This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 158.132.179.232

This content was downloaded on 17/04/2014 at 06:16

Please note that terms and conditions apply.

Future large scale accelerator projects for particle physics

View the table of contents for this issue, or go to the journal homepage for more

2013 Phys. Scr. 2013 014016

(http://iopscience.iop.org/1402-4896/2013/T158/014016)

Home Search Collections Journals About Contact us My IOPscience

IOP PUBLISHING PHYSICA SCRIPTA

Phys. Scr. T158 (2013) 014016 (10pp) doi:10.1088/0031-8949/2013/T158/014016

Future large scale accelerator projects forparticle physicsR Aleksan

CEA, DSM/IRFU/SPP, F-91191 Gif sur Yvette Cedex, France

E-mail: roy.aleksan@cea.fr

Received 1 October 2013Accepted for publication 30 October 2013Published 6 December 2013Online at stacks.iop.org/PhysScr/T158/014016

AbstractThe discovery of a new particle, the properties of which are compatible with the expectedBrout–Englert–Higgs scalar field in the Standard Model (SM), is the starting point of anintense program for studying its couplings. With this particle, all the components of the SMhave now been unraveled. Yet, the existence of dark matter, baryon asymmetry of the Universeand neutrino mass call for new physics at an energy scale, which is not determined so far.Therefore, new large scale accelerators are needed to investigate these mysteries throughultra-high precision measurements and/or the exploration of higher energy frontiers. In thefollowing, we discuss the various accelerator projects aimed at the achievement of the aboveobjectives. The physics reach of these facilities will be briefly described as well as their maintechnical features and related challenges, highlighting the importance of accelerator R&D notonly for the benefit of particle physics but also for other fields of research, and more generallyfor the society.

PACS numbers: 14.80.−j, 29.20.−c

(Some figures may appear in colour only in the online journal)

1. Introduction

A new particle with a mass around 125 GeV/c2 [1, 2]has recently been discovered by the ATLAS and CMSexperiments at the Large Hadron Collider (LHC) at CERN.Its measured mass and properties are compatible with theStandard Model (SM) prediction of a Brout–Englert–Higgs(H0) particle (also known as Higgs boson) responsible of theelectroweak symmetry breaking [3–5]. However, besides thisfundamental discovery, no other particle or phenomenon, notaccounted for by the SM, has been observed so far at theLHC, neither through precision measurements, nor throughdirect searches. This fact seems to push the energy scale ofany physics beyond the SM above several hundreds of GeV.

The higher-energy LHC run, which is expected to start in2015 at

√s ∼ 13–14 TeV, will extend the sensitivity to new

physics at a scale of 1 TeV or above. Fundamental discoveriesare therefore still possible by 2017–2018. However, even ifno new unexpected finding is unveiled with the coming LHCdata, there must be new phenomena, albeit at unknown energyscales, as suggested by the evidence for non-baryonic darkmatter, the cosmological baryon–antibaryon asymmetry and

possibly by non-zero neutrino masses, which call for physicsbeyond the SM. Therefore, in addition to the high-luminosityupgrade of the LHC, new particle accelerators are necessaryto explore the physics underlying these observations.

In the following, we will discuss the various possibilitiesfor future accelerators and examine their physics potentialthrough some selected studies that can be carried out at theseenvisioned infrastructures.

2. Higgs precision measurements

At present, one of the major questions at stake is whether theproperties of the newly observed particle are those expectedfor the SM Higgs boson. Thanks to the large number of Higgsbosons already produced or foreseen at the LHC, significantimprovements are to be expected in this area.

2.1. Prospects at LHC and its upgrades

The successful operation of the LHC has enabled each of theexperiments installed at the four interaction regions (IR) to

0031-8949/13/014016+10$33.00 1 © 2013 The Royal Swedish Academy of Sciences Printed in the UK

Phys. Scr. T158 (2013) 014016 R Aleksan

Figure 1. Delivered integrated luminosity by LHC to the ATLAS and CMS experiments.

Table 1. The measured signal strengths µ for a Higgs boson of massmH = 125.5 GeV (µ = 1 corresponds to the SM Higgs bosonhypothesis while µ = 0 to the background-only hypothesis).

ATLAS [6] CMS [7]

mH 125.5 ± 0.2(stat)+0.5−0.6(sys) 125.3 ± 0.4(stat) ± 0.5(sys)

µγ γ 1.55+0.33−0.28 0.77 ± 0.27

µZZ 1.43+0.40−0.25 0.92 ± 0.28

µww 0.98+0.31−0.28 0.68 ± 0.20

µbb 0.7 ± 0.7 1.15 ± 0.62

µττ 0.7 ± 0.7 1.10 ± 0.41

accumulate large integrated luminosities in the past 2 years asshown in figure 1.

Thanks to the corresponding data, many properties ofthe H0(125) particle have been studied, amongst which thesignal strengths µ for different fermions and vector bosonsfinal states, defined as the production rate for the finalstate normalized to the SM expectation. Within the presentprecisions, the measured values are compatible with thosepredicted for a SM Higgs particle (table 1).

Significant improvements are expected from theforthcoming LHC program. A first quantitative step will beachieved from 2015 on when the LHC will be operating at acenter-of-mass (c.m.) energy up to 14 TeV. Simultaneously,the instantaneous luminosity will gradually increase upto 2 × 1034 cm−2 s−1 enabling each ATLAS and CMS toaccumulate about 300 fb−1 by 2020. In parallel, a majorluminosity upgrade program (HL-LHC) is being carriedout and will be implemented around 2022–2023 with theobjective to accumulate up to 3000 fb−1 in 10–12 years.Such ameliorations will lead to significant improvementsconcerning the measurements of the Higgs properties asshown in tables 2 and 3 for some specific final states.

The anticipated higher energy and luminosity at HL-LHCwill enable many more measurements of the Higgs properties(e.g. the µµ, Zγ . . . decays as well as ttH and HHHcouplings). Although LHC can be labeled as the first Higgsfactory, one should not forget that it will also allow extensivesearches for new physics at the energy frontier to be carriedout.

2.2. Next generation of Higgs-factories beyond LHC

Despites the great LHC potential for improving themeasurement sensitivities and exploring the possible newphysics beyond the SM, it suffers from the fact that thecolliding particles are not point-like. As a consequencethe experimental environment is challenging and someparameters are difficult to measure in a model independentmanner. Thus several other possible accelerators are beinginvestigated. Besides the promising avenue of electron–positron colliders (Compact Linear Collider (CLIC) [10] andILC [11] for linear colliders and TLEP [12, 13] for circularcollider), γ γ and µ+µ− colliders [14] are considered. At eachof these facilities, Higgs particles are produced through cleanand well identified mechanisms (see figure 2) that enableprecise measurements of the Higgs properties, including thetotal H0 width, which is difficult to extract at LHC.

The H0 coupling to the electron is very small and thedirect Higgs production cross section is only around 1 fb ate+e− colliders operating at a c.m. energy corresponding tothe H0 mass. Although some very high luminosity colliderssuch as TLEP may be able to produce non-negligibleamount of H0 through e+e− annihilation, the unfavorablesignal over background ratio makes its detection difficult.However, at a c.m. energy around 250 GeV, H0 is producedin association with a Z0 vector boson with a cross sectionof about 200 fb, which can be further increased by 40%or so with achievable polarized beams. This process allowsone to tag the Higgs boson and infer its production bymeasuring the mass recoiling against the Z0 boson. Thetotal width as well as all partial widths, including theinvisible one, can thus be extracted in a model independentway. Theoretical uncertainties not being a limitation, themeasurement accuracies depend essentially only on howmuch integrated luminosity can be provided by the facilities.

Figure 3 shows the instantaneous luminosities expectedfor various projects. Integrated yearly luminosities can beobtained by multiplying these figures by a canonical 107 s peryear. The Higgs couplings to fermions and vector bosons canbe obtained with excellent accuracies as shown in table 4,which summarizes the uncertainties that are expected at thecurrently envisioned linear and circular colliders.

2

Phys. Scr. T158 (2013) 014016 R Aleksan

Table 2. The expected signal strengths µ for a Higgs boson with a mass of 125 GeV with 300 fb−1 accumulated at 14 TeV. The columnBRinv refers to the search for invisible Higgs decays and represents the 95% CL limit on the branching ratio. Different hypothesis have beenconsidered by the ATLAS [8] and CMS [9] studies. Hyph. a assumes that the systematic (experiment and theory) uncertainties areunchanged compared to the present ones. Hyph. b assumes that theory uncertainties are negligible. Hyph. c assumes that the experimentalsystematic uncertainties scale as the statistics while the ones from theory are halved.

Exper. Hyph. δµγ γ (%) δµZZ(%) δµww(%) µbb(%) µττ (%) BRinv(%)

ATLAS a 14 12 13 – 22 32b 9 6 8 – 16 23

CMS a 12 11 11 14 14 28c 6 7 6 11 8 17

Table 3. Identical to table 2 assuming a total integrated luminosity of 3000 fb−1. The ∗ in the ττ column indicates that only τ pairs producedvia the vector boson fusion process have been considered.

Exper. Hyph. δµγ γ (%) δµZZ(%) δµww(%) µbb(%) µττ (%) BRinv(%)

ATLAS a 10 10 9 – 19∗ 16b 4 4 5 – 12∗

CMS a 8 7 7 7 8 17c 4 4 4 5 5 6

Figure 2. Leading Feynman diagrams for Higgs boson production in e+e−, γ γ and µ+µ− collisions from left to right respectively.

Figure 3. Expected instantaneous luminosities as function of thec.m. energy at the proposed e+e− colliders. TLEP can accommodateseveral IR without significant luminosity loss per IR and the red lineindicates the TLEP luminosity summed over four interactionpoints.

At γ γ and µ+µ− colliders, Higgs bosons are produceddirectly with no accompanying particles. However onlymuon colliders enable the measurement of the H0 width byobserving its Breit–Wigner line shape, even though this latteris convoluted with the beam energy spread. In γ γ colliders,the photon beams are obtained through Compton backwardscattering of laser beams on primary high energy electrons(see figure 4). The energy of the back-scattered photons isabout 80% of that of the electron. Thus, Higgs particle can beproduced directly with two electron beams of about 80 GeVeach. However, since the c.m. energy spread is much largerthan the expected H0 width (∼4.2 MeV), it is not possible tomeasure this width in this way.

Table 4. Expected experimental uncertainties on Higgs couplingparameters. The values correspond to integrated luminositiesaccumulated over a period of about 7 years at the ILC and over 10years at TLEP.

HL-LHC ILC TLEPPhysical 3000 fb−1 exp−1 (250 + 350) 240 + 350quantity (%) (%) 4 IP (%)

10H/0H – 6.0 1.010inv/0H ∼7 2.9 0.451gHγ γ /gHγ γ 2.0 14.5 1.51gHgg/gHgg 3.0 4.4 0.81gHww/gHww 2.0 0.5 0.191gHZZ/gHZZ 2.0 0.9 0.151gHµµ/gHµµ <10 45 6.21gHττ/gHττ 2.0 2.9 0.541gHcc/gHcc ∼7 3.8 0.711gHbb/gHbb 4.0 2.4 0.42

Although γ γ and µµ colliders offer several advantagesfor Higgs studies, such as the operation at lower c.m. energy,there are serious technology challenges to overcome in orderto realize such facilities. For γ γ collider the development ofhigh-power high-repetition rate lasers and the development ofa practical interaction region and machine detector interfaceare outstanding issues, while for µµ colliders the rapid muoncooling in the six-dimensional phase space remains to bedemonstrated. This latter issue requires the development ofcooling channels in which series of high-gradient RF cavitieshave to operate in conjunction with very high-field (up to∼20 T) solenoid magnets, as is discussed later.

3

Phys. Scr. T158 (2013) 014016 R Aleksan

Table 5. Examples of experimental precisions, which are expected at future planned and proposed colliders.

Physical quantities Units LHC (3000 fb−1) ILC [11] TLEP [13] Assumption/caveat

1mW MeV ∼10 ∼7 <0.5 ∃ theory limitations1m t MeV 500–800 34 10 Current theory limit (100)1mH from EW fit GeV ∼20 +16.6 − 11.6 ∼10 Current theory errors1mH from EW fit GeV – +5.3 − 5.0 ∼1.4 Negligible theory errors1mH direct MeV ∼50 ∼35 ∼7Nν families n/a ±0.004 ±0.001 ∃ theory limitations

Figure 4. Schematic principle of γ γ colliders (left) and normalized Compton cross section as function of the scattered photon energy(right) for different circularly polarized laser photon (Pc = ±1) and electron polarizations λe. The rate of back-scattered photon at themaximum energy is reduced by a factor of 2 should electron polarization not be possible.

3. High precision electroweak test

In order to carry out precision measurements, twocharacteristics amongst those to be considered are to beunderlined:

(a) the ‘simplicity’ and ‘cleanliness’ of the initial state andthe interactions to be studied, i.e. the calculability of thephysical processes,

(b) the number of relevant events produced, i.e. acombination of the luminosity of the acceleratorand the interaction cross sections.

With regard to the former characteristic, it is generallyagreed that leptons (electrons, muons, neutrinos and theirantiparticles) are the probes of choice since they are point-likeobjects. However taking into account the requirement forhigh numbers of relevant events, which includes technologicalaspects, e+e− colliders offers the best potential, in particularif one needs to cover a relatively large range of c.m. energies.Past examples of such colliders are the LEP at CERN, whichhas enabled to study in detail the electroweak sector, andB-factories at KEK and SLAC, which have played a crucialrole in flavor and CP violation physics.

As an illustration, let us recall that the precisemeasurement of the properties of the Z and W bosons in the1990s led to the prediction of the top-quark mass, which wasdiscovered later at the Tevatron at FNAL. Similarly the verysame LEP data, with the addition of the improved W bosonand top-quark mass measurements at the Tevatron, have ledto the prediction of 94+25

−20 GeV [15] for the mass of the Higgsboson. The H0 mass, which has been recently measured at theLHC, is within 1.3 standard deviation of that prediction. This

alone highlights the importance of electroweak precision tests.From the ability of the proposed ILC and TLEP projects tooperate at the Z-pole, WW and t t thresholds one may estimatethe precisions that could be obtained for the Higgs boson massmeasurement and its SM prediction. Table 5 summarizes theerrors one would expect on the Higgs boson mass and someother important parameters.

The concept of linear collider avoids one of the maininherent problems with circular colliders, i.e. the synchrotronradiation. However, since the particle bunches are usedonly once, very small bunch sizes (σy < 8 nm) togetherwith a relatively large bunch charge (2 × 1010 particles perbunch) are needed to reach sufficiently high luminosity. Thiscombination leads to a strong influence of one beam on theother through the emission of significant γ -radiation at theinteraction point, known as the beamstrahlung effect.

This effect is much weaker at circular colliders as thebeam size is larger. Nevertheless, since the same bunch isused many times, the repetitive beam crossing induces alimitation in the beam lifetime [16] that depends on themomentum acceptance of the collider optics. The mitigationof this effect is obtained by using the top-up scheme pioneeredat B-factories, at which continuous refilling of the lostparticles is realized using the injection system. The mainremaining limitation is thus on the amount of overall powerthat can be afforded technically and economically since theenergy loss per turn scales as (E/me)

4 for a given bendingradius. For a given total radiation power, the maximumachievable luminosity at circular colliders drops rapidly as theenergy increases as show in figure 3. In contrast, the beamsize at linear colliders diminishes as the energy increases(σx,y ∝

√1/E) leading to higher achievable luminosity.

4

Phys. Scr. T158 (2013) 014016 R Aleksan

Table 6. Selected parameter list of proposed e+e− colliders at some specific operation c.m. energies.

Parameters Units ILC CLIC TLEP

c.m. energy GeV 250 350 500 500 1400 3000 91 240 350Luminosity/IP nb−1 7.5 1.0 1.8 2.3 3.7 5.9 560 48 13Number IP 1 1 1 1 1 1 4 4 4Length km 31 31 31 13.2 27.2 48.3 80 80 80Gradient MV m−1 31.5 31.5 31.5 80 100 100 20 20 20Rep. freq. Hz 5 5 5 50 50 50 CW CW CWN bunch 1312 1312 1312 354 312 312 4400 80 12e± /bunch 109 20 20 20 6.8 3.7 3.7 44 50 75Beam size x/y nm 729/8 683/6 474/6 200/3 60/1.5 40/1 ∼105/270 7 × 104/140 105/100Power MW 122 121 163 272 364 589 250 260 280

Facility Power (MW)

Ebeam

(GeV)Dist. (km)

Det. Vol (kt)

ESSnSB 5 2 360 500(a)

HyperK 0.75 30 295 560(a)

LBNO 0,77 400 2300 20(b)

LBNE-10 0.72 120 1290 10(b)

LBNE-PX 2.2 120 1290 34(b)

IDS-NF 4 10* 2000 100(c)

NuMAX 1 5* 1300 10(b)

Figure 5. Expected precision for the measurement of δ at present and future long-baseline ν-experiments. Results are shown as a functionof the fraction of possible values of δ measured with a given precision, defined as half of the confidence interval at 1 standard deviation(see [17, 18] for more detail). In the corresponding table, Ebeam refers to the energy of the proton (or muon when marked with ∗). The lettersin the column ‘Detector volume’ refers to detector technology: ‘a’ for water Cherenkov, ‘b’ for liquid argon, ‘c’ for magnetized iron andscintillators.

Table 6 shows some of the design parameters for the proposede+e− colliders.

While e+e− colliders are important tools enabling thestudy of a wide range of physics topics, the investigation ofneutrino properties requires dedicated ν-beams. Indeed, theobservation of mixing between the neutrino species and thelatest measurement of a relatively large mixing angle θ13 openup the possibility to observe CP violation in the lepton sector.This phenomenon would enable leptogenesis which in turncould explain the baryon asymmetry of the Universe. Theconventional technique to obtain intense ν-beams is basedon megawatt proton beams hitting a target. Large amountsof secondary particles are thus produced, amongst whichpions and kaons, which decay to neutrinos (predominantlyνµ). Several projects are studied around the world andtheir potential sensitivities for observing CP violation andmeasuring the CP-violating phase δ are shown in figure 5.Here, the main technical challenge resides in the developmentof multi-megawatt proton drivers and the correspondingtargets.

Such proton accelerators is the subject of intense R&Daround the world as they are useful in many areas ofscience (e.g. condensed matter, biology, chemistry) as well associetal applications, such as environment and safer energyproduction. One megawatt machines already exist (e.g. SNSin the USA or SINQ at PSI) and several other projects with

higher potential are either under study, in development orbeing constructed, such as the 5 mW linac for the EuropeanSpallation Source in Lund.

The pions and kaons produced from the high-power targetalso generate a large number of muons, which could becaptured an accumulated in a muon storage rings. In turn,these muons could be used to produce either intense νµ and νe

beams or to develop µ+µ− colliders (figure 6). In both cases,several major issues have to be overcome. Amongst those, thevery rapid acceleration of the muons to match the very shortµ lifetime and the 4(6)-dimension space–momentum coolingof the muons for satisfying the requirements of the NeutrinoFactory and the µ-collider represent outstanding challenges.

The concept of muon cooling is established since along time and is based on a series of sections composedof an ionization medium for energy loss followed by RFcavities for reacceleration. In addition high field solenoids areneeded in each section to maintain the muons focused in theforward direction. This method is subject to an active R&Dprogram within the MICE collaboration [19] but has not yetbeen demonstrated experimentally. Assuming that ionizationcooling is feasible at the required level, µ+µ− colliders canbe envisioned for a Higgs Factory or a multi-TeV leptoncollider facility. As examples, the main parameters of suchcolliders [14] are shown in table 7.

5

Phys. Scr. T158 (2013) 014016 R Aleksan

Figure 6. Schematic layouts of a neutrino factory (top) and a muon-collider (bottom).

Table 7. Selected parameter list of envisioned µ+µ− colliders atsome specific operation c.m. energies.

Parameters Units µ+µ− colliders

c.m. energy GeV 126 1500 3000Avg luminosity/IP nb−1 0.08 12,5 44Higgs per year (107 s) 13 500 37 500 200 000Number IP 1 2 2Length km 0.3 2.5 4.5Rep. freq. Hz 15 12 12N bunch 1 1 1µ± /bunch 109 4000 2000 2000Beam size x/y µm 75 6 3Proton driver power MW 4 4 4

4. Reaching very high energies

The discoveries of most of the fundamental particles havebeen achieved by exploring new energy domains using eitherelectron or proton probes. It is thus natural to investigatehow to access higher energies with these probes and reviewwhich technologies are the most promising. Many technicalissues are critical to reach higher energies. However, besidesthe size of the tunnel hosting the accelerator, two technicalparameters can be identified as key-drivers according the typeof accelerator (circular or linear) being considered:

(a) The maximum achievable magnetic field of the dipoles,which is the main limiting factor for circular pp colliders.

(b) The maximum achievable accelerating gradient of the RFsystem, which is the most important parameter for linearcolliders.

In the following we discuss briefly these two collideroptions and stress the main technology issues.

4.1. High field magnets

The maximum c.m. energy at pp collider relies on theachievable field within the accelerator dipoles. The highestfield in dipoles operating at particle accelerators is achievedwith superconducting magnets using NbTi conductorsreaching about 5 T. Such magnets have been used at HERAand are in operation at the LHC. Higher fields are expectedto be reached at the LHC as the currently used magnets willbe operated close to or higher than 8 T in 2015. The plannedluminosity upgrade of the LHC (HL-LHC) requires thedevelopment of even higher field (up to 13 T) magnets basedon a new conductor technology (Nb3Sn). This technologycould enable to increase further the maximum field (upto 15 T, see figure 7) but in order to reach 100 TeV c.m.collisions, a much larger ring than for the LHC will benecessary. The outcome of a pre-feasibility study is shown infigure 8 and a possible location of such a ring is visualized.

4.2. High gradient accelerating systems

Because of synchrotron radiation, it is generally agreedthat linear colliders are the preferred scheme to achievevery high energy in e+e− collisions, say above 500 GeV.The main limitation toward TeV or multi-TeV reach isdue to the maximum accelerating gradient that can berealized. The standard technology currently used relies onRF cavities and the largest total operating RF voltage in asingle accelerator was achieved already 25 years ago at theStanford Linear Collider (SLC). About 45 GV was realizedwith an average accelerating gradient of about 20 MV m−1

with S-band normal conducting structures. In limited sections,such as the initial acceleration of the positron bunches,40 MV m−1 gradients have been obtained at SLC. Since thensignificant progress has been made both using normal andsuperconducting RF systems.

6

Phys. Scr. T158 (2013) 014016 R Aleksan

Figure 7. Preliminary design for high field dipole using a combination of superconducting materials. Field up to 15 T can be obtained usinga combination of NbTi and Nb3Sn conductors. To reach 20 T, the use of high temperature superconductor is necessary.

Figure 8. The energy as function of the ring circumference (left) for different dipole fields. 100 TeV can be obtained either with dipolesoperating at 15 T in a 100 km ring or 20 T in 80 km. The figure in the right shows a possible location for an 80 km tunnel in the Geneva area.A pre-feasibility study has shown that the risks for such a tunnel may be manageable [20].

4.2.1. Superconducting accelerating structures. Theaccelerator with the until now largest number ofsuperconducting RF cavities was the LEP collider, inwhich some 280 multi-cell structures with a RF frequencyof 352.3 MHz have been deployed to operate the acceleratorat 104.5 GeV per beam. The total RF voltage was 3.6 GVand the average gradient achieved was about 7.5 MV m−1,while the maximum gradient approached 9 MV m−1. Toreach much higher beam energy at linear colliders whilekeeping a manageable accelerator size, much higher gradientare thus necessary. A very significant breakthrough hasbeen realized thanks to the R&D effort toward the ILC andgradients of 45 MV m−1 at 1.3 GHz have been reached in thebest cavities. Integrating the industrial constraint, an averageof 31.5 MV m−1 is planned for the production. Figure 9shows the actual yields of cavities reaching high acceleratinggradients. With such cavities, a c.m. energy up to 500 GeVcould be obtained with a 31 km accelerator. It could beenvisioned to increase even more the energy (up to 1 TeV) butfurther improvement of the gradient would be needed to limitthe size of the collider.

4.2.2. Normal conducting accelerating structures. Due tointrinsic limitations of bulk Niobium cavities, high frequencynormal conducting structures have been investigated in orderto reach even higher energies than what is practical atthe ILC. The CLIC project aims at 3 TeV collisions, with12 GHz RF structures in a 48 km tunnel. To achieve such achallenging objective, RF cavities operating with a gradientof 100 MV m−1 are necessary. Although the concept of CLIChas been laid out long ago, it is rather innovative comparedto the traditional way of accelerating beam by generating theRF power, mostly via klystrons. In CLIC the RF power isgenerated through the deceleration of a so-called low energydrive beam, the energy of which is extracted and transferredto the cavities of the main beam (figure 10). As mentionedabove another key feature of the CLIC design is the maximumaccelerating gradient that can be achieved in the cavities.This gradient has to be the highest possible for pulse lengthof 156 ns while keeping a low enough rate of electricalbreakdowns, i.e. sparks that can occur in the structure duringthe RF pulse, which can give undesired transverse kicks tothe beam. Because of the large number of RF structures(∼140 000) needed for a 3 TeV collider, it is estimated that

7

Phys. Scr. T158 (2013) 014016 R Aleksan

Figure 9. A 1.3 GHz superconducting niobium nine-cell cavity (left) being produced for the XFEL light source projects. Cavity yield(right) after up to two surface chemistry treatment as function of the year of production during the ILC R&D program. The blue (red) dotsrefer to gradients >28 MV m−1 (>35 MV m−1).

Figure 10. A schematic view (left) of the acceleration concept of the CLIC. The right diagram shows the actual accelerating gradientobtained with 12 GHz RF structures. The target breakdown rate of 3 × 10−7 m−1 perpulse was achieved with the latest prototype (TD24).

the probability of such an event should not exceed a level of3 × 10−7 m−1 perpulse.

4.2.3. Novel accelerating systems. To go significantlybeyond the present or envisioned accelerating gradients, novelmethods are mandatory. Many methods are studied to achievethis objective, amongst which plasma wakefield acceleration(PWA) seems to be a very promising avenue. The paradigm ofPWA embraces several methods but its concept resides in thegeneration of ultra-high electrical fields by ionizing a medium,for example He gaz. Electrical fields exceeding 100 GV m−1

can be obtained, however it is fair to say that in general thesegradients are so far been produced over rather limited lengths(few mm up to few cm). The medium can be ionized usingeither a drive particle beam (electrons [21, 22] or protons [23],see figure 11) or using a high power drive laser beam.

The accelerated beam can be obtained with two differentmethods that can be described in the following somewhatsimplified way. One uses a particle beam, which followsclosely the drive beam and gets accelerated in the wakefieldcreated by the latter. Alternatively, the electrons of theionized medium (expelled from a region surrounding the drivebeam) rush back in because of the restoring force of therelatively immobile plasma ions and get directly acceleratedwithin a special highly nonlinear ‘bubble’ regime [24]. Thislatter mode requires a laser pulse strong enough to blowout electrons to form an electron density ‘bubble’ which

Figure 11. Schematic view of beam driven wakefield acceleration.The proton beam (which can be also replaced by an electron beamor a laser beam) ionizes the medium and creates a high electricwakefield, which accelerates the witness beam.

is essential for producing collimated, quasi-monoenergeticelectron beams. The laser peak power P must be kept wellabove the critical power Pcr ≈ 2 × 10−7ω2

0/ne (GW), whereω0 is the laser frequency and ne is the plasma density. Forillustration, with a plasma density of 1017 e cm−3, a petawattlaser is required, which corresponds to lasers with a pulseenergy of 100 J and 100 fs duration.

Lasers able to reach such high power do exist; howevermany other parameters are essential to realize a collideruseful for particle physics. To name a few, the bunchcharge, the beam emittance, the energy spread and therepetition frequency are crucial ones. Although still not fullysatisfactory, most recent experiments [25] have shown thatthe former three are not out of reach. However a majorissue remains the repetition rate of the laser and its energy

8

Phys. Scr. T158 (2013) 014016 R Aleksan

Table 8. Summary of the accelerator systems used in the world for health, cancer therapy and industrial applications. The yearlyapproximate total market value of these accelerators is about 4 B$ (see [30] for the detail related to industrial accelerators). It should benoted that this table does not include all applications and therefore represents lower figures for the accelerator applications. For examplerelatively new applications such as Focused Ion Beams used in semiconductor industry and ion beam figuring used for preparing optical andnano-material surfaces are not listed.

System sold Approx.Total systems per year system price(2011) approx. Application approx. ($ M)

11 000 Cancer therapy 600 2.0–5.010 200 Ion implantation 600 1.5–2.57000 Electron cutting and welding 130 0.5–2.52600 Electron beam and x-ray irradiators 100 0.2–8.01000 Radioisotope production (incl. PET) 70 1.0–301500 Non-destructive testing (incl. security) 100 0.3–2.0250 Ion beam analysis (incl. AMS) 30 0.4–1.51500 Neutron generators (incl. sealed tubes) 70 0.1–3.035 050 Total 1700

efficiency. The petawatt lasers presently in operation (orplanned) have repetition rates of typically 1 shot per hourat full power where 10 kHz would be needed. Furthermorethe electrical wall-plug efficiency is very low, of the order of0.1%. Significantly better efficiencies (as high as 20–30%) incombination with a high repetition rate can be obtained withfiber laser but the pulse energy is very small. An interestingand promising solution has been recently proposed and testedby combining and synchronizing many fiber lasers [26].

Altogether, PWA using particle or laser beams as driversis a very interesting long term possibility but many obstacleshave to be surmounted and an ambitious and strong R&Dprogram has to be pursued.

5. A collaborative R&D effort for enabling largescale projects

Besides the technical challenges, which are discussedin previous sections, numerous other issues have to beovercome in order to realize the next generation of largescale accelerators. To name a few, one can mention thegeneration and preservation of ultra-low emittance, collisionsof nanometric beams, development and handling of newmaterials, efficient energy management, etc. A key and vitalelement in order to be in position to construct new ambitiousaccelerator projects is the ability to carry out a vibrantR&D program. Very significant resources are needed, whichare difficult to obtain in a single laboratory and even in asingle country. Therefore a large scale collaborative effort isrequired. Furthermore, the availability of, and the access to,large and costly R&D infrastructures are essential.

Several important initiatives in this direction have beencarried out or are being set up. Amongst those, one canidentify TTC [27], US-LARP [28] and TIARA [29]. Thelatter project aims at integrating national and internationalaccelerator R&D infrastructures into a single distributedaccelerator R&D facility. With such a facility, TIARAaims at establishing a sustainable framework for developingand supporting strong joint R&D programs in the fieldof accelerator Science and Technologies, for promotingeducation and training and for enhancing innovation incollaboration with industry.

6. Impacts of accelerator science and technologyfor society

Although particle accelerators have been developed forparticle physics and basic science in many differentresearch fields (e.g. nuclear, biological and chemical science,condensed matter and material science), they are also widelyused in the world for a broad range of societal, clinicaland industrial applications. As an illustration, table 8 showsa variety of application and the corresponding approximatenumbers of accelerators being used in the world for cancertherapy as well as some industrial applications.

Besides the applications above, particle acceleratorshave applications in cultural heritage, energy andenvironment, cargo scanning and security. Altogetherthe R&D in accelerator science and technology has astrong social–economic impact in society as is shown inthe Accelerator-for-Society Website [31] implemented byTIARA.

7. Conclusion

The past few years has seen extraordinary progresses inparticle physics, with the discovery of the Higgs boson asthe culminating point. These achievements have only beenpossible thanks to the development of very large scale particleaccelerators allowing the exploration of new territories. Theenergy and/or intensity frontiers could be surpassed thanks tothe ingenuity of scientists and to steady R&D activities.

However, still unresolved mysteries such as dark matter,baryon asymmetry of the Universe or the mass of the neutrinoscall for even more ambitious technological challenges. A newgeneration of large scale particle accelerators will thereforebe needed. Many new promising ideas enabling to go beyondboth the present energy and intensity frontiers by an order ofmagnitude have emerged and their technical feasibilities arebeing studied.

Several technical breakthroughs are necessary and theirsuccessful realization will depend on the ability of thescientific community to establish a strong and sustainableR&D program. In turn, these progresses will lead to majorimprovements for society in a broad range of areas, suchas health and medicine, energy and environment, culturalheritage, security and numerous industrial applications.

9

Phys. Scr. T158 (2013) 014016 R Aleksan

References

[1] Aad G et al (ATLAS Collaboration) 2012 Observation of anew particle in the search for the Standard Model Higgsboson with the ATLAS detector at the LHC Phys. Lett. B716 1–26

[2] Chatrchyan S et al (CMS Collaboration) 2012 Observation ofa new boson at a mass of 125 GeV with the CMSexperiment at the LHC Phys. Lett. B 716 30–61

[3] Englert F and Brout R 1964 Broken symmetry and the mass ofgauge vector mesons Phys. Rev. Lett. 13 321

[4] Higgs P W 1964 Broken symmetries, massless particles andgauge fields Phys. Lett. 12 132

[5] Higgs P W 1964 Broken symmetries and the masses of gaugebosons Phys. Rev. Lett. 13 508

[6] ATLAS Collaboration 2013 Combined coupling measurementsof the Higgs-like boson with the ATLAS detector using upto 25 fb−1 of proton–proton collision data ATLAS NoteATLAS-CONF-2013-034

[7] Chatrchyan S et al (CMS Collaboration) 2013 Observation ofa new boson with mass near 125 GeV in pp collisions at√

s = 7 and 8 TeV CERN-PH-EP/2013-035arXiv:1303.4571v2 [hep-ex]

[8] ATLAS Collaboration 2013 Projections for measurements ofHiggs boson cross sections, branching ratios and couplingparameters with the ATLAS detector at a HL-LHCATL-PHYS-PUB-2013-014

[9] CMS Collaboration 2013 Projected performance of anupgraded CMS detector at the LHC and HL-HLC:contribution to the snowmass process CMS NOTE-13-002arXiv:1307.7135v1 [hep-ex]

[10] Aicheler M et al (CLIC Collaboration) 2012 A multi-Tevlinear collider based on CLIC technology CLIC ConceptualDesign Report CERN-2012-007(http://project-clic-cdr.web.cern.ch/project-CLIC-CDR/CDR Volume1.pdf)

[11] International Linear Collider 2013 Technical Design Report(www.linearcollider.org/ILC/Publications/Technical-Design-Report)

[12] Koratzinos M et al 2013 TLEP: a high-performance circulare+e− collider to study the Higgs bosonTLEP-2013-001/CONF-ACC arXiv:1305.6498

[13] Bicer M et al 2013 First look at the physics case of TLEPTLEP-2013-005/PUB-GEN arXiv:1308.6176 [hep-ex]

[14] Delahaye J-P et al 2013 Enabling intensity and energyfrontier science with a muon accelerator facility

in the US, FERMILAB-CONF-13-307-APCarXiv:1308.0494

[15] Baak M et al 2012 The electroweak fit of the standard modelafter the discovery of a new boson at the LHC Eur. Phys.J. C 72 2205

[16] Telnov V I 2013 Limitation on the luminosity of e+e− storagerings due to beamstrahlung arXiv 1307.3915[physics.acc-ph]

[17] de Gouvea A et al 2013 Neutrinos arXiv:1310.4340v1 [hep-ex][18] Baussan E et al 2013 A very intense neutrino super beam

experiment for leptonic CP violation discovery based on theeuropean spallation source linac: a snowmass 2013 WhitePaper arXiv:1309.7022v2 [hep-ex]

[19] Bonesini M (on behalf of MICE Collaboration) 2013 Progressof the MICE experiment at RAL arXiv:1303.7363[physics.acc-ph]

[20] Osborne J et al 2012 Pre-feasibility study for an 80 km tunnelproject at CERN EDMS No: 1233485(https://indico.cern.ch/contributionDisplay.py?contribId=165&confId=175067)

[21] Blumenfeld I et al 2007 Energy doubling of 42 GeV electronsin a metre-scale plasma wakefield accelerator Nature445 741

[22] Hogan M J et al 2010 Plasma wakefield accelerationexperiments at FACET New J. Phys. 12 055030

[23] Caldwell A et al 2009 Proton-driven plasma-wakefieldacceleration Nature Phys. 5 363

[24] Pukhov A and Meyer-ter-Vehn J 2002 Laser wake fieldacceleration: the highly nonlinear broken-wave regimeAppl. Phys. B 74 355

[25] Wang X et al 2013 Quasi-monoenergetic laser-plasmaacceleration of electrons to 2 GeV Nature Commun.4 1988

[26] Mourou G et al 2013 The future is fibre acceleratorsNature Photon. 7 258

[27] TTC Collaboration TESLA Technology Collaboration(http://tesla-new.desy.de)

[28] US-LARP Collaboration US LHC Accelerator ResearchProgram

[29] TIARA Collaboration Test infrastructures and acceleratorresearch area (www.eu-tiara.eu/index.php)

[30] Hamm R W and Hamm M E 2012 Introduction to the BeamBusiness Industrial Accelerators and their Applications(Singapore: World Scientific)

[31] TIARA Collaboration Accelerators for society(www.accelerators-for-society.org/)

10