Periodic Reporting for period 1 - aMUSE (advanced Muon Campus in US and Europe contribution)
Reporting period: 2022-01-01 to 2024-06-30
aMUSE plans to strengthen and expand the collaboration between EU and US researchers to carry out cutting-edge searches for New Physics (NP) in the muon sector, while promoting the development of next-generation muon accelerators. The project is mainly based at the Muon Campus of FNAL in the US. Here, the Muon (g-2) experiment aims to solve the long-standing muon anomaly puzzle collecting twenty times more statistics than its predecessor. A high-profile discovery path for the search for charged Lepton Flavour Violation (cLFV) will be exploited by the Mu2e experiment, whose goal is to improve the discovery sensitivity for the as-yet-unseen muon-to-electron conversion by four orders of magnitude, exploring mass scales up to 10^4 TeV/c^2. aMUSE also promotes an ambitious extension of the Muon Campus activities. Its R&D programs will exploit the future phase of Mu2e-II, with an upgraded proton beam for a ten-fold increase in muon yield, designing and developing state-of-the-art detectors to face this challenge. At this high-intensity frontier, aMUSE will explore the design of a beam line extension to seed the birth of a new generation of experiments searching for cLFV muon decays as a possible alternate running to Mu2e-II. Its ultimate goal is to significantly improve sensitivity with respect to current or proposed facilities and to promote the integration of EU groups in developing accelerator and detector strategies. In this respect, aMUSE provides an excellent platform for an ambitious EU-US network to advance the development of muon beams. Low- and high-energy research is synergistic: muon cooling is fundamental to both high-energy muon collisions and low-energy high-intensity muon beams. The long-standing US expertise in the study of muon beam technologies is integrated with the experience of EU researchers, creating a unique opportunity for the advancement of a challenging and promising project.
The Muon (g-2) experiment concluded its data taking and published the measurement of the muon magnetic anomaly (a_mu) with data collected between 2018 and 2020. aMUSE researchers are currently involved in the analysis of the full dataset and on the magnetic field calibration to reduce its contribution to the overall a systematic error. The commissioning of the Mu2e detectors of interest to the project, the crystal calorimeter and the Stopping Target Monitor, is steadily progressing. Calibration techniques have been outlined and are being refined. The theory group is actively involved in the community to provide theoretical interpretations of Muon (g-2) results and insights for the Mu2e physics run. The aMUSE community is at the forefront of promoting an upgrade to the Fermilab Muon Campus to fully exploit the next-generation accelerator complex, with the development of beam line and detectors for the Mu2e-II upgrade and studies on a next-generation muon decay experiment at the newly proposed Advanced Muon Facility (AMF). The developments related to muon beams on the cooling techniques and on the machine-detector interface and detector optimization for a multi-TeV muon collider are in line with the scheduled plan, profiting of the new-born International Muon Collider Collaboration, where aMUSE researchers cover key roles. Application of research developments in particle physics to other fields is also progressing. Data taking campaigns involving research and industrial aMUSE partners to characterise irradiation damages have been performed, and the R&D studies for the identification of hazardous substances for mine-clearance and counterterrorism is advancing.
The Muon (g-2) experiment has succeeded in improving the BNL measurement of a_mu by completing the analysis of its 2018-2020 sample with four times the BNL statistics. Both statistical and systematic errors on a_mu were improved by a factor ~2, increasing to more than 5 sigma’s the discrepancy between this measurement and its Standard Model (SM) evaluation based on e+e- data. This exciting result motivated an intense work on the theory side to discriminate between BSM scenarios or independent SM evaluation carried out with lattice calculations. The existing tension in this sector is really puzzling and the experiment, including the aMUSE team, is working hard to complete both the analysis of the full data set (×20 BNL), aiming to a further factor two reduction on the errors and a test of concurring BSM scenarios with very light new particles, motivated by g-2 and dark matter open questions. The theory group is using these new developments to obtain more accurate predictions for cLFV observables that can be tested with the Mu2e experiment. In this respect, aMUSE aims to complete the construction and commissioning phase of the experiment and to start the first physics run with beam. Collecting 1/10 of the total number of Protons on Target (10^19), this first data taking period will allow to improve the cLFV sensitivity by a factor 1000, exploring mass scales not achievable with current, next-generation or even larger energy colliders.
The connection and synergy between the Muon Campus activity and its future upgrades (Mu2e-II and AMF) are focused on the development of a new generation of detectors to overcome the challenges foreseen for the next-generation cLFV experiments. For Mu2e-II, ongoing studies on innovative gas mixtures that could improve tolerance to very high energy deposits over time while preserving a very low material budget are a pre-requisite for the design of extremely light tracking systems, able to sustain a rate capability up to 10^8 tracks/sec on the detector. The ×10 increase in radiation level and ×3 increase in hit occupancy for calorimetry ask for a dramatic technology change with much faster crystals than CsI. The primary solution exploits the 280 nm fast component (0.9 ns) of BaF2 crystals while minimizing its long (600 ns) component above 350 nm. This is achieved by both Yttrium doping of crystals and coupling them to newly developed Solar-Blind SiPMs. The BaF2 emission is so fast and bright that application on TOF-PET can be also explored for the medical field. Moreover, the ten-fold increase in muon beam intensity gave rise to a completely different design of the pion production target, involving active cooling. Developments in the conceptual design of the AMF, with muons transported through a fixed-field alternating gradient storage ring, have triggered the conceptual design for muon surface beam detectors. Simulations and hardware tests have been performed, orienting toward a design consisting of a silicon-pixel positron tracker and a photon pair-conversion detector. Thanks also to aMUSE, the Muon Collider has become a priority in US, recognised in the P5 report.
In order to achieve our goals and build bridges towards other fields of excellence and applications for society, collaboration and connections with industries are also exploited. Partnerships arising from the synergies of our work on High Precision Crystals and Silicon Photomultipliers finds applications for mine clearance, medical physics and laser-plasma characterization.
The connection and synergy between the Muon Campus activity and its future upgrades (Mu2e-II and AMF) are focused on the development of a new generation of detectors to overcome the challenges foreseen for the next-generation cLFV experiments. For Mu2e-II, ongoing studies on innovative gas mixtures that could improve tolerance to very high energy deposits over time while preserving a very low material budget are a pre-requisite for the design of extremely light tracking systems, able to sustain a rate capability up to 10^8 tracks/sec on the detector. The ×10 increase in radiation level and ×3 increase in hit occupancy for calorimetry ask for a dramatic technology change with much faster crystals than CsI. The primary solution exploits the 280 nm fast component (0.9 ns) of BaF2 crystals while minimizing its long (600 ns) component above 350 nm. This is achieved by both Yttrium doping of crystals and coupling them to newly developed Solar-Blind SiPMs. The BaF2 emission is so fast and bright that application on TOF-PET can be also explored for the medical field. Moreover, the ten-fold increase in muon beam intensity gave rise to a completely different design of the pion production target, involving active cooling. Developments in the conceptual design of the AMF, with muons transported through a fixed-field alternating gradient storage ring, have triggered the conceptual design for muon surface beam detectors. Simulations and hardware tests have been performed, orienting toward a design consisting of a silicon-pixel positron tracker and a photon pair-conversion detector. Thanks also to aMUSE, the Muon Collider has become a priority in US, recognised in the P5 report.
In order to achieve our goals and build bridges towards other fields of excellence and applications for society, collaboration and connections with industries are also exploited. Partnerships arising from the synergies of our work on High Precision Crystals and Silicon Photomultipliers finds applications for mine clearance, medical physics and laser-plasma characterization.