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MEG Experiment

MEG Experiment. m ->e g branching ratio. Present limit: 1.2x10 -11. LFV process Forbidden in the SM Sensitive to SUSY-GUT, SUSY-seesaw etc. Our goal : Br( m ->e g )>10 -13 ~10 -14. Clear 2-body kinematics. 52.8 MeV. 180 °. 52.8 MeV.

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MEG Experiment

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  1. MEG Experiment

  2. m->eg branching ratio Present limit: 1.2x10-11 • LFV process • Forbidden in the SM • Sensitive to SUSY-GUT, SUSY-seesaw etc. • Our goal : Br(m->eg)>10-13~10-14 Clear 2-body kinematics 52.8 MeV 180° 52.8 MeV Michel decay (μ＋→e＋νeνμ) + random γ Background Rate ~ 10-14 Radiative muon decay (μ＋→e＋νeνμγ) Background Rate < 10-14 Only allowed after KamLAND Motivation & Event Signature

  3. Xe Detector Approved in 1999, at Paul Scherrer Institut, in Switzerland COBRA 252cm Timing Counter Physics run in 2006 Initial goal at 10-13, finally to 10-14 m+ beam : World’s most intense DC Beam 108m+/s g detector : 800liter liquid xenon scintillation detector with 830 PMTs e+ detector: solenoidal magnetic spectrometer with a gradient magnetic field (COBRA) m+ 262cm DriftChamber MEG Experiment & Detector

  4. Experimental Hall • The most powerful machine in the world. • Proton energy: 590MeV • Nominal operation current: 1.8mA. • Max > 2.0mA possible. Paul Scherrer Institut

  5. Small Prototype

  6. Small Prototype • 32 2-inch PMTs surround the active volume of 2.34 liter • g-ray sources of Cr,Cs,Mn, and Y • asource for PMT calibration • Operating conditions • Cooling & liquefaction using liquid nitrogen • Pressure controlled • PMT operation of 1.0x106 gain • Proof-of-Principle Experiment • PMT works in liquid xenon? • Light yield estimation is correct? • Simple setup to simulate and easy to understand. S.Mihara et al. IEEE TNS 49:588-591, 2002

  7. Small PrototypeEnergy resolution • Results are compared with MC prediction. • Simulation of g int. and energy deposition : EGS4 • Simulation of the propagation of scint. Light EGS cut off energy : 1keV Rayleigh Scattering Length: 29cm Wph = 24eV

  8. Small PrototypePosition and Timing resolutions • PMTs are divided into two groups by the y-z plane • gint. positions are calculated in each group and then compared with each other. • Position resolution is estimated as sz1-z2/√2 • The time resolution is estimated by taking the difference between two groups. • Resolution improves as ～1/√Npe

  9. Large Prototype

  10. Large Prototype • 70 liter active volume (120 liter LXe in use) • 372x372x496 mm3: 17.3 R.L. in depth • Development of purification system for xenon • Total system check in a realistic operating condition: • Monitoring/controlling systems • Sensors, liquid N2 flow control, refrigerator operation, etc. • Components such as • Feedthrough,support structure for the PMTs, HV/signal connectors etc. • PMT long term operation at low temperature • Performance test using • 10, 20, 40MeV Compton g beam • 60MeV Electron beam • g from p0 decay

  11. TERAS g Beam • Compton Spectrum • (Eg-Ec/2)2+(Ec/2)2 • Electron beam (TERAS, Tsukuba in Japan) • Energy: 764MeV • Energy spread: 0.48%(sigma) • Divergence: <0.1mrad(sigma) • Beam size: 1.6mm(sigma) • Laser photon • Energy: 1.17e-6x4 eV (for 40MeV) • Energy spread: 2x10-5 (FWHM) • Divergence: unknown • Beam size: unknown Collimator size 10MeV 20MeV 40MeV

  12. Energy Spectrum Fitting • Suppose Compton Spectrum around the edge • (E-Ec/2)2+Ec2/4 • Detector Response Function • Gaussian with Exponential tail • f(x) = N*exp{t/s2(t/2-(x-x0)}, x<x0+t • N*exp{-1/2((x-x0)/s)2}, x>x0+t • Convolution • Integration +/- 5s • Principle… Eg Npe Convolution of Compton Spectrum Response Function sE~1.9%

  13. 1. TERAS Beam Test Inverse Compton g Energy : 10, 20, 40 MeV Incident Position : Detector Center Estimate Energy Resolution using Compton Edge 40MeV g Energy 40MeV g Vertex distribution 1.6% in s 3.8mm in s

  14. 175° 80MeV q Eg Eg p0 Energy (MeV) 55MeV 170° 155° Opening angle (deg) 54.9MeV 82.9MeV Energy (MeV) p0 Beam Test at PSI Concept p- (at rest) + p -> p0 + n, p0(28MeV/c) -> g + g • Requiring q>170o • FWHM = 1.3 MeV • Requiring q > 175o • FWHM = 0.3 MeV Opening angle selection of two g’s  monochromatic g 180°

  15. Beam Test Setup H2 target+degrader LYSO Eff ~14% NaI LP S1 Eff(S1xLP)~88% beam

  16. 115cm 115cm • p- (at rest) + p -> p0 + n, • p0(28MeV/c) -> g + g • monochromatic g calibration • of around 52.8MeV • p- + p -> n(8.9MeV) + g (129MeV) • linearity check & neutron response LP Xe 8x8NaI g n p-p->np0, p0->2g p- LP Xe total charge To get Energy Resolution Select p0 events Select NaI energy Select incident position in Xe detector Remove too shallow and too deep events p-p->ng NaI Energy (MeV) p0 Beam Test at PSI

  17. Energy Resolutions CEX 2004 55 MeV 83 MeV to Xe • = 1.23 ±0.09 % FWHM=4.8 % 55 MeV to Xe EXe[Nph] 83 MeV σ= 1.00±0.08 % FWHM=5.2%

  18. Energy Resolution vs Energy PSI 2003 TERAS 2003 alpha Right  is a nice function of gamma energy

  19. Position Reconstruction • Localized Weight Method • Projection to x and y directions. • Peak point and distribution spread • Position reconstruction using the selected PMT

  20. Examples of Reconstruction (40 MeV gamma beam w/ 1 mm collimator)

  21. Position ReconstructionResolution

  22. g Reconstruction of the event depth • Using event broadness on the inner face • Necessary to achieve good timing resolution

  23. g MC simulation Data D D Short labs Previous Test D= • D: depth parameter D: 20~100  0~25cm D Long labs This Test D D

  24. Energy Resolution vs. Depth Parameter • For g incident at the detector center • D > 35, 45, 55….85 • Resolution: < 2% in sigma except shallow events (D<45). D Number of Photoelectrons

  25. Nfpmt(0.5) • Another depth parameter: Nfpmt(0.5) • Sort front-face PMTs in the order of the distance to the x-y weighted mean position. • Sum up PMT output in that order. • Stop summing when the sum exceeds the half of Qsum[Front]. •  Number of PMTs used in the SUM. QPMT 1.0 Shallow Deep 0.5 NPMT

  26. Timing/Z Resolution p- • Improving Z resolution is essential to improve timing resolution. • Intrinsic timing resolution can be evaluated by comparing left and right parts of the detector. • <T> = (TLTR)/2 NaI g S1 Xenon g LYSO tLP - tLYSO TL Left Right TR g

  27. Absolute timing, Xe-LYSO analysis high gain normal gain 103 psec 110 psec 55 MeV Normal gain High gain A few cm in Z

  28. Timing Resolution <TL>-<TR> • Estimated using Electron Beam (60MeV) data • Resolution improves in proportion to 1/sqrt(Npe). • For 52.8 MeV g, s~60 psec + depth resolution. • QE improvement and wave-form analysis will help to achieve better resolution. (Visit “The DRS chip” by S.Ritt) s=75.6+/-2.0ps 45 MeV Energy deposit by 60 MeV electron injection s Timing Resolution (psec) 52.8MeV g (nsec) 104 4x104 Number of Photoelectron

  29. PMT Development

  30. Photomultiplier R&D Ichige et al. NIM A327(1993)144 • Photocathode • Bialkali :K-Cs-Sb, Rb-Cs-Sb • Rb-Cs-Sb has less steep increase of sheet resistance at low temperature • K-Cs-Sb has better sensitivity than Rb-Cs-Sb • Multialkali :+Na • Sheet resistance of Multialkali dose not change so much. • Difficult to make the photocathod, noisy • Dynode Structure • Compact • Possible to be used in magnetic field up to 100G • Metal channel  Uniformity is not excellent

  31. PMT (HAMAMATSU R6041Q) • Features • 2.5-mmt quartz window • Q.E.: 6% in LXe (TYP) (includes collection eff.) • Collection eff.: 79% (TYP) • 3-atm pressure proof • Gain: 106 (900V supplied TYP) • Metal Channel Dynodethin and compact • TTS: 750 psec (TYP) • Works stably within a fluctuation of 0.5 % at 165K 32 mm 57 mm

  32. PMT DevelopmentSummary

  33. Motivation • Under high rate background, PMT output (old Type PMT, R6041Q) reduced by 10-20%. • This output deterioration has a time constant (order of 10min.): Related to the characteristics of photocathode whose surface resistance increases at low temperature. • Rb-Sc-Sb + Mn layer used in R6041Q • Not easy to obtain “high” gain. Need more alkali for higher gain. • Larger fraction of alkali changes the characteristic of PC at low temp. So, New Type PMTs, R9288 (TB series) were tested under high rate background environment. • K-Sc-Sb + Al strip used in R9288 • Al strip, instead of Mn layer, to fit with the dynode pattern Confirmed stable output. ( Reported in last BVR) But slight reduction of output in very high rate BG Add more Al Strip Al Strip Pattern • Low surface resistance R9288 ZA series Yasuko HISAMATSU MEG VRVS Meeting @PSI June 2004

  34. Gas Purification

  35. He H2O CO/N2 O2 Xe CO2 Possible Contaminants • Residue Gas Analysis • Vacuum level • LP Chamber 2.0x10-2Pa • Analyzing section 2.0x10-3Pa H2O is the most dangerous for Xe scintillation light

  36. Gas return To purifier Circulation pump Purification System • Xenon extracted from the chamber is purified by passing through the getter. • Purified xenon is returned to the chamber and liquefied again. • Circulation speed 5-6cc/minute

  37. Gas return To purifier Circulation pump Purification System • Xenon extracted from the chamber is purified by passing through the getter. • Purified xenon is returned to the chamber and liquefied again. • Circulation speed 5-6cc/minute • Enomoto Micro Pump MX-808ST-S • 25 liter/m • Teflon, SUS

  38. Simulated data Measured/MC Before purification: ~10 ppm After purification: ~10 ppb How much water contamination?

  39. Purification Performance • Xenon Detector Large Prototype • 3 sets of Cosmic-ray trigger counters • 241Am alpha sources on the PMT holder • Stable detector operation for more than 1200 hours Cosmic-ray events a events

  40. Xenon Purifier • Attenuation of Sci light • Scintillation light emission from an excited molecule • Xe+Xe*Xe2*2Xe + hn • Attenuation • Rayleigh scattering lRay~30-45cm • Absorption by impurity

  41. Absorption Length • Fit the data with a function : A exp(-x/ labs) • labs >100cm (95% C.L) from comparison with MC. • CR data indicate that labs > 100cm has been achieved after purification.

  42. Liquid Phase Purification

  43. In ~10 hours, λabs ~ 5m Liquid-phase Purification Performance

  44. PET vs MEG • 511 keV vs 52.8 MeV • Smaller number of photo-electron • Shallower detector • Read out from back vs from front • Smaller light collection • Efficient detection • Small amount of material in front • Efficient fiducial volume • Mild requirement on resolutions

  45. The End

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