High Energy Muon Collider Research Articles

Muonic force and nonstandard neutrino interactions at muon colliders

The discovery of neutrino oscillations implies that neutrinos are massive and mixed, necessitating an extension of the Standard Model, which may require the introduction of nonstandard neutrino interactions (NSIs). We investigate the potential of a high-energy muon collider to probe such NSIs with muons, specifically focusing on muonic forces. By analyzing the monophoton signal from the process μ+μ−→νν¯γ, we explore four-fermion contact interactions involving two muons and two neutrinos. Moreover, we examine minimal models that generate scalar and vector-mediated NSIs and study their phenomenology. Projected sensitivities for the strength of NSIs, εαβμμ, are presented at a 95% confidence level for a center-of-mass energy of 3 TeV and integrated luminosities of L=1 and 10 ab−1, showcasing the complementarity between a muon collider and other experimental probes. Published by the American Physical Society 2024

Searching for heavy neutral leptons at a future muon collider

As the planning stages for a high energy muon collider enter a more concrete era, an important question arises as to what new physics could be uncovered. A TeV-scale muon collider is also a vector boson fusion (VBF) factory with a very clean background, and as such it is a promising environment to look for new physics that couples to the electroweak (EW) sector. In this paper, we explore the ability of a future TeV-scale muon collider to search for Majorana and Dirac heavy neutral leptons (HNLs) produced via EW bosons. Employing a model-independent, conservative approach, we present an estimation of the production and decay rate of HNLs over a mass range between 200 GeV and 9.5 TeV in two benchmark collider proposals with s=3, 10 TeV, as well as an estimation of the dominant Standard Model (SM) background. We find that exclusion limits for the mixing between the HNLs and SM neutrinos can be as low as O(10−6). Additionally, we demonstrate that a TeV-scale muon collider allows for the ability to discriminate between Majorana and Dirac type HNLs for a large range of mixing values. Published by the American Physical Society 2024

Single axion-like particles production in the γγ channel at high energy muon colliders

Axion-like particles (ALPs), which are pseudo-scalar particles, appear in various new physics models with the spontaneous global symmetry breaking. The difference between ALPs and axion models is that for the former both masses and couplings of ALPs are free parameters. It appears that the ALPs with masses above 1[Formula: see text]GeV are beyond the reach of low-energy accelerators, existing cosmological and astrophysical constraints, while high-energy colliders provide unique possibilities to explore such particles. With the establishment of the International Muon Collider Collaboration, muon colliders have been widely concerned. In general, ALPs are mainly produced at muon colliders through the [Formula: see text] annihilation and electroweak vector-boson-fusion (VBF) processes. In this paper, we study the prospect of the production of heavy ALPs with mass greater than 10[Formula: see text]GeV via the massless [Formula: see text]-fusion process at the multi-TeV muon collider. Focusing on the couplings of ALPs to the Standard Model gauge bosons, we obtain the excluding and discovering capabilities for ALPs at the 3[Formula: see text]TeV and 10[Formula: see text]TeV muon colliders. Our numerical results suggest that the multi-TeV muon collider may be more sensitive to ALPs with the mass of 35–400[Formula: see text]GeV than LHC, and can provide the possibility to search for ALPs with masses greater than 3[Formula: see text]TeV. In addition, we also discuss the effect of narrow width approximation on the ALP search sensitivity.

Search for a leptoquark and vector-like lepton in a muon collider

The proposal for a high-energy muon collider offers many opportunities in the search for physics beyond the Standard Model (BSM). The collider by construction is likely to be more sensitive to the muon-philic models, primarily motivated by the BSM explanation of muon (g−2) excess and quark flavor anomalies. In this work, we explore the potential of the proposed muon collider in the context of such models and focus on one such model that extends the Standard Model (SM) with a leptoquark, a vector-like lepton, and a real scalar. In this model, we propose searches for TeV scale leptoquarks in 2μ+2b+E̸T channel. Notably, the leptoquark can be produced singly at the muon collider with a large cross-section. We have shown that a significant signal in this channel can be detected at a 3 TeV muon collider even with an integrated luminosity as low as ∼10 fb−1.

Higgs boson width and couplings at high energy muon colliders with forward muon detection

We propose a novel method using the ZZ-fusion channel and forward muon detection at high energy muon colliders to address the challenge of the Higgs coupling-width degeneracy. Our approach enables inclusive Higgs rate measurement to 0.75% at 10 TeV muon collider, breaking the coupling-width degeneracy. Results indicate the potential to refine Higgs coupling to subpercent levels and estimate its total width within (−0.41%, +2.1%). Key insights include the effectiveness of forward muon tagging in signal-background separation despite broad recoil mass distribution due to muon energy reconstruction and beam energy spread. The study emphasizes the significance of muon rapidity coverage up to |η(μ)|<6, enhancing measurement precision. Our findings highlight the unique capabilities of high energy lepton colliders for model-independent Higgs coupling determination and lay the groundwork for future advancements in muon collider technology and Higgs physics research. Published by the American Physical Society 2024

Top Yukawa coupling measurement at the muon collider

We have presented a detailed study about the prospects for the measurement of the top Yukawa coupling in the vector boson fusion production of a top quark pair at high energy muon colliders. By employing the effective W approximation and the high energy limit for the helicity amplitudes of the subprocess W+W−→tt¯, we have derived the energy scaling of the statistical signal significance in the presence of the anomalous couplings by focusing on the interference term only. The sensitivity on the top Yukawa coupling decreases as the bin energy increases. For the anomalous triple gauge boson couplings and the gauge-boson-fermion couplings with E2 energy growing behavior, the signal significance has mild increase at the beginning and starts to decrease for s^tt¯∼0.2sμ+μ−. The 95% CL on the anomalous top Yukawa coupling is projected to be 5.6% (1.7%) at a 10 (30) TeV muon collider, which is comparable to the sensitivity of 2% at the 100 TeV collider. Published by the American Physical Society 2024

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.

Top Yukawa coupling determination at high energy muon collider

The Top Yukawa coupling profoundly influences several core mysteries linked to the electroweak scale and the Higgs boson. We study the feasibility of measuring the Top Yukawa coupling at high energy muon colliders by examining the high energy dynamics of the weak boson fusion to top quark pair processes. A deviation of the Top Yukawa coupling from the Standard Model would lead to a modified VV→tt¯ process, violating unitarity at high energy. Our analysis reveals that utilizing a muon collider with a center-of-mass energy of 10 TeV and an integrated luminosity of 10 ab−1 allows us to investigate the Top Yukawa coupling with a precision surpassing 1.5%, more than one order of magnitude better than the precision from tt¯h channel at muon colliders. This precision represents a notable enhancement compared to the anticipated sensitivities of the High-Luminosity LHC (3.4%) and those at muon colliders derived from the tt¯H process. Published by the American Physical Society 2024

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.

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.

Searching for heavy neutral lepton and lepton number violation through VBS at high-energy muon colliders

High-energy muon collider can play as an emitter of electroweak gauge bosons and thus leads to substantial vector boson scattering (VBS) processes. In this work, we investigate the production of heavy neutral lepton (HNL) N and lepton number violation (LNV) signature through VBS at high-energy muon colliders. VBS induces LNV processes W±Z/γ → ℓ±N → ℓ±ℓ±W∓ → ℓ±ℓ±qq¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ q\\overline{q} $$\\end{document}′ with an on-shell HNL N at μ+μ− colliders. In analogy to neutrinoless double-beta decay with the HNL in t-channel, the LNV signature W+W+ → ℓ+ℓ+ can also happen via VBS at same-sign muon collider. They provide clean and robust LNV signatures to tell the nature of Majorana HNLs and thus have more advantageous benefits than direct μμ annihilation. We analyze the potential of searching for Majorana HNL and obtain the exclusion limits on mixing VℓN. Based on this same-sign lepton signature, we also obtain the sensitivity of muon collider to the Weinberg operator.

Going all the way in the search for WIMP dark matter at the muon collider through precision measurements

Dark Matter is a necessary ingredient for a complete theory of Nature, which has so far remained elusive in laboratory searches for new particles. Searches at current and future colliders are in principle a promising way to search for electroweak charged dark matter particles, but the sensitivity of experiments at the LHC and future pp colliders falls short to fully probe the whole mass range allowed for electroweak charged dark matter particles, which extends in principle up to the O(0.1) PeV. In this work we examine the effect of on-shell and off-shell propagation of electroweak charged thermal dark matter particles on integrated and differential rates of several Standard Model final states at the muon collider, considering candidates from weak 2-plet at the TeV scale up to 7-plet and 9-plet in the O(0.1) PeV ballpark. For fermionic WIMPs we find that all dark matter candidates with nle 5, corresponding to a thermal mass up to 14 TeV, can be probed at the high-energy muon collider for some center-of-mass energy at or below 14 TeV. For the n>5 WIMPs our results show that higher energy muon colliders offer a route to conclusively probe both scalar and fermionic WIMPs off-shell production all the way up to the perturbativity bound for WIMP dark matter at O(0.1) PeV. Our results bring WIMPs over the whole allowed mass range in the realm of collider searches and motivate research and development for the realization of a high energy muon collider.

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 mN and the mixing angle with ℓ-flavor Uℓ. In this work, we focus on the regime mN> O(100) GeV and study the projected sensitivities on the |Uℓ|2 − mN plane with the full-reconstructable HNL decay into a hadronic W and a charged lepton. The projected reach in |Uℓ|2 leads to the best sensitivities in the TeV realm.

Revealing the origin of neutrino masses through the Type II Seesaw mechanism at high-energy muon colliders

The future muon collider can play as an ideal machine to search for new physics at high energies. In this work, we study the search potential of the heavy Higgs triplet in the Type II Seesaw mechanism at muon colliders with high collision energy and high luminosity. The latest neutrino oscillation data are taken into account for realizing the leptonic decay modes of the charged Higgs bosons (H±±, H±) in the Type II Seesaw. We show the impact of neutrino mass and mixing parameters on the purely leptonic decays. The pair production of doubly charged Higgs H++H−− is through direct μ+μ− annihilation and vector boson fusion (VBF) processes at muon collider. The associated production H±±H∓ can only be induced by VBF processes. We simulate both the purely leptonic and bosonic signal channels of charged Higgs bosons in Type II Seesaw, together with the Standard Model backgrounds. We show the required luminosity for the discovery of the charged Higgses and the reachable limits on the leptonic decay branching fractions.

Shining light on magnetic monopoles through high-energy muon colliders

The search for magnetic monopoles has been a longstanding concern of the physics community for nearly a century. However, up to now, the existence of elementary magnetic monopoles remains an open question. The electroweak 't Hooft-Polyakov monopoles have been predicted with mass at the order of 10 TeV. This mass scale is unreachable at current colliders. Recently, the muon colliders have gained much attention in the community due to technological developments. The advantages of the muon beam encourage us to raise the high-energy option and consider the high-energy muon collider as a unique opportunity to search for magnetic monopoles. This letter discusses the production of magnetic monopoles via the annihilation process and proposes the search for magnetic monopoles at future high-energy muon colliders.

Pair production of the vectorlike top partner at future muon collider

Motivated by the recent interests in future high-energy muon colliders, we study the search potential of heavy singlet vectorlike top partner (VLQ-T) through the s-channel process μ+μ−→TT¯ at such facilities with s=6 TeV. We focus on the final signals via the T→Wb decay channel where one of the W bosons decays leptonically and the other decays hadronically. By carrying out a full simulation for the signals and the relevant Standard Model backgrounds, the 2σ exclusion limit and 5σ discovery prospects are, respectively, obtained on the VLQ-T mass and the branching ratio of T→Wb. The numerical results show that the future high-energy muon collider will be a promising hunting ground for such new particles.

Electroweak ALP searches at a muon collider

A high-energy muon collider with center-of-mass energy around and above 10 TeV is also a vector boson fusion (VBF) machine, due to the significant virtual electroweak (EW) gauge boson content of high-energy muon beams. This feature, together with the clean environment, makes it an ideal collider to search for TeV-scale axion-like particles (ALP) coupling to Standard Model EW gauge bosons, which current and other future colliders have limited sensitivities to. We present detailed analyses of heavy ALP searches in both the VBF and associated production channels at a muon collider with different running benchmarks. We also show projected constraints on the ALP couplings in the effective field theory, including an operator with its coefficient not determined by the mixed Peccei-Quinn anomaly. We demonstrate that a muon collider could probe new ALP parameter space and push the sensitivities of the couplings between the ALP and EW gauge bosons by one order of magnitude compared to HL-LHC. The projected limits and search strategies for ALPs could also be applied to other types of resonances coupling to EW gauge bosons.

High precision higgs from high energy muon colliders

Muon colliders are an exciting possibility for reaching the highest energies possible on the shortest timescale. They potentially combine the greatest strengths of e+e− and pp colliders by bridging the energy versus precision dichotomy. In this paper we study the sensitivity of Higgs properties that can be achieved with a future 3 or 10 TeV muon collider from single Higgs production. The results presented here represent the first comprehensive picture for the precision achievable including backgrounds and using fast detector simulation with Delphes. Additionally, we compare the results of fast detector simulation with available full simulation studies that include the muon collider specific Beam Induced Background, and show the results are largely unchanged. We comment on some of the strengths and weaknesses of a high energy muon collider for Higgs physics alone, and demonstrate the complementarity of such a collider with the LHC and e+e− Higgs factories. Furthermore, we discuss some of the exciting avenues for improving future results from both theoretical and detector R&D that could be undertaken.

Probing heavy triplet leptons of the type-III seesaw mechanism at future muon colliders

We study the search potential of heavy triplet leptons in a type-III seesaw mechanism at future high-energy and high-luminosity muon colliders. The impact of up-to-date neutrino oscillation results is taken into account on neutrino parameters and the consequent decay patterns of heavy leptons in a type-III seesaw. The three heavy leptons in the Casas-Ibarra parametrization with diagonal unity matrix are distinguishable in their decay modes. We consider the pair production of charged triplet leptons ${E}^{+}{E}^{\ensuremath{-}}$ through both ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ annihilation and vector boson fusion (VBF) processes. The leading-order framework of electroweak parton distribution functions is utilized to calculate the VBF cross sections. The charged triplet leptons can be probed for masses above $\ensuremath{\sim}1\text{ }\text{ }\mathrm{TeV}$. The ${E}^{+}{E}^{\ensuremath{-}}\ensuremath{\rightarrow}ZZ{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ channel can be further utilized to fully reconstruct the three triplet leptons and distinguish neutrino mass patterns. The pair production of heavy neutrinos $N$ and the associated production ${E}^{\ifmmode\pm\else\textpm\fi{}}N$ are only induced by VBF processes and lead to a lepton-number-violating (LNV) signature. We also study the search potential of LNV processes at future high-energy muon colliders with $\sqrt{s}=30\text{ }\text{ }\mathrm{TeV}$.