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1. Probing lepton number violation: a comprehensive survey of dimension-7 SMEFT

   https://doi.org/10.1007/JHEP05(2024)154  

   Fridell, Kåre; Gráf, Lukáš; Harz, Julia; Hati, Chandan (May 2024, Journal of High Energy Physics)

   NA (Ed.)

   A<sc>bstract</sc> Observation of lepton number violation would represent a groundbreaking discovery with profound consequences for fundamental physics and as such, it has motivated an extensive experimental program searching for neutrinoless double beta decay. However, the violation of lepton number can be also tested by a variety of other observables. We focus on the possibilities of probing this fundamental symmetry within the framework of the Standard Model Effective Field Theory (SMEFT) beyond the minimal dimension-5. Specifically, we study the bounds on ∆L= 2 dimension-7 effective operators beyond the electron flavor imposed by all relevant low-energy observables and confront them with derived high-energy collider limits. We also discuss how the synergy of the analyzed multi-frontier observables can play a crucial role in distinguishing among different dimension-7 SMEFT operators. 

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2. Neutron star cooling with lepton-flavor-violating axions

   https://doi.org/10.1103/PhysRevD.109.103005  

   Zhang, Hong-Yi; Hagimoto, Ray; Long, Andrew J (May 2024, Physical Review D)

   The cores of dense stars are a powerful laboratory for studying feebly coupled particles such as axions. Some of the strongest constraints on axionlike particles and their couplings to ordinary matter derive from considerations of stellar axion emission. In this work we study the radiation of axionlike particles from degenerate neutron star matter via a lepton-flavor-violating coupling that leads to muon-electron conversion when an axion is emitted. We calculate the axion emission rate per unit volume (emissivity) and by comparing with the rate of neutrino emission, we infer upper limits on the lepton-flavor-violating coupling that are at the level of $\left|g_{ae\mu }\right|\lesssim {10}^{-6}$. For the hotter environment of a supernova, such as SN 1987A, the axion emission rate is enhanced and the limit is stronger, at the level of $\left|g_{ae\mu }\right|\lesssim {10}^{-11}$, competitive with laboratory limits. Interestingly, our derivation of the axion emissivity reveals that axion emission via the lepton-flavor-violating coupling is suppressed relative to the familiar lepton-flavor-preserving channels by the square of the plasma temperature to muon mass ratio, which is responsible for the relatively weaker limits. Published by the American Physical Society2024 

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3. Interesting Directions in Flavor Physics

   Datta, Alakabha (June 2024, Andromeda Publishing and Academic Services)

   Flavor physics continues to be an interesting avenue to look for beyond the standard model (SM) physics. Recent results from flavor physics, both in the quark and lepton sectors, hint at possible new physics. In this work we focus on some flavor physics results, mainly in b decays, and speculate on possible new physics interpretations of these results. We also present a model that can connect some of the B anomalies to the MiniBooNe anomaly and the muon g − 2 measurement. 

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4. Neutrino mass and mixing models with eclectic flavor symmetry ∆(27) ⋊ T′

   https://doi.org/10.1007/JHEP05(2023)144  

   Ding, Gui-Jun; King, Stephen F.; Li, Cai-Chang; Liu, Xiang-Gan; Lu, Jun-Nan (May 2023, Journal of High Energy Physics)

   A bstract The Kähler potentials of modular symmetry models receive unsuppressed contributions which may be controlled by a flavor symmetry, where the combination of the two symmetry types is referred to as eclectic flavor symmetry. After briefly reviewing the consistency conditions of eclectic flavor symmetry models, including those with generalised (g)CP, we perform a comprehensive bottom-up study of eclectic flavor symmetry models based on Ω(1) ≅ ∆(27) ⋊ T ′, consisting of the flavor symmetry ∆(27) in a semi-direct product with the modular symmetry T ′. The modular transformations of different ∆(27) multiplets are given by solving the consistency condition. The eight nontrivial singlets of ∆(27) are related by T ′ modular symmetry, and they have to be present or absent simultaneously in any Ω(1) model. The most general forms of the superpotential and Kähler potential invariant under Ω(1) are discussed, and the corresponding fermion mass matrices are presented. Based on the eclectic flavor group Ω(1), two concrete lepton models which can successfully describe the experimental data of lepton masses and mixing parameters are constructed. For the two models without gCP, all six mixing parameters vary in small regions. A nearly maximal atmospheric mixing angle θ 23 and Dirac CP phase δ CP are obtained in the first model. After considering the compatible gCP symmetry and the assumption of $$ \mathfrak{R}\tau $$ R τ = 0 in the first model, the μ − τ reflection symmetry is preserved in the charged lepton diagonal basis. As a consequence, the atmospheric mixing angle and Dirac CP phase are predicted to be maximal, and two Majorana CP phases are predicted to be π . 

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5. Test of lepton universality in beauty-quark decays

   https://doi.org/10.1038/s41567-021-01478-8  

   Aaij, R.; Beteta, C. Abellán; Ackernley, T.; Adeva, B.; Adinolfi, M.; Afsharnia, H.; Aidala, C. A.; Aiola, S.; Ajaltouni, Z.; Akar, S.; et al (March 2022, Nature Physics)

   Abstract The standard model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown that a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton–proton collision data collected with the LHCb detector at CERN’s Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the standard model, such as a new fundamental interaction between quarks and leptons. 

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