The Electromagnetic calorimeter of the MEG Experiment m

The Electromagnetic calorimeter of the MEG Experiment m e+g XIII International Conference on Calorimetry in High Energy Physics Pavia Italy, 26 -30 May 2008 G. Gallucci, INFN Pisa 1

MEG Experiment • 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 BR (m -> e + g) 10 -55 Impossible to measure • SUSYGUT theories SUSY SU(5) BR 10 -14 10 -13 SUSY SO(10) BR 10 -12 10 -11 MEG Experiment sensitivity 10 -13 @ 90% CL 2

Event Signature and background Accidental contribution more important than correlated one y = Eg / mm Bacc Rm dx (dy)2 dw 2 eg dteg (ln(dy) + 7. 33 ) x = Ee/ 3 mm A very good electromagnetic calorimeter

Photon detector: calorimeter (1) Operated for 2 Months in 2007 4

Photon detector: calorimeter (2) 800 liters of Liquid Xenon Refrigerator HV Signals Cooling pipe Vacuum for thermal insulation Al Honeycomb window (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% 5

Liquid Xenon Density Liquefaction temperature Energy per scintillation photon Radiative length Decay time Wavelenght of emission peak Rayleigh diffusion Refractive index (on emission peak) 2. 95 g/cm 3 165 °K 19. 6 e. V, 23. 6 e. V 2. 77 cm 4. 2, 22, 45 nanosecond 175 nanometer 40 cm 1. 56 Scintillation process: 1) Xe* + Xe 2* 2 Xe + hn 2) Xe+ + Xe 2+ + e Xe + Xe** Xe* + heat 1) 1. High number of scintillation photons ( 40 k g/Me. V) 2. Fast time response 3. High density, compact detector 1. Ultraviolet scintillation light 2. Absorpiotn lenght from H 2 O and O 2 6

PMTs • R 9869 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 % 7

Cryogenic tools Gas phase-purifier High pressure storage LXe storage tank Detector Liquid phase-purifier 8

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 g a Gamma Ray Alpha particle g a 9

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/s 2). 10

Alpha, QE and Absorption lenght 25 alpha sources 241 Am on 5 wires. Absorption lenght evaluation Quantum Efficiency Ratio Data/MC vs distance fitted with an exponential curve. Evaluation in cold gas or Liquid Wire thickness 50 mm Alpha average path 40 mm Shadow effect (Rings) l > 3 m @95 % 11 C. L.

Cockroft-Walton Accelerator Cockroft-Walton accelerator Reaction Peak energy s peak g-lines Li(p, )Be 440 ke. V 5 mb (17. 6, 14. 6) Me. V >16. 1 Me. V >11. 7 Me. V 4. 4 Me. V s. E = 3. 6 % 12

Charge exchange reaction p 0 Decay - + p 0 + n 0 g + g m( 0) 135 Me. V/c 2 p( 0) 2. 9 Me. V/c 54. 9 Me. V < Eg < 83. 9 Me. V A - beam against a LH 2 target Na. I Crystals “grid” 13 It is possible to move tha Na. I in different positions around f angle

Spectra of photons from muon radiative decay Red = computed from theory y ar in lim e Pr im el Pr Pile-up Blue = measured from Michel runs y ar in Integrated spectra 14

Energy Resolution and Linearity Gamma Linearity Energy resolution on 54. 9 Me. V 52. 8 Me. V CW Measured in 0 runs Risolutions (FWHM) Gamma Energy (on 55 Me. V) 4. 8 % Gamma Position (mm) 15. 0 Gamma Time (nanosec) 0. 15 15

Position and Time Resolution s=64 psec Time resolution Position Resolution (FWHM) Gp 15 millimeters Using Pb collimator holes and edges 16

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. 17