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The MEG Experiment at PSI: a sensitive search for m e g decay

This paper outlines the MEG experiment, which aims to search for the e+e- decay signature and explore its connection with neutrino oscillations. It discusses the physics motivations, experimental setup, and the sensitivity of the experiment.

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The MEG Experiment at PSI: a sensitive search for m e g decay

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  1. The MEG Experiment at PSI: a sensitive searchform egdecay Fabrizio Cei INFN and University of Pisa XV Incontri sulla Fisica delle Alte Energie Lecce, 23-25 April 2003

  2. Physics motivations: SUSY indications; Connection with neutrino oscillations. The   e signature: Signal & Background. The experimental setup: The muon beam; The positron spectrometer; The timing counter; The Liquid Xenon e.m. calorimeter; Trigger & Electronics. Conclusions: Sensitivity; Time profile. Outline Fabrizio Cei

  3. Physics Motivations In theStandard Modelwith massive Dirac neutrinos Lepton Flavour Violation processes(asm  eg, t  eg, m  eee, m  e) are predicted atimmeasurably small levels. However,Super Symmetric Theories predict such processesatmuch more reasonable rates. Since the SM background is negligible, processes like m  egareclear evidences for Super Symmetry. Problem:are such rates experimentally observable? Fabrizio Cei

  4. SUSY Indications LFV processes especially sensitive toSSM grand unified theories (SUSY-GUT). LFV induced by finite slepton mixing through radiative corrections. • Some predictions: • SUSY SU(5) BR (m  eg)  10-14  10-13 • SUSY SO(10) BRSO(10) 100 BRSU(5) • (R. Barbieri et al., Phys. Lett. B338(1994) 212 • R. Barbieri et al.,Nucl. Phys. B445(1995) 215) Fabrizio Cei

  5. Experimental Bound Predictions of m  eg BR in SU(5) Models J. Hisano et al., Phys. Lett. B391 (1997) 341 Goal of MEG Experiment Small (< 10) tan b values are highly disfavoured by recent combined LEP data. (ALEPH, DELPHI, L3 & OPAL Collaborations, hep-ex/0107030) Fabrizio Cei

  6. Experimental bound Our goal Connection with n-oscillations Additional contributiontoslepton mixingfromV21, matrix element responsible for solar neutrino deficit. (J. Hisano & N. Nomura, Phys. Rev. D59 (1999) 116005) tan(b) = 30O tan(b) = 0O Largely favoured and confirmed by Kamland All solar n experiments combined Fabrizio Cei

  7. Previous m egSearches Other LFV searches The MEG experiment aims to gain two orders of magnitude in theupper limit (not a simple experimental challenge!). MEG Fabrizio Cei

  8. The MEG Collaboration INFN & Lecce UniversityG. Cataldi, C. Chiri, P. Creti, F. Grancagnolo, M. Panareo, S. Spagnolo INFN & Pisa UniversityA. Baldini*, C. Bemporad, F.Cei, M.Grassi, F. Morsani, D. Nicolo’, R. Pazzi, F. Raffaelli, F. Sergiampietri, G. Signorelli INFN & Pavia UniversityA.de Bari, P. Cattaneo, G. Cecchet, G. Nardo’, M. Rossella INFN & Genova UniversityS. Dussoni, F. Gatti, D. Pergolesi, R. Valle INFN Roma I D. Zanello ICEPP, University of TokyoT. Mashimo, S. Mihara, T. Mitsuhashi, T. Mori*, H. Nishiguchi, W. Ootani, K. Ozone, T. Saeki, R. Sawada, S. Yamashita KEK, TsukubaT. Haruyama, A. Maki, Y. Makida, A. Yamamoto, K. Yoshimura Osaka UniversityY. Kuno Waseda UniversityT. Doke, J. Kikuchi, H. Okada, S. Suzuki, K. Terasawa, M. Yamashita, T. Yoshimura PSI, VilligenJ. Egger, P. Kettle, M. Hildebrandt, S. Ritt Budker Institute, NovosibirskL.M. Barkov, A.A. Grebenuk, D.G. Grigoriev, B, Khazin, N.M. Ryskulov * Spokepersons Fabrizio Cei

  9. e+ +g e+ +g n n n n e+ + Signal and Background Background Signal eg “Accidental” enn egn n ee  gg eZ  eZg “Prompt” m egn n (muon radiative decay) qeg= 180° Ee= Eg=52.8 MeV te = tg g Fabrizio Cei

  10. Use a high-intensity muon beam and m-decay at rest; Measure g time, energy and angle by using afast & high-resolution e.m. calorimeter; Measure e+ momentum by using a high-resolution spectrometer; Measure e+ time by using fast counters (scintillator bars); Use a trigger scheme based on the e+-g coincidence. Experimental Strategy Fabrizio Cei

  11. A simulated event 52.8 MeV photon 52.8 MeV positron Fabrizio Cei

  12. Required Performances Experimental sensitivity limited by spill-in of background (especially accidental) into signal region high resolution measurements are needed. The accidental BR is To obtainBR (m  eg) 10-13we must haveBRacc 3•10-14and this requires: FWHM Fabrizio Cei

  13. Detector Building Switzerland Drift Chambers, Readout & DAQ Russia Manpower, Muons transport solenoid Italy e+ counter (Ge + Pv), Trigger (Pi, Le), Splitter (Le), LXe Calorimeter (Pi, Le) Japan LXe Calorimeter,Superconducting Solenoid Fabrizio Cei

  14. The most powerful machine in the world; Proton energy590 MeV; Nominal operation current1.8 mA The Paul Scherrer Institute Experimental Hall Fabrizio Cei

  15. Primary proton beam The Muon Beam • Itexists; • It providescontinuous >108 +/s(with e+ contamination) on 5x5 mm2; • Two separate configurationsof thepE5beam line(U & Z); • Muon momentum29 MeV/c. Fabrizio Cei

  16. Beam Line Tests Goal: optimize the beam elements in order to achieve: a) a good m/e separation mass selection device (Wien filter); b) a good coupling between beam and spectrometer solenoidal magnet; c) a high intensity of stopping muons in a spot  5x5 mm2  degrader to reduce the muon momentum before a 150 mm target. Fabrizio Cei

  17. Results • Rm(total) • Rm(after filter) • m/e separation • Spot size sV 6.5 mm, sH 5.5 mm(U) Fabrizio Cei

  18. Uniform field Gradient field Gradient field Uniform field The Positron Spectrometer COnstantBendingRAdius(COBRA) spectrometer • Constant bending radiusindependent of emission angles • High pT positronsquickly swept out Fabrizio Cei

  19. Magnetic Field Longitudinal Profile (R = 0) Radial Profile Needed < 50 G: OK) Fabrizio Cei

  20. Central coil Solenoids and Coils • Bc =1.26 Tcurrent =359 A; • Five coilswith three different diameters; • Compensation coils to suppress the stray field around the Liquid Xenon detector; • High-strength aluminium stabilized superconductor • thin magnet (1.46 cm Aluminium, 0.2 X0); • “Crash” Tests completed; • Winding completed @TOSHIBA; • Magnet readyto be shipped atPSIwithin this year. Fabrizio Cei

  21. Positron Tracker • 17 chamber sectors aligned radially with 10° intervals and 20 wires each; • Two staggered arrays of drift cells; • Chamber gas: He-C2H6 mixture(50/50) to reduce multiple scattering; • Vernier pattern made of 15 mm kapton foilsto measure z-positionby charge division. (X,Y) ~ 200 mm(drift time);(Z) ~ 300 mm;(t) ~ 10 ns Fabrizio Cei

  22. Full-size prototype @ PSI; test in November (cosmic muons & 90Sr source, magnetic field); Improved vernier strips structure (uniform resolution); Summary of Drift Chamber simulation. Drift Chambers R & D First results with a small-size prototype @Tokyo university (90Sr source, no magnetic field) Fabrizio Cei

  23. Positron Timing Counter BC404 scintillator • Two layers of scintillation counters read by PMTs at right angles with each other. • Outer: mainly timing measurementInner: mainly trigger information • Goal: timing resolution100 ps FWHM Fabrizio Cei

  24. Timing Counter R & D Timing resolution measurement: COsmic Ray TESt Facility (CORTES) 4 small counters + 8 MSGC + 1scintillator bar (1 cm thick) Measured resolution t improves as ~1/√Npe2 cm thick to get 100 ps FWHM resolution New design in progress; expected full simulation of geometryand light collection and prototype tests in October/November; final design and building in 2004. Fabrizio Cei

  25. 800 lof Liquid Xenon equipped with800 PMTs; Homogeneous detector; Onlyscintillationlight; Largelight yield (~ NaI). H.V. Refrigerator Signals Cooling pipe Vacuum for thermal insulation Al Honeycomb Liq. Xe window PMT filler Plastic 1.5m Liquid Xenon Calorimeter Liquid Xenon properties Experimental check Fabrizio Cei

  26. LXe Calorimeter Performances • Full MC simulation of Liquid Xenon behaviour and calorimeter response. • Positionresolution: • corresponding to: • angular resolution • (practically independent of light absorption length). • Z-coordinate resolution(depth of interaction point; important for timing • resolutionpreliminary measurements later): • Energy resolution strongly depends on optical properties of Liquid Xenon • reconstruction algorithmsneeded tocorrect for non homogeneities. Fabrizio Cei

  27. Energy Reconstruction FWHM(E)/E (%) FWHM  4.1 % Asymptotic value FWHM  2.5 % for labs labs = 100 cm Fabrizio Cei

  28. Liquid Xenon Calorimeter Prototype The Large Prototype(LP) • 40 x 40 x 50 cm3; • 228 PMTs, 100 litres Liquid Xenon (the largestin the World). • Purposes: • Test cryogenic operations on a long term and on a large volume; • Measure the Liquid Xenon properties; • Check the reconstruction methods; • Measure the Energy, Position and Timing resolutions with: • a) Cosmic rays; • b) -sources; • c) 60 MeV eˉ from KSR storage ring; • d) 40 MeV  from TERAS • Compton Backscattering; • e) e+and50 MeV from p° atPSI. Planned this year Fabrizio Cei

  29. -source LEDs The LP from “inside” FRONT face -sources and LEDs for PMTcalibration and monitoring Fabrizio Cei

  30. Measurement of LXe Optical Properties • First testsshowed a number ofscintillation photonsmuch lowerthan expected. • ItimprovedwithXenon purification: Oxysorb + gas getter + re-circulation(took time). • There was a strong absorptiondue to contaminants(mainly H2O, ~ 3 ppm). March 2002 Now labs> 1 m @ 90% C.L. Fabrizio Cei

  31. Improvement of labs with time We observed an improvement both in comparisonwithMC simulationandGXe data No Monte Carlo !! Increase with time Fabrizio Cei

  32. -trigger with 5106 gain; Geometrical cuts to exclude -sources; Energy scale: -source 208Tl (2.59±0.06) MeV 40K (1.42 ± 0.06) MeV Other lines ?? uniform on the front face; few 10 min (with non-dedicated trigger); nice calibrationfor low energy ’s. 40K (1.461 MeV) 208Tl (2.614 MeV) Radioactive Background in LP Never seen before ! Fabrizio Cei

  33. Timing Resolution Measurements our goal Dt = (Dtz2 + Dtsc2)1/2 = (802 + 602)1/2 ps = 100 ps (FWHM) DtzTime-jitter due tog interaction point (MC:) Dtsc Scintillation timeandphoton statistics Measurement of Dtsc2with60 MeV electron beam@ Kyoto Sincrotron Ring (N.B. Electron energy spectrum degraded by materials in front of the detector) 52.8MeV • weighted average of the PMT TDCs time-walk corrected; • Dtscvsnumber of photoelectrons; • extrapolation @ 52.8MeV is ok; • new PMT with improved QE:5 10 % 5 % 10 % Fabrizio Cei

  34. PMT characterization: Cryogenic Test Facility in Pisa • Full design and mechanical drawingscompleted; • Cryostat delivered at the beginning of april; • Orders made for dry UHV pumping group, leak detector, UHV components, cryogenic bottle, PMT’s …; some of them already received; • Laboratoryin preparation. Fabrizio Cei

  35. 2 boards LXe inner face (312 PMT) . . . . . . 10 boards 20 boards 1 board LXe lateral faces (488 PMT: 4:1 fan-in) 1 board 2 x 48 Type1 Type1 Type1 Type1 Type1 Type1 Type1 Type1 Type1 12 boards 2 boards 3 16 16 3 16 3 . . . Timing counters (160 PMT) Type2 Type2 Type2 Type2 Type2 Type2 2 VME 6U 1 VME 9U 2 x 48 4 x 48 20 x 48 12 x 48 10 x 48 Trigger • Based on simple quantities: •  energy (QSUM); • e+ -  coincidence • in time and direction; • LXe & Timing Counter • information (no DC). • Built on a FADC-FPGA • architecture; • More complex algorithms • implementable. Fabrizio Cei

  36. Trigger Performances • Beam rate108 s-1 • Fast LXeenergy sum > 45 MeV2103 s-1 g interaction point (PMT of max charge) e+ hit point in timing counter • time correlationg – e+200 s-1 • angular correlationg – e+20 s-1 Trigger Status • Design and simulation of type1 board completed; • Prototype boarddeliveredby late spring. Fabrizio Cei

  37. Readout Electronics • Waveform digitising for all channels; • Custom domino sampling chip designed @ PSI; • Cost per DSC ~ 1 US$; • 2.5 GHz sampling speed @ 40 ps timing resolution; • Sampling depth 1024 bins; • Readout similar to trigger. Prototypes delivered in autumn Fabrizio Cei

  38. Sensitivity Cuts @ 1.4 FWHM Fabrizio Cei

  39. Revised document Now LoI Proposal Planning R & D Assembly Data Taking 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Conclusions and Time Schedule • The experiment may provide a clean indication of New Physics (SUSY); • Measurements and detector simulation make us confident that we can reach a SES of 4 • 10-14 for meg ( BR  10-13); • Final prototypes will be ready and measured within November 2003: • Large Prototype for photon energy, position and timing resolutions; • Full scale Drift Chamber; • -Transport and degrader-target. • Experiment approved by INFN-CSN1 at beginning of April. http://meg.psi.ch http://meg.pi.infn.it http://meg.icepp.s.u-tokyo.ac.jp Tentative time schedule Fabrizio Cei

  40. Lxe Calorimeter Calibration • p-p  p0n • p0 (28 MeV/c)  g g • 54.9 MeV < E(g) < 82.9 MeV q Eg Requiring q > 170o FWHM = 1.3 MeV Requiring q > 175o FWHM = 0.3 MeV Eg p0 g Eg p- 170o 1.3 MeV for q>170o 0.3 MeV for q>175o 82.9 MeV 54.9 MeV g 175o • Am-Beg source4.43 MeV • p-p  g n • E(g) = 129.4 MeV q Eg Fabrizio Cei

  41. m  egnn Ee > 0.85; Eg > 0.8; qeg > 120o 150 ps 1 5 ~ 6 te-tg 150 ps 1 0.3 ~ 0.4 te-tg LXe Calorimeter Calibration (2) Crystal box PRD 38 (1988) 2077 108 m/s Accidental background R.Tribble’s talk at Univ. of Tokyo Oct. 1999 107 m/s Signal 1/10 Background <1/100 Accidental background Fabrizio Cei

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