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Search for m e g with the MEG experiment at PSI: results and prospects

Search for m e g with the MEG experiment at PSI: results and prospects. A.M. Baldini March 23rd for the MEG collaboration. Hep-ex:0908.2594v1 18 Aug 2009. Most recent m +  e + g Experiments (Hincks and Pontecorvo 1948). Two orders of magnitude improvement

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Search for m e g with the MEG experiment at PSI: results and prospects

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  1. Search for meg with the MEG experiment at PSI: results and prospects A.M. Baldini March 23rd for the MEG collaboration Hep-ex:0908.2594v1 18 Aug 2009

  2. Most recent m+  e+g Experiments (Hincks and Pontecorvo 1948) Two orders of magnitude improvement tough experimental challenge!

  3. e+ +g e+ +g n n n n e+ + Signal and background background signal eg accidental en n egn n ee  g g eZ  eZ g physical egn n qeg = 180° Ee= Eg=52.8MeV Te = Tg g

  4. The sensitivity is limited by theaccidental background The n. of acc. backg events (nacc.b.) depends quadratically on the muon rate and on how well we measure the experimental quantities: e-g relative timing and angle, positron and photon energy Effective BRback (nback/Rm T) Integral on the detector resolutions of the Michel and radiative decay spectra

  5. Required Performances BR (meg) 10-13 reachable BRacc.b. 2 10-14 and BRphys.b.  0.1 BRacc.b.with the following resolutions FWHM Need of a DC muon beam

  6. Experimental method • Detector outline • Stopped beam of 3 107 /sec in a 150 mm target • Solenoid spectrometer & drift chambers fore+ momentum • Scintillation counters for e+ timing • Liquid Xenon calorimeter for  detection (scintillation) • Method proposed in 1998: PSI-RR-99-05: 10-14 possibility • MEG proposal: september 2002: 10-13 goal: A. Baldini and T. Mori spokespersons: Italy, Japan, Switzerland, Russia

  7. Detectors responsibilities Switzerland Drift Chambers Beam Line DAQ Russia LXe Tests Beam line Italy e+ counter Trigger LXe Calorimeter Japan LXe Calorimeter,Spectrometer’s magnet USA(UCI) Calibrations/Target/DC pressure system

  8. APD Cooled Support TC Final Design APD F.E. Board Fibers (longitudinal position): mainly needed for trigger • A PLASTIC SUPPORT STRUCTURE ARRANGES THE SCINTILLATOR BARS AS REQUESTED • THE BARS ARE GLUED ONTO THE SUPPORT • INTERFACE ELEMENTS ARE GLUED ONTO THE BARS AND SUPPORT THE FIBRES • FIBRES ARE GLUED AS WELL • TEMPORARY ALUMINIUM BEAMS ARE USED TO HANDLE THE DETECTOR DURING INSTALLATION • PTFE SLIDERS WILL ENSURE A SMOOTH MOTION ALONG THE RAILS APD PM Muon beam direction Divider Board Main Support Scintillator Slab Scintillator Housing PM-Scintillator Coupler

  9. 800 l of Liquid Xe 848 PMT immersed in LXe Only scintillation light High luminosity Unsegmented volume H.V. Refrigerator Signals Cooling pipe Vacuum for thermal insulation Al Honeycomb Liq. Xe window PMT filler Plastic 1.5m The Liquid Xe calorimeter Experimental check In a Large Prototype

  10. The liquid xenon calorimeter

  11. m radiative decay Laser 20 cm 3 cm 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 alpha Xenon Calibration PMT QE & Att. L Cold GXe LXe e+ g e- Nickel g Generator Proton Acc Li(p,)Be LiF target at COBRA center 17.6MeV g ~daily calib. Can be used also for initial setup 9 MeV Nickelγ-line on off quelle K Bi NaI Illuminate Xe from the back Source (Cf) transferred by comp air  on/off Tl Li(p, 1) at 14.6 MeV F Polyethylene 0.25 cm Nickel plate Li(p, 0) at 17.6 MeV

  12. Calorimeter energy Resolution and uniformity at 55 MeV by means of Another (movable) detector (NaI ) is placed at 180° wrt the LXe calorimeter Energy resolution on the calorimer Entrance face sR = 1.5% FWHM = 4.6 %

  13. CW beam line

  14. LiF target LITHIUM  - spectrum + FLUORINE  - spectrum Automatic insertion/Extraction from the experiment center (target)

  15. First physics run in 2008 • First 3 months physics data taking (september-december 2008) • Xe LY increase • DCHs instability on part of the chambers after some months of operation: reduction of efficiency to 30% • APD: noise on DCHs turned off CW Calibration each three days during 2008 run

  16. RD RD RD RD RD RD 2008 run : 1014 muons stopped in target We also took RMD data once/week at reduced beam intensity Programmed beam shutdowns Air test in COBRA Cooling system repair

  17. 2008 data analysis: blind analysis: Eg vs Dtge window

  18. Sidebands are used to MEASURE accidental background distributions g Energy NO unwanted backgrounds Radiative decay + In flight positron annihilation + resolution + pileup: in agreement with MCs

  19. Radiative Muon decays (low photon energy)

  20. DCH resolutions from 2008 data Tracks with two turns in the spectrometer are used to detetrmine the Angular resolutions s(Df) =14 mrad s(f)=10 mrad The edge of Michel positrons used to determine momentum resolution score = 374 keV (60%) stail1 = 1.06 MeV (33%) stail2 = 2 MeV (7%) s(Dq) = 25 mrad s(q) = 18 mrad

  21. Probability distribution functions • Signal: from data except positron angular resolutions which is based on MC • Background: from sidebands (D timing flat) • Radiative decay: MC based on theoretical distributions + experimental resolutions • Analysis cuts

  22. Likelihood analysis: accidentals + radiative + signal PDFs to fit data + Feldman Cousins Best fit in the signal region Agreement of 3 different analyses

  23. Kinematical distributions with a different analysis

  24. Normalization: measured Michel eventssimultaneous with the normal MEG trigger dove: pre-scaling 107 • Independent of instantaneous beam rate • Nearly insensitive to positron acceptance and efficiency factors associated with DCH and TC

  25. 90% CL limit • 90 % C.L. NSig 14.6 corresponds to BR(m→eg)  2.8 x 10-11 • Computed sensitivity1.3 x 10-11 • Statistical fluctuation ~5% • From side bands analysis we expected 0.9 (left) and 2.1 (right) x 10-11 • Bad luck

  26. Xenon purification New (2009) custom liquid phase purification system : Oxysorb-like + “silent” pump (piston-type) • 50 cc / cycle, 60 rpm operation • 180 liter/h liquid circulation

  27. 2 months of data taking in 2009:

  28. Hit map 2008 2009 • Problem on DCHs  problem in HV distribution cards • All chambers repaired before start of 2009 beam time

  29. 2009 run • Smoother: LXe clean, DCHs working properly • Shorter run: another experiment (muonic atom Lamb shift) having good results • Transverse (fibers+APD) timing counter still missing: noise induced in DCHs • Preliminary DCHs resolutions though improved are not yet at the proposal level. Synchronization between different electronic channels measuring timing not yet at good level

  30. Prospects • 2 months of stable data taking at the end of 2009 • Improvement in sensitivity due to stable conditions: 6 * 10-12 for 2009 data (analysing now): ready this summer • Started running in stable conditions at the end of 2009: continue at least until 2012 (no competitor) • Data taking now paused due to accelerator maintenance will resume next month • Start thinking of possibile improvements/upgrades now Planning R & D Assembly Data Taking 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 http://meg.psi.ch More details at

  31. Present: 2009 analysis Likelihood analysis A.M. Baldini PSI February 17° 2010

  32. Pm (Y. Kuno et al., MEG TN1, 1997 and references) Det. 1 Det. 2 • For suitable geometry big  factors can be obtained • This is not the case for MEG (detailed calculations are necessary ) • In some theories (minimal SU(5) model) the positron has a definite helicity  Pm is less effective

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