Publications

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Renormalization-group improvement in hadronic $τ$ decays in 2018

Diogo Boito, Pere Masjuan, Fabio Oliani

SciPost Phys. Proc. 1, 049 (2019) · published 22 February 2019 | · pdf

One of the main sources of theoretical uncertainty in the extraction of the strong coupling from hadronic tau decays stems from the renormalization group improvement of the series. Perturbative series in QCD are divergent but are (most likely) asymptotic expansions. One needs knowledge about higher orders to be able to choose the optimal renormalization-scale setting procedure. Here, we discuss the use of Pad\'e approximants as a model-independent and robust method to extract information about the higher-order terms. We show that in hadronic \boldmath $\tau$ decays the fixed-order expansion, known as fixed-order perturbation theory (FOPT), is the most reliable mainstream method to set the scale. This fully corroborates previous conclusions based on the available knowledge about the leading renormalon singularities of the perturbative series.

1 citation
Sensitivity on the electromagnetic dipole moments of the tau-lepton at the CLIC

Maria de los Ángeles Hernández-Ruíz, Ahmet A. Billur, Murat Köksal, Alejandro Gutiérrez-Rodríguez

SciPost Phys. Proc. 1, 045 (2019) · published 22 February 2019 | · pdf

In this paper we established model independent bounds on the anomalous magnetic and electric dipole moments of the tau-lepton using the process $\gamma\gamma \to \tau^+\tau^-}$. We use data collected with the future $e^+ e^-$ linear collider such as the CLIC at $\sqrt{s}$ = 380, 1500, 3000 GeV, and we consider systematic uncertainties of $\delta_{sys}$ = 0$\%$, 3$\%$, 5$\%$. The theory predictions are a very good prospect for probing the dipole moments of the tau-lepton at the future $e^ + e^-$ linear collider at the $\gamma\gamma$ mode.
Prospects of measurements on the anomalous magnetic and electric dipole moments of the $\tau$-neutrino in $pp$ collisions at the LHC

Alejandro Gutiérrez-Rodríguez, Murat Köksal, Ahmet A. Billur, Maria de los Ángeles Hernández-Ruíz

SciPost Phys. Proc. 1, 044 (2019) · published 22 February 2019 | · pdf

In this paper the production cross-section $pp\rightarrow (\gamma, Z) \to \nu_\tau \bar \nu_\tau \gamma+X$ in $pp$ collisions at $\sqrt{s}=13, 14, 33\hspace{0.8mm}TeV$ is presented. Furthermore, we estimate bounds at the $95\%$ C.L. on the dipole moments of the $\tau$-neutrino using integrated luminosity of ${\cal L}= 100, 500, 1000, 3000 \hspace{0.8mm}fb^{-1}$ collected with the ATLAS detector at the LHC and we consider systematic uncertainties of $\delta_{sys} = 0\%, \, 5\%, \,10\%$. We found that the current and future LHC bounds are weaker than other experiments reported in the literature. However, it is shown that the process under consideration is a good prospect for probing the dipole moments of the $\tau$-neutrino at the LHC.
Tau-decay determination of the strong coupling

Antonio Pich

SciPost Phys. Proc. 1, 036 (2019) · published 21 February 2019 | · pdf

We review the current status of the determination of the strong coupling from tau decay. Using the most recent release of the ALEPH data, a very comprehensive phenomenological analysis has been performed, exploring all strategies previously considered in the literature and several complementary approaches. Once their actual uncertainties are properly assessed, the results from all adopted methodologies are in excellent agreement, leading to a very robust and reliable value of the strong coupling, $\alpha_s^{(n_f=3)}(m_\tau^2) = 0.328\pm 0.013$, which implies $\alpha_s^{(n_f=5)}(M_Z^2) = 0.1197\pm 0.0015$.

3 citations
Light sterile neutrinos: a critical overview

Ivan Esteban

SciPost Phys. Proc. 1, 026 (2019) · published 20 February 2019 | · pdf

Light sterile neutrinos, first proposed after the results of the LSND experiment, have been a polemical topic for the last decades after seemingly contradictory data appeared from different experiments. In this overview, I review the experimental hints that point towards sterile neutrinos as well as their statistical compatibility. Even though the muon neutrino disappearance experiments strongly rule out vanilla sterile neutrinos, each oscillation channel remains internally mostly consistent. In any case, in the near future a series of independent and precise experiments should finally settle down this issue.
LHC searches motivated by recent $B$-anomalies

Darius Alexander Faroughy

SciPost Phys. Proc. 1, 021 (2019) · published 19 February 2019 | · pdf

We discuss the physics case for the LHC motivated by the $B$-physics anomalies. After correlating semi-tauonic $B$ decays to di-tau production at the LHC, and discussing the possible models solving the $B$-anomalies, we show how existing LHC data in $\tau\bar\tau$ tails exclude most beyond the SM scenarios except for a handful of leptoquark (LQ) models. We analyze the impact of LHC searches for some of these LQ solutions using current data. In particular, we focus on the well known $U_1$ vector LQ as well as the GUT-inspired scalar LQs, $R_2$ and $S_3$. By exploiting the complementarity between di-tau searches and the lepton flavor violating decays $B\to K\mu\tau$ and $\tau\to\mu\phi$ we argue that these model can be cornered by the LHC, Belle~II and LHCb in the near future.

2 citations
Monte Carlo, fitting and Machine Learning for Tau leptons

Vladimir Cherepanov, Elzbieta Richter-Was, Zbigniew Was

SciPost Phys. Proc. 1, 018 (2019) · published 19 February 2019 | · pdf

Status of tau lepton decay Monte Carlo generator TAUOLA, and its main recent applications are reviewed. It is underlined, that in recent efforts on development of new hadronic currents, the multi-dimensional nature of distributions of the experimental data must be taken with a great care. Studies for H to tau tau; tau to hadrons indeed demonstrate that multi-dimensional nature of distributions is important and available for evaluation of observables where tau leptons are used to constrain experimental data. For that part of the presentation, use of the TAUOLA program for phenomenology of H and Z decays at LHC is discussed, in particular in the context of the Higgs boson parity measurements with the use of Machine Learning techniques. Some additions, relevant for QED lepton pair emission and electroweak corrections are mentioned as well.

2 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

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