the meg experiment: status and perspectives donato nicolò università di pisa & infn (on the...

The MEG experiment:status and perspectives

Donato NicolòUniversità di Pisa & INFN

(on the behalf of MEG collaboration)

Nufact 09Chicago, 20-25 July 2009

April 18, 2023The MEG experiment ... 2

Outlook• The MEG experiment• The Run 2008

– Data summary– Detector performance – Analysis strategy

• Perspectives for 2009– Improvements– Roadmap– Sensitivity plan

• Conclusions(see W.Marciano’s talks for theoretical issues)

The experiment

•Physics goal•Signature & background•Detector layout

Next goal

Current experimental limit

Physics goal•LFV induced by finite slepton mixing

through radiative corrections

R. Barbieri et al., Phys. Lett. B338(1994) 212

R. Barbieri et al., Nucl. Phys. B445(1995) 215

•Additional contribution to slepton mixing from V21 (the matrix element responsible for solar neutrino deficit) in MSSM models with right-handed Majorana neutrinos (see-saw)J. Hisano, N. Nomura, Phys. Rev. D59 (1999)

• Experimental hint: δaμ = 307∙10-11 (3.7σ)

tan()<3 excluded at 95% C.L. by combined LEP results (hep-ex/0107030)

Minimal SU(5) SUSY-SUGRA

pure SUSY effect (no SM contamination)

4The MEG experiment ...

Signal & background


Detector layoutThe detector

• Liquid Xenon calorimeter for detection (scintillation)

- fast (τ ~ 20÷40 ns)

- high light yield (70% NaI)

• Thin wall quasi-solenoidal spectrometer & drift chambers (X0=2∙10-3) for e+ momentum

• Scintillation counters for e+ timing

The PSI beam

The worldwide most intense DC beam (>108m/s)

of surface muons (28 MeV/c)

stopped on a thin target

Matter effects must beminimized in order not to spoil the resolution

April 18, 20236The MEG experiment ...

Run 2008 summary• Beam time & intensity

– from 12 September to 14 December (total time = 7∙106 s)

– μ-stop rate on target = 3.0∙107 s-1 (@Ip = 2 mA)

– overall live-time = 49%, dead-time due to• 17% DAQ• 16% beam shutdown• 11% detector calibration (see below)• 7% dedicated, low-intensity runs for Radiative decays

• Trigger & DAQ– signal event rate ~5 s-1

– additional pre-scaled triggers enabled for • detector monitoring• efficiency and background computation

– overall DAQ rate ~6.5 s-1

– max. 30 ev/s (limited by VME readout & Domino digitizer on-line calibration)


Detector performance

• monitoring & calibration• γ-energy & timing• e+-momentum & normalization• evidence of radiative decays

9

Calibration toolsLED

PMT Gain

Higher V with light att.

Can be repeated frequently

alpha

PMT QE & Att. L

Cold GXe

LXe

Laser

Laser

(rough) relative timing calib.

< 2~3 nsec< 2~3 nsec

(n,n Ni 9 MeV Nickel γ-line

NaIPolyethylene

0.25 cm Nickel plate

3 cm 20 cm

Neutron pulsed generator to induce (n,

(p,reactions

Li(p,)Be

LiF target at COBRA center

17.6MeV

~daily calib.

Can be used also for initial setup

KBi

TlF

Li(p, 0) at 17.6 MeV

Li(p, 1) at 14.6 MeV

- + p 0 + n

0 (55MeV, 83MeV)

- + p + n (129MeV)

10 days to scan all volume precisely

(faster scan possible with less points)

LH2 target

e+

e-

ee

Lower beam intensity < 107

Is necessary to reduce pile-ups

Better t, makes it possible to take data with higher beam intensity

A few days ~ 1 week to get enough statistics

MEG detectorMEG detectorstandardstandardcalibrationscalibrations

MEG detectorMEG detectorstandardstandardcalibrationscalibrations

0

TO BE IM

PLEMENTED

TO BE IM

PLEMENTED

TO B

E IM

PLEM

ENTE

D

TO B

E IM

PLEM

ENTE

D

April 18, 2023The MEG experiment ...


α-sources on wires• 241Am sources on =100m wires to

determine PMT QEs

monitor absorption length

Rα = 7 mm

Rα = 40 μm

GAS

LIQUID

The MEG experiment ...

(p,γ) reactions• Makes us of a Cockcroft-Walton accelerator to deliver

tunable-energy protons to a Li2B4O7 target– Li: high rate, higher energy photon– B: two (lower energy) time-coincident photons

Reaction Eres res -lines

Li(p,)Be 440 keV 5 mb (17.6, 14.6) MeV

B(p,)C 163 keV 2 10-1 mb (4.4, 11.7, 16.1) MeV

>16.1 MeV >11.7 MeV

4.4 MeV

ΔE/E = 8.5% (FWHM)

April 18, 202311


B(p,γ)C reaction• 2 simultaneous lines to exploit the (LXe-TC) coincidence

4.4 MeV

11.6 MeV

“Energy” deposit in TC

Ene

rgy

dep

osit

in L

Xe

tLXe - tTC

• Daily calibrations of LXe with α and (p,γ) on Li

• γ-events– LY smaller than expected (although improving during the Run)– τscint shorter than expected (measured ~30 ns, expected 50),

improving• α-events

– both in agreement with expectations N2 contamination?

liquid-phase purification gas-phase purification

Monitoring of LY


expected LYNpe = 30000

γ from pion-CEX


• liquid H-target to rate enhancement• beam polarity and settings to be changed as well

to be done quite seldom

γ energy & timing resolution


• not yet as expected but • also affected by pile-up (background level much higher than in normal μ-beam)• by unfolding pile-up distribution one obtains ΔE/E=5.1% (σR=1.8%)

γ-energy spectrum


LXe-alone energy spectrum exhibits no difference with expectations both in absolute rate and spectral shape detection efficiency and background are under control

DC operation in Run 2008• operated in He+ethane 50%/50%mixture and immersed in He-

atm• at turn-on (July 08) the system was fine 30/32 planes OK (@1850 V)

• HV deterioration observed during the Run; at the end– 11/32 planes OK (@1850 V)– 7/32 planes off-nominal voltage (1700÷1800 V)

• body of evidence for He-diffusion inside the HV distribution frame


RUN

24000

27000

29000

33000

35000

37500

40000

DC performance


The rate of events with a reconstructed track decreases with the Run going on absolute e+-efficiency getting lower and lower

e+-momentum resolution


obtained from a fit of the edge of Michel spectrum(with a slight dependence on the emission angle)

twice worse than expected

DC relative efficiency

18/04/23MEG normalization... 20

• no quality cut• quality cut

pe > 50 MeV/c

0.459.87εε

M

S

• Relative efficiency (i.e. fraction of signal/Michel events) is almost constant during the run (in spite of DC deterioration)• average ratio agrees with the expected fraction of e+ with p>50 MeV

it is possible to normalize the signal pdf by counting the number of Michel

MC

Radiative decays• The number of observed

events is compatible withestimated efficiencies

• also the angular distribution agrees with expectations

• also seen in normal data (with kinematical cuts applied)σ(Δteγ) = (159±9) ps

(extrapolated to 143 ps @52.8 MeV)

• contribution from tracking e+ time-of-flight uncertainty

• mainly limited by tracking


cos(θeγ

)

(Δteγ)

Analysis strategy• Decided to adopt a blind-box likelihood analysis strategy

– blinding observables are Eγ and Δteγ


pdfs:• signal: from detector response function

• accidental: from event distribution in data sidebands

• RD: from RD data distribution and trigger simulation (angular cut)

Plan for Run 2009

• Bug fixing & other improvements• Beam and livetime• Prospects

DC HV• DC dismounted and operated in pure He-atm

(“aquarium”)• replacement of the glue for HV cable protection• Fill HV vias in the PCB close to GND plane with araldite• Test of all DCs after mounting

• successful!April 18, 2023The MEG experiment ... 24

LXe up-to-date• LXe tank re-filled after several cycles of gas-phase

purification• at detector turn-on we found:

– LY(γ) improved by 30% w.r.t. end of Run2008 and in agreement with expectations

– same LY(α) as Run2008

– τγ /τα > 2 as expected N2 contamination removal


20082009 prospectssigma goal 2008* 2009

Eγ (%) 1.5% 1.9% < 1.7%

tγ (ps) 65 85 < 80

xγ (mm) 2÷4 5÷6 5

pe (%) 0.35 0.8 < 0.5

te (ps) 45 55 50

θe (mrad) 4.5 9÷18 10

decay vtx (mm)

0.9 3 < 2

Δteγ 80 145 100

background 0.1÷0.3 ? 0.6÷3


* analysis still going on, values may slightly change

Conclusions• Run 2008

– data suffering from detector instabilities– however we proved our sensitivity to μ→eγ (observation

of RD events in normal data taking)– data analysis close to the end, box to be opened in 1

week– plan to submit a paper by middle August

• Run 2009– problems with DC HV distribution have been fixed

• efficiency and resolution improved

– also LXe reached the optimal performance – other major improvements

• new 2Gs digitizer (DRS4), linear in its dynamic range• possibility to reduce both DAQ and calibration deadtime

• need to run in stable condition until 2011 to reach the goal


Backup slides

Pile-up rejection


•reconstruction of the main cluster•replacement of Npe for pile-up cluster with expected values

TC status• New fast electronics for the shaping of fiber-coupled

APDs– Stereo reconstruction of TC hit point– useful at both trigger and off-line stage

• Implementation of a Nd-laser– precise tool for LXe-TC timing


QE measurement


Obtained by comparison of measured vs. expected number of photoelectrons from each α-source


Xe + radiation

Xe* Xe+

Xe+ + Xe

Xe2+ + e-

Xe + Xe**

Xe** → Xe*

Xe* + Xe

Xe2*

2Xe + hv

Scintillation mechanism

XeXe XeXe

XeXe XeXe

ee ee

ee

excitation ionization

recombination

excimer

=175nm, 14 nm FWHM

excitation probability dE/dx


Features• Compact

– Z=54, ρ=2.95 g/cm3 (X0=2.7 cm), RM=4.1 cm @ T=165 K

• High light yield – LY=42000 phe/MeV ≈ 0.7 LY(NaI) for m.i.p.’s

• Fast– 1=4ns, 3=22ns, rec=45ns

• Particle ID– 2

– LY= 1.2 x LYmip

• n = 1.65 ( nquartz) good optical coupling with PMTs

• No self-absorption (λAbs=∞) position-independent energy response homogeneous calorimeter


Light absorption•Due to the presence of contaminants (mainly oxygen and water moisture in the VUV region)•measurements of optical properties available in the literature often contradictory

Absorption coefficients (λ-1) @ 1 ppm in LXe


Additional quenching factors

• Ionization quenching e-capture by electro-negative impurities (namely O2)(WARP collaboration, submitted to NIM A, and references therein)

• Non-radiative collitional reactions

Xe2* + N2 2 Xe + N2

1/t’j = 1/j + k[N2] (shorter decay-time)

A’j = Aj/(1 + j k[N2]) (lower light intensity)

used in LAr to shorten the long decay-time component(WARP collaboration, arXiv:0804.1217v1 [nucl-ex])

In both cases, quenching of scintillation light is expected,More significant in the case of lightly ionizing particles


E vs abs• Absorption R(E) position-dependent

• Energy resolution dominated by shower-fluctuations

E=52.8 MeV

MC

need to verify optical properties on a large-size prototype


The “Large Prototype”


Design•40 x 40 x 50 cm3, 100 l LXe

(same depth, 1/10 of the final volume)

the world-wide largest at that time

• Equipped with 240 PMTs

(HAMAMATSU R6041+R9288TB)

- K-Cs-Sb photocathode

- Quartz window (suited for VUV)

• Gas purification system

(getter+Oxysorb) to keep impurity

content < 1ppb


Absorption measurement•Use of -sources

Nphe vs distance

•Increase of both

light yield and

absorption length

observed during

the purification cycle

removal of water

•Use of -sources

Nphe vs distance

•Increase of both

light yield and

absorption length

observed during

the purification cycle

removal of water

λabs > 125 cm (68% CL) o λabs > 95 cm (95 % CL)


E @ signal window-p0n, 0 54.9 MeV < E< 82.9 MeV

E

170o

175o

170o FWHM = 1.3 MeV

175o FWHM = 0.3 MeV55 MeV in LXe

83 MeV in LXe


Performances

(E)/E =

4.8%

R=(1.230.09)%

(T) = 125

ps unprecedented at these energies!

TLXe – Tref (ns)

ELXe(MeV)


The final detector


Conceptual view•4, single-vessel detector consisting in

•800 l LXe

•viewed by 848 PMTs

•C-shaped, /4 = 9%

•located out of the

spectrometer field (B < 100 G)

•19 X0 depth (containment > 99%)

•0.4 X0 front material

(see R.Valle’s talk for further details on the overall detector)


Setting the detector up• Detector instrumentation

– completed in Aug 2007– all PMTs mounted– sources

• -wires • LEDs• Laser fibers

– sensors• PT100 temperature sensors• Surface level meter

• LXe transfer – completed by 20 September– 15 days needed (10 l/h speed)

• Purification system– Liquid-phase circulation– dedicated pump, 70 l/h – molecular sieves (>25 g water

absorption)


Absorption Length• Data/MC ratio vs

distance fitted with an exponential curve

• Slope compatible with no absorption

• Obtained on 25

Nov (after 180h

purification) > 3 m @95 % C.L.> 3 m @95 % C.L.


Li(p,γ)Be reaction• 1-morning data taking twice a week with a LiF target• Clear 17.6 MeV peak on the 14.8 MeV broad resonance• Improvement of light yield • Consistent with absorption length measurement with -

sources

Purification


Troubles in 2007 run• Light yield

– smaller than expected (~ ½) in the case of -events– in agreement with expectations in the case of -events

• Scintillation decay time– shorter in the case of -events– OK in the case of -events

possible contamination from O2 and/or N2 impurity

= 21 ns

= 34 ns


Performances in 2007 Run

• From 0-decay events

up = 2.4%FWHM = 6.5%

55MeV

t = 115ps

Intrinsic resolution on a 12-PMT sample


Solutions for 2008 run• Impurity removal

– use of a O2-getter cartridge– developed for LAr use at CERN– mounted at the outlet of the liquid-phase

purifier with by-pass valves– (in parallel) restoring of gas-phase

circulation through a Zr-getter• Avoid to use inner Nitrogen cooling pipe


Perspectives for 2008 run


Conclusions•LXe scintillation is an established technique for e.m. calorimetry

with unprecedented performances @ 50 MeV

•Validation by test on a 100l prototype

•Main concerns on the final detector experienced on 2007 Run

are related to O2, N2 contamination

•Standard solutions adopted to fix that problem

•We are confident to reach goal resolutions by 2008 Run


E @ low energy

• Event selection of environmental radioactivity (non-dedicated trigger) topological cuts to exclude -induced events

• Identified lines

• 208Tl (2.59±0.06) MeV

• 40K (1.42 ± 0.06) MeV

• Low energy calibration

40K (1.461 MeV)

208Tl (2.614 MeV)

looks like a NaI ...


E and background

Background suppression improvements of detector performances

(in particular for E)

signal window

-ratee+-energy energy back-to-back timing

in-flight e+-annihilation

radiative decay

)2(4

33.7ln2

222

tE

E

E

E

E

ER

N

NBR e

e

eaccacc

α-sources, a closer view


the meg experiment: status and perspectives donato nicolò università di pisa & infn (on the...

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