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1. Probing muon g-2 at a future muon collider

   https://doi.org/10.21468/SciPostPhys.16.3.072  

   Arakawa, Jason; Rajaraman, Arvind; Sui, Taotao; Tait, Tim_M P (January 2024, SciPost Physics)

   The4.2\sigma $4.2\sigma$ 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} ${(g-2)}_{\mu }$ 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. 

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2. Lepton flavor violation: From muon decays to muon colliders

   https://doi.org/10.1103/bg4z-dmgb  

   Asadi, Pouya; Bagherian, Hengameh; Fraser, Katherine; Homiller, Samuel; Lu, Qianshu (January 2026, Physical Review D)

   We investigate the unique potential of a high-energy muon collider to probe lepton-flavor-violating signals arising from physics beyond the Standard Model (SM). Low-energy, precision searches for charged lepton flavor violation (LFV) are projected to dramatically improve their sensitivity in the coming years and could provide the first evidence of new physics. We interpret the sensitivity of these searches in terms of a set of LFV operators in the SM effective field theory. The same operators are then probed at the TeV scale via new, high-energy processes only available at a high-energy muon collider, such as $\mu \mu \to \mu \tau$ or the scattering of a muon of an electroweak gauge boson into LFV final states. We find that, for most operators, a muon collider could confirm signals if they are seen at future low-energy experiments, whereas for certain flavor combinations it extends the reach to scales well beyond those accessible at lower energies. We also project the sensitivity of a muon collider to lepton-flavor-violating decays of the SM Higgs boson and demonstrate improved sensitivity to $h\to e\tau$ and $h\to \mu \tau$ by an order of magnitude compared to the High-Luminosity LHC. The importance of having multiple, complementary probes is illustrated by considering both various combinations of operators and relative sizes of flavor-violating transitions between generations under various assumptions for the flavor structure of new physics. 

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3. Muons in air showers with IceCube: muon density at ground and high-energy muon multiplicity

   https://doi.org/10.22323/1.484.0035  

   Verpoest, Stef; IceCube-Collaboration, for the (March 2025, Sissa Medialab)

   Various measurements of muons in air showers using ground-based particle detector arrays have indicated a discrepancy between observed data and predictions from simulations. The IceCube Neutrino Observatory can offer unique insights into this issue. Its surface array, IceTop, measures the muon density at large lateral distances, while the deep in-ice detector provides information on high-energy muons. Recent analyses have determined the surface muon density and the high-energy (Eμ≳ 500 GeV) muon multiplicity in near-vertical air showers for primary energies ranging from 2.5 PeV to 100 PeV. In this contribution, we present the results and discuss their consistency with predictions from current hadronic interaction models. 

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4. Building the case for forward muon detection at a muon collider

   https://doi.org/10.1103/PhysRevD.111.053010  

   Ruhdorfer, Maximilian; Salvioni, Ennio; Wulzer, Andrea (March 2025, Physical Review D)

   We survey the opportunities offered by the detection of the forward muons that accompany the creation of neutral effective vector bosons at a muon collider, in different kinematic regimes. Vectors with relatively low energy produce the Higgs boson and the extended muon angular coverage enables studies of the Higgs properties, such as the measurement of the inclusive production cross section and the branching ratio to invisible final states. New heavy particles could be produced by vectors of higher energy, through Higgs portal interactions. If the new particles are invisible, the detection of the forward muons is essential in order to search for this scenario. The angular correlations of the forward muons are sensitive to the quantum interference between the vector-boson helicity amplitudes and can be exploited for the characterization of vector-boson scattering and fusion processes. This is illustrated by analyzing the $CP$ properties of the Higgs coupling to the $Z$ boson. Our findings provide a physics case and a set of benchmarks for the design of a dedicated forward muon detector. Published by the American Physical Society2025 

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5. Muon Collider Forum report

   https://doi.org/10.1088/1748-0221/19/02/T02015  

   Black, KM; Jindariani, S; Li, D; Maltoni, F; Meade, P; Stratakis, D; Acosta, D; Agarwal, R; Agashe, K; Aimè, C; et al (February 2024, Journal of Instrumentation)

   Abstract 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. 

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