The Electromagnetic calorimeter of the MEG Experiment XIII International Conference on Calorimetry in High Energy Physics Pavia Italy, 26-30 May 2008 G. Gallucci, INFN Pisa

MEG Experiment sensitivity  10-13 @ 90% CL Reserch of rare muon decay m -> e + g with lepton flavour violation at Paul Scherrer Institut (Villigen, Switzerland) In the Standard Model BR(m -> e + g ) = 0 Leptonic flavour and number conservation Neutrino oscillation SUSYGUT theories SUSY SU(5) BR  10-14  10-13 SUSY SO(10) BR  10-12 10-11 BR (m -> e + g)  10-55 Impossible to measure MEG Experiment sensitivity  10-13 @ 90% CL

Event Signature and background Accidental contribution more important than correlated one A very good electromagnetic calorimeter y = Eg / mm x = Ee/ mm Bacc  Rm dx (dy)2 dw2eg dteg (ln(dy) + 7.33 )

Photon detector: calorimeter (1) Operated for 2 Months in 2007

Photon detector: calorimeter (2) Refrigerator 800 liters of Liquid Xenon (the biggest Xenon calorimeter in the world) External structure made in steel except front part in alluminum honeycomb and carbon fibers Internal PMTs supported structure made in alluminum and plastic (peek) for inner face 846 PMTs installed with photocatodic coverage  30% Solid angle coverage  10% HV Signals Cooling pipe Vacuum for thermal insulation Al Honeycomb window

Scintillation process: Liquid Xenon Density 2.95 g/cm3 Liquefaction temperature 165 °K Energy per scintillation photon 19.6 eV, 23.6 eV Radiative length 2.77 cm Decay time 4.2, 22, 45 nanosecond Wavelenght of emission peak 175 nanometer Rayleigh diffusion  40 cm Refractive index (on emission peak) 1.56 Scintillation process: 1) Xe* + Xe  Xe2*  2 Xe + hn 2) Xe+ + Xe  Xe2+ Xe2+ + e  Xe + Xe** Xe**  Xe* + heat  1) Ultraviolet scintillation light Absorpiotn lenght from H2O and O2 High number of scintillation photons ( 40k g/MeV) Fast time response High density, compact detector

Average Quantum efficiency PMTs R9869 Hamamatsu Photocatodic surface 96% K - Cs – Sb , 4% Al to reduce Photocatodic impedence at low temperature Compact structure with 12 amplification stages in order to operate into a low magnetic field Last two stages have a Zener dyode to stabilyze Voltage Average Quantum efficiency 15 %

Cryogenic tools Detector Gas phase-purifier High pressure storage LXe storage tank Liquid phase-purifier

Data Acquisition System Each PMT read from 2 types of waveform digitizer to reject pile-up and to subtract pedestal Trigger  100 MHz DRS  2 GHz Gamma Ray g g Alpha particle a a

Runs with different Led amplitude to compute PMTs gains (Nphe~1/s2). Leds and PMTs gains In order to monitor the calorimeter stability and response, Pmts characteristics (gains and QE): There are 36 leds mounted in 12 different positions with different attenuations. Runs with different Led amplitude to compute PMTs gains (Nphe~1/s2).

Alpha, QE and Absorption lenght 25 alpha sources 241Am on 5 wires. Absorption lenght evaluation Ratio Data/MC vs distance fitted with an exponential curve. Quantum Efficiency Evaluation in cold gas or Liquid Wire thickness  50 mm Alpha average path  40 mm Shadow effect (Rings) l > 3 m @95 % C.L.

Cockroft-Walton Accelerator Reaction Peak energy s peak g-lines Li(p,)Be 440 keV 5 mb (17.6, 14.6) MeV >16.1 MeV >11.7 MeV 4.4 MeV sE = 3.6 %

Charge exchange reaction p- + p  p0 + n p0 Decay p0  g + g m(p0)  135 MeV/c2 p(p0)  2.9 MeV/c 54.9 MeV < Eg < 83.9 MeV A p- beam against a LH2 target NaI Crystals “grid” It is possible to move tha NaI in different positions around f angle

Spectra of photons from muon radiative decay Red = computed from theory Preliminary Blue = measured from Michel runs Pile-up Preliminary Integrated spectra

Energy Resolution and Linearity Gamma Linearity Energy resolution on 54.9 MeV 52.8 MeV CW Measured in 0 runs Risolutions (FWHM) Gamma Energy (on 55 MeV) 4.8 % Gamma Position (mm) 15.0 Gamma Time (nanosec) 0.15

Position and Time Resolution s=64 psec Position Resolution (FWHM) Gp  15 millimeters Using Pb collimator holes and edges

Conclusions The Calorimeter performances are suitable for MEG experiment goals. The detector is now in the purification phase till the first week of June. After the purification, we will start to take physics data.