Liquid Xenon Detector for the MEG Experiment Toshiyuki

Liquid Xenon Detector for the MEG Experiment Toshiyuki Iwamoto 4/Oct/2006 10 th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD 06) Siena 4/Oct/2006 Siena 2006 1

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. Clear 2 -body kinematics Ee = E = 52. 8 Me. V Back to back (qe = 180°) Simultaneous (Te = T ) • The present experimental limit – ~ 1. 2 x 10 -11 by MEGA experiment 4/Oct/2006 Siena 2006 2

Sensitivity Background Radiative m decay : m+ e+ nm ne < 10 -14 Accidental overlap: m decay m e n n + random To reject these background, energy, timing and vertex resolution pile-up rejection from waveform analysis Single event sensitivity ~ 4 x 10 -14 Background event ~ 4 x 10 -14 90% C. L. sensitivity ~ 1 x 10 -13 MEGA photon detector requirement DE = 5% (FWHM) DT = 150 ps (FWHM) DX = 9 mm (FWHM) 4/Oct/2006 MEG Siena 2006 3

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

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

Photon Detector 800 liter liquid xenon, 846 2” PMTs in liquid High light yield, fast response and good uniformity (~40000 ph/Me. V, t=4. 2, 22, 45 ns) Low temperature (~160 K), l = 178 nm : development of PMT, refrigerator Xenon purity : remove H 2 O Performance test of Xe detector 2. 3 liter prototype confirmed the principle of Xe 68. 6 liter prototype 10, 20, and 40 Me. V inverse Compton scattering energy, timing and vertex resolution 55, 83 Me. V, 129 Me. V from p-p->np 0, n reaction energy, and timing resolution 4/Oct/2006 Siena 2006 6

Improvement PMT Q. E. : ~15% Photocathode : K-Cs-Sb Metal channel dynode, 12 stages Silica window (for UV light) Purification Gas & Liquid phase purification tested Metal getter (zirconium) by gas ~0. 5 l/h Molecular sieves by liquid ~100 l/h Compact, and low heat load Tolerance up to 100 G magnetic field Al strip to keep surface resistance at low temp. Zener diode for high rate background 1 m 57 mm 4/Oct/2006 2. 5 m Siena 2006 7

Prototype test, 10~40 Me. V 68. 6 l liquid xenon prototype detector 228 PMTs 40 Me. V g Energy s ~ 4 mm 1. 6% in s 4/Oct/2006 Siena 2006 Vertex distribution 8

LYSO Na. I pm p- (at rest) + p -> p 0 + n, p 0(28 Me. V/c) -> g + g (54. 9 Me. V<E <82. 9 Me. V) Almost monochromatic is available p- + p -> n + g (129 Me. V) Energy, timing resolution neutron response 4/Oct/2006 83 Me. V 175° 55 Me. V Opening angle (q) 170° phe Xenon detector Prototype LXe H 2 target Energy (Me. V) Prototype test, 55~129 Me. V Siena 2006 9 ENa. I + ELYSO (Me. V)

Energy distribution @ 55 Me. V s= 1. 23 ± 0. 09 % FWHM=4. 8 % Timing distribution Energy Resolution (s) [%] Photon detector performance 5% Energy resolution vs. Energy 1% 110 psec 110 - 64 (LYSO) - 61 (Beam) = 65 psec 4/Oct/2006 Siena 2006 Expected Performance Energy : 5% (FWHM) Timing : 150 psec (FWHM) Vertex : 9 mm (FWHM) 10

Detector operation 250 l gas xenon tank x 8 purifier Refrigerator 4/Oct/2006 Siena 2006 1000 l liquid xenon storage tank 11

Calibration method • Gain monitor : LED • Q. E. : 241 Am a source • Energy 3 x 3 Na. I array – 55, 83, and 129 Me. V from p-p p 0 n, p 0 2 – Ni thermal n capture 9 Me. V – Proton accelerator • Li(p, )Be 17. 6 Me. V • B(p, )C 16. 1 Me. V • Timing – Laser – radiative m decay – 55 Me. V from p-p p 0 n reaction (Pb+Scintillator) LH 2 target • Vertex – 55 Me. V from p-p p 0 n reaction (collimator) 4/Oct/2006 Siena 2006 12

Detector Construction 4/Oct/2006 Siena 2006 13

Detector Construction top outer side inner The performance of all PMTs checked in the liquid xenon before installation 4/Oct/2006 Siena 2006 14

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 15

Summary • MEG experiment will search for m->e 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 16