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Download New results of the MEG experiment: search for e with sensitivity to BR
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New results of the MEG experiment: search for m+e+g with sensitivity to BR 10-13 A. Baldini INFN Pisa: PANIC July 28th 2011 The Standard Model of particle interactions was explicitily built on the non observation of this decay (Lepton Flavour) Even with the introduction of the recently discovered neutrino masses SM predicts the non observability of me g (BR 10-54) m e g is an extremely powerfull test for New Physics models Lab. Year Upper limit Experiment or Auth. PSI 1977 < 1.0 10-9 A. Van der Schaaf et al. TRIUMF 1977 < 3.6 10-9 P. Depommier et al. LANL 1979 < 1.7 10-10 W.W. Kinnison et al. LANL 1986 < 4.9 10-11 Crystal Box LANL 1999 < 1.2 10-11 MEGA PSI ~2012 ~ 10-13 MEG meg predicted by several models ( mainly (but not only) SUSY GUTS) @ rates just below the MEGA results … MSSM with large tanb Just one recent example B physics constraints G.Isidori Signal and background signal meg background 4(5) Measured variables Physical: RMD e+ m+ g megnn n qeg = 180° Ee = Eg = 52.8 MeV Te = Tg e+ m+ n g 1/10 of the accidental one Accidental (Dominant) menn megnn ee g g eZ eZ g n n e+ m+ g We have to measure positrons and photons kinematic variables in the most precise way in order to reduce backgrounds Detector outline Liq. Xe Scintillation Detector Liq. Xe Scintillation Detector Thin Superconducting Coil g Stopping Target Muon Beam e+ g Timing Counter e+ Drift Chamber Drift Chamber 1m 1. Stopped pE5 beam of 3 107 m /sec in a 150 mm target 2. Solenoidal spectrometer & drift chambers for e+ momentum (DCH) 3. Scintillation counters for e+ timing (TC) 4. Liquid Xenon calorimeter for g detection (LXe) (scintillation) Part of One half of the collaborators... MEG Physics analysis PDF= Probabilty Distribution Function Signal Radiative Bkg Accidental Bkg Blind analysis 9 PDFs Michel positrons Mott scattering device double turn tracks g variables 0.3% + 4.4 g from Am/Be source, 9 MeV g from n capture (n generator) in Ni Linearity Straight line fit g e+ variables + Mott scattering device MEG Physics runs 2008: 3 months of data taking with low efficient Drift Chambers (DCH) system and varying xenon light yield as a function of (gaseous) purification Sensitivity = 1.3 x 10-11, 90%UL = 2.8 x 10-11 Published : NPB 834 (2010) 1 2009 and 2010 DCH system stable (HV distribution system problem corrected) + Xenon purified and stable. . 2010: twice the statistics of 2009 Sensitivity: average UL obtained by 100 Toy MC experiments generated with no signal 2009 and 2010 data sets Slightly worse DC noise Conditions Twice the statistics of 2009 data 2009 preliminary results presented at ICHEP 2010 Recent analysis improvements Better understanding of the spectrometer • alignement by using cosmic rays tracks and millipede algorithm (CMS) • relative spectrometer-target aligned by using reconstructed holes in the target • geometrical correlation between positron momentum and angular error taken into account Better evaluation of the relative alignement of spectrometer and calorimeter Usage of the background measured in the sidebands in the likelihood FC statistical method + profile likelihood for estimating the confidence intervals Relative alignement of g and e+ Spectrometer – target alignement Update of 2009 data Events in the signal region: 2009 updated 2009 likelihood analysis 0.17 <9.6x Sensitivity = 3.3 x 10-12 BR =0 probability = 8% Nsig best value = 3.4 2010 data Events in the side bands Events in the signal region 2010 data unblinded on July 5° 2011 2010 Sensitivity = 2.2 x 10-12 2009 +2010 Summary of analysis results 3.2 1.7 .96 MEG result Sensitivity = 1.6 x 10-12 Systematic errors included (2% effect on UL) Larger contributions from relatice angle offset, correlation in positron kinematical variables, normalization Profile likelihood curves not directly used for computing confidence intervals Summary • 8% probability statistical fluctuations happen! •The combined analysis of 2009 and 2010 data is consistent with a null result and gives an UL (90% CL) of 2.4 x 10-12 : a constraint 5 times better than the previous best one to the existence of meg • Paper on the arXive containing the results shown here: http://xxx.lanl.gov/abs/1107.5547 • 2011 data taking started (noise problem in DC cured) • MEG will continue to run this and next year to reach a sensitivity of few times x 10-13 Backup slides BR<1.5 x 10-11 90%CL
