1. Upgrade of LXe gamma-ray detector in MEG experiment
Daisuke Kaneko, ICEPP, Univ. of Tokyo, on behalf of the MEG collaboration
Abstract
We are searching for the μ → e +γ decay in MEG experiment at Paul Scherrer Institute in Switzerland, and already updated the most stringent upper limit for the branching ratio. In order to achieve to higher sensitivity, we are planning to upgrade of the experiment, where an upgrade of the liquid xenon gamma-ray detector is included. It was turn out by simulation, that energy and position resolution will be significantly improved especially for events γ-ray convert at shallow part of detector. It is needed to develop MPPCs which are available in MEG detector, because conventional MPPC have little sensitivity to VUV range photon, from xenon scintillation.
μ → e +γ decay
MEG experiment
muon
beam
positron bent by
COBRA magnet
Stopping
target
μ → e +γ is a good probe for new physics,
because cLFV is suppressed in SM.
B ~ O (10-50)
Many theories such as SUSY-GUT predict
μ → e +γ in the reachable branching ratio.
B ~ O (10-12 ~ 15 )
Performed in PSI (Switzerland) in pursuit of most intense DC μ-beam
Signal is 52.8MeV γ-ray and 52.8MeV e+ emitted back-to-back.
γ-ray → Liquid Xenon Detector
positron → Drift Chamber (tracker) → Timing Counter
Main back-ground is accidental event, occasionally coincident γ& e+.
Therefore, good resolution is needed to achieve high sensitivity.
Latest physics result → H.Natori’s talk
Positron Spectrometer upgrade→ M.Nishimura’s poster
cLFV search has a long history, but no signal has been observed yet.
S. Antush et al. JHEP, 11:090, 2006.
Expectation of μ → e +γ branching ratio , from SUSY-seesaw model.
recent θ13 measurement
Achieved Upper Limit and Aiming Sensitivity Goal
2.4× → 6× → 6×10-14
Phys. Rev. Lett., 107:171801, 2011
Current UL (90% C.L.)
Goal of 1st stage of MEG
Aimed in next stage
← feature of COBRA magnet
e+ quickly swept out. (left) Radius independent from angle,(right)
Expected performance
LXe detector upgrade concept
Energy resolution is expected to　significantly be improved. →
Position resolutions improve especially event such that γ convert near to incident surface. ↓
Detection efficiency improve by about 10%.
Replace current 2” PMT with
・MPPC
・Smaller, square PMT
・Flat panel PMT
Modification in lateral
PMT arrangement
・Slant angle
　　　for better uniformity
・Wider inner face
　　　reduce energy leakage
PMT
MPPC γ
MPPC Package drawing
MPPCs on PCB
CG image
Cross-section of PCB
Arrangement for detector inner surface
Depth [cm]
position resolution [mm]
Horizontal
Red : PMT
Blue : MPPC
MPPC
(12mm)
Requirement for Upgrade
Commercial MPPCs don’t have sensitivity to xenon scintillation light (λ~178nm)
Developing with Hamamatsu
Result of UV sensitive MPPC Test
PDE(photon detection efficiency)
measurement
Gain is calculated from 1p.e peak charge, and corrected by removing effect of optical cross-talk & after-pulse.
charge
1 p.e.
2 p.e.
3 p.e.
pedestal
charge
If C.T or A.P occur, charge of 1 p.e. event become larger.
Testing Facility
↑ Flashing LED at very low intensity. Difference from poisson statistics is effect from C.T. & A.P .
xenon
chamber
refrigerator
gas xenon
recovery tank
control panel
getter
Test bench used in LXe
Sample MPPCs are mounted on board and immersed in liquid xenon. PMT, LEDs for calibration and Pt-100 RTDs also attached.
Temperature dependence
“F” type
“G” type
“H” type
Thermal noise could be reduced at low temperature.
Typically, dark count rate is few Hz in LXe temp:165K.
Shift of Break-down voltage and quench resistance by temperature are observed.
102kΩ
272kΩ
Xenon handling system
Resistance obtained by fitting I-V curve with MPPC model function.
3x3 mm
DAQ with DRS
We take waveform with waveform-digitizer DRS4, the same system as MEG experiment.
Currently, the most sensitive MPPC sample have ~10% PDE at nominal operation voltage.