The analysis of a combined data set, totaling 3.6 × 10(14) stopped muons on target, in the search for the lepton flavor violating decay μ(+) → e(+)γ is presented. The data collected by the MEG ...experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and yield a new upper limit on the branching ratio of this decay of 5.7 × 10(-13) (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.
We present a new result based on an analysis of the data collected by the MEG detector at the Paul Scherrer Institut in 2009 and 2010, in search of the lepton-flavor-violating decay μ(+)e(+)γ. The ...likelihood analysis of the combined data sample, which corresponds to a total of 1.8×10(14) muon decays, gives a 90% C.L. upper limit of 2.4×10(-12) on the branching ratio of the μ(+)→e(+)γ decay, constituting the most stringent limit on the existence of this decay to date.
A new timing detector measuring ∼50MeV/c positrons is under development for the MEG II experiment, aiming at a time resolution σt∼30ps. The resolution is expected to be achieved by measuring each ...positron time with multiple counters made of plastic scintillator readout by silicon photomultipliers (SiPMs). The purpose of this work is to demonstrate the time resolution for ∼50MeV/c positrons using prototype counters. Counters with dimensions of 90×40×5mm3 readout by six SiPMs (three on each 40×5mm2 plane) were built with SiPMs from Hamamatsu Photonics and AdvanSiD and tested in a positron beam at the DAΦNE Beam Test Facility. The time resolution was found to improve nearly as the square root of the number of counter hits. A time resolution σt=26.2±1.3ps was obtained with eight counters with Hamamatsu SiPMs. These results suggest that the design resolution is achievable in the MEG II experiment.
The saturation of SiPMs is a potential issue for scintillator calorimeters with SiPM readout. When a large number of particles hit a scintillator, the SiPM output can be saturated due to the limited ...number of pixels. In order to convert the SiPM output into the number of incident photons correctly, it is necessary to understand the behavior of the SiPM saturation. The saturation curve is usually measured by directly injecting a fast laser pulse (∼400 nm) to a SiPM . However, the effect of the time constant of the scintillation light emission, which is not negligible compared to the recovery time of the SiPM pixel, is not included in this method. We propose a new method to measure the SiPM saturation with scintillation light excited by an UV laser. The measured saturation curve can directly be used for the saturation correction of the calorimeter response, since the effect of the cell recovery during the scintillation emission is included. A fast fs UV pulse laser with a wavelength of 190 nm is used. The 190 nm laser causes scintillation excitation, whereas it is invisible to the SiPM . The laser is injected to a plastic scintillator coupled to a SiPM . For comparison, the same test is carried out also by injecting a fast laser pulse with a wavelength of 470 nm. The saturation curve is obtained for a wide range of Npe, and a large over-saturation is observed for the 190 nm injection, which is significantly larger than the 470 nm injection. This difference is the effect of the time constant of the scintillation emission, and can have a big impact on the saturation correction.
The electro-magnetic calorimeter concept using plastic scintillator (Sc-ECAL) is being developed for detectors at future electron-positron colliders such as ILC . The Sc-ECAL is based on 5×45×2mm3 ...plastic scintillator strips readout by SiPMs. This can realize a virtual 5×5mm2 cell segmentation which is required for particle flow calorimetry at ILC detectors. However, there are some issues such as signal/noise ratio and ghost hits. A scintillator strip design based on double SiPM readout is being developed where a twice longer strip (5×90×2mm3) is read out by two SiPMs, one at each end. The performance of Sc-ECAL can be improved in various aspects by this method. The performance of the double SiPM readout was tested with a prototype setup. The position dependence of the light yield for a 90 mm-long scintillator strip was measured. A higher light yield and reasonably uniform response were observed with the sum of the two SiPM readouts. The hit position was reconstructed by using the charge and time difference between the two readouts. A position resolution of 20 mm was obtained.
This paper discusses the effects of radiation damage to SiPMs on the performances of plastic scintillator counters with series-connected SiPM readout, focusing on timing measurements. The ...performances of a counter composed of a 120×40×5mm3 scintillator tile read out by two sets of six SiPMs from AdvanSiD connected in series attached on the short sides are presented, for different combinations of SiPMs at various levels of irradiation. Firstly, six SiPMs were equally irradiated with electrons from 90Sr sources up to a fluence of Φe−≈3×1012cm−2. The timing resolution of the counter gradually deteriorated by the increase in dark current. The dark current and the deterioration were reduced when the counter was cooled from 30 °C to 10 °C. Secondly, 33 SiPMs were irradiated with reactor neutrons. The fluence levels ranged from Φeq≈8.7×108cm−2 to Φeq≈5.5×1013cm−2. The characteristics of counters read out by series-connected SiPMs with non-uniform damage levels were investigated. The signal pulse height, the time response, and the timing resolution depend on the hit position in the counter when SiPMs’ irradiation is not uniform.
We have developed a new laser-based time calibration system for highly segmented scintillator counters like the MEG II pixelated Timing Counter (pTC), consisting of 512 centimetre-scale scintillator ...counters read out by silicon photomultipliers (SiPMs). It is difficult to apply previous laser-based calibration methods for conventional metre-scale Time-Of-Flight detectors to the MEG II pTC from the implementation and the accuracy points of view. This paper presents a new laser-based time calibration system which can overcome such difficulties. A laser pulse is split into each scintillator counter via several optical components so that we can directly measure the time offset of each counter relative to the laser-emitted time. We carefully tested all the components and procedures prior to the actual operation. The laser system was installed into the pTC and thoroughly tested under the real experimental condition. The system showed good stability and being sensitive to any change of timing larger than ∼10ps. Moreover, it showed an uncertainty of 48ps in the determination of the time offsets, which meets our requirements. The new method provides an example of the implementation of a precise timing alignment for the new type of detectors enabled by the advance of SiPM technology.
A high resolution Timing Counter for the MEG II experiment De Gerone, M.; Bevilacqua, A.; Biasotti, M. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
07/2016, Letnik:
824
Journal Article
Recenzirano
The development of a Timing Counter detector designed for the MEGII upgrade of the MEG experiment, which strives to improve the sensitivity on the μ+→e+γ decay of an order of magnitude, is presented. ...It is based on two sets of counters (sectors) arranged on a semi-cylindrical structure; each sector consists of 256 counters. Each counter consists of tile of fast scintillator with a dual-side read-out based on SiPM arrays in series connection. The high granularity has two advantages: optimized size for achieving high resolution (75ps) for the single counter, and a signal e+ crosses several counters, so that resolution improves by averaging multiple time measurements. A prototype has been built and tested both in BTF and PSI facilities in order to prove the multi-hit scheme in MEG-like beam conditions. A 35ps resolution with eight hits has been obtained with a e+ beam at 100kHz. The first sector will be tested in the MEG II pre-engineering run planned at the end of 2015.
The MEG II experiment (Baldini et al., 2018) 1 is designed to improve the sensitivity to the μ+→e+γ decay. A crucial component is the Pixelated Timing Counter (pTC), dedicated to the measurement of ...the positron time to reduce the combinatorial background (Cattaneo et al., 2014; Nishimura et al., 2016) 3,4. The detector consists of 512 scintillation counters, each performing a precise measurement of the positron crossing time. This approach requires that the time offsets of the counters are calibrated and regularly monitored over the lifetime of the experiment. The pTC time calibration and monitor system will use a laser diode to deliver pulses to each pixel. The system components have been tested in laboratory, the results demonstrate that a calibration and monitoring resolution satisfying the requirements is within reach.
The MEG II experiment is designed to improve by an order of magnitude the sensitivity of 4.2×10−13 reached by MEG on the search for μ+→e+γ decay. A pixelated Timing Counter (pTC) has been developed ...to improve the time resolution by measuring the positron time information independently with several counters. We constructed and installed the pTC and performed commissioning runs at the πE5 beam line at PSI. A resolution of 38.5 ps is obtained with commissioning run data.
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