Periodic Reporting for period 1 - COMETBMP (Designing, Optimising and Validating the Beam Measurement Programme at the New Intense Muon Beam Line for the COMET Muon-to-Electron Search Experiment)
Reporting period: 2022-03-31 to 2024-03-30
The COMET experiment will search for muon-to-electron conversion in aluminium, Al + μ- -> Al + e-, at J-PARC in Japan, with a single-event sensitivity of up to O(10^-17) [1]. The conversion process violates charged lepton flavour conservation. Its branching ratio is estimated to be lower than O(10^-54) in the standard model, including neutrino scintillation. However, several models beyond the standard model (BSM) estimate it to be enhanced to maximally O(10^-15). Since the standard model's branching ratio is negligibly small, any observation of the process implies a new BSM physics discovery.
The experiment aims to make the first observation or further improve upon the current upper limit provided by the previous experiment [2]. The synchrotron accelerator at J-PARC provides the world's most intense proton beam to a fixed target, generating a large number of muons. Particularly, the low-momentum portion should be transported to a muon stopping target to be measured by detectors. However, the intense proton beam also introduces a significant number of background particles into the detectors. To address both challenges, we have installed the Muon Transport Solenoid (TS) after the proton-beam target section. This component of the beamline is curved and surrounded by a series of solenoid magnets. The curved solenoidal magnetic field enables us to transport the secondary muon beam while effectively eliminating other background particles at the same time.
The COMET collaboration plans to execute Beam Measurement Programmes (BMP) differently from Phase-I, which is the main phase for measuring the conversion process, in order to test muon transport and study beam backgrounds. The BMP consist of two phases: (1) Phase-α focuses on TS commissioning with a simple setup, and (2) Phase-I BMP aims to investigate beam backgrounds using the same beamline as in Phase-I. One crucial aspect of the Phase-I BMP is the design of a beam blocker (BB), which will be installed before the detectors to shield them from intense background beam flux and assist in effectively measuring important particles to assess TS performance.
The project contributed to both Phase-α and the Phase-I BMP. I served as the Phase-α working group leader. One of the most significant accomplishments was the successful demonstration that the TS transports beam muons as anticipated. For the Phase-I BMP, we tested the performance of a tungsten BB using pion, muon, and electron beams at the Paul Scherrer Institut (PSI) in Switzerland. The experimental data are currently being compared with our simulation tool's expectations. This comparison aims to validate the simulation tool for further refinement of the BB design. Additionally, the experiment included successful tests for two other detectors for Phase-α and Phase-I.
[1] The COMET Collaboration, 2020, Progress of Theoretical and Experimental Physics.
[2] Bertl et al., The European Physical Journal C - Particles and Fields.
The experiment aims to make the first observation or further improve upon the current upper limit provided by the previous experiment [2]. The synchrotron accelerator at J-PARC provides the world's most intense proton beam to a fixed target, generating a large number of muons. Particularly, the low-momentum portion should be transported to a muon stopping target to be measured by detectors. However, the intense proton beam also introduces a significant number of background particles into the detectors. To address both challenges, we have installed the Muon Transport Solenoid (TS) after the proton-beam target section. This component of the beamline is curved and surrounded by a series of solenoid magnets. The curved solenoidal magnetic field enables us to transport the secondary muon beam while effectively eliminating other background particles at the same time.
The COMET collaboration plans to execute Beam Measurement Programmes (BMP) differently from Phase-I, which is the main phase for measuring the conversion process, in order to test muon transport and study beam backgrounds. The BMP consist of two phases: (1) Phase-α focuses on TS commissioning with a simple setup, and (2) Phase-I BMP aims to investigate beam backgrounds using the same beamline as in Phase-I. One crucial aspect of the Phase-I BMP is the design of a beam blocker (BB), which will be installed before the detectors to shield them from intense background beam flux and assist in effectively measuring important particles to assess TS performance.
The project contributed to both Phase-α and the Phase-I BMP. I served as the Phase-α working group leader. One of the most significant accomplishments was the successful demonstration that the TS transports beam muons as anticipated. For the Phase-I BMP, we tested the performance of a tungsten BB using pion, muon, and electron beams at the Paul Scherrer Institut (PSI) in Switzerland. The experimental data are currently being compared with our simulation tool's expectations. This comparison aims to validate the simulation tool for further refinement of the BB design. Additionally, the experiment included successful tests for two other detectors for Phase-α and Phase-I.
[1] The COMET Collaboration, 2020, Progress of Theoretical and Experimental Physics.
[2] Bertl et al., The European Physical Journal C - Particles and Fields.
(1) Management and conduct of Phase-α.
As the Phase-α working group (WG) leader, I assumed responsibility for guiding the team through the project's various stages. Initially comprising a few core members, the WG expanded to include over 30 international researchers and students from around the world by the time of the experiment. I organised regular WG meetings and collaborated with members to determine the detector setup and experimental run plans. Managing several subgroups for detector developments, I provided leadership and direction to ensure the project's success. The entire WG's activities received high praise from the collaboration.
The Phase-α experiment took place over two weeks in February and March 2023 at J-PARC, marking the first commissioning opportunity for the TS. During the experiment, I oversaw data-taking and managed shift personnel, ensuring smooth operations and data quality.
(2) Range Counter development for Phase-α
The Range Counter (RC) played a crucial role in Phase-α by measuring the momentum spectrum of the negative muon beam transported by the TS. At the onset of the project in June 2022, I conducted an experiment to evaluate the first prototype of the RC using a slow negative muon beam at the Materials and Life Science Experimental Facility of J-PARC [3]. This experiment confirmed the RC's principle idea of counting negative muons and finalised its design. During Phase-α, the actual RC performed successfully, contributing to the first measurement of negative muons and their momentum spectrum.
Following the completion of the Phase-α experiment, another small experiment was conducted in November 2023 at PSI to assess the performance of the RC used in Phase-α.
(3) Conduct of a test-beam campaign at Paul Scherrer Institut in Switzerland for COMET including the BB development
The test-beam experiment conducted in November 2023 at PSI incorporated sub-experiments aimed at evaluating three detectors crucial to COMET: the BB, RC, and a hodoscope detector. Refer to (2) for details of the RC part.
A simple BB prototype was constructed using tungsten plates and subjected to testing with low-momentum electron, muon, and pion beams at PSI. The primary objectives were to assess its stopping power against various momenta of the primary beams of interest in COMET and to observe the secondary particles generated from the halted primary beams. While the expected stopping power was achieved, discrepancies emerged between the measured number and phase space of the generated secondary particles and the simulation estimates, deviating by up to a few tens of percent or more, as initially suspected. Consequently, modifications will be made to the software.
Additionally, the hodoscope detector, serving as the final prototype of the COMET Cylindrical Trigger Hodoscope, underwent successful evaluation.
[3] https://mlfinfo.jp/en/proposals/2022A.html(opens in new window)
As the Phase-α working group (WG) leader, I assumed responsibility for guiding the team through the project's various stages. Initially comprising a few core members, the WG expanded to include over 30 international researchers and students from around the world by the time of the experiment. I organised regular WG meetings and collaborated with members to determine the detector setup and experimental run plans. Managing several subgroups for detector developments, I provided leadership and direction to ensure the project's success. The entire WG's activities received high praise from the collaboration.
The Phase-α experiment took place over two weeks in February and March 2023 at J-PARC, marking the first commissioning opportunity for the TS. During the experiment, I oversaw data-taking and managed shift personnel, ensuring smooth operations and data quality.
(2) Range Counter development for Phase-α
The Range Counter (RC) played a crucial role in Phase-α by measuring the momentum spectrum of the negative muon beam transported by the TS. At the onset of the project in June 2022, I conducted an experiment to evaluate the first prototype of the RC using a slow negative muon beam at the Materials and Life Science Experimental Facility of J-PARC [3]. This experiment confirmed the RC's principle idea of counting negative muons and finalised its design. During Phase-α, the actual RC performed successfully, contributing to the first measurement of negative muons and their momentum spectrum.
Following the completion of the Phase-α experiment, another small experiment was conducted in November 2023 at PSI to assess the performance of the RC used in Phase-α.
(3) Conduct of a test-beam campaign at Paul Scherrer Institut in Switzerland for COMET including the BB development
The test-beam experiment conducted in November 2023 at PSI incorporated sub-experiments aimed at evaluating three detectors crucial to COMET: the BB, RC, and a hodoscope detector. Refer to (2) for details of the RC part.
A simple BB prototype was constructed using tungsten plates and subjected to testing with low-momentum electron, muon, and pion beams at PSI. The primary objectives were to assess its stopping power against various momenta of the primary beams of interest in COMET and to observe the secondary particles generated from the halted primary beams. While the expected stopping power was achieved, discrepancies emerged between the measured number and phase space of the generated secondary particles and the simulation estimates, deviating by up to a few tens of percent or more, as initially suspected. Consequently, modifications will be made to the software.
Additionally, the hodoscope detector, serving as the final prototype of the COMET Cylindrical Trigger Hodoscope, underwent successful evaluation.
[3] https://mlfinfo.jp/en/proposals/2022A.html(opens in new window)
(a) Phase-α
The Phase-α experiment finished successfully. Preliminary results indicate that the observed momentum spectrum aligns with theoretical expectations. The project has effectively demonstrated that the TS, a state-of-the-art component of COMET, transports muon beams as designed. This achievement represents a significant milestone for the COMET collaboration. Consequently, its scientific implications will resonate across all COMET users and activities.
(b) Range Counter development
Parts of the RC are large and very thin (0.5 mm) plastic scintillators to detect slow muons without impeding their passage. This unconventional design posed a novel challenge: achieving a sufficiently high detection efficiency for our objectives. Nevertheless, the RC operated successfully in Phase-α, and its performance was appropriately assessed at PSI. The findings will be documented in a published paper.
(c) Beam blocker prototype test
The evaluation of the BB prototype provided an opportunity to validate our simulation tool based on Geant4. Geant4 is known as one of the most reliable simulators for particle physics tracking and processes [4]. However, given that the energy scale we deal with in COMET is out of the range extensively validated by many experiments, it was imperative to verify that the software can accurately reproduce experimental results. The comparison findings will serve as contributions to Geant4 as well.
[4] https://geant4.web.cern.ch(opens in new window)
The Phase-α experiment finished successfully. Preliminary results indicate that the observed momentum spectrum aligns with theoretical expectations. The project has effectively demonstrated that the TS, a state-of-the-art component of COMET, transports muon beams as designed. This achievement represents a significant milestone for the COMET collaboration. Consequently, its scientific implications will resonate across all COMET users and activities.
(b) Range Counter development
Parts of the RC are large and very thin (0.5 mm) plastic scintillators to detect slow muons without impeding their passage. This unconventional design posed a novel challenge: achieving a sufficiently high detection efficiency for our objectives. Nevertheless, the RC operated successfully in Phase-α, and its performance was appropriately assessed at PSI. The findings will be documented in a published paper.
(c) Beam blocker prototype test
The evaluation of the BB prototype provided an opportunity to validate our simulation tool based on Geant4. Geant4 is known as one of the most reliable simulators for particle physics tracking and processes [4]. However, given that the energy scale we deal with in COMET is out of the range extensively validated by many experiments, it was imperative to verify that the software can accurately reproduce experimental results. The comparison findings will serve as contributions to Geant4 as well.
[4] https://geant4.web.cern.ch(opens in new window)