Search Results (11)

The anomalous magnetic moment of the tau lepton, $a_{\tau }$, represents a fundamental test of the Standard Model (SM) and a high-sensitivity probe for New Physics in the third generation of leptons. Due to the tau’s extremely short lifetime, traditional spin-precession measurements remain inaccessible, necessitating innovative experimental strategies at high-energy colliders. This review provides a comprehensive overview of the current experimental landscape, highlighting the recent paradigm shift from LEP-era constraints to the unprecedented precision reached at the LHC. We emphasize the importance of Ultra-Peripheral Heavy-Ion Collisions (UPCs), which act as a “photon-photon collider” of extreme intensity. By leveraging the $Z^4$ enhancement of the coherent photon flux in Lead–Lead ($PbPb$) interactions, these collisions provide a theoretically robust “quasi-static” environment. To interpret these developments, we first establish the general theoretical framework within the Standard Model Effective Field Theory (SMEFT). This allows us to critically compare the UPC results with the latest measurements from proton–proton collisions—including the recent CMS observation of the $\gamma \gamma \to \tau \tau$ process and the ATLAS constraints from the high-mass Drell–Yan tail—evaluating their complementarity and the challenges related to Effective Field Theory validity at the TeV scale. Finally, we outline the future prospects for $a_{\tau }$ at Belle II and the Future Circular Collider (FCC) stages. While FCC-hh in $PbPb$ mode provides a theoretically clean environment, its sensitivity remains limited to $\mathcal{O}\left({10}^{-2}\right)$. Conversely, the next generation of lepton facilities, specifically Belle II and FCC-ee, aims for the $\mathcal{O}\left({10}^{-5}\right)$ level, required to probe SM electroweak loop corrections. Long-term projections for a high-energy Muon Collider suggest a potential reach of $\mathcal{O}\left({10}^{-6}\right)$. Full article

After rapid approval and installation, the SND@LHC Collaboration was able to gather data successfully in 2022 and 2023. Neutrino interactions from ν_μs originating at the LHC IP1 were observed. Since muons constitute the major background for neutrino interactions, the muon flux entering the acceptance was also measured. To improve the rejection power of the detector and to increase the fiducial volume, a third Veto plane was recently installed. The energy resolution of the calorimeter system was measured in a test beam. This will help with the identification of ν_e interactions that can be used to probe charm production in the pseudo-rapidity range of SND@LHC (7.2 < η < 8.4). Events with three outgoing muons have been observed and are being studied. With no vertex in the target, these events are very likely from muon trident production in the rock before the detector. Events with a vertex in the detector could be from trident production, photon conversion, or positron annihilation. To enhance SND@LHC’s physics case, an upgrade is planned for HL-LHC that will increase the statistics and reduce the systematics. The installation of a magnet will allow the separation of ν_μ from ${\overline{\nu }}_{\mu }$ Full article

This experiment to search for the one of the charged lepton flavor-violating processes, muon-electron conversion, DeeMe, is being conducted at the J-PARC MLF H-Line in Japan. This experiment utilizes a pulsed proton beam from the Rapid Cycling Synchrotron (RCS). A graphite target is bombarded with a pulsed proton beam, negative pion production and pion-in-flight-decay to negative muon; then, the creation of muonic atoms is caused in the same pion production target. A converted electron is expected to be emitted after 1 ∼ 2 micro second-delayed timing. And two-body reaction of the new process, ${\mu }^{-}+(\mathrm{A},\mathrm{Z})\to e^{-}+(\mathrm{A},\mathrm{Z})$, results in 105 MeV monoenergetic electron. Thus, 1 ∼ 2 micro second-delayed 105 MeV monoenergetic electron is a searched signal. Electrons around 105 MeV are transported by the H-Line and analyzed using the dipole magnet (0.4 T) and four multi-wire proportional chambers (MWPCs). However, the burst pulse reaching ${10}^8$ charged particles/pulse attributable to the RCS pulse leads to significant dead time for the MWPC. Thus, the HV switching scheme is introduced to handle the prompt burst. The target single event sensitivity is ${10}^{-13}$. The H-Line construction was completed, and commissioning went well. The overview of the experiment and the current status are described in this article. Full article

In many modern particle physics experiments, high-rate data handling is one of the most critical challenges due to the increase in particle intensity required to achieve higher statistics. We will tackle the challenge in the COMET experiment by developing the sub-microseconds ultra-fast machine learning (ML) algorithm implemented inside FPGAs to search for the lepton flavour violation process, a $\mu$-e conversion, using the world’s most intense muon beam. Our previous study showed that a trigger algorithm based on a gradient-boosted decision tree will realise the sufficient trigger performance within 3.2 $\mathsf{\mu }$s with a cut-based event classification. In this paper, we further investigated neural network algorithms as event classifications. For the feasibility test, a multi-layer perceptron (MLP) model was implemented inside the FPGA, and the preliminary results are presented. Full article

Charged lepton flavour violation processes provide a well-motivated experimental probe into new physics beyond the Standard Model. Muon to electron conversion is one example that the COMET experiment aims to measure with increased sensitivity over previous searches. Taking a staged approach, the COMET experiment will measure muon to electron conversion with sensitivities of $\mathcal{O}\left({10}^{-15}\right)$ and $\mathcal{O}\left({10}^{-17}\right)$ in Phase-I and Phase-II, respectively. An important initial low-intensity beam run, Phase-$\alpha$, is also planned to begin in 2023 with Phase-I following in 2024. This article summarises the COMET experiment and the recent progress made towards the beginning of physics runs. Full article

The Mu2e experiment at Fermilab will search for the neutrinoless ${\mu }^{-}\to e^{-}$ conversion in the field of an aluminum nucleus. The Mu2e data-taking plan assumes two running periods, Run I and Run II, separated by an approximately two-year-long shutdown. This paper presents an estimate of the expected Mu2e Run I search sensitivity and includes a detailed discussion of the background sources, uncertainties of their prediction, analysis procedures, and the optimization of the experimental sensitivity. The expected Run I $5\sigma$ discovery sensitivity is $R_{\mu e}=1.2\times {10}^{-15}$, with a total expected background of $0.11\pm 0.03$ events. In the absence of a signal, the expected upper limit is $R_{\mu e}<6.2\times {10}^{-16}$ at 90% CL. This represents a three order of magnitude improvement over the current experimental limit of $R_{\mu e}<7\times {10}^{-13}$ at 90% CL set by the SINDRUM II experiment. Full article

The Mu2e experiment at Fermi National Accelerator Laboratory will search for the charged-lepton flavour-violating neutrinoless conversion of negative muons into electrons in the Coulomb field of an Al nucleus. The conversion electron with a monoenergetic 104.967 MeV signature will be identified by a complementary measurement carried out by a high-resolution tracker and an electromagnetic calorimeter, improving by four orders of magnitude the current single-event sensitivity. The calorimeter—composed of 1348 pure CsI crystals arranged in two annular disks—has a high granularity, 10% energy resolution and 500 ps timing resolution for 100 MeV electrons. The readout, based on large-area UV-extended SiPMs, features a fully custom readout chain, from the analogue front-end electronics to the digitisation boards. The readout electronics design was validated for operation in vacuum and under magnetic fields. An extensive radiation hardness certification campaign certified the FEE design for doses up to 100 krad and 10${}^{12}$ n${}_{1\mathrm{MeVeq}}$/cm${}^2$ and for single-event effects. A final vertical slice test on the final readout chain was carried out with cosmic rays on a large-scale calorimeter prototype. Full article

The Mu2e experiment at Fermilab will search for the standard model-forbidden, charged lepton flavour-violating conversion of a negative muon into an electron in the field of an aluminium nucleus. The distinctive signal signature is represented by a mono-energetic electron with an energy near the muon’s rest mass. The experiment aims to improve the current single-event sensitivity by four orders of magnitude by means of a high-intensity pulsed muon beam and a high-precision tracking system. The electromagnetic calorimeter complements the tracker by providing high rejection power in muon to electron identification and a seed for track reconstruction while working in vacuum in presence of a 1 T axial magnetic field and in a harsh radiation environment. For 100 MeV electrons, the calorimeter should achieve: (a) a time resolution better than 0.5 ns, (b) an energy resolution <10%, and (c) a position resolution of 1 cm. The calorimeter design consists of two disks, each loaded with 674 undoped CsI crystals read out by two large-area arrays of UV-extended SiPMs and custom analogue and digital electronics. We describe here the status of construction for all calorimeter components and the performance measurements conducted on the large-sized prototype with electron beams and minimum ionizing particles at a cosmic ray test stand. A discussion of the calorimeter’s engineering aspects and the on-going assembly is also reported. Full article

Lepton-flavor violation (LFV) has been discovered in the neutrino sector by neutrino oscillation experiments. The minimal extension of the Standard Model (SM) to include neutrino masses allows LFV in the charged sector (CLFV) at the loop level, but at rates that are too small to be experimentally observed. Lepton-number violation (LNV) is explicitly forbidden even in the minimally extended SM, so the observation of an LNV process would be unambiguous evidence of physics beyond the SM. The search for the LNV and CLFV process ${\mu }^{-}+N(A,Z)\to e^{+}+N^{\prime }(A,Z-2)$ (referred to as ${\mu }^{-}\to e^{+}$) complements $0\nu \beta \beta$ decay searches, and is sensitive to potential flavor effects in the neutrino mass-generation mechanism. A theoretical motivation for ${\mu }^{-}\to e^{+}$ is presented along with a review of the status of past ${\mu }^{-}\to e^{+}$ experiments and future prospects. Special attention is paid to an uncertain and potentially dominant background for these searches, namely, radiative muon capture (RMC). The RMC high energy photon spectrum is theoretically understudied and existing measurements insufficiently constrain this portion of the spectrum, leading to potentially significant impacts on current and future ${\mu }^{-}\to e^{+}$ work. Full article

Charged Lepton Flavor Violation is expected to be one of the most powerful tools to reveal physics beyond the Standard Model. The COMET experiment aims to search for the neutrinoless coherent transition of a muon into an electron in the field of a nucleus. Muon-to-electron conversion has never been observed, and can be, and would be, clear evidence of new physics if discovered. The experimental sensitivity of this process, defined as the ratio of the muon-to-electron conversion rate to the total muon capture rate, is expected to be significantly improved by a factor of 100 to 10,000 in the coming decade. The COMET experiment will take place at J-PARC with single event sensitivities of the orders of 10⁻¹⁵ and 10⁻¹⁷ in Phase-I and Phase-II, respectively. The ambitious goal of the COMET experiment is achieved by realizing a high-quality pulsed beam and an unprecedentedly powerful muon source together with an excellent detector apparatus that can tolerate a severe radiation environment. The construction of a new beam line, superconducting magnets, detectors and electronics is in progress towards the forthcoming Phase-I experiment. We present the experimental methods, sensitivity and backgrounds along with recent status and prospects. Full article

Superconducting magnets are widely used in accelerator science applications. Muon production solenoids are applications that have recently attracted considerable public attention, after the approval of muon-related physics projects such as coherent muon to electron transition or muon-to-electron-conversion experiments. Based on its characteristics, muon production solenoids tend to be subjected to high radiation exposure, which results in a high heat load being applied to the solenoid magnet, thus limiting the superconducting magnet operation, especially for low-temperature superconductors such as niobium titanium alloy. However, the use of high-temperature superconductors may extend the operation capabilities owing to their functionality at higher temperatures. This study reviews the characteristics of high temperature superconductor magnets in high-radiation environments and their potential for application to muon production solenoids. Full article