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1. Search for charged lepton flavor violating $Z$ and $Z^{\prime }$ boson decays in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$

   https://doi.org/10.1103/8lw4-6lyk  

   Hayrapetyan, A; Makarenko, V; Tumasyan, A; Adam, W; Andrejkovic, J W; Benato, L; Bergauer, T; Dragicevic, M; Giordano, C; Hussain, P S; et al (December 2025, Physical Review D)

   A search for flavor violating decays of the $Z$ boson to charged leptons is performed using data from proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ collected with the CMS detector at the LHC, corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ . Each of the decays $Z\to e\mu$ , $Z\to e\tau$ , and $Z\to \mu \tau$ is considered. The data are consistent with the backgrounds expected from standard model processes. For the $Z\to e\mu$ channel the observed (expected) 95% confidence level upper limit on the branching fraction is $1.9\left(2.0\right)\times {10}^{-7}$ , which is the most stringent direct limit to date on this process; the corresponding limits for the $Z\to e\tau$ and $Z\to \mu \tau$ channels are $13.8\left(11.4\right)\times {10}^{-6}$ and $12.0\left(5.3\right)\times {10}^{-6}$ , respectively. Additionally, the $e\mu$ final state is used to search for lepton flavor violating decays of $Z^{\prime }$ resonances in the mass range from 110 to $500\mathrm{GeV}$ . No significant excess is observed above the predicted background levels. 

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2. Expanding the landscape of exotic muon decays

   https://doi.org/10.1103/cb52-r75c  

   Greljo, Admir; Palavrić, Ajdin; Tunja, Mirsad; Zupan, Jure (April 2026, Physical Review D)

   We chart new-physics models that produce exotic, high-multiplicity muon decays featuring prompt or displaced $e^{+}{e}^{-}$ pairs and/or photons, with or without missing energy, such as $\mu \to 5e$ , $\mu \to 7e$ , etc. Starting from an effective-field-theory perspective, we estimate the reach on the ultraviolet scale and identify conditions under which lower-multiplicity modes are suppressed or occur at comparable rates. We then construct explicit realizations in minimal dark-sector models with light, feebly interacting particles, such as flavor-protected scalars, dark photons, inelastic dark matter, and axionlike particles. The predicted novel signatures can be probed at MEG II and Mu3e, as well as during calibration runs of COMET and Mu2e. A future discovery would provide valuable insights into short-distance dynamics and the mechanism of lepton-flavor symmetry breaking. 

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3. Search for the $Z$ Boson Decay to $\tau \tau \mu \mu$ in Proton-Proton Collisions at $\sqrt{s}=13\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevLett.133.161805  

   Hayrapetyan, A; Tumasyan, A; Adam, W; Andrejkovic, J W; Bergauer, T; Chatterjee, S; Damanakis, K; Dragicevic, M; Hussain, P S; Jeitler, M; et al (October 2024, Physical Review Letters)

   The first search for the $Z$ boson decay to $\tau \tau \mu \mu$ at the CERN LHC is presented, based on data collected by the CMS experiment at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of $138{\mathrm{fb}}^{-1}$ . The data are compatible with the predicted background. For the first time, an upper limit at the 95% confidence level of 6.9 times the standard model expectation is placed on the ratio of the $Z\to \tau \tau \mu \mu$ to $Z\to 4\mu$ branching fractions. Limits are also placed on the six flavor-conserving four-lepton effective-field-theory operators involving two muons and two tau leptons, for the first time testing all such operators. © 2024 CERN, for the CMS Collaboration2024CERN 

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4. Long-lived axionlike particles from tau decays

   https://doi.org/10.1103/51gx-m32t  

   Ema, Yohei; Fox, Patrick J; Hostert, Matheus; Menzo, Tony; Pospelov, Maxim; Ray, Anupam; Zupan, Jure (December 2025, Physical Review D)

   Axionlike particles (ALPs) are well-motivated examples of light, weakly coupled particles in theories beyond the Standard Model. In this work, we study long-lived ALPs coupled exclusively to leptons in the mass range between $2m_e$ and $m_{\tau }-m_e$ . For anarchic flavor structure the leptophilic ALP production in tau decays or from ALP-tau bremsstrahlung is enhanced thanks to derivative couplings of the ALP and can surpass production from electron and muon channels, especially for ALPs heavier than $m_{\mu }$ . Using past data from high-energy fixed-target experiments such as CHARM and BEBC we place new constraints on the ALP decay constant $f_a$ , reaching scales as high as $\mathcal{O}\left({10}^8\right)\mathrm{GeV}$ in lepton-flavor-violating channels and $f_a\sim \mathcal{O}\left({10}^2\right)\mathrm{GeV}$ in lepton-flavor-conserving ones. We also present projections for the event-rate sensitivity of current and future detectors to ALPs produced at the Fermilab Main Injector, the CERN SPS, and in the forward direction of the LHC. We show that SHiP will be sensitive to $f_a$ values that are over an order of magnitude above the existing constraints. 

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5. 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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