Muon Collider Research Articles

Direct production of SM-singlet scalars at the muon collider

The present work proposes a minimal extension of the Standard Model (SM) where a gauge-singlet scalar (ϕ) can be directly produced at the muon colliders without relying on its mixing with any other doublet state present in the theory. The New Physics (NP) interactions include a TeV-scale scalar leptoquark of electromagnetic charge 1/3 arising naturally in a grand unifying gauge formulation. Within the proposed framework, the SM-singlet scalar can effectively couple to various SM fields at the one-loop level, out of which the μ¯μϕ and γγϕ couplings are crucial to produce it at the future muon colliders. Assuming O(1) NP couplings, the decay widths and production cross-section of the singlet scalar have been discussed in detail over the considered parameter space. Depending on the resonance scale, di-lepton and/or di-gluon channels can be significant to test/falsify the model.

Probing Zee-Babu states at muon colliders

Probing Zee-Babu states at muon colliders

Lepton flavor violation by two units

Charged lepton flavor violation arises in the Standard Model Effective Field Theory at mass dimension six. The operators that induce neutrinoless muon and tauon decays are among the best constrained and are sensitive to new-physics scales up to 107GeV. An entirely different class of lepton-flavor-violating operators violates lepton flavors by two units rather than one and does not lead to such clean signatures. Even the well-known case of muonium–anti-muonium conversion that falls into this category is only sensitive to two out of the three ΔLμ=−ΔLe=2 dimension-six operators. We derive constraints on many of these operators from lepton flavor universality and show how to make further progress with future searches at Belle II and future experiments such as Z factories or muon colliders.

Doubly-charged scalars of the minimal left-right symmetric model at muon colliders

We investigate the prospects of probing the doubly-charged scalars of the minimal Left-Right Symmetric model (MLRSM) at a muon collider. We assess its capability by studying the production of doubly-charged scalars and their subsequent decay into four charged lepton final states containing the same-charge lepton pairs. We find that the channels with same-charge electron and muon pairs, i.e., (e±e±μ∓μ∓ and its charge conjugated pairs), have the largest sensitivity due to the lowest Standard Model background. Besides, we show that the possibility of using fully polarized initial muon beams in the muon collider can enhance the detection sensitivity of doubly-charged scalars of the MLRSM. Furthermore, we show that one can put exclusion limits on the magnitudes of triplet Yukawa couplings that are directly related to the neutrino sector of the MLRSM for the mass range 1.1−5 TeV of the doubly-charged scalars.

Discriminating Majorana and Dirac heavy neutrinos at lepton colliders

In this paper we investigate how well the nature of heavy neutral leptons can be determined at a future lepton collider, after its potential discovery. Considered in a simplified model are prompt decays of the neutrino in the mass range from 100 GeV to 10 TeV. We study event selection and application of multivariate analyses to determine whether such a newly discovered particle is of the Dirac or Majorana nature. Combining lepton charge and kinematic event variables, we find that the nature of a heavy neutrino, whether it is a Dirac or a Majorana particle, can be determined at 95% C.L. almost in the whole discovery range. We will briefly speculate about other than the studied channels and the robustness of this statement in more general models of heavy neutral leptons, particularly on the complementarity of high-energy electron-positron vs. muon colliders on resolving the flavor structure of heavy neutrinos.

Probing muon g-2 at a future muon collider

The 4.2\sigma4.2σ discrepancy in the (g-2)(g−2) of the muon provides a hint that may indicate that physics beyond the standard model is at play. A multi-TeV scale muon collider provides a natural testing ground for this physics. In this paper, we discuss the potential to probe the BSM parameter space that is consistent with solving the (g-2)_{\mu}(g−2)μ discrepancy in the language of the SMEFT, utilizing the statistical power provided by fitting event rates collected running at multiple energies. Our results indicate the importance of including interference between the BSM and the SM amplitudes, and illustrates how a muon collider running at a handful of lower energies and with less total collected luminosity can better significantly constrain the space of relevant SMEFT coefficients than would be possible for a single high energy run.

Fast timing detectors for the muon system of a muon collider experiment: requirements from simulation and prototype performance

Looking at the future path of high-energy physics, a muon collider offers incomparable potential for discovery in the multi-TeV energy range. However, its development must address some relevant technological challenges, which arise from the short muon lifetime, 2.2 μs, and from the difficulty of producing large numbers of muons in groups with small emittance. The first one in particular leads to the production of the so-called Beam-induced Background (BIB), which affects the design of the machine and detector.The purpose of this contribution is to describe the expected performance of the muon system of a multipurpose muon collider detector designed to reconstruct the products of multi-TeV collisions with extreme accuracy.We are proposing a design of the muon system fully based on Micropattern Gaseous Detectors (MPGD), which foresees the combination of tracking layers based on Triple-GEM detectors, with a timing layer based on a new generation MPGD called Picosec. Picosec was developed in the last few years by the RD51 collaboration, with the aim of obtaining a fast timing (sub ns) MPGD.A dedicated R&D is ongoing to optimize such a technology for the application in a muon collider experiment, both from the detector and the mechanics point of views.This contribution will present the results obtained during the R&D of the Picosec technology for the muon collider both from laboratory tests and test beams performed in 2023. Special attention will be given to the performance obtained with different gas mixtures. Moreover, we will discuss the plans for the R&D dedicated to the muon collider.

First results on MPGD prototypes in test beam for MPGD-based HCAL at a future Muon Collider experiment

The Multi-TeV Muon Collider will allow significant advancement in particle physics and in the understanding of its Standard Model for the era after the High-Luminosity LHC. The Muon Collider physics program involves precise Higgs boson sector measurements and TeV-scale new physics exploration. These goals demand accurate full-event reconstruction. The Particle Flow algorithm, which utilizes tracking, calorimeter, and muon detectors, is ideal for identifying and precisely estimating particle momenta/energies and can accomplish this task. Tracking detectors measure charged particle momenta, while calorimeters provide energy measurements for photons and neutral hadrons. Therefore, combining an exceptional tracking system with high-granularity calorimeters is essential. A major challenge is discerning μμ collision products from beam-induced-background, due to muon decay. To address this, an innovative hadronic calorimeter (HCAL) using Micro Pattern Gas Detectors (MPGDs) is proposed. MPGDs provide robust technology for high radiation and ensure precise spatial measurements. Dedicated studies are needed to assess and optimize the performance of an MPGD-based HCAL, including the development of medium-scale prototypes for performance measurements.This article describes the studies for a hadronic calorimeter based on MPGDs, relying on advanced technologies such as μ-rwell, resistive MicroMegas, and RPWELL. To assess the performance of MPGD detectors, a test beam was conducted in July 2023 at the Super Proton Synchrotron at CERN. This test beam aimed to evaluate the performance of MPGD detectors with 1×1 cm2 pad readout under beam irradiation. Preliminary results from this test are presented.

Using k-means assistant event selection strategy to study anomalous quartic gauge couplings at muon colliders

The search for new physics beyond the Standard Model is one of the central problems of current high energy physics interest. As the luminosities of current and near-future colliders continue to increase, the search for new physics has increased the requirements for processing large amounts of data. Meanwhile, quantum computing which is rapidly evolving, has great potential to become a powerful tool to help search for new physics signals. Since the k-means algorithm is known to be able to be accelerated with the help of quantum computing, we investigate and propose an event selection strategy based on k-means algorithm to search for new physics signals. Taking the case of tri-photon processes at the muon colliders as an example, the event selection strategy is shown to be effective in helping to search for the signals of dimension-8 operators contributing to anomalous quartic gauge couplings. Compared with traditional event selection strategy, the expected constraints are generally tighter.

Phenomenology of heavy neutral gauge boson at muon collider

Phenomenology of heavy neutral gauge boson at muon collider

Muon Collider Forum report

A multi-TeV muon collider offers a spectacular opportunity in the direct exploration of the energy frontier. Offering a combination of unprecedented energy collisions in a comparatively clean leptonic environment, a high energy muon collider has the unique potential to provide both precision measurements and the highest energy reach in one machine that cannot be paralleled by any currently available technology. The topic generated a lot of excitement in Snowmass meetings and continues to attract a large number of supporters, including many from the early career community. In light of this very strong interest within the US particle physics community, Snowmass Energy, Theory and Accelerator Frontiers created a cross-frontier Muon Collider Forum in November of 2020. The Forum has been meeting on a monthly basis and organized several topical workshops dedicated to physics, accelerator technology, and detector R&D. Findings of the Forum are summarized in this report.

Precision Higgs width and couplings with a high energy muon collider

The interpretation of Higgs data is typically based on different assumptions about whether there can be additional decay modes of the Higgs or if any couplings can be bounded by theoretical arguments. Going beyond these assumptions requires either a precision measurement of the Higgs width or an absolute measurement of a coupling to eliminate a flat direction in precision fits that occurs when ghVV/ghVVSM\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\left|{g}_{hVV}/{g}_{hVV}^{SM}\\right| $$\\end{document} > 1, where V = W±, Z. In this paper we explore how well a high energy muon collider can test Higgs physics without having to make assumptions on the total width of the Higgs. In particular, we investigate off-shell methods for Higgs production used at the LHC and searches for invisible decays of the Higgs to see how powerful they are at a muon collider. We then investigate the theoretical requirements on a model which can exist in such a flat direction. Combining expected Higgs precision with other constraints, the most dangerous flat direction is described by generalized Georgi-Machacek models. We find that by combining direct searches with Higgs precision, a high energy muon collider can robustly test single Higgs precision down to the O.1%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O}\\left(.1\\%\\right) $$\\end{document} level without having to assume SM Higgs decays. Furthermore, it allows one to bound new contributions to the width at the sub-percent level as well. Finally, we comment on how even in this difficult flat direction for Higgs precision, a muon collider can robustly test or discover new physics in multiple ways. Expanding beyond simple coupling modifiers/EFTs, there is a large region of parameter space that muon colliders can explore for EWSB that is not probed with only standard Higgs precision observables.

Aiming for tops of ALPs with a muon collider

Future muon colliders with center-of-mass energy of O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{O} $$\\end{document}(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.

Measuring lepton number violation in heavy neutral lepton decays at the future muon collider

The future muon collider has the potential to discover feebly interacting particles in a wide range of masses above the electroweak scale. It is particularly suitable to search for heavy neutral leptons (HNLs), as their production cross section σ∼mW−2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sigma \\sim {m}_W^{-2} $$\\end{document} is not suppressed by the new physics scale. We demonstrate that with the capacity to observe up to 105 events in the previously unexplored TeV mass range, the muon collider provides the means to measure the fraction of lepton number violating (LNV) processes with precision at the level of a percent. This capability enables elucidating the nature of HNLs, allowing us to differentiate between Majorana, Dirac, and non-minimal scenarios featuring multiple degenerate HNLs. We link the observed fraction of LNV processes to the parameters of the model with three degenerate HNLs, which could be responsible for generating baryon asymmetry in the Universe. Additionally, we present a simple estimate for the number of signal events, as well as analyze the feasibility of vector boson fusion processes in searches for HNLs.

Magnets for a Muon Collider

The renewed interest for a muon collider has motivated a thorough analysis of the accelerator technology required for this collider option at the energy frontier. Magnets, both normal and superconducting, are among the crucial technologies throughout the accelerator complex, from production, through acceleration and collision. In this paper we initiate a catalog of magnet specifications for a muon collider at 10TeV center-of-mass. We take the wealth of work performed within the scope of the US-DOE Muon Accelerator Program as a starting point, update it with present demands for the increased energy reach, and focus on the magnet types and variants with the most demanding performance. These represent well the envelope of issues and challenges to be addressed by future design and development. We finally give a first and indicative selection of suitable magnet technology, taking into account both established practices as well as the perspective evolution in the field of accelerator magnets.

A two harmonics circuit for the powering of the very fast RCS (Rapid Cycling Synchrotron) of the muon collider accelerator

This paper analyses the application of a two harmonics circuit with additional active filter for the powering of the four RCS stages of the muon acceleration to the ultimate 10 TeV energy level.

Leptonic di-flavor and di-number violation processes at high energy colliders* *This study was supported in part by the National Natural Science Foundation of China (NNSFC) (12047503, 12075301, 11821505, 11705045). It was also Supported in part by the Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-SYS006)

The leptonic di-flavor violation (LFV) processes and and leptonic di-number violation (LNV) processes ( ) at same-sign high energy colliders are studied. The new physics (NP) factors that may play roles in these processes are highlighted by cataloging them into three types. Taking into account the experimental constraints, the processes at colliders are computed, and the results are presented properly. The results lead to the conclusion that observing the NP factors through the LFV and LNV processes at TeV-energy colliders has significant advantages that cannot be achieved elsewhere. Therefore, once the techniques for muon acceleration and collision are developed successfully, the option of building same-sign high energy muon colliders should be seriously considered.

Boosting dark matter searches at muon colliders with machine learning: The mono-Higgs channel as a case study

Abstract The search for dark matter (DM) candidates at high-energy colliders is one of the most promising avenues to understand the nature of this elusive component of the universe. Several searches at the Large Hadron Collider (LHC) have strongly constrained a wide range of simplified models. The combination of the bounds from the LHC with direct-detection experiments exclude the most minimal scalar-singlet DM model. To address this, lepton portal DM models are suitable candidates where DM is predominantly produced at lepton colliders since the DM candidate only interacts with the lepton sector through a mediator that carries a lepton number. In this work, we analyze the production of DM pairs in association with a Higgs boson decaying into two bottom quarks at future muon colliders in the framework of the minimal lepton portal DM model. It is found that the usual cut-based analysis methods fail to probe heavy DM masses for both the resolved (where the decay products of the Higgs boson can be resolved as two well-separated small-R jets) and the merged (where the Higgs boson is clustered as one large-R jet) regimes. We have then built a search strategy based on boosted-decision trees (BDTs). We have optimized the hyperparameters of the BDT model to both have a high signal-to-background ratio and to avoid overtraining effects. We have found very important enhancements of the signal significance with respect to the cut-based analysis by factors of 8–50 depending on the regime (resolved or merged) and the benchmark points. Using this BDT model on a 1D parameter space scan, we found that future muon colliders with $\sqrt{s}=3$ TeV and ${\cal L} = 1~{\rm ab}^{-1}$ can exclude DM masses up to 1 TeV at the 95% confidence level.

Muon accelerators—muon lifetime measurements as window to Planck scale physics

A prominent effective description of particles interacting with the quantum properties of gravity is through modifications of the general relativistic dispersion relation. Such modified dispersion relations lead to modifications in the relativistic time dilation. A perfect probe for this effect, which goes with the particle energy cubed E 3 over the quantum gravity scale E QG and the square of the particle mass M 2 would be a very light unstable particle for which one can detect the lifetime in the laboratory (lab) as a function of its energy measured in the lab to very high precision. In this article we conjecture that a muon collider or accelerator would be a perfect tool to investigate the existence of an anomalous time dilation, and with it the fundamental structure of spacetime at the Planck scale.

Study of the gluonic quartic gauge couplings at muon colliders

The potential of muon colliders opens up new possibilities for the exploration of new physics beyond the Standard Model. It is worthwhile to investigate whether muon colliders are suitable for studying gluonic quartic gauge couplings (gQGCs), which can be contributed by dimension-8 operators in the framework of the Standard Model effective field theory, and are intensively studied recently. In this paper, we study the sensitivity of the process to gQGCs. Our results indicate that the muon colliders with c.m. energies larger than 4 TeV can be more sensitive to gQGCs than the Large Hadron Collider.