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1. 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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2. Observation of the $J/\psi \to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decay in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevD.109.L111101  

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

   The $J/\psi \to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$decay has been observed with a statistical significance in excess of five standard deviations. The analysis is based on an event sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS experiment in 2018 and corresponding to an integrated luminosity of $33.6{\mathrm{fb}}^{-1}$. Normalizing to the $J/\psi \to {\mu }^{+}{\mu }^{-}$decay mode leads to a branching fraction of $[{10.1}_{-2.7}^{+3.3}\left(\mathrm{stat}\right)\pm 0.4\left(\mathrm{syst}\right)]\times {10}^{-7}$, a value that is consistent with the standard model prediction. © 2024 CERN, for the CMS Collaboration2024CERN 

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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. Measurement of the branching fraction of the decay $B^{-}\to D^0\rho (770{)}^{-}$ at Belle II

   https://doi.org/10.1103/PhysRevD.109.L111103  

   Adachi, I; Aggarwal, L; Aihara, H; Akopov, N; Aloisio, A; Anh_Ky, N; Asner, D M; Atmacan, H; Aushev, V; Aversano, M; et al (June 2024, Physical Review D)

   We measure the branching fraction of the decay $B^{-}\to D^0\rho (770{)}^{-}$using data collected with the Belle II detector. The data contain 387 million $B\overline{B}$pairs produced in $e^{+}{e}^{-}$collisions at the $\mathrm{\Upsilon }\left(4S\right)$resonance. We reconstruct $8360\pm 180$decays from an analysis of the distributions of the $B^{-}$energy and the $\rho (770{)}^{-}$helicity angle. We determine the branching fraction to be $(0.939\pm 0.021\left(\mathrm{stat}\right)\pm 0.050\left(\mathrm{syst}\right))\%$, in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two. Published by the American Physical Society2024 

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5. Test of Lepton Flavor Universality with $B_s^0\to \varphi {\ell }^{+}{\ell }^{-}$ Decays

   https://doi.org/10.1103/PhysRevLett.134.121803  

   Aaij, R; Abdelmotteleb, A_S W; Abellan_Beteta, C; Abudinén, F; Ackernley, T; Adefisoye, A A; Adeva, B; Adinolfi, M; Adlarson, P; Agapopoulou, C; et al (March 2025, Physical Review Letters)

   Lepton flavor universality in rare $b\to s$transitions is tested for the first time using $B_s^0$meson decays. The measurements are performed using $pp$collision data collected by the LHCb experiment between 2011 and 2018, corresponding to a total integrated luminosity of $9{\mathrm{fb}}^{-1}$. Branching fraction ratios between the $B_s^0\to \varphi e^{+}{e}^{-}$and $B_s^0\to \varphi {\mu }^{+}{\mu }^{-}$decays are measured in three regions of dilepton mass squared, $q^2$, with $0.1<q^2<1.1$, $1.1<q^2<6.0$, and $15<q^2<19{\mathrm{GeV}}^2/c^4$. The results agree with the standard model expectation of lepton flavor universality. © 2025 CERN, for the LHCb Collaboration2025CERN 

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