Research Summary
0. Keyword Highlight
Elementary particle physics
photo-sensor (SiPM) and photo-detector (pTC)
Calibration system (laser-based system and residual minimization)
β-ray irradiation test
Positron reconstruction (Kalman Filter technique, pattern recognition algorithm ...)
1. Standard Model and Particle Physics Experiment
Keyword: Elementary particle physics, standard model
Standard model is one of the most fundamental theory of physics, which well explains the matters and interactions origin with 17 elementary particles (12 fermions and 5 bosons). However, there are some experimental results which are not consistent with standard model e.g. neutrino oscillation. Standard model is not a perfect theory.
Fig. 1 Elementary particles in the standard model. Flavor violation (FV) can be occurred in quark / neutrino sector (with tiny neutrino mass). But in the charged lepton sector, FV has not been observed yet.
2. MEG II Experiment
Keyword: cLFV, μ -> eγ decay, new physics
MEG II experiment is the upgraded experiment of MEG (Mu to E + Gamma, μ -> eγ). We search for the new physics through muon decay. This decay, called charged Lepton Flavor Violation (cLFV), is practically prohibited in standard model, but predicted to occur within experimental reach ratio in many beyond standard model theories. Thus, to find cLFV means to find the new physics!
Achievement: [A-2]
3. MEG II pixelated Timing Counter (pTC)
Keyword: Photo-detector, photo-sensor, scintillation detector, timing resolution
To find the cLFV, we need to detect the positron and gamma ray from muon precisely. pTC measures the positron decay time at the accuracy of O(30 ps) by using its pixelated-structure. When charged particle like positron passes the scintillator material, the scintillation photons are emitted and we detect these photons with small size silicon photo-sensors (SiPM).
MEG II pTC is mainly made and operated by Japanese (Tokyo) and Italian (Pavia, Genova) team. We managed pTC production / operation / commissioning / data analysis and so on.
Achievement: [A-3,6][B-3,7,9][C-4]
Fig 2. MEG II Experiment detectors. Liquid Xenon detector determines the gamma-ray's timing and vertex. Positron track is caught by cylindrical drift chamber (DC, CDCH). Positron timing is determined by the pixelated timing counter (TC, pTC).
Fig 3. The photo-sensor array (left). 6 SiPMs are connected in series.These arrays are attached to the both side of a scintillator.
Counters which compose pTC (right). 512 counters are used for actual data taking.
4. Calibration
Keyword: pulse laser, residual minimization, Millepede-II (DESY)
To obtain the best performance from data, calibration is one of the most important step. We have to know all 512 of counter's time offset. To know the time offset, we have developed 2 complementary method; one is the "laser-based" method, the other is the "tracking-based" method.
Achievement: [A-5][B-6,8]
Fig. 4 Laser- calibration system from achievement [B-9] . By using the optical switch and the optical splitter, the pulse laser can be divided and illuminate the counters simultaneously. Time offset of each counter is monitored relative to the laser-synchronized pulse (SYNC signal).
Fig 5. Track-based calculation. We can get the measured time by analyzing the pTC data, and we can get the time-of-flight (TOF) by reconstructing the track inside TC. We define the chi2 as written in the picture, and then minimize it. The minimization calculation is done by using Millepede II (provided by DESY, www.desy.de/~kleinwrt/MP2).
Fig. 6 The comparison between laser-based calibration and track-based (residual minimization) calibration from achievement [B-9]. These 2 methods are complementary.
5. Radiation Damage Study
Keyword: β ray irradiation, timing measurement, GEANT 4
SiPM is used in a wide range of purposes, not only for particle physics experiment but also for medical equipments like PET (Positron Emission Tomography), etc.
But when particles are irradiated on the SiPM, the silicon part was damaged and the performance is degraded. This is known as radiation damage effect, and many studies have been done since it is one of the most crucial problem for experiment.
We studied radiation damage effect on scintillation detector with series connected SiPMs readout, especially focusing on timing, and succeeded to evaluate its effect.
We repeated 'irradiation step' and 'measurement step' 4 times. At each irradiation step we irradiated the β ray from 37MBq Sr90 sources for 70 hours. And then we measured the dark current and timing resolution with the dedicated setup. The total dose from the source is simulated and calculated by using the GEANT 4 simulation (https://geant4.web.cern.ch/).
Achievement: [A-4][B-1,4,5][C-1,3,6]
Fig. 6 Left: Alignment jigs for sensors. These are made by the 3-D printer with the help of Genova group.
Right: Timing measurement setup. After the β ray irradiation on 6 SiPMs, we measure the timing. The waveform digitizer (DRS, developed in PSI, https://www.psi.ch/en/drs) is used for data taking.
6. Development of Positron Reconstruction Algorithm
Keyword: Kalman filter, Pattern recognition algorithm
One of the biggest improvement of MEG II from MEG is the reconstruction efficiency of positron. To achieve the target sensitivity, we need to reconstruct positron with 2 times better efficiency under 2 times intense muon beam. This is very challenging but now we are trying to achieve the target value by using the detector information efficiently.
For track reconstruction, we mainly use Kalman filter technique. It is the efficient recursive algorithm and suitable to particle physics experiment. The tracking or track finding based on Kalman filter technique are often called 'local' method, since tracking starts from the certain local point (seed).
On the other hand, these days pattern recognition with machine learning technique becomes popular and popular. This method is often called 'global' method.
In MEG II experiment, the tracking strategy is 'local' method. The positron analysis team (mainly Italian, Russian and Japanese people work together) leads the development of the algorithm.
Achievement: [B-10, 11][C-5]
Fig. 7 The analysis flow of positron reconstruction (preliminary). Positron's track information is determined by tracking detector (cylindrical drift chamber, CDCH), and positron's timing is determined by pixelated timing counter (pTC, TC). In particle physics experiment, ROOT (https://root.cern.ch/, C++ based software toolkit, library) is widely used .
Fig 8. The positron reconstruction visualization under actual beam rate environment (7e7 μ/s). The yellow pixel shows the timing counter which succeeded hit reconstruction. The yellow circle shows the CDCH hits (drift circle) with the one stereo direction (The purple line shows the other stereo direction). If you have more interests, please check achievements [C-5].
Fig. 9 The track reconstruction inside the timing counter without CDCH information (TC Independent Track). In right picture, yellow circles show the TC hits, blue pixels show the TC counters. Blue projections show the forward propagation of Kalman filter, purple projections show backward propagation, red projections show the smoothed track. The kalman filter calculation and visualization is based on GENFIT (http://genfit.sourceforge.net/Main.html).
TC is not a tracking detector. We cannot get the 3-D position and 3-D momentum information. So in principal, track reconstruction without CDCH information is not possible. But we have developed and succeeded the track reconstruction - These results were reported in achievement [C-7].