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The MEG experiment: status and perspectives

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

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  1. 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

  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 MEG experiment ...

  3. The experiment Physics goal Signature & background Detector layout

  4. 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 ...

  5. Signal & background The MEG experiment ...

  6. 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 ...

  7. 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 ...

  8. Detector performance monitoring & calibration γ-energy & timing e+-momentum & normalization evidence of radiative decays

  9. 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 ...

  10. 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 ...

  11. (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 ...

  12. 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 ...

  13. 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 ...

  14. 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 ...

  15. 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 ...

  16. 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 ...

  17. 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 ...

  18. 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 ...

  19. 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 ...

  20. 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...

  21. 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 ...

  22. 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 ...

  23. Plan for Run 2009 Bug fixing & other improvements Beam and livetime Prospects

  24. 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 ...

  25. 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 ...

  26. 20082009 prospects *analysis still going on, values may slightly change The MEG experiment ...

  27. 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 ...

  28. Backup slides

  29. Pile-up rejection • reconstruction of the main cluster • replacement of Npe for pile-up cluster with expected values The MEG experiment ...

  30. 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 ...

  31. QE measurement Obtained by comparison of measured vs. expected number of photoelectrons from each α-source The MEG experiment ...

  32. 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 ...

  33. 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 ...

  34. 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 ...

  35. 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 ...

  36. 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 ...

  37. The “Large Prototype” The MEG experiment ...

  38. 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 ...

  39. 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 ...

  40. Eg 170o 175o q sE @ signal window p-pp0n, 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 ...

  41. Performances sR=(1.230.09)% TLXe – Tref (ns) ELXe(MeV) s(Tg) = 125 ps s(Eg)/Eg = 4.8% unprecedented at these energies! The MEG experiment ...

  42. The final detector The MEG experiment ...

  43. 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 ...

  44. 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 ...

  45. 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 ...

  46. 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 ...

  47. 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 ...

  48. 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 ...

  49. 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 ...

  50. Perspectives for 2008 run The MEG experiment ...

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