MEG II 実験液体キセノンガンマ線検出器 における再構成法の開発 Development of the event reconstruction method for MEG II liquid xenon gamma-ray detector 小川真治、 他 MEG II 日本物理学会 第 70 回年次大会 1

Upgrade of LXe detector for MEG II Replace PMT of inner face to MPPC – Photon collection effeciency becomes uniform Change PMT alignment of lateral face – Energy leak decreases 2 MEG MEG II (CG) MEG MEG II wider log scale MEG MEG II Imaging power improves

Upgrade of LXe detector for MEG II Improvement of position and energy resolution was expected. 10% improvement of detection efficiency was also expected. These result was summarized in “MEG Upgrade Proposal (arXiv: ) ”. 3 depth ≧ 2cm 60 % of events depth ＜ 2cm 40 % of events σ up 2.4% ↓ 1.1% σ up 1.7% ↓ 1.0%

Event reconstruction study with MC We recalculated the resolution of LXe detector with the realistic settings. – Measured characteristics of final version MPPC are used for this study. – Effect of crosstalk and afterpulse of MPPC are included. 4 PDEGainCrosstalk Prob. Afterpulse Prob. Proposal17%0.5 ×10 6 not simulated This study27%0.8 × % 27% PDE 0.8M Gain 15% Crosstalk Prob.

MC simulation Waveform of each sensor for signal gamma is made in MC simulation. This study considers – Reflectivity at the surface of MPPC – Crosstalk, after pulse, saturation of MPPC (including the change of waveform) – Noises on the waveform (same level with MEG I) – Digitization of waveform (dynamic range, sampling frequency) This study does not consider – Effect of the pileup. – Non-uniformity of gain, PDE, crosstalk etc… – Degradation of statistical term of energy resolution which we observed for VUV. 5 gem4 bartender waveform analysis Event Reconstruction Event Reconstruction MEG II analyzer hit of scitilation photon 1p.e. waveform simulated waveform # of p.e. timing

Waveform analysis In waveform analysis, charge (# of p.e.) and timing are calcultaed. Charge is calculated from fixed integration range (w/o digital high-pass filter). Constant fraction method is used for timing calculation (with 25% of pulse height threshold) For timing calculation of MPPC, sum waveform of 16 adjacent channels are used. (As we can not extract timing precisely from low p.e. waveform) 6 constant fraction time integration range 200ns pulse height * 0.25 simulated waveform

Waveform analysis for over range channel As PDE and gain increases, 35% of event have at least one channel in which pulse height becomes higher than dynamic range of waveform digitizer (950mV). We can avoid over range by decreasing amp gain but it leads to worse SN ratio, and it may result in worse resolution. (Quantitative estimation has not done yet.) However, appropriate waveform analysis to these over range channel is important for reconstruction as they have large # of p.e. 7 Number of over range channel per event 35% event have over range channel some channels become over range Pulse height vs #ofp.e. black:MPPC red:PMT

Waveform analysis for over range channel For these over range channel, TOT (Time Over Threshold) method are used for charge calculation in MEG I. Same method can be used for MEG II. Relation between TOT and charge are calculated beforehand. In this study, over range channels are not used for timing calculation mV TOT simulated waveform (over range ch) (#of p.e. from wf analysis) /(#of p.e. of MC truth) Red:Over Range channel (Npe>5000) Blue:Normal channel (Npe~4000) DRS dynamic range 0.2% rms 4% rms

Position reconstruction Position is reconstructed by fitting # of p.e. distribution with the solid angle from conversion point to each MPPCs. We estimated position resolution by comparing with MC truth. 9 u v w γ u v w

Position resolution Improvement of position resolution for shallow event from MEG I can be seen as we expected in proposal. 10

Energy reconstruction Energy is reconstructed by the summation of # of p.e. from all channels taking into acount of different coverage for each channel. Calibration with solid angle for shallow event is not used in MEG II. Resolution becomes %. (a little better than proposal) 11 depthabs resrel res w<2cm327 keV0.62% w>2cm280 keV0.53% Reconstructed Eγ (0<w<2cm) Reconstructed Eγ (w>2cm) (MeV)

Timing reconstruction Timing is reconstructed by fitting the time at each sensor, taking into acount of TOF from conversion point. Calibrations of time walk effect and calibrairon with position are done. Preliminary result shows 71ps resolution (similar resolution with MEG I). Optimization and improvement of the analysis are on going. 12 Preliminary 71ps resolution t γ (rec) - t γ (MC)

Summary Resolution of MEG II LXe detector are calculated with realistic settings from measured characteristics of final version MPPC. For position and energy, same or a little better resolution is confirmed. For timing, preliminary result shows similar resolution with MEG I. 13 ResolutionMEG IMEG II (Proposal) MEG II (this study) u (mm) v (mm) w (mm)653.7 E γ (w<2cm)2.4%1.1%1.0% E γ (w>2cm)1.7%1.0%0.9% t γ (ps) % contribution are assumed (from unsolved difference between MC and real detector in MEG I)

Prospect Calculation of the resolution with different settings – Type of series connection (4-seg was assumed for this study) – Noise level – Amp gain Estimation of the effect of the pileup. More optimization and improvement of reconstruction method for MEG II will be done. 14

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Total nphe for signal gamma Total nphe becomes higher thanks to higher PDE. PDE:17% -> 27% nphe:1.25×10 5 nphe:1.85×10 5 Due to 1.6 times degradation for VUV, (stat. term)=0.23% (stat. term)=0.38% Small enough but may not negligible. 16 MC truth without CTAP,ST red:PMT blue:MPPC black:PMT+MPPC Total # of p.e. for one signal gamma

MC nphe Maximum number of nphe in one signal gamma event. Due to higher PDE, effect of saturation can become bigger. 17 DRS OR 25% of pixels 35% of event 13% of event

CTAP,saturation in MC CTAP,Saturation of MPPC are simulated in bartender 18

# of p.e. from waveform analysis We checked wheather # of p.e. is calculated properly or not by comparing waveform analysis result with MC truth. Charge is calculated properly for most channels (including over range channel). 19 (#of p.e. from wf analysis)/(#ofp.e. of MC truth) TOT is used black:MPPC red:PMT For some channel,calculation is not done propoerly. 4% rms # of p.e.

reco.u u(reco)-u(MC) 20

reco.v v(reco)-v(MC) 21

reco.w w(reco)-w(MC) 22

reco.pos pos(reco)-pos(MC) at all depth 23 u sigma :2.7mm v sigma :2.3mm w sigma :3.7mm

reco.E vs reco.w After the energy calibration as a function of w 24

Tres vs recow Worse timing resolution for shallow event. 25 0<w<1cm1<w<2cm 2<w<3cm 3<w<5cm 5<w<7cm7<w<10cm 10<w<20cm 84ps69ps 65ps69ps 86ps Preliminary

Eres vs CTAP Energy resolution with different CTAP 26 Old setting Degradation of Eres due to CTAP were observed with Eres from MC truth.