Status and results of the MEG experiment search

Status and results of the MEG experiment: search for + e+ with sensitivity to BR 10 -13 A. Baldini INFN Pisa: Venezia: March 16 th 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 e (BR 10 -54) e 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

e predicted by several models ( mainly (but not only) SUSY GUTS) @ rates just below the MEGA results (Paradisi’s talk later) … MSSM with large tanb Just one recent example G. Isidori

Signal and background signal e background 4(5) Measured variables Physical: RMD e + + e nn n qe = 180° Ee = E = 52. 8 Me. V Te = T e + + n 1/10 of the accidental one Accidental (Dominant) enn e nn ee e. Z n n e + + We have to measure positrons and photons kinematic variables in the most precise way in order to reduce backgrounds

Detector outline reminder 1. Stopped p. E 5 beam of 3 107 /sec in a 150 m target 2. Solenoidal spectrometer & drift chambers for e+ momentum (DCH) 3. Scintillation counters for e+ timing (TC) 4. Liquid Xenon calorimeter for detection (LXe) (scintillation)

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: DCH system stable (HV distribution system problem corrected) + Xenon purified and stable. 2 months of data taken in stable conditions. Sensitivity: average UL obtained by 100 Toy MC experiments generated with no signal

2009 analysis

PDF= Probabilty Distribution Function

10 Michel positrons Mott scattering device double turn tracks

variables

+ 4. 4 from Am/Be source, 9 Me. V from n capture (n generator) in Ni

e+ variables + Mott scattering device

2009:

BR<1. 5 x 10 -11 90%CL

Latest on analysis improved to compute the background in the analisys box We plan to present a combined 2009+2010 analysis asap + Adopted a common FC + profile likelyhood analysis approach (PDG)

Relative alignement of and e+ ? ?

Delayed start: DCHs construction, MEG target accidentally broken Premature end: BTS solenoid magnet problem on beginning of november

Improvements for this year (2011) and later • TC counter now complete with fibers+APDs • Improved DAQ and Trigger efficiencies • New HV system for reducing noise of Drift Chambers system • New measurement of positrons resolutions with Mott scattering device • Radiative Decays for better e alignement • Improvements in E reconsruction by means of detailed understanding of light transmission in calorimeter • Improvements in magnetic field knowledge • beam intensity optimiztion • Increased positron acceptance

Summary • With the 2009 data MEG has established an upper limit (90% CL) on BR e of 1. 5 x 10 -11 with an estimated sensitivity of 6. 1 x 10 -12 • The analysis methods and systematics were recently improved. It was however decided not to present a re-analysis of 2009 data only but a combined analysis of 2009+2010 data in order to get a clearer picture of the origin of what we observed in 2009 • The expected sensitivity of 2009+2010 data is roughly 1. 5 x 10 -12 • 2011 data taking will soon start • Continue to run to reach the expected sensitivity of few times x 10 -13

Backup slides