Publications

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Revisiting the flavor violating decays of the Tau and Mu leptons in the Standard Model with massive neutrinos

Gerardo Hernández Tomé

SciPost Phys. Proc. 1, 017 (2019) · published 19 February 2019 | · pdf

The flavor violating leptonic decays of the $\tau$ and $\mu$ leptons into three lighter charged leptons are revisited in the framework the Standard Model with massive neutrinos. In contrast to the previous prediction, we have found strongly suppressed rates for the $\tau^-\to \mu^- \ell^+ \ell^-$ ($\ell=\mu, e$) decays. Our results are in good agreement with the approximation of neglecting masses and momenta of the external particles in the loop integrals made in the first computation for the $\mu^- \to e^-e^+ e^-$ decay.
Test lepton flavor universality with (semi)leptonic $D$ decays at BESIII

S. F. Zhang on behalf of the BESIII Collaboration

SciPost Phys. Proc. 1, 016 (2019) · published 19 February 2019 | · pdf

Using 2.93~fb$^{-1}$ data taken at $\sqrt{s}=3.773$~GeV, 0.482~fb$^{-1}$ data taken at $\sqrt{s}=4.009$~GeV and 3.19~fb$^{-1}$ data taken at $\sqrt{s}=4.178$~GeV with the BESIII detector, precision measurements of the branching fractions of $D^0\to K^-\mu^+\nu_\mu$, $D^+\to\bar K^0\mu^+\nu_\mu$, $D^0\to\pi^-\mu^+\nu_\mu$, $D^+\to\pi^0\mu^+\nu_\mu$, $D^+\to\ell^+\nu_\ell$ and $D_s^+\to\ell^+\nu_\ell$ are performed. Combining the known branching fractions of $D\to\bar Ke^+\nu_e$, $D\to\pi e^+\nu_e$ and $D^+_s\to\tau^+\nu_\tau$, we have tested the lepton flavor universality with $D\to\bar K\ell^+\nu_\ell$, $D\to\pi\ell^+\nu_\ell$, $D^+\to\ell^+\nu_\ell$ and $D_{s}^+\to\ell^+\nu_\ell$ decays. We have also tested lepton flavor universality in different $q_{\ell\nu_\ell}^2$ intervals for $D\to\bar K\ell^+\nu_\ell$~($\ell=e$ or $\mu$). Besides, hadronic form factor $f_+^K(0)$, $D$ meson decay constant $f_{D_s^+}$ and $f_{D^+}$, and quark mixing matrix element $|V_{cs}|$ and $|V_{cd}|$ are also extracted with the most precise accuracies to date.

3 citations
Lepton flavour violation and neutrino masses

Ana M. Teixeira

SciPost Phys. Proc. 1, 015 (2019) · published 19 February 2019 | · pdf

Neutrino oscillations provided the first evidence for the violation of flavour in the lepton sector, and established a clear need to consider extensions of the Standard Model. Many new phenomena can emerge from these New Physics (NP) constructions, among which processes violating lepton number and charged lepton flavour, all clear signals of New Physics. Following a short overview of the status of experimental searches, we comment on the prospects of several models of massive neutrinos, from minimal constructions to complete NP models, to the above mentioned observables.
Charged lepton flavour violating processes

Adrian Signer

SciPost Phys. Proc. 1, 014 (2019) · published 19 February 2019 | · pdf

Charged lepton flavour violating processes are naturally present in many extensions of the Standard Model. After a brief overview on the experimental situation, an effective-field-theory framework is described that allows to interpret and compare the various experiments in a consistent way. The usefulness of this approach is then illustrated in the context of a specific model with a doubly charged scalar.
Probing lepton universality with (semi)-leptonic B decays

Giovanni Banelli, Robert Fleischer, Ruben Jaarsma and Gilberto Tetlalmatzi-Xolocotzi

SciPost Phys. Proc. 1, 013 (2019) · published 19 February 2019 | · pdf

The most recent measurements of the observables $R_{D^{(*)}}$ are in tension with the Standard Model offering hints of New Physics in $b\rightarrow c \ell \bar{\nu}_{\ell}$ transitions. Motivated by these results, in this work we present an analysis on their $b\rightarrow u \ell \bar{\nu}_{\ell}$ counterparts (for $\ell=e, ~\mu, ~\tau$). Our study has three main objectives. Firstly, using ratios of branching fractions, we assess the effects of beyond the Standard Model scalar and pseudoscalar particles in leptonic and semileptonic $B$ decays ($B^-\rightarrow \ell^- \bar{\nu}_{\ell}$, $\bar{B}\rightarrow \pi \ell \bar{\nu}_{\ell}$ and $\bar{B}\rightarrow \rho \ell \bar{\nu}_{\ell}$). Here a key role is played by the leptonic $B$ processes, which are highly sensitive to new pseudoscalar interactions. In particular, we take advantage of the most recent measurement of the branching fraction of the channel $B^-\rightarrow \mu^-\bar{\nu}_{\mu}$ by the Belle collaboration. Secondly, we extract the CKM matrix element $|V_{ub}|$ while accounting simultaneously for New Physics contributions. Finally, we provide predictions for the branching fractions of yet unmeasured leptonic and semileptonic $B$ decays.

1 citation
Tau polarimetry in B meson decays

Rodrigo Alonso, Jorge Martin Camalich, Susanne Westhoff

SciPost Phys. Proc. 1, 012 (2019) · published 19 February 2019 | · pdf

This article summarizes recent developments in $B\to D^{(\ast)}\tau\nu$ decays. We explain how to extract the tau lepton's production properties from the kinematics of its decay products. The focus is on hadronic tau decays, which are most sensitive to the tau polarizations. We present new results for effects of new physics in tau polarization observables and quantify the observation prospects at BELLE II.

5 citations
Experimental status of LNU in B decays in LHCb

Sean Benson on behalf of the LHCb collaboration

SciPost Phys. Proc. 1, 011 (2019) · published 19 February 2019 | · pdf

In the Standard Model, the three charged leptons are identical copies of each other, apart from mass differences. Experimental tests of this feature in semileptonic decays of b-hadrons are highly sensitive to New Physics particles which preferentially couple to the 2nd and 3rd generations of leptons. A review of the latest lepton universality tests in semileptonic $b\to c$ transitions at LHCb is reported.
Theory Status - Puzzles in B Meson Decays and LFU?

Svjetlana Fajfer

SciPost Phys. Proc. 1, 010 (2019) · published 18 February 2019 | · pdf

Currently B meson puzzles motivate many studies of New Physics due to the observed deviations from the Standard Model predictions. There are two B meson puzzles, RD(∗) and RK(∗). The first one denotes the deviations in the decays driven by the charged current in the ratio of the decay widths for B → D(∗)τν and B → D(∗)μν, while the second one is related to the ratio of the decay widths for B → K (∗) μ+ μ− and B → K (∗) e+ e− transition. Also, the measured muon anomalous magnetic moment differs from the SM predictions. Usually, the effective Lagrangian approach containing New Physics effects is used to analyse RD(∗) and RK(∗). Among many models of New Physics, various leptoquark models are suggested to resolve both B meson anomalies. If New Physics is confirmed in B decays a number of processes at low and high energies should confirm its presence.

3 citations
NLO prediction for the decays $\tau \to \ell \ell'\ell' \nu \bar \nu$ and $\mu \to e e e \nu \bar\nu$

Matteo Fael

SciPost Phys. Proc. 1, 009 (2019) · published 18 February 2019 | · pdf

These proceedings present the differential decay rates and the branching ratios of the tau and muon decays $\tau \to \ell \ell' \ell' \nu \bar\nu$ (with $\ell,\ell'=\mu,e$) and $\mu \to e e e \nu \bar \nu$ in the Standard Model at NLO. These five-body leptonic decays are a tool to study the Lorentz structure of weak interactions and to test lepton flavour universality. They are also a source of SM background to searches for the lepton-flavour-violating decays $\mu \to e e e$ and $\tau \to \ell \ell' \ell'$. Even if the shift in the branching ratios induced by radiative corrections turns out to be small and of order 1% - mainly due to a running effect of the fine structure constant - locally in the phase space these corrections can reach the 5 - 10% level, depending on the applied cuts. We found for instance that in the phase space region where the neutrino energies are small, and the momenta of the three charged leptons have a similar signature as in $\mu \to eee$ and $\tau \to \ell \ell'\ell'$, the NLO corrections decrease the leading-order prediction by about 10 - 20%.
Non-standard interactions in $\tau^- \to (\pi^-\eta,\pi^-\pi^0)\nu_{\tau}$ decays

Gabriel López Castro

SciPost Phys. Proc. 1, 008 (2019) · published 18 February 2019 | · pdf

Originally thought as clean processes to study the hadronization of the weak currents, semileptonic tau lepton decays can be useful to set constraints on non-standard (NS) weak interactions. In this talk we summarize our recent studies on the effects of NS interactions in $\tau^- \to (\pi^-\pi^0,\pi^-\eta)\nu_{\tau}$ decays. We find that experimental data on these decays provide strong constraints on NS scalar and tensor interactions, respectively. Further improved measurements at Belle II and a better knowledge of necessary tensor and scalar form factors will allow to set limits on these NS interactions that are similar or better to contraints obtained from other low-energy processes

4 citations

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