Liquid Xenon Detector for the MEG Experiment Toshiyuki Iwamoto 4/Oct/2006 10th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD06) Siena 4/Oct/2006 Siena 2006

MEG Physics Lepton flavor violating process already violated in neutrino sector Forbidden in the Standard Model Sensitive to the new physics models, SUSY GUT, or SUSY seesaw etc. The present experimental limit ~ 1.2x10-11 by MEGA experiment Clear 2-body kinematics Ee = Eg = 52.8 MeV Back to back (qeg = 180°) Simultaneous (Te = Tg) 4/Oct/2006 Siena 2006

Sensitivity Background Radiative m decay : m+  e+ nm ne g < 10-14 Accidental overlap: m decay m  e n n + random g To reject these background, energy, timing and vertex resolution pile-up rejection from waveform analysis Single event sensitivity ~ 4x10-14 Background event ~ 4x10-14 90% C.L. sensitivity ~ 1x10-13 photon detector requirement DEg = 5% (FWHM) DTg = 150ps (FWHM) DXg = 9mm (FWHM) MEGA MEG 4/Oct/2006 Siena 2006

MEG detector Approved in 1999 at PSI Physics run in 2007 4/Oct/2006 Siena 2006

MEG Detector 590 MeV proton Ring Cyclotron Drift chamber COBRA magnet w/ graded B field Most intense DC muon beam (108/s) H. Nishiguchi, 5th Oct “The MEG positron spectrometer” Timing counter Photon detector 4/Oct/2006 Siena 2006

Photon Detector 800 liter liquid xenon, 846 2” PMTs in liquid High light yield, fast response and good uniformity (~40000ph/MeV, t=4.2, 22, 45ns) Low temperature (~160K), l = 178nm : development of PMT, refrigerator Xenon purity : remove H2O Performance test of Xe detector 2.3 liter prototype confirmed the principle of Xe 68.6 liter prototype 10, 20, and 40 MeV inverse Compton scattering g energy, timing and vertex resolution 55, 83MeV, 129MeV g from p-p->np0,ng reaction energy, and timing resolution 4/Oct/2006 Siena 2006

Improvement Purification Gas & Liquid phase purification tested PMT Q.E. : ~15% Photocathode : K-Cs-Sb Metal channel dynode, 12 stages Silica window (for UV light) Compact, and low heat load Tolerance up to 100G magnetic field Al strip to keep surface resistance at low temp. Zener diode for high rate background Purification Gas & Liquid phase purification tested Metal getter (zirconium) by gas ~0.5l/h Molecular sieves by liquid ~100l/h 1m 57mm 2.5m 4/Oct/2006 Siena 2006

Prototype test, 10~40MeV g 68.6l liquid xenon prototype detector 228 PMTs 40MeV g Energy 1.6% in s s ~ 4mm Vertex distribution 4/Oct/2006 Siena 2006

Prototype test, 55~129MeV g H2 target Prototype LXe LYSO g g NaI p- Opening angle (q) Energy (MeV) 83MeV H2 target Prototype LXe LYSO g g 175° 170° NaI 55MeV p- m phe Xenon detector p- (at rest) + p -> p0 + n, p0(28MeV/c) -> g + g (54.9MeV<Eg<82.9MeV) Almost monochromatic g is available p- + p -> n + g (129MeV) Energy, timing resolution neutron response 4/Oct/2006 Siena 2006 ENaI + ELYSO (MeV)

Photon detector performance Energy distribution @ 55MeV 5% 1% Energy resolution vs. Energy = 1.23 ±0.09 % FWHM=4.8 % Energy Resolution (s) [%] Timing distribution 110 psec Expected Performance Energy : 5% (FWHM) Timing : 150 psec (FWHM) Vertex : 9mm (FWHM) 110 - 64 (LYSO) - 61 (Beam) = 65psec 4/Oct/2006 Siena 2006

Detector operation 250l gas xenon tank x 8 purifier Refrigerator 1000l liquid xenon storage tank 4/Oct/2006 Siena 2006

Calibration method Gain monitor : LED Q.E. : 241Am a source 3x3 Energy 55, 83, and 129 MeV g from p-pp0n, p02g Ni thermal n capture 9MeV Proton accelerator Li(p,)Be 17.6MeV B(p,)C 16.1MeV Timing Laser radiative m decay 55 MeV g from p-pp0n reaction (Pb+Scintillator) Vertex 55 MeV g from p-pp0n reaction (collimator) 3x3 NaI array LH2 target 4/Oct/2006 Siena 2006

Detector Construction 4/Oct/2006 Siena 2006

Detector Construction top outer side inner The performance of all PMTs checked in the liquid xenon before installation 4/Oct/2006 Siena 2006

Schedule Cryostat will come to PSI in the end of this year PMT installation into holders almost done Liquid xenon detector assembly will be done in the beginning of next year, and then liquefaction, purification and stable operation test will be continued. physics run starts in 2007 Positron spectrometer run and calibration run will be done in this year 4/Oct/2006 Siena 2006

Summary MEG experiment will search for m->eg with better sensitivity than previous experiment The performance of liquid xenon detector has been tested by using prototype, and the real detector is being constructed Physics run of MEG experiment will start in 2007. Positron spectrometer data and calibration data will be taken in this year. 4/Oct/2006 Siena 2006