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Register a new userBeta decays as sensitive probes of lepton flavor universality
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Nuclear $beta$ decays as well as the decay of the neutron are well-established low-energy probes of physics beyond the Standard Model (SM). In particular, with the axial-vector coupling of the nucleon $g_A$ determined from lattice QCD, the comparison between experiment and SM prediction is commonly used to derive constraints on right-handed currents. Further, in addition to the CKM element $V_{us}$ from kaon decays, $V_{ud}$ from $beta$ decays is a critical input for the test of CKM unitarity. Here, we point out that the available information on $beta$ decays can be re-interpreted as a stringent test of lepton flavor universality (LFU). In fact, we find that the ratio of $V_{us}$ from kaon decays over $V_{us}$ from $beta$ decays (assuming CKM unitarity) is extremely sensitive to LFU violation (LFUV) in $W$-$mu$-$ u$ couplings thanks to a CKM enhancement by $(V_{ud}/V_{us})^2sim 20$. From this perspective, recent hints for the violation of CKM unitarity can be viewed as further evidence for LFUV, fitting into the existing picture exhibited by semi-leptonic $B$ decays and the anomalous magnetic moments of muon and electron. Finally, we comment on the future sensitivity that can be reached with this LFU violating observable and discuss complementary probes of LFU that may reach a similar level of precision, such as $Gamma(pitomu u)/Gamma(pito e u)$ at the PEN and PiENu experiments or even direct measurements of $Wtomu u$ at an FCC-ee.
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We propose a novel strategy to test lepton flavor universality (LFU) in top decays, applicable to top pair production at colliders. Our proposal exploits information in kinematic distributions and mostly hinges on data-driven techniques, thus having very little dependence on our theoretical understanding of top pair production. Based on simplified models accommodating recent hints of LFU violation in charged current B meson decays, we show that existing LHC measurements already provide non-trivial information on the flavor structure and the mass scale of such new physics (NP). We also project that the measurements of LFU in top decays at the high-luminosity LHC could reach a precision at the percent level or below, improving the sensitivity to LFU violating NP in the top sector by more than an order of magnitude compared to existing approaches.
The study of lepton flavor universality violation (LFUV) in semitauonic $b$-hadron decays has become increasingly important in light of longstanding anomalies in their measured branching fractions, and the very large datasets anticipated from the LHC and Belle II. In this review, we undertake a comprehensive survey of the experimental environments and methodologies for semitauonic LFUV measurements at the $B$-factories and LHCb, along with a concise overview of the theoretical foundations and predictions for a wide range of semileptonic decay observables. We proceed to examine the future prospects to control systematic uncertainties down to the percent level, matching the precision of Standard Model (SM) predictions. Furthermore, we discuss new perspectives and caveats on combinations of the LFUV data and revisit the world averages for the ${cal R}(D^{(*)})$ ratios. Here we demonstrate that different treatments for the correlations of uncertainties from $D^{**}$ excited states can vary the current $3sigma$ tension with the SM within a $1sigma$ range. Prior experimental overestimates of $D^{**}tau u$ contributions may further exacerbate this. The precision of future measurements is also estimated; their power to exploit full differential information, and solutions to the inherent difficulties in self-consistent new physics interpretations of LFUV observables, are briefly explored.
Lepton flavor universality can be tested in the semileptonic decays $Lambda_bto Lambda_c^{(ast)}$ where $Lambda_c^{(ast)}$ denotes either the ground state $Lambda_c(2286)$ (with $J^P=1/2^+$) or its orbital excitations $Lambda_c(2595)$ (with $J^P=1/2^-$) and $Lambda_c(2625)$ (with $J^P=3/2^-$). We calculate the differential decay rates as well as the branching fractions of these decays for both tauonic and muonic modes with form factors obtained from a covariant confined quark model previously developed by us. We present results for the rate ratios of the tauonic and muonic modes which provide important tests of lepton flavor universality in forthcoming experiments.
A recent proposal for explaining discrepancies in angular observables in the rare decay B --> K*mu+mu- with a gauged L_mu - L_tau current carried with it the prediction of lepton flavor universality violation in related B-meson decays. This prediction gained empirical support with a subsequent hint for lepton flavor universality violation in the B --> K l+l- decay by LHCb. In this short paper we fully quantify the prediction including the associated uncertainties. We also provide new predictions for a variety of additional observables sensitive to lepton flavor universality violation in B-meson decays.
In the present work, we study the OZI-allowed three body open flavor decay properties of higher vector charmonium and bottomonium states with an extended quark pair creation model. For the bottomonium system, we get that (i) the $BBpi$ and $B^*B^*pi$ partial decay widths of the $Upsilon(5S)$ state are consistent with the experiment, and the $BB^*pi$ partial decay width of the $Upsilon(5S)$ state is smaller but very close to the Belles experiment. Meanwhile, (ii) the $BB^*pi$ and $B^*B^*pi$ decay widths of $Upsilon(11020)$ can reachs $2sim3$ MeV. In addition, (iii) for the most of higher vector charmonium states, the partial decay widths of the $DD^*pi$ and $D^*D^*pi$ modes can reach up to several MeV, which may be observed in future experiments.
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