Muon Collider Research Articles

Charged resonances and MDM bound states at a multi-TeV muon collider

A multi-TeV muon collider proves to be very efficient not only for the search for new heavy neutral particles, but also for the discovery of charged bosons of the W′ type. We find that, by analyzing the associated production with a Standard Model W, charged resonances can be probed directly up to multi-TeV mass values close to the collision energy, and for very small couplings with the SM fermions, of the order of 10−3 − 10−4 times the SM weak coupling. Additionally, charged bound states of WIMP Minimal Dark Matter, specifically a Majorana fermionic 5-plet, can be discovered with low statistics by running above the kinematic threshold, at a center-of-mass energy just slightly above the mass of the MDM bound state. This opens up a very interesting possibility for the discovery of WIMPs, complementary to the search for the resonant production of the neutral MDM bound state component, which relies on an on-peak search. For 5-plet MDM, indeed, the proposed search strategy is more efficient than the WIMP searches based on mono-X, missing-mass and disappearing tracks signatures.

A boosted muon collider

A muon collider could produce the heavier Standard Model particles with a boost, for example in resonant processes such as μ−μ+→ h or μ−μ → Z. We propose machine configurations that produce the boost (asymmetric beam energies, tilted beams) and estimate how much the luminosity is reduced or perhaps enhanced. The feasibility of the proposed configurations, as well as an estimation of the beam-induced backgrounds and beam energy spread, needs to be evaluated in order to derive more solid conclusions on the physics potential of such boosted collider configurations. If achievable, the boost can provide new interesting observational opportunities. For example it can significantly enhance the sensitivity to long-lived new particles decaying in a far-away detector, such as dark higgses or sterile neutrinos produced in h or Z decays.

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.

LePDF: Standard Model PDFs for high-energy lepton colliders

The emission of collinear radiation off an elementary lepton can be factorised from the hard scattering process by introducing Parton Distribution Functions of a Lepton (LePDF), which, contrary to protons, can be derived from first principles. In case of multi-TeV lepton colliders, such as the muon colliders currently being proposed, the complete structure of Standard Model interactions must be taken into account. In this work we solve numerically the corresponding DGLAP equations at the double-log order and provide public files with LePDFs for both muons and electrons, including polarisation effects. We discuss several interesting aspects of the resulting PDFs and compare them with the Effective Vector Approximation, showing that the latter fails to describe well the vector bosons PDFs at small momentum fractions, unless it is extended to higher orders.

Bell inequalities and quantum entanglement in weak gauge boson production at the LHC and future colliders

Quantum entanglement of weak interaction gauge bosons produced at colliders can be explored by computing the corresponding polarization density matrix. To this end, we consider the Higgs boson decays Hrightarrow W W^* and Hrightarrow Z Z^*, in which W^* and Z^* are off-shell states, and the WW, WZ and ZZ di-boson production in proton collisions. The polarization density matrix of the di-boson state is determined by the amplitude of the production process and can be experimentally reconstructed from the angular distribution of the momenta of the final states into which the gauge bosons decay. We show that a suitable instance of the Bell inequality is violated in Hrightarrow Z Z^* to a degree that can be tested at the LHC with future data. The same Bell inequality is violated in the production of WW and ZZ boson pairs for invariant masses above 900 GeV and scattering angles close to pi /2 in the center of mass frame. LHC data in this case are not sufficient to establish the violation of the Bell inequality. We also analyze the prospects for detecting Bell inequality violations in di-boson final states at future e^+e^- and muon colliders. A further observable that provides a lower bound on the amount of polarization entanglement in the di-boson system is computed for each of the examined processes. The analytic expressions for the polarization density matrices are presented in full in an Appendix. We also provide the unitary matrices required in the optimization procedure necessary in testing the Bell inequalities.

Probe of axion-like particles in vector boson scattering at a muon collider

We have examined the sensitivity of the axion-like particles (ALP) couplings to electroweak gauge bosons in the diphoton production at a future muon collider. The collisions at the μ + μ − energies of 3 TeV, 14 TeV, and 100 TeV are addressed. The differential cross sections versus the invariant mass of the final photons and total cross section versus minimal diphoton invariant mass are presented. We have derived the exclusion regions for the ALP-gauge boson coupling. The obtained bounds are much stronger than the current experimental bounds in the ALP mass region 10 GeV to 10 TeV. The partial-wave unitarity constraints on the ALP-gauge boson coupling are estimated. We have shown that the unitarity is not violated in the region of the ALP coupling studied in the present paper.

Beam test, simulation, and performance evaluation of PbF2 and PWO-UF crystals with SiPM readout for a semi-homogeneous calorimeter prototype with longitudinal segmentation

Crilin (Crystal Calorimeter with Longitudinal Information) is a semi-homogeneous, longitudinally segmented electromagnetic calorimeter based on high-Z, ultra-fast crystals with UV-extended SiPM readout. The Crilin design has been proposed as a candidate solution for both a future Muon Collider barrel ECAL and for the Small Angle Calorimeter of the HIKE experiment. As a part of the Crilin development program, we have carried out beam tests of small (10 × 10 × 40 mm3) lead fluoride (PbF2) and ultra-fast lead tungstate (PbWO4, PWO-UF) crystals with 120 GeV electrons at the CERN SPS to study the light yield, timing response, and systematics of light collection with a proposed readout scheme. For a single crystal of PbF2, corresponding to a single Crilin cell, a time resolution of better than 25 ps is obtained for>3 GeV of deposited energy. For a single cell of PWO-UF, a time resolution of better than 45 ps is obtained for the same range of deposited energy. This timing performance fully satisfies the design requirements for the Muon Collider and HIKE experiments. Further optimizations of the readout scheme and crystal surface preparation are expected to bring further improvements.

Muon collider signatures for a Z′ with a maximal μ − τ coupling in [formula omitted

The gauged U(1)Lμ−Lτ model is a candidate model for explaining the muon g-2 anomaly because the Z′ in the model has a natural normal coupling to muon. Due to other experimental data constraints the viable mass range for the usual Z′ is constrained to be lower than a few hundred MeV. It has been shown that if the Z′ has a maximal off-diagonal mixing, (μ¯γμτ+τ¯γμμ)Zμ′, a large mass for Z′ is possible. This class of models has a very interesting signature for detection, such as μ−μ+→τ−τ+ pair, μ−μ+→μ±μ±τ∓τ∓ at a muon collider. In this work we study in detail these processes. We find that in the parameter space solving the muon g-2 anomaly, t-channel τ−τ+ pair production can easily be distinguished at more than 5σ level from the s-channel production as that predicted in the standard model. The smoking gun signature of doubly same sign μ±μ±+τ∓τ∓ pairs production can have a 5σ sensitivity, at a muon collider of 3 TeV with O(fb−1) luminosity.

Type-II see-saw at μ+μ− collider

Doubly-charged Higgs bosons have extensively been searched at the LHC. In this work, we study the sensitivity reach of the doubly-charged scalar (H±±) in muon collider for the well-known Type-II seesaw scenario. First, we perform a cut-based analysis to predict the discovery prospect in the muon collider operating with 3 TeV center of mass energy. In addition to this, we have also performed a multivariate analysis and mentioned the cut-based result along with the result obtained from the multivariate analysis. We find that the cut-based analysis is more significant as compared to the multivariate analysis in the large doubly-charged scalar mass region. We predict that a doubly-charged scalar mass, MH±±, up to 1450 GeV can be probed with 5σ significance for center of mass s=3 TeV and integrated luminosity L=1000fb−1.

Investigation of anomalous triple gauge couplings in μγ collision at multi-TeV muon colliders

The pursuit of discovery in particle physics has always required study at the highest possible energies. Multi-TeV muon colliders, which have important advantages, offer unprecedented potential for investigating new physics beyond the Standard Model. We extensively assume a range of center-of-mass energies from 3 to 30 TeV and a range of integrated luminosities from 1 to 90 ab−1 for future multi-TeV muon colliders. We perform a phenomenological study of the anomalous WWγ couplings via the process [Formula: see text] using a model-independent analysis in the effective field theory framework at multi-TeV muon colliders. The sensitivity estimates obtained for the anomalous WWγ couplings at systematic uncertainties of δsys = 0%, 3%, 5% are compared with the experimental results. Our best sensitivity limits at 95% confidence level for cWWW/Λ2, [Formula: see text], c B /Λ2, c W /Λ2, and [Formula: see text] couplings are [−0.030; 0.025], [−0.014; 0.014], [−0.185; 0.187], [−0.187; 0.189], and [−1.177; 1.097] TeV−2 at the multi-TeV muon collider with center-of-mass energy of 30 TeV and integrated luminosity of 90 ab−1, respectively.

Can the Higgs still account for the g−2 anomaly?

In this paper, we use an Effective Field Theory (EFT) approach to evaluate the viability of the Higgs to account for the [Formula: see text] anomaly. Although the SM contribution of the Higgs to the muon’s magnetic dipole moment is negligible, using a bottom-up EFT, we show that given the current level of experimental limits on the Higgs sector, the Higgs can still yield a viable solution to the [Formula: see text] anomaly if its couplings to the rest of the SM particles are allowed to deviate from their SM predictions. Such a solution would only require an [Formula: see text] fine-tuning. Further, applying unitarity arguments, we show that such a solution would indicate a scale of New Physics (NP) of [Formula: see text]5–8[Formula: see text]TeV, which could be lowered to [Formula: see text]3.4–4 TeV if the Higgs couplings to the [Formula: see text] and [Formula: see text] are assumed to conform to their SM predictions. We show that such a scenario could yield significant enhancement to the di-Higgs production in muon colliders, thus providing further motivation for its consideration. A key takeaway of this study is to highlight the importance of measuring the [Formula: see text] coupling in future experiments.

Vector-like lepton searches at a muon collider in the context of the 4321 model* *Supported in part by the National Natural Science Foundation of China (12150005, 12075004, . 12061141002) and MOST (2018YFA0403900)

A feasibility study is performed on the search for vector-like leptons (VLLs) at a muon collider in the context of the "4321 model", an ultraviolet-complete model with rich collider phenomenology and the potential to explain several recent existing B physics measurements or anomalies. Pair production and decays of VLLs lead to an interesting final state topology with multi-jets and multi-tau leptons. In this study, we perform a Monte Carlo investigation with various machine learning techniques and examine the projected sensitivity on VLLs over a wide mass range at a TeV-scale muon collider. We find that a 3 TeV muon collider with only of data can be sensitive over the mass range of a VLL up to 1450 GeV in the "4321 model."

Exploring muonphilic ALPs at muon colliders

Axion-like particles (ALPs) are new particles that extend beyond the standard model (SM) and are extensively investigated. When considering ALPs within an effective field theory framework, their couplings with SM particles can be studied independently. It is a daunting task to search for GeV-scale ALPs coupled to muons in collider experiments because their coupling is proportional to the muon mass. However, a recent study by Altmannshofer, Dror, and Gori (2022) highlighted the importance of a four-point interaction, W-μ--a, as well as interactions from the chiral anomaly, whose couplings are not dependent on the muon mass. These interactions provide a new opportunity to explore muonphilic ALPs (μALPs) at the GeV scale. We explore various μALP production channels at muon colliders with μALPs decaying into a pair of muons. In particular, we find that a pair of neutrinos accompanied by a μALP is the most effective channel to search for μALPs in the electrowek violating (EWV) scenario. In contrast, a photon plus a μALP becomes a better channel to search for μALPs in the electroweak preserving (EWP) scenario because there is no W-μ--a interaction in this situation. Most importantly, we find that the future bounds for μALPs in the EWV scenario are considerably stronger than those in the EWP scenario and the existing bounds for exploring μALPs with GeV .

Searching for anomalous quartic gauge couplings at muon colliders using principal component analysis

Searching for new physics (NP) is one of the areas of high-energy physics that requires the most processing of large amounts of data. At the same time, quantum computing has huge potential advantages when dealing with large amounts of data. The principal component analysis (PCA) algorithm may be one of the bridges connecting these two aspects. On the one hand, it can be used for anomaly detection, and on the other hand, there are corresponding quantum algorithms for PCA. In this paper, we investigate how to use PCA to search for NP. Taking the example of anomalous quartic gauge couplings in the tri-photon process at muon colliders, we find that PCA can be used to search for NP. Compared with the traditional event selection strategy, the expected constraints on the operator coefficients obtained by PCA based event selection strategy are even better.

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.

Lepton portal dark matter at muon colliders: Total rates and generic features for phenomenologically viable scenarios

Lepton portal dark matter at muon colliders: Total rates and generic features for phenomenologically viable scenarios

Lepton flavor physics at μ+μ+ colliders

We discuss sensitivities to lepton flavor violating (and conserving) interactions at future muon colliders, especially at μ+μ+ colliders. Compared with the searches for rare decays of μ and τ, we find that the TeV-scale future colliders have better sensitivities depending on the pattern of hierarchy in the flavor mixings. As an example, we study the case with the type-II seesaw model, where the flavor mixing parameters have direct relation to the neutrino mass matrix. At a μ+μ+ collider, the number of events of the μ+μ+ → μ+τ+ process can be larger than mathcal{O}(100) with the center of mass energy sqrt{s} = 2 TeV, and with an integrated luminosity mathcal{L} = 1 ab−1, while satisfying bounds from rare decays of μ and τ. We discuss impacts of the overall mass scale of neutrinos as well as CP violating phases to the number of expected events.

Invisible Higgs boson decay from forward muons at a muon collider

We propose to probe the Higgs boson decay to invisible particles at a muon collider by observing the forward muons that are produced in association with the Higgs in the $Z$-boson fusion channel. An excellent sensitivity is possible in line of principle, owing to the large number of produced Higgs bosons, provided a forward muon detector is installed. We find that the resolution on the measurement of the muon energy and angle will be the main factor limiting the actual sensitivity. This poses tight requirements on the forward muon detector design.

Optimal search reach for heavy neutral leptons at a muon collider

Neutrinos are the most elusive particles known. Heavier sterile neutrinos mixing with the standard neutrinos might solve the mystery of the baryon asymmetry of the universe. In this letter, we show that among all future energy frontier accelerators, muon colliders will provide the farthest search reach for such neutrinos for mass ranges above the Z pole into the multi-TeV regime, becoming the optimal machine for this kind of studies. We compare the performance of muon with electron colliders of the same machine energy and briefly discuss the complementarity in flavor space between the two types of accelerators.

Probe of the anomalous neutral triple gauge couplings in photon-induced collision at future muon colliders

The anomalous ZZγ and Zγγ neutral triple gauge couplings occurred by dimension-eight operators are investigated through the process μ+μ−→μ+γ⁎μ−→μ+Z(νν¯)μ− at the muon collider with s=3, 6, 10 and 14 TeV. The charged lepton pseudo-rapidity, the charged lepton transverse momentum and the transverse missing energy distributions are taken in consideration for the final state of the process in the analysis. The sensitivities of the anomalous couplings are obtained at 95% Confidence Level with integrated luminosity of Lint=1, 4, 10 and 20 ab−1, respectively, according to center-of-mass energies of muon collider taking into account the effects of systematic uncertainties 0%, 3% and 5%. The best limits of anomalous CBB/Λ4, CBW/Λ4, CB˜W/Λ4 and CWW/Λ4 couplings without systematic uncertainty at center-of-mass energy of 14 TeV and integrated luminosity of 20 ab−1 are found to be [-0.01026; 0.00636] TeV−4, [-0.02482; 0.03053] TeV−4, [-0.01830; 0.02510] TeV−4, [-0.06981; 0.07387] TeV−4, respectively.