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The MEG experiment: status and perspectives. Donato Nicolò Università di Pisa & INFN (on the behalf of MEG collaboration) Nufact 09 Chicago, 20-25 July 2009. Outlook. The MEG experiment The Run 2008 Data summary Detector performance Analysis strategy Perspectives for 2009 Improvements
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The MEG experiment:status and perspectives Donato Nicolò Università di Pisa & INFN (on the behalf of MEG collaboration) Nufact 09 Chicago, 20-25 July 2009
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 MEG experiment ...
The experiment Physics goal Signature & background Detector layout
Current experimental limit Next goal 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 toslepton mixingfrom 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(b)<3excluded at 95% C.L.by combined LEP results (hep-ex/0107030) Minimal SU(5) SUSY-SUGRA pure SUSY effect(no SM contamination) The MEG experiment ...
Signal & background The MEG experiment ...
Detector layout The PSI beam The worldwide most intenseDC beam (>108m/s) of surface muons (28 MeV/c) stopped on a thin target • The detector • Liquid Xenon calorimeter for detection (scintillation) • - fast (τ ~ 20÷40 ns) • - high light yield (70% NaI) • Thin wall quasi-solenoidalspectrometer & drift chambers (X0=2∙10-3) fore+ momentum • Scintillation counters for e+ timing Matter effects must be minimized in order not to spoil the resolution The 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) The MEG experiment ...
Detector performance monitoring & calibration γ-energy & timing e+-momentum & normalization evidence of radiative decays
LED Laser Lower beam intensity < 107 Is necessary to reduce pile-ups Better st, makes it possible to take data with higher beam intensity A few days ~ 1 week to get enough statistics g e m (rough) relative timing calib. < 2~3 nsec n n PMT Gain Higher V with light att. Can be repeated frequently p0 gg p- + p p0 + n p0 gg (55MeV, 83MeV) p- + p g + n (129MeV) 10 days to scan all volume precisely (faster scan possible with less points) LH2 target Laser alpha MEG detector standard calibrations PMT QE & Att. L Cold GXe LXe e+ g e- (n,g) on Ni (p, g) reactions Li(p,)Be LiF target at COBRA center 17.6MeV g ~daily calib. Can be used also for initial setup 9 MeV Nickelγ-line Neutron pulsed generator to induce (n, g) K Bi NaI Tl Li(p, 1) at 14.6 MeV F Polyethylene 20 cm 3 cm 0.25 cm Nickel plate Li(p, 0) at 17.6 MeV Calibration tools TO BE IMPLEMENTED TO BE IMPLEMENTED The MEG experiment ...
α-sources on wires • 241Am sources on =100mm wires to • determine PMT QEs • monitor absorption length GAS Rα= 7 mm LIQUID Rα= 40 μm The MEG experiment ...
(p,γ) reactions • Makes us of a Cockcroft-Walton accelerator to deliver tunable-energy protons to a Li2B4O7target • Li: high rate, higher energy photon • B: two (lower energy) time-coincident photons >11.7 MeV 4.4 MeV >16.1 MeV ΔE/E = 8.5% (FWHM) The MEG experiment ...
B(p,γ)C reaction • 2 simultaneous lines to exploit the (LXe-TC) coincidence 4.4 MeV Energy deposit in LXe 11.6 MeV “Energy” deposit in TC tLXe - tTC The MEG experiment ...
Monitoring of LY • Daily calibrations of LXe with αand(p,γ) on Li • γ-events • LY smaller than expected (although improving during the Run) • τscintshorter than expected (measured ~30 ns, expected 50), improving • α-events • both in agreement with expectations N2 contamination? expected LY Npe = 30000 liquid-phase purification gas-phase purification The MEG experiment ...
γ from pion-CEX • liquid H-target to rate enhancement • beam polarity and settings to be changed as well to be done quite seldom The MEG experiment ...
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%) The MEG experiment ...
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 The MEG experiment ...
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 • 11/32planes OK (@1850 V) • 7/32 planes off-nominal voltage (1700÷1800 V) • body of evidence for He-diffusion inside the HV distribution frame The MEG experiment ...
DC performance The rate of events with a reconstructed track decreases with the Run going on absolute e+-efficiency getting lower and lower The MEG experiment ...
e+-momentum resolution obtained from a fit of the edgeof Michel spectrum (with a slight dependence on the emission angle) twice worse than expected The MEG experiment ...
DC relative efficiency • 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 pdfby counting the number of Michel MC pe > 50 MeV/c • quality cut • no quality cut MEG normalization...
Radiative decays • The number of observed events is compatible with estimated 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) The MEG experiment ...
Analysis strategy • Decided to adopt a blind-box likelihood analysis strategy • blinding observables are EγandΔteγ • pdfs: • signal: from detector response function • accidental: from event distribution in data sidebands • RD: from RD data distribution and trigger simulation (angular cut) The MEG experiment ...
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 viasin the PCB close to GND plane with araldite • Test of all DCs after mounting • successful! The MEG experiment ...
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 The MEG experiment ...
20082009 prospects *analysis still going on, values may slightly change The MEG experiment ...
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 The MEG experiment ...
Pile-up rejection • reconstruction of the main cluster • replacement of Npe for pile-up cluster with expected values The MEG experiment ...
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 The MEG experiment ...
QE measurement Obtained by comparison of measured vs. expected number of photoelectrons from each α-source The MEG experiment ...
e Xe Xe e e Xe Xe Scintillation mechanism Xe + radiation Xe* Xe+ Xe+ + Xe Xe2+ + e- Xe+ Xe** Xe** → Xe* Xe* + Xe Xe2* 2Xe + hv excitation ionization excitation probability dE/dx recombination excimer l=175nm, 14 nm FWHM The MEG experiment ...
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 • t1=4ns, t3=22ns, trec=45ns • Particle ID • tg 2 ta • LYa= 1.2 x LYmip • n = 1.65 ( nquartz) good optical coupling with PMTs • No self-absorption (λAbs=∞) position-independent energy response homogeneous calorimeter a g The MEG experiment ...
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 The MEG experiment ...
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/tj + k[N2](shorter decay-time) A’j = Aj/(1 + tj 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 The MEG experiment ...
sE vs labs • Absorption R(E) position-dependent • Energyresolution dominatedby shower-fluctuations MC Eg=52.8 MeV need toverify optical properties on a large-size prototype The MEG experiment ...
The “Large Prototype” The MEG experiment ...
Design • 40 x 40 x 50 cm3, 100 lLXe • (same depth, 1/10 of the final volume) • the world-wide largest at that time • Equipped with240 PMTs • (HAMAMATSU R6041+R9288TB) • - K-Cs-Sb photocathode • - Quartz window (suited for VUV) • Gas purification system • (getter+Oxysorb) to keep impurity • content < 1ppb The MEG experiment ...
Absorption measurement • Use ofa-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) oλabs> 95 cm (95 % CL) The MEG experiment ...
Eg 170o 175o q sE @ signal window p-pp0n, p0 g g 54.9 MeV < E(g) < 82.9 MeV q>170oFWHM = 1.3 MeV q>175o FWHM = 0.3 MeV 83 MeV in LXe 55 MeV in LXe The MEG experiment ...
Performances sR=(1.230.09)% TLXe – Tref (ns) ELXe(MeV) s(Tg) = 125 ps s(Eg)/Eg = 4.8% unprecedented at these energies! The MEG experiment ...
The final detector The MEG experiment ...
Conceptual view • 4p, single-vesseldetector consisting in • 800 l LXe • viewed by 848 PMTs • C-shaped, W/4p = 9% • located out of the • spectrometer field (B < 100 G) • 19 X0depth (containment > 99%) • 0.4 X0front material • (see R.Valle’s talk for further details on the overall detector) The MEG experiment ...
Setting the detector up • Detector instrumentation • completed in Aug 2007 • all PMTs mounted • sources • a-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) The MEG experiment ...
Absorption Length • Data/MC ratio vs distance fitted with an exponential curve • Slope compatible with no absorption • Obtained on 25 Nov (after 180h purification) l> 3 m @95 % C.L. The MEG experiment ...
Purification 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 The MEG experiment ...
g a ta= 21 ns tg= 34 ns Troubles in 2007 run • Light yield • smaller than expected (~ ½) in the case of g-events • in agreement with expectationsin the case ofa-events • Scintillation decay time • shorterin the case of g-events • OK in the case ofa-events possible contamination from O2 and/or N2 impurity a The MEG experiment ...
Performances in 2007 Run • From p0-decay events Intrinsic resolution on a 12-PMT sample 55MeV g st = 115ps s up = 2.4% FWHM = 6.5% The MEG experiment ...
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 The MEG experiment ...
Perspectives for 2008 run The MEG experiment ...