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1. Distinguishing Charged Lepton Flavor Violation Scenarios with Inelastic $\mu \to e$ Conversion

   https://doi.org/10.1103/PhysRevLett.133.261801  

   Haxton, W C; Rule, Evan (December 2024, Physical Review Letters)

   The Mu2e and COMET experiments are expected to improve existing limits on charged lepton flavor violation (CLFV) by roughly 4 orders of magnitude. $\mu \to e$conversion experiments are typically optimized for electrons produced without nuclear excitation, as this maximizes the electron energy and minimizes backgrounds from the free decay of the muon. Here we argue that Mu2e and COMET will be able to extract additional constraints on CLFV from inelastic $\mu \to e$conversion, given the ${}^{27}\mathrm{Al}$target they have chosen and backgrounds they anticipate. We describe CLFV scenarios in which inelastic CLFV can induce measurable distortions in the near-endpoint spectrum of conversion electrons, including cases where certain contributing operators cannot be probed in elastic $\mu \to e$conversion. We extend the nonrelativistic EFT treatment of elastic $\mu \to e$conversion to include the new nuclear operators needed for the inelastic process, evaluate the associated nuclear response functions, and describe several new-physics scenarios where the inelastic process can provide additional information on CLFV. Published by the American Physical Society2024 

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2. Nuclear-level effective theory of $\mu \to e$ conversion: Inelastic process

   https://doi.org/10.1103/PhysRevC.111.025501  

   Haxton, W C; Rule, Evan (February 2025, Physical Review C)

   Mu2e and COMET will search for electrons produced via the neutrinoless conversion of stopped muons bound in 1s atomic orbits of ${}^{27}\mathrm{Al}$, improving existing limits on charged lepton flavor violation (CLFV) by roughly four orders of magnitude. Conventionally, $\mu \to e$conversion experiments are optimized to detect electrons originating from transitions where the nucleus remains in the ground state, thereby maximizing the energy of the outgoing electron. Clearly, detection of a positive signal in forthcoming experiments would stimulate additional work—including subsequent conversion experiments using complementary nuclear targets—to further constrain the new physics responsible for CLFV. Here we argue that additional information can be extracted without the need for additional experiments, by considering inelastic conversion in ${}^{27}\mathrm{Al}$. Transitions to low-lying nuclear excited states can modify the near-endpoint spectrum of conversion electrons, with the ratio of the elastic and inelastic responses being sensitive to the underlying CLFV operator. We extend the nuclear effective theory of $\mu \to e$conversion to the inelastic case, which adds five new response functions to the six that arise for the elastic process. We evaluate these nuclear response functions in ${}^{27}\mathrm{Al}$and calculate the resulting conversion-electron signal, taking into account the resolution anticipated in Mu2e/COMET. We find that ${}^{27}\mathrm{Al}$is an excellent target choice from the perspective of the new information that can be obtained from inelastic $\mu \to e$conversion. Published by the American Physical Society2025 

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3. Search for charged-lepton flavor violation in top quark production and decay in pp collisions at $$ \sqrt{s} $$ = 13 TeV

   https://doi.org/10.1007/JHEP06(2022)082  

   Tumasyan, A.; Adam, W.; Andrejkovic, J. W.; Bergauer, T.; Chatterjee, S.; Dragicevic, M.; Escalante Del Valle, A.; Frühwirth, R.; Jeitler, M.; Krammer, N.; et al (June 2022, Journal of High Energy Physics)

   A bstract Results are presented from a search for charged-lepton flavor violating (CLFV) interactions in top quark production and decay in pp collisions at a center-of-mass energy of 13 TeV. The events are required to contain one oppositely charged electron-muon pair in the final state, along with at least one jet identified as originating from a bottom quark. The data correspond to an integrated luminosity of 138 fb − 1 , collected by the CMS experiment at the LHC. This analysis includes both the production (q → e μ t) and decay (t → e μ q) modes of the top quark through CLFV interactions, with q referring to a u or c quark. These interactions are parametrized using an effective field theory approach. With no significant excess over the standard model expectation, the results are interpreted in terms of vector-, scalar-, and tensor-like CLFV four-fermion effective interactions. Finally, observed exclusion limits are set at 95% confidence levels on the respective branching fractions of a top quark to an e μ pair and an up (charm) quark of 0 . 13 × 10 − 6 (1 . 31 × 10 − 6 ), 0 . 07 × 10 − 6 (0 . 89 × 10 − 6 ), and 0 . 25 × 10 − 6 (2 . 59 × 10 − 6 ) for vector, scalar, and tensor CLFV interactions, respectively. 

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4. Lepton Universality Test with MUSE at PSI

   https://doi.org/10.1088/1742-6596/2391/1/012015  

   Kohl, Michael (December 2022, Journal of Physics: Conference Series)

   Abstract Lepton universality (LU) typically refers to the lepton coupling, which is considered to be the same fore,μ, andτleptons, if the interaction is electroweak according to the Standard Model, and it is hence a compelling probe for New Physics. The same principle of universal electroweak lepton interaction leads to the expectation that lepton scattering yields are equal foreandμbeams under the same kinematic condition. The mere mass difference betweeneandμaffects kinematic quantities (such as the relation between scattering angle andQ²), and the lepton mass dependence of elastic cross sections for leptons scattered from structured and pointlike objects are taken into account. By comparinge⁺,e⁻,μ⁺, andμ⁻scattering yields, two-photon exchange (TPE) effects, universal or not, can be separated from the general LU test of thee/μyield ratio. With its separable mixed beams ofe⁺/μ⁺ande⁻/μ⁻, respectively, the MUSE experiment at PSI is not only designed to measure the proton charge radius with four lepton species, but is also uniquely suited to probe TPE and LU, while benefitting from partial cancellations of certain shared systematics. An overview will be given of the MUSE experiment, the sensitivity, and the present status. 

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5. Charged lepton flavor violation at the EIC

   https://doi.org/10.1007/JHEP03(2021)256  

   Cirigliano, Vincenzo; Fuyuto, Kaori; Lee, Christopher; Mereghetti, Emanuele; Yan, Bin (March 2021, Journal of High Energy Physics)

   null (Ed.)

   A bstract We present a comprehensive analysis of the potential sensitivity of the Electron-Ion Collider (EIC) to charged lepton flavor violation (CLFV) in the channel ep → τX , within the model-independent framework of the Standard Model Effective Field Theory (SMEFT). We compute the relevant cross sections to leading order in QCD and electroweak corrections and perform simulations of signal and SM background events in various τ decay channels, suggesting simple cuts to enhance the associated estimated efficiencies. To assess the discovery potential of the EIC in τ - e transitions, we study the sensitivity of other probes of this physics across a broad range of energy scales, from pp → eτX at the Large Hadron Collider to decays of B mesons and τ leptons, such as τ → eγ , τ → eℓ + ℓ − , and crucially the hadronic modes τ → eY with Y ∈ π, K, ππ, Kπ, …. We find that electroweak dipole and four-fermion semi-leptonic operators involving light quarks are already strongly constrained by τ decays, while operators involving the c and b quarks present more promising discovery potential for the EIC. An analysis of three models of leptoquarks confirms the expectations based on the SMEFT results. We also identify future directions needed to maximize the reach of the EIC in CLFV searches: these include an optimization of the τ tagger in hadronic channels, an exploration of background suppression through tagging b and c jets in the final state, and a global fit by turning on all SMEFT couplings, which will likely reveal new discovery windows for the EIC. 

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