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At least 325 records · Page 18

Muon capture on deuteron using local chiral potentials

The muon capture reaction μ – + d → n + n + ν μ in the doublet hyperfine state is studied using nuclear potentials and consistent currents derived in the chiral effective field theory, which are local and expressed in coordinate space (the so-called Norfolk models). Only the largest contribution due to the 1 S 0 nn scattering state is considered. Particular attention is given to the estimate of theoretical uncertainty, for which four sources have been identified: 1) the model dependence, 2) the chiral-order convergence for the weak nuclear current, 3) the uncertainty in the single-nucleon axial form factor, and 4) the numerical technique adopted to solve the bound and scattering A = 2 systems. This last source of uncertainty has turned out to be essentially negligible. For the 1 S 0 doublet muon capture rate Γ D ( 1 S 0 ), we obtain Γ D ( 1 S 0 ) = 255.8(0.6) (4.4)(2.9) s –1 , where the three errors come from the first three sources of uncertainty. The value for Γ D ( 1 S 0 ) obtained within this local chiral framework is compared with previous calculations and found in very good agreement.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Evidence for Higgs boson decay to a pair of muons

Evidence for Higgs boson decay to a pair of muons is presented. This result combines searches in four exclusive categories targeting the production of the Higgs boson via gluon fusion, via vector boson fusion, in association with a vector boson, and in association with a top quark-antiquark pair. The analysis is performed using proton-proton collision data at $ \sqrt{s} $ = 13 TeV, corresponding to an integrated luminosity of 137 fb$^{−1}$, recorded by the CMS experiment at the CERN LHC. An excess of events over the back- ground expectation is observed in data with a significance of 3.0 standard deviations, where the expectation for the standard model (SM) Higgs boson with mass of 125.38 GeV is 2.5. The combination of this result with that from data recorded at $ \sqrt{s} $ = 7 and 8 TeV, corresponding to integrated luminosities of 5.1 and 19.7 fb$^{−1}$, respectively, increases both the expected and observed significances by 1%. The measured signal strength, relative to the SM prediction, is $ {1.19}_{-0.39}^{+0.40}{\left(\mathrm{stat}\right)}_{-0.14}^{+0.15}\left(\mathrm{syst}\right) $. This result constitutes the first evidence for the decay of the Higgs boson to second generation fermions and is the most precise measurement of the Higgs boson coupling to muons reported to date.[graphic not available: see fulltext]

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Aiming for tops of ALPs with a muon collider

Future muon colliders with center-of-mass energy of $\mathcal{O}$ (1 – 10) TeV can provide a clean high-energy environment with advantages in searches for TeV-scale axion-like particles (ALPs), pseudo-Nambu-Goldstone bosons associated with spontaneously broken global symmetries, which are widely predicted in physics beyond the Standard Model (SM). We exploit ALP couplings to SM fermions, and guided by unitarity constraints, build a search strategy focusing on the ALP decay to top quark pairs at muon colliders. It is found that a large parameter space of TeV-scale ALPs with TeV-scale decay constants can be probed by utilizing the ALP-top quark coupling.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Unified framework for B -anomalies, muon g – 2 and neutrino masses

We present a model of radiative neutrino masses which also resolves anomalies reported in B -meson decays, \( {R}_{D^{\left(\ast \right)}} \) and \( {R}_{K^{\left(\ast \right)}} \) , as well as in muon g – 2 measurement, Δ a μ . Neutrino masses arise in the model through loop diagrams involving TeV-scale leptoquark (LQ) scalars R 2 and S 3 . Fits to neutrino oscillation parameters are obtained satisfying all flavor constraints which also explain the anomalies in \( {R}_{D^{\left(\ast \right)}} \) , \( {R}_{K^{\left(\ast \right)}} \) and Δ a μ within 1 σ . An isospin-3/2 Higgs quadruplet plays a crucial role in generating neutrino masses; we point out that the doubly-charged scalar contained therein can be produced in the decays of the S 3 LQ, which enhances its reach to 1.1 (6.2) TeV at \( \sqrt{s} \) = 14 TeV high-luminosity LHC ( \( \sqrt{s} \) = 100 TeV FCC-hh). We also present flavor-dependent upper limits on the Yukawa couplings of the LQs to the first two family fermions, arising from non-resonant dilepton ( pp → ℓ + ℓ – ) processes mediated by t -channel LQ exchange, which for 1 TeV LQ mass, are found to be in the range (0 . 15 – 0 . 36). These limits preclude any explanation of \( {R}_{D^{\left(\ast \right)}} \) through LQ-mediated B -meson decays involving ν e or ν μ in the final state. We also find that the same Yukawa couplings responsible for the chirally-enhanced contribution to Δ a μ give rise to new contributions to the SM Higgs decays to muon and tau pairs, with the modifications to the corresponding branching ratios being at (2–6)% level, which could be tested at future hadron colliders, such as HL-LHC and FCC-hh.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

Supersymmetric gauged $ \mathrm{U}{(1)}_{L_{\mu }-{L}_{\tau }} $ model for electron and muon (g - 2) anomaly

Minimal gauged U 1 L μ - L τ models can provide for an additional source for the muon anomalous magnetic moment however it is difficult to accommodate the discrepancy in the electron magnetic moment in tandem. Owing to the relative sign between the discrepancies in these quantities, it seems unlikely that they arise from the same source. We show that a supersymmetric (SUSY) gauged U 1 L μ - L τ model can accommodate both the muon and electron anomalous magnetic moments in a very simple and intuitive scenario, without utilizing lepton flavor violation. The currently allowed parameter space in this kind of a scenario is constrained from the latest LHC and various low energy experimental data,e.g., recent COHERENT data, CCFR, Borexino, BaBaR, supernova etc. These constraints, in conjunction with the requirement to explain both lepton magnetic moments, lead to an upper bound on the first generation slepton mass, a lower bound on the second generation slepton mass and constricts the allowed range for the new gauge boson mass and coupling. The scheme can be probed at the ongoing COHERENT and Coherent CAPTAIN-Mills experiments and at future experiments, e.g., DUNE, BELLE-II etc.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Heavy neutral leptons at muon colliders

The future high-energy muon colliders, featuring both high energy and low background, could play a critical role in our searches for new physics. The smallness of neutrino mass is a puzzle of particle physics. Broad classes of solutions to the neutrino puzzles can be best tested by seeking the partners of SM light neutrinos, dubbed as heavy neutral leptons (HNLs), at muon colliders. We can parametrize HNLs in terms of the mass m N and the mixing angle with ℓ-flavor U ℓ . In this work, we focus on the regime m N > O(100) GeV and study the projected sensitivities on the |U ℓ | 2 – m N plane with the full-reconstructable HNL decay into a hadronic W and a charged lepton. The projected reach in |U ℓ | 2 leads to the best sensitivities in the TeV realm.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Search for vector-like leptons with long-lived particle decays in the CMS muon system in proton-proton collisions at $$\sqrt{\text{s}}$$ = 13 TeV

Abstract A first search is presented for vector-like leptons (VLLs) exclusively decaying into a light long-lived pseudoscalar boson and a standard model τ lepton. The pseudoscalar boson is assumed to have a mass below the τ + τ − threshold, so that it decays exclusively into two photons. It is identified using the CMS muon system. The analysis is carried out using a data set of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment in 2016–2018, corresponding to an integrated luminosity of 138 fb −1. Selected events contain at least one pseudoscalar boson decaying electromagnetically in the muon system and at least one hadronically decaying τ lepton. No significant excess of data events is observed compared to the background expectation. Upper limits are set at 95% confidence level on the vector-like lepton production cross section as a function of the VLL mass and the pseudoscalar boson mean proper decay length. The observed and expected exclusion ranges of the VLL mass extend up to 700 and 670 GeV, respectively, depending on the pseudoscalar boson lifetime.

Chekhovsky, V. [Yerevan Physics Institute]↗

Search for vector-like leptons with long-lived particle decays in the CMS muon system in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A first search is presented for vector-like leptons (VLLs) exclusively decaying into a light long-lived pseudoscalar boson and a standard model τ lepton. The pseudoscalar boson is assumed to have a mass below the τ + τ − threshold, so that it decays exclusively into two photons. It is identified using the CMS muon system. The analysis is carried out using a data set of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment in 2016–2018, corresponding to an integrated luminosity of 138 fb −1 . Selected events contain at least one pseudoscalar boson decaying electromagnetically in the muon system and at least one hadronically decaying τ lepton. No significant excess of data events is observed compared to the background expectation. Upper limits are set at 95% confidence level on the vector-like lepton production cross section as a function of the VLL mass and the pseudoscalar boson mean proper decay length. The observed and expected exclusion ranges of the VLL mass extend up to 700 and 670 GeV, respectively, depending on the pseudoscalar boson lifetime.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Naturalness and the muon magnetic moment

We study a predictive model for explaining the apparent deviation of the muon anomalous magnetic moment from the Standard Model expectation. There are no new scalars and hence no new hierarchy puzzles beyond those associated with the Higgs; the only new particles at the TeV scale are vector-like singlet and doublet leptons. Interestingly, this simple model provides a calculable example violating the Wilsonian notion of naturalness: despite the absence of any symmetries prohibiting its generation, the coefficient of the naively leading dimension-six operator for (g – 2) vanishes at one-loop. While effective field theorists interpret this either as a surprising UV cancellation of power divergences, or as a delicate cancellation between matching UV and calculable IR corrections to (g – 2) from parametrically separated scales, there is a simple explanation in the full theory: the loop integrand is a total derivative of a function vanishing in both the deep UV and IR. The leading contribution to (g – 2) arises from dimension-eight operators, and thus the required masses of new fermions are lower than naively expected, with a sizeable portion of parameter space already covered by direct searches at the LHC. The viable parameter space free of fine-tuning for the muon mass will be fully covered by future direct LHC searches, and all of the parameter space can be probed by precision measurements at planned future lepton colliders.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Search for pairs of scalar leptoquarks decaying into quarks and electrons or muons in $ \sqrt{s}$ = 13 TeV pp collisions with the ATLAS detector

A search for new-physics resonances decaying into a lepton and a jet performed by the ATLAS experiment is presented. Scalar leptoquarks pair-produced in pp collisions at s = 13 TeV at the Large Hadron Collider are considered using an integrated luminosity of 139 fb-1, corresponding to the full Run 2 dataset. They are searched for in events with two electrons or two muons and two or more jets, including jets identified as arising from the fragmentation of c- or b-quarks. The observed yield in each channel is consistent with the Standard Model background expectation. Leptoquarks with masses below 1.8 TeV and 1.7 TeV are excluded in the electron and muon channels, respectively, assuming a branching ratio into a charged lepton and a quark of 100%, with minimal dependence on the quark flavour. Upper limits on the aforementioned branching ratio are also given as a function of the leptoquark mass.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Searching for leptoquarks at future muon colliders

A high energy muon collider can provide new and complementary discovery potential to the LHC or future hadron colliders. Leptoquarks are a motivated class of exotic new physics models, with distinct production channels at hadron and lepton machines. We study a vector leptoquark model at a muon collider with √s = 3, 14 TeV within a set of both UV and phenomenologically motivated flavor scenarios. We compute which production mechanism has the greatest reach for various values of the leptoquark mass and the coupling between leptoquark and Standard Model fermions. We find that we can probe leptoquark masses up to an order of magnitude beyond √s with perturbative couplings. Additionally, we can also probe regions of parameter space unavailable to flavor experiments. In particular, all of the parameter space of interest to explain recent low-energy anomalies in B meson decays would be covered even by a √s = 3 TeV collider.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Search for charged-lepton flavour violation in top quark interactions with an up-type quark, a muon, and a $τ$ lepton in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A search for charged-lepton flavour violation (CLFV) in top quark (t) production and decay is presented. The search uses proton-proton collision data corresponding to 138 fb$^{-1}$ collected with the CMS experiment at $\sqrt{s}$ = 13 TeV. The signal consists of the production of a single top quark via a CLFV interaction or top quark pair production followed by a CLFV decay. The analysis selects events containing a hadronically decaying $τ$ lepton and a muon of opposite electric charge, as well as at least three jets, one of which is identified as originating from the fragmentation of a bottom quark. Machine learning classification techniques are used to distinguish signal from standard model background events. The results of this search are consistent with the standard model expectations. The upper limits at 95% confidence level on the branching fraction $\mathcal{B}$ for CLFV top quark decays to a muon, a $τ$ lepton, and an up or a charm quark are set at $\mathcal{B}$(t $\to$ $μτ$u) $\lt$ (0.04, 0.08, and 0.12) $\times$ 10$^{-6}$, and $\mathcal{B}$(t $\to$ $μτ$c) $\lt$ (0.81, 1.71, and 2.05) $\times$ 10$^{-6}$ for scalar, vector, and tensor-like operators, respectively.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Full Detector Simulation with Unprecedented Background Occupancy at a Muon Collider

In recent years, a Muon collider has attracted a lot of interest in the high-energy physics community, thanks to its ability of achieving clean interaction signatures at multi-TeV collision energies in the most cost-effective way. Estimation of the physics potential of such an experiment must take into account the impact of beam-induced background on the detector performance, which has to be carefully evaluated using full detector simulation. Tracing of all the background particles entering the detector region in a single bunch crossing is out of reach for any realistic computing facility due to the unprecedented number of such particles. To make it feasible a number of optimisations have been applied to the detector simulation workflow. This contribution presents an overview of the main characteristics of the beam-induced background at a Muon collider, the detector technologies considered for the experiment and how they are taken into account to strongly reduce the number of irrelevant computations performed during the detector simulation. Special attention is dedicated to the optimisation of track reconstruction with the conformal tracking algorithm in this high-occupancy environment, which is the most computationally demanding part of event reconstruction.

97 MATHEMATICS AND COMPUTING↗

Isotope dependence of muon-to-electron conversion

The lepton-flavor-violating conversion of a muon into an electron in the field of a nucleus is one of the most sensitive probes of physics beyond the Standard Model and the experiments Mu2e, COMET, and DeeMe will explore uncharted terrain in the near future. An observation of this μ - → e - conversion process opens up the possibility to distinguish the underlying operator or new-physics model by exploiting the target-nucleus dependence of the conversion rate. To facilitate the choice of optimal targets we provide μ - → e - conversion rates for all stable isotopes and estimate nuclear-structure uncertainties. Our results enable studies of mixed or enriched target materials that are particularly promising for distinguishing scenarios in which the muon converts either on protons or neutrons.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Hadronic uncertainties versus new physics for the W boson mass and Muon g – 2 anomalies

There are now two single measurements of precision observables that have major anomalies in the Standard Model: the recent CDF measurement of the W mass shows a 7σ deviation and the Muon g – 2 experiment at FNAL confirmed a long-standing anomaly, implying a 4.2σ deviation. Doubts regarding new physics interpretations of these anomalies could stem from uncertainties in the common hadronic contributions. We demonstrate that these two anomalies pull the hadronic contributions in opposite directions by performing electroweak fits in which the hadronic contribution was allowed to float. The fits show that including the g – 2 measurement worsens the tension with the CDF measurement and conversely that adjustments that alleviate the CDF tension worsen the g – 2 tension beyond 5σ. This means that if we adopt the CDF W mass measurement, the case for new physics in either the W mass or muon g – 2 is inescapable regardless of the size of the SM hadronic contributions. Lastly, we demonstrate that a mixed scalar leptoquark extension of the Standard Model could explain both anomalies simultaneously.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

Muon colliders to expand frontiers of particle physics

Muon colliders offer enormous potential for the exploration of the particle physics frontier but are challenging to realize. As such, a new international collaboration is forming to make such a muon collider a reality.

43 PARTICLE ACCELERATORS↗

The MuFusE large-volume diamond anvil cell for exploring muon-catalyzed fusion at higher pressures and temperatures

A new large-volume diamond anvil cell (DAC) has been developed for the Muon-catalyzed Fusion (μCF) Experiment (MuFusE), enabling the compression and heating of deuterium–tritium (d–t) mixtures to pressures and temperatures needed to advance μCF research. The MuFusE DAC achieves the large sample volumes necessary for high-precision fusion measurements while integrating cryogenic loading, all-metal sealing, and flexible bellows to maintain a secure environment during cell compression. Combined with remote pneumatic actuation and secondary containment, the DAC safely managed a 25 Ci tritium inventory while providing a clear optical path for in situ measurements of sample pressure and composition via laser spectroscopy. Utilizing 5 mm diameter diamond anvils oriented in the path of a high-intensity muon beam, the apparatus achieved a stable sample volume of 19.2 mm 3 at liquid density, pressures up to 933 MPa and temperatures up to 400 K—benchmarks that significantly exceed previously reported limits for static d–t targets.

Kalow, J. D. [Acceleron Fusion, Inc., Cambridge, M↗

The Muon Smasher’s Guide

In this work, we lay out a comprehensive physics case for a future high-energy muon collider, exploring a range of collision energies (from 1 to 100 TeV) and luminosities. We highlight the advantages of such a collider over proposed alternatives. We show how one can leverage both the point-like nature of the muons themselves as well as the cloud of electroweak radiation that surrounds the beam to blur the dichotomy between energy and precision in the search for new physics. The physics case is buttressed by a range of studies with applications to electroweak symmetry breaking, dark matter, and the naturalness of the weak scale. Additionally, we make sharp connections with complementary experiments that are probing new physics effects using electric dipole moments, flavor violation, and gravitational waves. An extensive appendix provides cross section predictions as a function of the center-of-mass energy for many canonical simplified models.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗