New physics in the kinematic distributions of B¯→D(*)τ−(→ℓ−ν¯ℓντ)ν¯τ

We investigate the experimentally accessible kinematic distributions of the $\overline{B}\ensuremath{\rightarrow}{D}^{(*)}{\ensuremath{\tau}}^{\ensuremath{-}}(\ensuremath{\rightarrow}{\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}}{\ensuremath{\nu}}_{\ensuremath{\tau}}){\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}}$ decays. Specifically, we study the decay rates as functions of the $B\ensuremath{\rightarrow}{D}^{(*)}$ transferred squared momentum, the energy of the final charged lepton and the angle of its 3-momentum with respect to the 3-momentum of the recoiling ${D}^{(*)}$. The angular distribution allows to introduce new observables, like a forwar…

$SU(2)\times U(1)$ gauge invariance and the shape of new physics in rare $B$ decays

New physics effects in $B$ decays are routinely modeled through operators invariant under the strong and electromagnetic gauge symmetries. Assuming the scale for new physics is well above the electro-weak scale, we further require invariance under the full Standard-Model gauge symmetry group. Retaining up to dimension-6 operators, we unveil new constraints between different new-physics operators that are assumed to be independent in the standard phenomenological analyses. We illustrate this approach by analyzing the constraints on new physics from rare $B_{q}$ (semi-)leptonic decays.

New physics in the kinematic distributions of $\bar B\to D^{(*)}\tau^-(\to\ell^-\bar\nu_\ell\nu_\tau)\bar\nu_\tau$

We investigate the experimentally-accessible kinematic distributions of the $\bar B\to D^{(*)}\tau^-(\to\ell^-\bar\nu_\ell\nu_\tau)\bar\nu_\tau$ decays. Specifically, we study the decay rates as functions of the $B\to D^{(*)}$ transferred squared momentum, the energy of the final charged lepton and the angle of its 3-momentum relative to the 3-momentum of the recoiling $D^{(*)}$. The angular distribution allows to introduce new observables, like a forward-backward asymmetry, which are complementary to the total rates. We present analytic formulas for the observable 3-fold 5-body differential decay rates, study the predictions in the Standard Model and investigate the effects in different ne…

Reassessing the discovery potential of the $B \to K^{*} \ell^+\ell^-$ decays in the large-recoil region: SM challenges and BSM opportunities

We critically examine the potential to disentangle Standard Model (SM) and New Physics (NP) in $B \to K^* \mu^+\mu^-$ and $B\to K^* e^+ e^-$ decays, focusing on $(i)$ the LHCb anomaly, $(ii)$ the search for right-handed currents, and $(iii)$ lepton-universality violation. Restricting ourselves to the large-recoil region, we advocate a parameterisation of the hadronic matrix elements that separates model-independent information about nonperturbative QCD from the results of model calculations. We clarify how to estimate corrections to the heavy-quark limit that would generate a right-handed (virtual) photon in the $b\to s\gamma$ contribution to the decay. We then apply this approach to the di…

Weak decays of unstable $b$-mesons

We investigate the decays of the excited $(b\bar q)$ mesons as probes of the short-distance structure of the weak $\Delta B=1$ transitions. These states are unstable under the electromagnetic or strong interactions although their widths are typically suppressed by phase space. As compared to the pseudoscalar $B$ meson, the purely leptonic decays of the vector $B^*$ are not chirally suppressed and are sensitive to different combinations of the underlying weak effective operators. An interesting example is $B^*_s\to \ell^+\ell^-$, which has a rate that can be accurately predicted in the standard model. The branching fraction is $\mathcal{B}\sim10^{-11}$, irrespective of the lepton flavor and …

Lepton universality violation and lepton flavor conservation in $B$-meson decays

Anomalies in (semi)leptonic $B$-meson decays present interesting patterns that might be revealing the shape of the new physics to come. In order to understand the experimental data, we explore symmetry arguments that lead to the hypothesis of minimal flavor violation. In particular, under the assumption of negligible neutrino mass effects in charged lepton processes, the presence of lepton universality violation without lepton flavor violation naturally arises. This can account for a deficit of $B^+\to K^+\mu\mu$ over $B^+\to K^+ee$ decays with new physics coupled predominantly to muons and a new physics scale of a few TeV. A prediction of this scenario is the modification of processes invo…

Global effective-field-theory analysis of new-physics effects in (semi)leptonic kaon decays

We analyze the decays $K\to\pi\ell\nu$ and $P\to\ell\nu$ ($P=K,\pi$, $\ell=e,\,\mu$) using a low-energy Effective-Field-Theory approach to parametrize New Physics and study the complementarity with baryon $\beta$ decays. We then provide a road map for a global analysis of the experimental data, with all the Wilson coefficients simultaneously, and perform a fit leading to numerical bounds for them and for $V_{us}$. A prominent result of our analysis is a reinterpretation of the well-known $V_{ud}-V_{us}$ diagram as a strong constraint on new physics. Finally, we reinterpret our bounds in terms of the $SU(2)_L\times~U(1)_Y$-invariant operators, provide bounds to the corresponding Wilson coeff…

New Physics in $s\to u\ell^-\bar\nu$: Interplay between semileptonic kaon and hyperon decays

We review a novel model-independent approach to the analysis of new-physics effects in the $s\to u\ell^-\bar\nu$ transitions. We apply it to (semi)leptonic kaon decays and study their complementarity with pion and hyperon $\beta$ decays or with collider searches of new physics.

Non-standard semileptonic hyperon decays

We investigate the discovery potential of semileptonic hyperon decays in terms of searches of new physics at teraelectronvolt scales. These decays are controlled by a small $SU(3)$-flavor breaking parameter that allows for systematic expansions and accurate predictions in terms of a reduced dependence on hadronic form factors. We find that muonic modes are very sensitive to non-standard scalar and tensor contributions and demonstrate that these could provide a powerful synergy with direct searches of new physics at the LHC.

Lepton universality violation with lepton flavor conservation in B-meson decays

Anomalies in semileptonic B-meson decays present interesting patterns that might be revealing the shape of the new physics to come. Under the assumption that neutrino and charged lepton mass terms are the only sources of flavor violation and given the hierarchy between the two, we find that charged lepton universality violation without charged lepton flavor violation naturally arises. This can account for a deficit of B + → K + μμ over B + → K + ee decays with new physics coupled predominantly to muons and a new physics scale of a few TeV. A generic prediction of this scenario is a large enhacement of tauonic B decay rates that, in particular, could accommodate an excess in B → D (∗) τ ν. F…

SU(3)-breaking corrections to the hyperon vector coupling f1(0) in covariant baryon chiral perturbation theory