the trap are counted as a function of the trap depth and an exponential fit is performed. A more

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Likelihood fit: T = 237+- 21 K

Figure 12. Antiproton temperature measurement in the trap H2trap and H6

trap obtained after100 sec of cooling time with a small number of residual electrons left in the trap. The plotshows the counts due to annihilation of antiprotons on the scintillator as a function of thevacuum potential well on axis and the fit as described in the text. No corrections for spacecharge potential have been applied.

refined analysis must include the variation of the space charge potential. Note that neglectingthese refinements results in an overestimate of the temperature. Figure 12 shows as exampleof the temperature obtained in the trap formed with the electrodes H2

trap and H6trap without

space charge correction. After applying similar manipulations, we measured the values of thetemperature depending on the trap region. In the trap region using the electrodes located belowthe Ps formation target we measured an p temperature of 1000 K.

In the present setup the value of the antiproton temperature only influences the time offlight of the resulting antihydrogen from the production point to the trap electrodes where itannihilates. It is necessary that this time of flight be longer than the dead time (many hundredsns) induced in the detectors by the prompt annihilations of the positrons. With the measured penergy of 0.1 eV the time for p to fly 5 mm is 1 µsec and well matches the detector requirements.

3.5. Storage time of cooled antiprotons in 1 T trapWe discussed above that in the mixed antiproton and electrons plasma during the RWcompression the electrons are driving the dynamics, and compression of the electrons inducescompression of antiprotons. We observed a similar dynamical effect in the expansion during thestorage of a mixed antiproton and electron plasma in the Htrap without any RW: the relativelyfast expansion of the electrons induces expansions and losses also of the antiprotons. By reducing,as described, the number of electrons we achieved the necessary cooling and also a considerablylonger storage time of the mixed plasma in the non standard Htrap trap. As an example, figure13 shows the number of surviving antiprotons as a function of the trapping time in the trap withencap H5

trap and H9trap with all the electrons initially used for the initial stage of the cooling

(that is about 107 e−) and with reduced numbers of electrons as used during the final stage ofthe cooling.

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Figure 13. Storage time of antiprotons with variable number of electrons. The left panel showsthe ratio between the scintillator counts measured dumping antiprotons from the Htrap dividedby the number of not cooled antiprotons in the 5 T (they provide a normalization to compensatefor shot-to-shot variations). Without removing electrons or with a number of electrons that isstill too high, the electron expansion induces losses of antiprotons. With less electrons (as theright panel shows) the expansion is slower and the storage time is longer. Note that also in theconditions of the right panel we have electrons in the trap.

3.6. Antihydrogen production procedureAll the procedures of manipulation of antiprotons described here were used during theantihydrogen production protocol that we have developed during the last months of the runin 2017. We typically stack 2 AD shots in the 5 T trap and we compress, cool and transferthem exactly as described before. The overall manipulation procedure takes about 500 seconds.In the mean time we accumulated positrons and, when antiprotons are ready in the Htrap, weinjected positrons into the Ps production target and fired the laser as described in section 2.2.

We have collected significant sets of data under the same conditions with laser on and laseroff and the analysis is in progress.

4. Results obtained with the vertex detector in 2017In 2017, for the first time, the fiber vertex detector surrounding the production trap in the 1Tmagnet was used as a diagnostic tool for antiproton manipulations and antihydrogen production.The detector which was first installed in 2014 was under the responsibility of a group whose lastmembers left the collaboration in 2016. The responsibility was taken over by other members ofthe collaboration who performed hardware and software consolidations and commissioning of thedetector prior and during the 2017 beamtime leading to the first reconstructions of antiprotonannihilations in the 1T production trap.

4.1. FACT : Fast Annihilation Cryogenic Tracking detectorThe detector is a barrel-shaped fiber detector consisting of 794 scintillating fibers curved aroundthe symmetry axis of the production trap and arranged in 2 layers of two rows each (each

row consisting of roughly 200 fibers) to allow tracks reconstructions, see Fig 14. Each fiber ismechanically coupled to a clear fiber transporting the scintillation light to a multi-pixel photoncounter (MPPC) located at room temperature in the isolation vacuum of the AEgIS cryostat.The MPPCs are mounted in groups of 48 onto custom-made PCBs which are connected totheir analogue and digital readout electronics via a vacuum feedthrough. Each MPPC signal isconnected to a linear amplifier which produces a pulse with an amplitude of roughly 7 mV for asingle photoelectron. The energy deposited in a single fiber by a charged pion from an antiprotonannihilation will produce an amplified signal in the range of 100-200 mV corresponding to∼ 15-25 photoelectrons. The amplified analog signal is discriminated and the TTL outputis connected to an FPGA which samples the discriminator output at 200 MHz. The biasvoltage and comparator threshold for each MPPC can be adjusted by means of a dual channeldigital potentiometer, which is programmed via the FPGA. A total of 18 FPGA’s read outthe discriminated MPPC signals and communicate through Ethernet with the acquisition PC.Developments made in 2017 on the software and hardware tools currently permit the continuousread-out of roughly 500 ms (versus 50 µs previously) of data buffered in the FPGA’s, allowingto use FACT for antiproton manipulation diagnostics.

Figure 14. Sketch of the FACT detector layout. The scintillating fibers are located inside the1 T magnet at LHe temperature allowing for a detection of the charged pions close to theirproduction region on the wall of the production trap.

4.2. Antiproton manipulation diagnosticsThe design of FACT allows for reconstruction of the axial and radial positions of a vertex witha time resolution of 5 ns. Antiproton annihilations at given known positions were used in a firststage to commission and calibrate the detector. Fig. 15 a) shows tracks in a two dimensionalplane (x,z) as reconstructed by FACT, produced by charged pions from antiproton annihilationsonto a micro-channel plate (MCP) located at the downstream end of the detector. The redhorizontal lines indicate the position of the FACT fibers in the (x, z) coordinate system. Theaxial position of the MCP is reconstructed with a precision of ∼ 4 mm as illustrated by Fig. 15b) showing the axial position of tracks that intercept the axis of symmetry of the productiontrap (i.e. at r=0). Fig. 15 a) indicates areas of lower efficiencies which can be corrected by either

adjusting the gain of the MPPCs or the comparator threshold. Comparisons with Monte Carlosimulations to assess and correct for the effect of the geometry of the detector gave an overallefficiency of FACT for pions below the expected ∼ 100% which calls for further fine adjustmentsof the detector using cosmic rays during the winter shutdown period before the restart of thedata taking period in 2018. For this purpose, further improvements on the readout scheme arecurrently being implemented to allow for continuous acquisition of FACT data. The proceduredeveloped will also allow for daily or weekly (depending on the available time for cosmic dataaccumulation) calibrations of the detector during data taking periods.

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Figure 15. a) two dimensional representation of tracks reconstructed by the FACT detector.The 2 layers consisting of two rows of fibers each are indicated by the red horizontal lines. TheMCP is located ∼ 138 mm from the FACT center as correctly reconstructed. b) Intercept ofthe tracks with the r=0 axis.

In 2017, FACT has been used for precise diagnostics of the geometrical position and timeof antiproton losses in the trap. Fig.16 shows the axial position of the reconstructed tracksintercepts at r=0 as a function of time when antiprotons are ejected from the trap towards theMCP. In this case annihilations on the MCP are followed in time by radial losses at the centerof the trap (z∼ 0 cm) due to rearrangement of the plasma following the loss of on-axis particlestowards the MCP. Analysis is also underway to estimate the temperature of the antiprotons atdifferent positions in the production trap using FACT data.

4.3. Antihydrogen detectionDuring antihydrogen formation procedures, the injection of positrons onto the positroniumconversion target produces a high number of gamma rays that are emitted close to the centerof FACT that momentarily “blinds” the detector. Given the high number of photoelectronsproduced the recovery time of the detector is of the order of 500 ns, hindering the detection offast antihydrogen atoms (with energies of O(eV)) which would have annihilated on the wallsof the trap within this time. Formation of colder antihydrogen can be detected by comparingnormalized (to the antiproton and positron numbers) numbers of tracks and vertices in FACTat a later time (typically within a time window of 500 ns - 10 µs after the positron arrival)for data acquired with or without positronium excitation lasers (as ground-state positroniumwill not lead to the formation of a detectable amount of antihydrogen atoms in the currentAEgIS configuration). A large amount of antihydrogen trials were recorded throughout the lastthree months of data-taking despite the important hardware drawbacks related to positroniumexcitation mentioned earlier. Detailed analysis of the data is currently underway to determineif the formation procedures developed in 2017 enabled a detectable amount of antihydrogenproduction.

Figure 16. Axial position of the reconstructed track intercept with the r=0 axis as a function oftime. The largest amount of tracks are originating from the MCP but a second peak is observedat the trap center (z∼0) at later times.

The main AEGIS antihydrogen detector has been successfully commissioned in 2017 followingextensive consolidations earlier in the year. The detector has proved to be an essential tool forantiproton diagnostics in the production trap. Final calibration and optimization of the detectorefficiency will be performed in early 2018 to optimally detect antihydrogen formation in 2018.

5. Secondary beam line measurements

Evaluations of possible antihydrogen detector technologies were continued parasitically also in2017 in the improved small-scale dedicated vacuum chamber incorporating an electrostaticextraction line in the secondary beam line of the AEgIS zone. The goal was to continueto characterize and evaluate different technologies that will constitute the elements of theantihydrogen detector which must meet a difficult challenge: separation of antihydrogenannihilations at the level of the detector from antiproton or antihydrogen annihilations upstream,as well as from a very large flux of pions from the initial antiproton capture process. Tests ofemulsion layers had been carried out in 2015; in 2016, we tested a thin (50 µm) silicon stripdetector, along with further possible technologies (CsI-based calorimeters, a high-resolutionplastic detector Cr-39, and other silicon detectors). In 2017, further tests were carried out withthree technologies: the silicon-based Timepix detector; further exposures of CR-39; and diamonddetectors.

5.1. Antiproton tagging with the Timepix3 detectorThe position detector for antihydrogen, able to operate in cryogenic and vacuum environmentis one of the key elements for the gravity experiment. For this reason, the studies with theTimepix3 this year were focused on its operation in cryogenic environment. A new set-up wasbuilt, which consists of a simple cryostat with cold finger, suitable for tests at 77 K, an OFHC(Oxygen-free high thermal conductivity) Cu plate, which hosted the Timepix3 detector and thecables, and a vacuum chamber which hosted the detector. Few pictures of the set-up are shownin fig. 17.

Figure 17. Pictures from the set-up for the cryogenic tests (77 K), showing the cryostat withthe Cu plate (top), the Cu support to which the Timepix3 PCB was glued (bottom left) andthe vacuum chamber (bottom right).

The Timepix3 ASICs were glued to separate PCBs with a Master Bond EP37-3FLFAO, two-component cryogenically compatible glue. The same glue was used to fix each of the PCBs to aT-shape Cu support (fig. 17, bottom left).

Three Timepix3 chips were first tested at LN (Liquid Nitrogen) temperature without anybeam present. The temperature of the Cu support (T-piece) always reached 77 K, after aboutan hour of cooling down and starting from room temperature. The temperature of the chip itselfreached only ∼ 130 K (-140 C). A threshold equalization was performed each time after the cooldown (or the warm up) of the chip. This procedure allows having all pixels correspond to theglobally set threshold in an equal manner. The threshold equalization is used to compensatethe pixel-to-pixel threshold variations, which are due to local transistor threshold voltages andcurrent mismatches, or due to more global effects like on-chip power drops. In a threshold scan,the ThrDAC of the individual pixels is set. First, it is set to its minimum value, DAC=0x0,which results in a Gaussian distribution (blue line in fig. 18). Second, it is set to its maximumvalue, DAC=0xF, resulting in a Gaussian distribution at higher threshold (red line). Then, foreach pixel, the threshold which is closest to the average of the Gaussian mean values is chosen.This results in a narrow distribution at the noise level (green line).

The results from the equalization at room temperature in fig. 18) show that the thresholdvariation after equalization at room temperature is ∼ 30 e- rms, whereas the corresponding valueat 130 K is ∼150 e- rms. The four bit range for the threshold equalization of th Timepix3 is notenough to equalize all the pixels properly at cryogenic temperature: the number of masked pixels(which could not be equalized and are discarded) is 1643, which is ∼2.5% of the total numberof pixels. The five-fold increase in the threshold mismatch results in a raise of the minimumthreshold form < 1000 e- (which is the usual value at room temperature), to ∼ 1500 e- at LNtemperature. However, for our application, when antiproton annihilations are detected with theTimepix3, the energy deposits per pixel are usually > 4000 e-, so this effect should not be anissue.

! !At!room!temperature!σeq=30.7!e2!!Pixels!masked!=!18!!

At!77!K!(130!K!on!the!chip)!σeq=148!e2!!Pixels!masked!=!1643!

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! !!

Figure 18. (Left) Noise equalization of the Timepix3 chip at room temperature (left) andat 130 K (right). (Right) Low energy (∼ 5 keV) antiproton annihilations observed with theTimepix3 (300 µm thick Si sensor), at a chip temperature of 190 K (-85C).

After the tests in the vacuum chamber, one Timepix3 ASIC, which was bonded to 300 µmsilicon sensor, was also tested with low energy antiprotons, in the secondary AEgIS beam line,using the same set-up. Even though the temperature of the T-shape Cu support reached 77 K,the Timepix3 chip only cooled down to ∼190 K (-85 C) due to a sub-optimal thermal couplingglue. Data were taken with ∼ 5 keV antiprotons annihilating in the detector at this temperature.A typical frame is shown in fig. 18(right). A significantly lower deposit of energy is observed, e.g.a max of 40 keV compared to ∼500 keV at room temperature. The calibration that was usedfor these measurements was performed during the tests at lower temperature (130 K) withoutbeam, so the real deposited energy might be slightly different.

For the next set of cryogenic measurements the set-up will be modified to allow a cool down ina controlled manner, by introducing heating resistors. The cool down will be then performed insteps and the behaviour of the chip will be monitored at different temperatures. A new cryogenicglue resistive down to 4 K will be employed instead of the current one. This upgrade will allowa better evaluation of the lowest temperature at which the Timepix3 is fully operational.

5.2. Antiproton tagging with a diamond detectorBetween the end of November and the beginning of December, measurements on a diamonddetector were carried out at the AEgIS low energy beam line in view of evaluating alternativesfor tagging of low energy antiprotons. Diamond detectors (Figure 19) were provided by CividecInstrumentation and consisted of a 500 µm diamond Knopf detector with gold metallization.The readout chain also included a current amplifier and computer readout system with anoscilloscope application, also provided by the owner institute.

Data analysis is under way. The preliminary plot in Figure 19 shows the response ofthe detector to the exposure to antiprotons. It is possible to distinguish an initial burst ofsignals followed by lower numbers of energetic individual signals. This can be interpretedas a first burst of pions (corresponding to pions produced by annihilation of the AD pulse- pulse length ∼ 200 ns - of antiprotons) and then individual pulses from individual lowenergy particles (energy-degraded antiprotons with longer time-of-flight, nuclear fragments fromupstream annihilations). Some pulses, however, saturated the amplifier. These are probably dueto antiprotons annihilating inside the detector, while most of the annihilations were expectedon the gold electrode surface. This suggests that the metallization of the detector was too thickfor very low energy antiprotons (∼ keV ) to reach the active volume of the detector. More tests

Figure 19. Left: The gold plated Knopf detector. Right: Diamond detector response toantiproton exposure. A 200 ns long antiproton extraction pulse from the AD is detected startingat t ∼ 58000. Later individual pulses correspond to low energy particles (either antiprotonsdegraded down to keV energies annihilating directly on the detector, or low energy nuclearfragments from upstream annihilations).

will follow, including a sensor with a thinner metallization, in order to enhance the likelihoodof annihilation inside the diamond detector.

6. Auxiliary developmentsIn parallel to the work in the main apparatus, a number of activities targeting the steps thatwill become important once Rydberg antihydrogen atoms will have been produced are beingpursued actively by the groups involved in AEgIS using equipment not incorporated inside theAEgIS apparatus. Two specific developments concern reaching lower initial temperatures forantiprotons via active sympathetic cooling, as well as enhancing the de-excitation cascade fromRydberg antihydrogen to ground state antihydrogen.

6.1. Work towards sympathetic cooling of antiprotons

The temperature of the antihydrogen produced by charge exchange with positronium isdominated by the temperature of the antiprotons prior to H formation. Hence, the pre-coolingof p to temperatures well below 1 K is an essential ingredient to the production of H cold enoughfor a precision gravity measurement.

The MPIK group is pursuing a technique proposed about 10 years ago [9] that uses laser-cooled atomic anions to sympathetically cool antiprotons. The main challenge of this coolingscheme lies in the identification of an anion with a suitable transition for efficient laser cooling.Until now, only three atomic elements have been identified whose anions have a bound–boundelectric-dipole transition potentially amenable to laser cooling. Since 2011, the MPIK group,funded by an ERC Starting Grant (UNIC 259209) have been experimentally investigating themost promising candidate anion lanthanum. This transition, between the 5d26s2 3Fe

2 groundstate and the 5d6s26p 3Do

1 excited state, was predicted by theoretical calculations to be moresuitable than a transition in the previous candidate osmium due to a significantly highertransition rate. In their initial investigation using collinear laser spectroscopy [10], they were ableto measure the transition frequency/wavelength to high precision and characterize the hyperfinestructure of the ground and excited states.

During 2017, the experimental apparatus was modified to include transverse laser access.

Using a combination of transverse and collinear spectroscopy, the cross-section of the transitionwas successfully measured to be σ = 1.0(1) × 10−12 cm2, corresponding to a transition rate of7800 Hz [11]. This means that cooling an ensemble of La− ions from 100 K to the Dopplertemperature TD = 0.17 µK will require the absorption of 8.4 × 104 photons, and hence take3.7 s in saturation. In addition, theoretical calculations using a novel relativistic high-precisiontreatment to determine yet unmeasured energy levels, transition rates, branching ratios, andlevel lifetimes were performed [11]. For instance, all relevant total branchings from the excitedstate are shown in Fig. 20 (left).

This combined experimental and theoretical work concludes the full characterization of thepotential laser cooling transition in La−. Further experimental activities in 2017 concentratedon the installation and commissioning of a newly constructed Paul trap (see Fig. 20 (right))with its associated high-voltage platform and RF electronics. Initial trapping and manipulationof different test anions (including Au− and Os−) have been successful. First tests of laserinteraction with the trapped ions are ongoing. After the successful demonstration of anionremoval from the Paul trap by two-photon detachment, resonant excitation of both Os− andLa− in the Paul trap will be carried out.

Figure 20. (Left) Partial energy level diagram of La− (energies to scale). The relevant decaybranches from the 3Do

1 excited state of the laser cooling transition, as well as from the 3Do2 excited

state of the repumping transition, are indicated. Thicknesses of blue arrows are indicative ofbranching ratios, but not to scale. (Right) Photograph of a new Paul trap incorporated into theapparatus for the demonstration of anion laser cooling. The rods have a diameter of 14.8 mmand a length of about 140 mm.

In parallel to the above effort, a second anionic species is being investigated, the anionicmolecule C−

2 , whose energy levels are already very well known, and which has a smaller massratio to antiprotons than lanthanum beneficial for sympathetic cooling. For C−

2 , the proposedsympathetic cooling scheme in Penning traps was recently theoretically investigated in [12].Using optical dipole forces on the X → A transition of C−

2 a Sisyphus cooling technique wassimulated to reach antiproton temperatures of mK within about 10 s of cooling time starting atcryogenic trap temperatures.

The setup to demonstrate the production and cooling of C−2 (molecular source, mass selector,

diagnostic lasers, traps) is undergoing commissioning in a test lab at building 275 at CERN.Within 2017, for the C−

2 source, a pulsed supersonic expansion valve with a discharge has beenoptimized for its flux of anions. A subsequent acceleration in a pulsed tube and an Einzel-lenstelescope has been commissioned for beam steering and focusing. Currently, the accelerated

anions are being investigated downstream of a permanent magnet mass selector to yield a pureC−2 beam suitable for the injection into a Penning trap. In order to show initial cooling of

C−2 , a Paul trap has been constructed and is currently prepared to be put under vacuum. The

Paul trap is designed with a digital drive for fast switching of the trapping fields to allow forTOF measurements to read-out the anions’ kinetic energy in the trap. For laser cooling on theX → A transition, the necessary IR-DFB diode lasers and their precision frequency lock to a gascell have been built and commissioned. To demonstrate resonant laser interaction and to inferthe internal and external energies at which C−

2 is created by the source, the C−2 beam will be

characterized in fly-by using fluorescence detection and by Doppler-sensitive photodetachmentof excited C−

2 . For the former, a large solid-angle parabolic mirror in front of a IR photodiodehas been assembled, which is currently being attached to the photodiode’s liquid nitrogen cold-finger. For the latter, to reach sufficient photodetachment laser power a high-finesse cavity andlaser lock has been set up. To operate all systems (Paul trap, mass-filtered C−

2 beam, fly-byfluorescence and photodetachment) in parallel, a 90-degree quadrupole beam bender has beenmanufactured. It is currently being electrically tested before it will be installed in the beampipe between the source part and the acceleration stage.

The next subsequent steps are: to commission the mass-filtered C−2 beam for Penning trap

injection; to measure the kinetic energies of produced C−2 and to trap C−

2 in the Paul trap beforeinteracting them with the lasers to establish cooling.

6.2. Work towards stimulated de-excitation of Rydberg antihydrogen atomsIn all currently running AD experiments, antihydrogen atoms are typically created in severalRydberg states. Spontaneous decay of such samples is especially low because of the presence ofhigh angular momentum m that allow decays only to the nearest manifold (n→ n− 1). We arecurrently studying a stimulated emission cascade that requires to have a light source emittingbetween 100 GHz (for n = 40 → 39) and 2100 GHz (for n = 15 → 14); spontaneous emissionshould allow to reach the ground state in tens of microseconds from the later states, which isthe typical timing we are looking for.

At Laboratoire Aime Cotton, Daniel Comparat is currently investigating several sources forsuch studies. The current setup, tested using Rydberg Cs atoms, is shown in Fig. 21. THzradiation is produced by a Sciencetech ST75 mercury lamp, which is known to be one of the mostefficient sources in the 0.1-5 THz (or 3-150 cm−1 or 60-3000 µm) region [13]. The lamp createsa hot black body source of temperature T that enhances the spontaneous emission n → n − 1of frequency νn by a factor n = 1

e−hνn/kBT−1. The light is filtered in order to send to the atom

only the useful 10 mW/cm2 (in the 0.1-5 THz band) Radiance power. This is the equivalentof a ∼ 800 K source whose radiation was sent to the atomic cloud, and that, if used in AEgIS,should allow to de-excite Rydberg anti-hydrogen atoms down to the ground state in few tens ofmicroseconds.

More detailed measurements will follow, and an adaptation of the design to the AEgISgeometry is under way for possible tests during 2019.

Figure 21. Setup at the Laboratoire Aime Cotton. The system is tested on a cesium beamexcited by an OPO pulsed laser to n ∼ 40. A schematic of the chamber is shown (top left)together with a photograph of the real experimental setup. The top-right plot is the firstmeasurement of the population of the 40d level versus time (0 corresponds to the moment atwhich the radiation is turned on). The ”no lamp” evolution corresponds to the natural decay ofthe Rydberg states; the more rapid decrease of the ”lamp” data corresponding to the evolutionof the 40d population in the presence of broadband THz radiation indicates stimulated de-excitation. The coloured curves correspond to fits for different effective black body temperatureas seen by the atoms; the optimal fit indicates that THz radiation mimicking a ∼ 800 K sourceis being produced.

7. Summary and plans for 2018 and beyond

In the last months of 2017, all elements required for the pulsed production of antihydrogenatoms were functional and an extended period of attempting to form antihydrogen atoms tookplace; the resulting data is being analyzed.

In 2018, our efforts will continue to focus on validating all the steps needed for the pulsedproduction of antihydrogen atoms by working on two programs in parallel. Improved sensitivityand understanding of the involved processes will be helped by the installation - during theshutdown period in January to March - of additional diagnostics. With the advances made in2017, we are well positioned to - on one hand - form and characterize antihydrogen atoms insidethe main apparatus as well as - on the other hand - to continue to thoroughly investigate theformation and laser excitation of positronium in our apparatus (in the external test set-up aswell as in the main apparatus). Parasitic activities related to validating different technologies(antihydrogen detection) and studies of systematics with antiprotons are also foreseen to

continue, albeit again at a reduced rate, since most of the candidate antihydrogen detectiontechnologies have now been tested.

In order to continue to focus on implementing and validating the different steps required toform a pulsed antihydrogen beam in the AEgIS apparatus, we request a pro-rata allocation ofthe antiproton beam time in 2018.

Given the strong (spatial, safety) limitations of the current experimental area, discussionswere started in 2016 with the ELENA team, with different departments within CERN andwith affected experiments (ASACUSA and GBAR) to prepare for a move into a new largerexperimental area inside the AD that the ELENA facility has made possible to envisage. Thesediscussions had converged on a schedule for moving the AEgIS apparatus into this new zoneduring LS2, with preparatory work in the new zone that could start from late 2019 (oncethe zone itself can be delimited). Detailed planning for this move has commenced and anECR (https://edms.cern.ch/document/1891119/0.1) incorporating all necessary steps has beencompleted and is in the process of validation.

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