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MEG experiment at the Paul Scherrer Institute
Nuclear Physics A 844 (2010) 150c–154c www.elsevier.com/locate/nuclphysa
MEG experiment at the Paul Scherrer Institute S. Miharaa a
Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan
The MEG experiment searches for a lepton-flavor violating muon decay, μ+ →e+ γ, with a single event sensitivity of 10−13 by employing an innovative detector system and world’s most intense DC muon beam provided at the Paul Scherrer Insitute. We completed detector construction in 2008 and started physics data acquisition. A preliminary report on the detector performance and possible reach of the experiment using 2008 data will be presented. 1. INTRODUCTION The MEG (Mu-E-Gamma) experiment is designed to improve the signal sensitivity by more than two orders of magnitude than the current experimental bound Br(μ+ →e+ γ)≤ 1.2×10−11 (90% C.L.), set by the MEGA experiment [1]. The μ+ →e+ γ decay is strictly forbidden in the standard model because the lepton-flavor number is not conserved in the reaction. Recently many experiments confirmed that neutrinos have their mass although they are tiny, known as neutrino oscillation phenomena [2]. However even if we take into account this effect in the standard model, the expected decay branching ratio of the μ+ →e+ γ decay is so small that there is no possibility to observe it in a laboratory. On the other hand, if we consider extensions of the standard model such as supersymmetry [3,4], the decay branching ratio can be enhanced as large as that can be observed in an experiment. Since the process itself is strictly forbidden in the standard model because of non-conservation of the lepton-flavor number, observation of the event(s) will certainly be a stunning evidence of the new physics. The MEG experiment was proposed in 1999 based on these considerations. After 9 years preparation of the detector system, the experiment has finally started data acquisition in 2008 at the 590MeV proton ring cyclotron facility at the Paul Scherrer Institute (PSI) in Switzerland. 2. MEG DETECTOR The μ+ →e+ γ decay is characterized by a pair of gamma and positron in the final state with a respective energy of half of muon mass 52.8MeV, emitted back to back in time coincidence. There are two main background sources for this process. One is a radiative muon decay, μ+ →e+ γ ν¯μ νe , where the positron and gamma can be misidentified as those of a μ+ →e+ γ event when two neutrinos carry small amount of energy. The other is an accidental overlap of uncorrelated events, which dominates the background. If a positron 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.05.026
S. Mihara / Nuclear Physics A 844 (2010) 150c–154c
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Figure 1. Schematic of the MEG experiment. a) details of the positron spectrometer and photon detector. b) The MEG beam transport system.
from a normal muon decay μ+ →e+ ν¯μ νe (Michel decay) overlaps accidentally a gamma emitted in a radiative muon decay or produced in an annihilation in flight of a positron, the event looks similar to the signal and can be misidentified. In order to suppress these background events and improve the sensitivity of the experiment, it is required to optimize the detector performance for measuring the positron and gamma at 52.8MeV. The MEG detector is designed in such a way and composed of a muon transport system, a positron spectrometer, and a gamma detector. Fig. 1 shows a schematic view of the beam line components in the PiE5 area showing also the MEG detector which will be described in the following sections. 2.1. Muon beam The MEG experiment uses world’s highest intensity muon beam provided at the PiE5 beam line of the cyclotron facility at PSI. Surface muons of 28MeV/c produced at the primary target are delivered to the stopping target located at the center of the spectrometer magnet through a chain of beam line magnets followed by a beam transport solenoid
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S. Mihara / Nuclear Physics A 844 (2010) 150c–154c
(BTS) magnet. The beam line contains an electro-static separator to suppress positron contamination without loosing muon beam intensity. A collimator system is equipped in front of the BTS magnet to suppress the beam halo that can stop outside the muon stopping target and possibly cause background. A degrader is also installed in the BTS magnet to realize a muon stopping rate of 3×107 μ+ /sec on the stopping target. The beam intensity has been monitored by measuring Michel decay positrons.
2.2. Positron detector The positron spectrometer consists of a super-conducting solenoid magnet, a positron tracking drift chamber system, and time measuring counters. The super-conducting solenoid magnet has a gradient magnetic field, ranging from 1.27 Tesla at the center to 0.49 Tesla at either end, so that low momentum particles can be swept out quickly. The field is adjusted by a series of coils with different radii and current densities so that particles with same momentum have a same bending radius in the magnet. This enables us to evaluate the particle momentum easily at the trigger level and help to control the trigger rate in a safe manner. The positron tracking drift chamber system is located at the center of the magnet and holds a muon stopping target. The system consists of 16 drift chamber modules. Each module has two layers of anode wires staggered by half of a drift-cell size and four layers of cathode readout with a Vernier pattern structure. The chambers are operated with a helium:ethane (50:50) gas mixture for realizing low-mass construction. The time measuring counters are placed at each end of the spectrometer magnet to measure the arrival time of positrons at the ends of their trajectory. Each array consists of 15 plastic scintillator bars read-out by fine-mesh photomultiplier tubes (PMTs).
2.3. Photon detector The photon detector uses liquid xenon of 900 liters as a scintillating material. The scintillation light from the liquid is observed by 846 PMTs surrounding the active volume. The active volume is not segmented and is viewed directly by photomultipliers submersed in the liquid. The liquid is recondensed by a pulse-tube refrigerator equipped on the top of the cryostat. There is also a liquid nitrogen cooling system to help the refrigerator cooling. Impurity in the liquid such as water, nitrogen, and oxygen must be suppressed sufficiently otherwise either of them can cause absorption of scintillation light and/or affect emission of scintillation light. This is done by using purification systems developed in the collaboration [5]. The gamma energy is reconstructed by counting up observed charge by all PMTs. The gamma interaction position in the detector is evaluated by using light distribution on PMTs. The time information is obtained by taking a weighted average of signal arrival time of PMTs. All electronics signals from the MEG detector is digitized with the in-house designed waveform digitizer boards [6]. The sampling speed for the drift chamber anode and cathode signals is 500MHz, while that of the PMT signals from the photon detector and timing counters is 1.6GHz. This allows us to achieve better capability of a pile-up rejection compared to conventional method.
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103
Number of events / (0.2MeV)
Number of events / (0.5MeV)
S. Mihara / Nuclear Physics A 844 (2010) 150c–154c
102
10
1 40
104 103 102 10 1
45
50
55
60 Eγ [MeV]
40
45
50
55
60 Ee [MeV]
Figure 2. Energy distributions of gamma (left) and positron (right) in the side band.
3. MEG STATUS IN 2008 Physics data was collected between September and December 2008, corresponding to 9.5×1013 muon stops on the target. Various kinds of calibration data were taken concurrently for monitoring the detector stability and evaluating its performance. For example, once a week acquisition of radiative-muon-decay data was conducted for an entire day at reduced beam intensity. Two charge exchange (CEX) runs (π − p → π 0 n → γγn) were performed one at the beginning and one at the end of the data-taking period. These CEX runs are for evaluating the photon detector performance at 54.9MeV, which is close to the signal gamma energy. Dalitz decays (π 0 → γe+ e− ) were also collected and used to study the detector time synchronization and resolution. Data analysis is performed in three steps by employing the blinding-box analysis method. First a simple selection is applied to reduce the data size to 16% using information of gamma energy and relative timing between the gamma and positron. In this pre-selection stage data falling in a box where the signal events should exist, the blinding box, is separated from others and is not used for optimizing the analysis. Then analysis procedure is optimized by using data outside the box. Detector performance is also evaluated by using this side-band data. The single event sensitivity using 2008 data is estimated to be 3-5×10−12 taking into account gamma and positron detection efficiencies, data acquisition efficiencies, and analysis efficiency. Evaluation of detector resolutions is in progress by using real data with a help of Monte Carlo simulations. The number of background events in the signal region is estimated by two different methods. One is to use the side-band data where the number is estimated by extrapolation. The other is to use individual spectra of gammas and positrons. Fig. 2 shows their energy distributions in the side band. Since it is known that the background is dominated by accidental overlaps, that can be estimated reliably by simply multiplying both spectra taking the relative time resolution into account. Consis-
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S. Mihara / Nuclear Physics A 844 (2010) 150c–154c
tency check of these evaluations will be carefully done before finalizing the analysis. The collaboration plans to publish the result in summer 2009. 4. SUMMARY The MEG experiment searching for a lepton-flavor violating muon decay, μ+ →e+ γ, has finally started physics data acquisition in 2008. The single event sensitivity using data taken in 2008 is estimated to be 3-5×10−12 . Detailed analysis is in progress by employing the blinding-box analysis method. At the begging of 2009 several maintenance work has been conduced to improve the detector performance. More statistics is foreseen in 2009 and future to reach the target sensitivity of 10−13 . REFERENCES 1. 2. 3. 4. 5. 6.
M. L. Brooks et al. [MEGA Collaboration], Phys. Rev. Lett. 83 (1999) 1521. T. Schwetz, M. A. Tortola and J. W. F. Valle, New J. Phys. 10 (2008) 113011. R. Barbieri, L. Hall and A. Strumia, Nucl. Phys. B 455 (1995) 219. J. Hisano, D. Nomura and T. Yanagida, Phys. Lett. B 437 (1998) 351. S.Mihara et al., Cryogenics 46 (2006) 688-693. S.Ritt, IEEE NSS, 4 (2007) 2485.
MEG experiment at the Paul Scherrer Institute S. Miharaa a
Institute of Particle and Nuclear Studies, KEK, Tsukuba 305-0801, Japan
The MEG experiment searches for a lepton-flavor violating muon decay, μ+ →e+ γ, with a single event sensitivity of 10−13 by employing an innovative detector system and world’s most intense DC muon beam provided at the Paul Scherrer Insitute. We completed detector construction in 2008 and started physics data acquisition. A preliminary report on the detector performance and possible reach of the experiment using 2008 data will be presented. 1. INTRODUCTION The MEG (Mu-E-Gamma) experiment is designed to improve the signal sensitivity by more than two orders of magnitude than the current experimental bound Br(μ+ →e+ γ)≤ 1.2×10−11 (90% C.L.), set by the MEGA experiment [1]. The μ+ →e+ γ decay is strictly forbidden in the standard model because the lepton-flavor number is not conserved in the reaction. Recently many experiments confirmed that neutrinos have their mass although they are tiny, known as neutrino oscillation phenomena [2]. However even if we take into account this effect in the standard model, the expected decay branching ratio of the μ+ →e+ γ decay is so small that there is no possibility to observe it in a laboratory. On the other hand, if we consider extensions of the standard model such as supersymmetry [3,4], the decay branching ratio can be enhanced as large as that can be observed in an experiment. Since the process itself is strictly forbidden in the standard model because of non-conservation of the lepton-flavor number, observation of the event(s) will certainly be a stunning evidence of the new physics. The MEG experiment was proposed in 1999 based on these considerations. After 9 years preparation of the detector system, the experiment has finally started data acquisition in 2008 at the 590MeV proton ring cyclotron facility at the Paul Scherrer Institute (PSI) in Switzerland. 2. MEG DETECTOR The μ+ →e+ γ decay is characterized by a pair of gamma and positron in the final state with a respective energy of half of muon mass 52.8MeV, emitted back to back in time coincidence. There are two main background sources for this process. One is a radiative muon decay, μ+ →e+ γ ν¯μ νe , where the positron and gamma can be misidentified as those of a μ+ →e+ γ event when two neutrinos carry small amount of energy. The other is an accidental overlap of uncorrelated events, which dominates the background. If a positron 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.05.026
S. Mihara / Nuclear Physics A 844 (2010) 150c–154c
151c
Figure 1. Schematic of the MEG experiment. a) details of the positron spectrometer and photon detector. b) The MEG beam transport system.
from a normal muon decay μ+ →e+ ν¯μ νe (Michel decay) overlaps accidentally a gamma emitted in a radiative muon decay or produced in an annihilation in flight of a positron, the event looks similar to the signal and can be misidentified. In order to suppress these background events and improve the sensitivity of the experiment, it is required to optimize the detector performance for measuring the positron and gamma at 52.8MeV. The MEG detector is designed in such a way and composed of a muon transport system, a positron spectrometer, and a gamma detector. Fig. 1 shows a schematic view of the beam line components in the PiE5 area showing also the MEG detector which will be described in the following sections. 2.1. Muon beam The MEG experiment uses world’s highest intensity muon beam provided at the PiE5 beam line of the cyclotron facility at PSI. Surface muons of 28MeV/c produced at the primary target are delivered to the stopping target located at the center of the spectrometer magnet through a chain of beam line magnets followed by a beam transport solenoid
152c
S. Mihara / Nuclear Physics A 844 (2010) 150c–154c
(BTS) magnet. The beam line contains an electro-static separator to suppress positron contamination without loosing muon beam intensity. A collimator system is equipped in front of the BTS magnet to suppress the beam halo that can stop outside the muon stopping target and possibly cause background. A degrader is also installed in the BTS magnet to realize a muon stopping rate of 3×107 μ+ /sec on the stopping target. The beam intensity has been monitored by measuring Michel decay positrons.
2.2. Positron detector The positron spectrometer consists of a super-conducting solenoid magnet, a positron tracking drift chamber system, and time measuring counters. The super-conducting solenoid magnet has a gradient magnetic field, ranging from 1.27 Tesla at the center to 0.49 Tesla at either end, so that low momentum particles can be swept out quickly. The field is adjusted by a series of coils with different radii and current densities so that particles with same momentum have a same bending radius in the magnet. This enables us to evaluate the particle momentum easily at the trigger level and help to control the trigger rate in a safe manner. The positron tracking drift chamber system is located at the center of the magnet and holds a muon stopping target. The system consists of 16 drift chamber modules. Each module has two layers of anode wires staggered by half of a drift-cell size and four layers of cathode readout with a Vernier pattern structure. The chambers are operated with a helium:ethane (50:50) gas mixture for realizing low-mass construction. The time measuring counters are placed at each end of the spectrometer magnet to measure the arrival time of positrons at the ends of their trajectory. Each array consists of 15 plastic scintillator bars read-out by fine-mesh photomultiplier tubes (PMTs).
2.3. Photon detector The photon detector uses liquid xenon of 900 liters as a scintillating material. The scintillation light from the liquid is observed by 846 PMTs surrounding the active volume. The active volume is not segmented and is viewed directly by photomultipliers submersed in the liquid. The liquid is recondensed by a pulse-tube refrigerator equipped on the top of the cryostat. There is also a liquid nitrogen cooling system to help the refrigerator cooling. Impurity in the liquid such as water, nitrogen, and oxygen must be suppressed sufficiently otherwise either of them can cause absorption of scintillation light and/or affect emission of scintillation light. This is done by using purification systems developed in the collaboration [5]. The gamma energy is reconstructed by counting up observed charge by all PMTs. The gamma interaction position in the detector is evaluated by using light distribution on PMTs. The time information is obtained by taking a weighted average of signal arrival time of PMTs. All electronics signals from the MEG detector is digitized with the in-house designed waveform digitizer boards [6]. The sampling speed for the drift chamber anode and cathode signals is 500MHz, while that of the PMT signals from the photon detector and timing counters is 1.6GHz. This allows us to achieve better capability of a pile-up rejection compared to conventional method.
153c
103
Number of events / (0.2MeV)
Number of events / (0.5MeV)
S. Mihara / Nuclear Physics A 844 (2010) 150c–154c
102
10
1 40
104 103 102 10 1
45
50
55
60 Eγ [MeV]
40
45
50
55
60 Ee [MeV]
Figure 2. Energy distributions of gamma (left) and positron (right) in the side band.
3. MEG STATUS IN 2008 Physics data was collected between September and December 2008, corresponding to 9.5×1013 muon stops on the target. Various kinds of calibration data were taken concurrently for monitoring the detector stability and evaluating its performance. For example, once a week acquisition of radiative-muon-decay data was conducted for an entire day at reduced beam intensity. Two charge exchange (CEX) runs (π − p → π 0 n → γγn) were performed one at the beginning and one at the end of the data-taking period. These CEX runs are for evaluating the photon detector performance at 54.9MeV, which is close to the signal gamma energy. Dalitz decays (π 0 → γe+ e− ) were also collected and used to study the detector time synchronization and resolution. Data analysis is performed in three steps by employing the blinding-box analysis method. First a simple selection is applied to reduce the data size to 16% using information of gamma energy and relative timing between the gamma and positron. In this pre-selection stage data falling in a box where the signal events should exist, the blinding box, is separated from others and is not used for optimizing the analysis. Then analysis procedure is optimized by using data outside the box. Detector performance is also evaluated by using this side-band data. The single event sensitivity using 2008 data is estimated to be 3-5×10−12 taking into account gamma and positron detection efficiencies, data acquisition efficiencies, and analysis efficiency. Evaluation of detector resolutions is in progress by using real data with a help of Monte Carlo simulations. The number of background events in the signal region is estimated by two different methods. One is to use the side-band data where the number is estimated by extrapolation. The other is to use individual spectra of gammas and positrons. Fig. 2 shows their energy distributions in the side band. Since it is known that the background is dominated by accidental overlaps, that can be estimated reliably by simply multiplying both spectra taking the relative time resolution into account. Consis-
154c
S. Mihara / Nuclear Physics A 844 (2010) 150c–154c
tency check of these evaluations will be carefully done before finalizing the analysis. The collaboration plans to publish the result in summer 2009. 4. SUMMARY The MEG experiment searching for a lepton-flavor violating muon decay, μ+ →e+ γ, has finally started physics data acquisition in 2008. The single event sensitivity using data taken in 2008 is estimated to be 3-5×10−12 . Detailed analysis is in progress by employing the blinding-box analysis method. At the begging of 2009 several maintenance work has been conduced to improve the detector performance. More statistics is foreseen in 2009 and future to reach the target sensitivity of 10−13 . REFERENCES 1. 2. 3. 4. 5. 6.
M. L. Brooks et al. [MEGA Collaboration], Phys. Rev. Lett. 83 (1999) 1521. T. Schwetz, M. A. Tortola and J. W. F. Valle, New J. Phys. 10 (2008) 113011. R. Barbieri, L. Hall and A. Strumia, Nucl. Phys. B 455 (1995) 219. J. Hisano, D. Nomura and T. Yanagida, Phys. Lett. B 437 (1998) 351. S.Mihara et al., Cryogenics 46 (2006) 688-693. S.Ritt, IEEE NSS, 4 (2007) 2485.