SciPost: SciPost Physics Proceedings: issue detail

SciPost Physics Proceedings issue 1

The 15th International Workshop on Tau Lepton Physics (TAU2018)

Event dates: from 2018-09-24 to 2018-09-28.

This is the fifteenth workshop of TAU series on physics of the tau lepton, it’s neutrino, and related processes. The goal of the workshop is to get theorists and experimentalists together to discuss recent progress in the field. New results from high energy machines (e.g LHC experiments: ATLAS, CMS, LHCb), low energy machines (BES-III and other) and B-factories (including Belle-2) will be presented as well as recent results from g-2 and neutrino oscillation experiments. The program also includes related subjects such as hadron cross section measurement at e+e- colliders, lepton universality puzzles in meson decays, searches for Lepton Flavour Violation in τ and μ decays and review talks on the future experiments.

(Guest) Fellows responsible for this Issue

Prof. Olga Igonkina
Dr Marcus Matthias Morgenstern

HFLAV $τ$ branching fractions fit and measurements of Vus with $τ$ lepton data

Alberto Lusiani

SciPost Phys. Proc. 1, 001 (2019) · published 18 February 2019 | · pdf

We report the status of the Heavy Flavour Averaging Group (HFLAV) averages of the $\tau$ lepton measurements We then update the latest published HFLAV global fit of the $\tau$ lepton branching fractions (Spring 2017) with recent results by BABAR. We use the fit results to update the Cabibbo-Kobayashi-Maskawa (CKM) matrix element Vus measurements with the $\tau$ branching fractions. We combine the direct $\tau$ branching fraction measurements with indirect predictions using kaon branching fractions measurements to improve the determination of Vus using $\tau$ branching fractions. The Vus determinations based on the inclusive branching fraction of $\tau$ to strange final states are about $3\sigma$ lower than the Vus determination from the CKM matrix unitarity.
Latest Belle results on Tau decays

Yifan Jin, Denis Epifanov, Hiroaki Aihara

SciPost Phys. Proc. 1, 002 (2019) · published 18 February 2019 | · pdf

We present preliminary results of a latest measurement at Belle, the branching fraction of rare tau decay, $\tau^- \rightarrow \pi^- \nu_{\tau} \ell^+ \ell^-$ ($\ell = e$ or $\mu$), using a 562 $\rm fb^{-1}$\ data set. The branching fraction of $\tau^- \rightarrow \pi^- e^+ e^- \nu_\tau$ is measured to be $\mathbf{ \mathcal{B}(\tau^-\rightarrow \pi^- e^+ e^- \nu_\tau) = (2.11 \pm 0.19 \pm 0.30) \times 10^{-5}}$, where the first uncertainty is statistical and the second is systematic. An upper limit is set on the branching fraction of $\tau^- \rightarrow \pi^- \mu^+ \mu^- \nu_\tau$, $\mathbf{\mathcal{B}(\tau^-\rightarrow \pi^- \mu^+ \mu^- \nu_\tau) < 1.06 \times 10^{-5}}$, at 90\% confidence level.
Prospects for $\tau$ lepton physics at Belle II

Michel Hernandez Villanueva on behalf of the Belle II collaboration

SciPost Phys. Proc. 1, 003 (2019) · published 18 February 2019 | · pdf

The Belle II experiment is an upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric $e^+e^-$ collider. The accelerator has already successfully completed the first phase of commissioning and first electron positron-collisions in Belle II were observed in April 2018. The design luminosity of SuperKEKB is 8x10$^{35}$ cm$^{-2}$s$^{-1}$ and the Belle II experiment aims to record 50 ab$^{-1}$ of data, a factor of 50 more than the Belle experiment. Belle II has a broad program of $\tau$ physics, in particular, precision measurements of Standard Model parameters and searches of lepton flavor and lepton number violations (LFV and LNV), benefiting from the large cross-section of the pairwise $\tau$ lepton production in $e^+e^-$ collisions. In this talk, we will review the $\tau$ lepton physics program of Belle II.
$τ$ lepton mass measurement at BESIII

Jianyong Zhang on behalf of BESIII collaboration

SciPost Phys. Proc. 1, 004 (2019) · published 18 February 2019 | · pdf

In order to obtain the mass of $\tau$ lepton at BESIII precisely, a beam energy measurement system was built at BEPCII. A scenario for high precision $\tau$ mass measurement was put forth before data taken. More than 130 $pb^{-1}$ $\tau$ mass scan data were collected in April 2018, and the uncertainty of $m_{\tau}$ is expected to be less than 100 keV.
Implications of tau data for CP violation in K decays

Antonio Rodríguez-Sánchez

SciPost Phys. Proc. 1, 005 (2019) · published 18 February 2019 | · pdf

The $D=6$ contribution of the Operator Product Expansion (OPE) of the VV-AA correlator of quark currents can be related to hadronic matrix elements associated to CP violation in non-leptonic kaon decays. We use those relations to find an updated value for $\langle(\pi\pi)_{I=2}|\mathcal{Q}_{8}|K\rangle$ in the chiral limit using the updated ALEPH spectral function. Taking instead values of the matrix elements from the lattice to obtain the $D=6$ vacuum elements provides a new short-distance constraint that allows for an inclusive determination of $f_{\pi}$ and an updated value for the $D=8$ condensate.
Current Status of inclusive hadronic tau determinations of |V_us|

K. Maltman, P.A. Boyle, R.J. Hudspith, T. Izubuchi, A. Juttner, C. Lehner, R. Lewis, H. Ohki, A. Portelli, M. Spraggs, J.M. Zanotti

SciPost Phys. Proc. 1, 006 (2019) · published 18 February 2019 | · pdf

We review the status of the determination of |V_{us}| from both flavor-breaking finite-energy sum rules based on inclusive non-strange and strange hadronic tau decay data and the recent lattice-based analysis of inclusive strange hadronic tau decay data. In particular, we update the results from these analysis frameworks taking into account recent improvements to a number of strange branching fractions reported by HFLAV at CKM2018 and this meeting. We find that inclusive tau decay data yields results for |V_{us}| compatible within errors with the expectations of three-family unitarity.
A no-go theorem for non-standard explanations of the $\tau\to K_S\pi\nu_\tau$ CP asymmetry

Vincenzo Cirigliano, Andreas Crivellin, Martin Hoferichter

SciPost Phys. Proc. 1, 007 (2019) · published 18 February 2019 | · pdf

A new tensor interaction is the only possibility to explain the $2.8\sigma$ tension observed by the BaBar collaboration in the CP asymmetry in $\tau\to K_S\pi\nu_\tau$ with physics beyond the Standard Model (BSM) realized above the electroweak scale. However, the strong phase generated by the interference between vector and tensor phases is suppressed by at least two orders of magnitude due to Watson's final-state-interaction theorem, and the strength of the CP-violating tensor interaction is strongly constrained by bounds from the neutron electric dipole moment and $D$-$\bar{D}$ mixing. As a result, a confirmation of the tension at Belle II would point to light BSM physics.
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
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%.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Measurements of Higgs-boson decays to leptons with the ATLAS detector

Lara Katharina Schildgen on behalf of the ATLAS Collaboration

SciPost Phys. Proc. 1, 019 (2019) · published 19 February 2019 | · pdf

Since its discovery in 2012, the Higgs boson has served as an important probe for precision measurements of the Standard Model and for searches for new physics beyond the Standard Model. One major goal of the LHC is the precise measurement of the Higgs Yukawa couplings to fermions. The latest ATLAS results of measurements of the Higgs boson decaying to leptons are presented, namely the cross-section measurement of the Higgs boson decay to two tau leptons and the searches for the di-muon decay and lepton-flavour-violating decays of the Higgs boson.
Search for lepton flavour violation with the ATLAS experiment

Brian Le on behalf of the ATLAS collaboration

SciPost Phys. Proc. 1, 020 (2019) · published 19 February 2019 | · pdf

Lepton flavour violation (LFV) is a striking signature of potential beyond the Standard Model physics. The search for LFV with the ATLAS detector is reported in searches focusing on the decay of the Higgs boson, the Z boson and of a heavy neutral gauge boson, Z', using pp collisions data with a center of mass energy of 8 TeV and 13 TeV.
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.
Searches for additional Higgs bosons decaying to tau leptons at the LHC

Claudio Caputo on behalf of the ATLAS an CMS collaborations

SciPost Phys. Proc. 1, 022 (2019) · published 20 February 2019 | · pdf

The searches for additional Higgs bosons decaying to tau leptons in scenarios beyond the standard model will be summarised, from the pp collision data collected by the ATLAS and CMS experiments at LHC Run-2.
SUSY Searches with Taus at the LHC

Feng Lyu on behalf of ATLAS and CMS Collaborations

SciPost Phys. Proc. 1, 023 (2019) · published 20 February 2019 | · pdf

The supersymmetric partner of the tau lepton, the stau, is predicted to be relatively light in a range of SUSY models and may be a key for dark matter. This talk presents recent ATLAS and CMS results from proton-proton collisions at sqrt(s) = 13 TeV corresponding to an integrated luminosity of 36.1 fb^-1 and 35.9 fb^-1 delivered by the LHC and recorded by the ATLAS and CMS detectors respectively in 2015 and 2016, to search for direct stau pair production, and indirect stau production mediated by other SUSY particles.
Neutrinoless double beta decay overview

Laura Cardani

SciPost Phys. Proc. 1, 024 (2019) · published 20 February 2019 | · pdf

Neutrinoless Double Beta Decay is a hypothesised nuclear process in which two neutrons simultaneously decay into protons with no neutrino emission. The prized observation of this decay would point to the existence of a process that violates a fundamental symmetry of the Standard Model of Particle Physics, and would allow to establish the nature of neutrinos. Today, the lower limits on the half-life of this process exceed 10$^{25}$-10$^{26}$ yr. I will review the current status of the searches for Double Beta Decay and the perspectives to enhance the experimental sensitivity in the next years.
Solar neutrino physics with Borexino

Andrea Pocar, M. Agostini, K. Altenmüller, S. Appel, V. Atroshchenko, Z. Bagdasarian, D. Basilico, G. Bellini, J. Benziger, G. Bonfini, D. Bravo, B. Caccianiga, F. Calaprice, A. Caminata, L. Cappelli, S. Caprioli, M. Carlini, P. Cavalcante, F. Cavanna, A. Chepurnov, K. Choi, L. Collica, D. D’Angelo, S. Davini, A. Derbin, X. F. Ding, A. Di Ludovico, L. Di Noto, I. Drachnev, K. Fomenko, A. Formozov, D. Franco, F. Gabriele, C. Galbiati, M. Gschwender, C. Ghiano, M. Giammarchi, A. Goretti, M. Gromov, D. Guffanti, T. Houdy, E. Hungerford, Aldo Ianni, Andrea Ianni, A. Jany, D. Jeschke, S. Kumaran, V. Kobychev, G. Korga, T. Lachenmaier, M. Laubenstein, E. Litvinovich, P. Lombardi, L. Ludhova, G. Lukyanchenko, L. Lukyanchenko, I. Machulin, G. Manuzio, S. Marcocci, J. Maricic, J. Martyn, E. Meroni, M. Meyer, L. Miramonti, M. Misiaszek, V. Muratova, B. Neumair, M. Nieslony, L. Oberauer, V. Orekhov, F. Ortica, M. Pallavicini, L. Papp, Ö. Penek, L. Pietrofaccia, N. Pilipenko, A. Porcelli, G. Raikov, G. Ranucci, A. Razeto, A. Re, M. Redchuk, A. Romani, N. Rossi, S. Rottenanger, S. Schönert, D. Semenov, M. Skorokhvatov, O. Smirnov, A. Sotnikov, L. F. F. Stokes, Y. Suvorov , R. Tartaglia, G. Testera, J. Thurn, E. Unzhakov, A. Vishneva, R. B. Vogelaar, F. von Feilitzsch, S. Weinz, M. Wojcik, M. Wurm, O. Zaimidoroga, S. Zavatarelli, K. Zuber, G. Zuzel

SciPost Phys. Proc. 1, 025 (2019) · published 20 February 2019 | · pdf

We present the most recent solar neutrino results from the Borexino experiment at the Gran Sasso underground laboratory. In particular, refined measurements of all neutrinos produced in the {\it pp} fusion chain have been made. It is the first time that the same detector measures the entire range of solar neutrinos at once. These new data weakly favor a high-metallicity Sun. Prospects for measuring CNO solar neutrinos are also discussed.
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.
Recent results from reactor antineutrino experiments

Hyunkwan Seo

SciPost Phys. Proc. 1, 027 (2019) · published 20 February 2019 | · pdf

The smallest neutrino mixing angle $\theta_{13}$ has been successfully measured by the disappearance of reactor antineutrinos at RENO, Daya Bay, and Double Chooz. The oscillation frequency is also measured based on energy and baseline dependent disappearance probability of reactor antineutrinos. Recent results fi nd a variation in the observed reactor antineutrino flux as a function of the reactor fuel evolution. We report more precisely measured values of $\theta_{13}$ and $\Delta m^2_{ee}$ and results on the evolution of observed reactor antineutrino yield and spectrum.
Latest results of the OPERA experiment on nu-tau appearance in the CNGS neutrino beam

N. Agafonova, A. Alexandrov, A. Anokhina, S. Aoki, A. Ariga, T. Ariga, A. Bertolin, C. Bozza, R. Brugnera, A. Buonaura, S. Buontempo, M. Chernyavskiy, A. Chukanov, L. Consiglio, N. D'Ambrosio, G. De Lellis, M. De Serio, P. del Amo Sanchez, A. Di Crescenzo, D. Di Ferdinando, N. Di Marco, S. Dmitrievsky, M. Dracos, D. Duchesneau, S. Dusini, T. Dzhatdoev, J. Ebert, A. Ereditato, J. Favier, R. A. Fini, F. Fornari, T. Fukuda, G. Galati, A. Garfagnini, V. Gentile, J. Goldberg, S. Gorbunov, Y. Gornushkin, G. Grella, A. M. Guler, C. Gustavino, C. Hagner, T. Hara, T. Hayakawa, A. Hollnagel, K. Ishiguro, A. Iuliano, K. Jakovcic, C. Jollet, C. Kamiscioglu, M. Kamiscioglu, S. H. Kim, N. Kitagawa, B. Klicek, K. Kodama, M. Komatsu, U. Kose, I. Kreslo, F. Laudisio, A. Lauria, A. Longhin, P. Loverre, M. Malenica, A. Malgin, G. Mandrioli, T. Matsuo, V. Matveev, N. Mauri, E. Medinaceli, A. Meregaglia, S. Mikado, M. Miyanishi, F. Mizutani, P. Monacelli, M. C. Montesi, K. Morishima, M. T. Muciaccia, N. Naganawa, T. Naka, M. Nakamura, T. Nakano, K. Niwa, S. Ogawa, A. Olchevsky, N. Okateva, K. Ozaki, A. Paoloni, L. Paparella, B. D. Park, L. Pasqualini, A. Pastore, L. Patrizii, H. Pessard, C. Pistillo, D. Podgrudkov, N. Polukhina, M. Pozzato, F. Pupilli, M. Roda, T. Roganova, H. Rokujo, G. Rosa, O. Ryazhskaya, A. Sadovsky, O. Sato, A. Schembri, I. Shakiryanova, T. Shchedrina, E. Shibayama, H. Shibuya, T. Shiraishi, S. Simone, C. Sirignano, G. Sirri, A. Sotnikov, M. Spinetti, L. Stanco, N. Starkov, S. M. Stellacci, M. Stipcevic, P. Strolin, S. Takahashi, M. Tenti, F. Terranova, V. Tioukov, S. Tufanli, A. Ustyuzhanin, S. Vasina, P. Vilain, E. Voevodina, L. Votano, J. L. Vuilleumier, G. Wilquet, B. Wonsak, C. S. Yoon

SciPost Phys. Proc. 1, 028 (2019) · published 20 February 2019 | · pdf

OPERA is a long-baseline experiment designed to search for $\nu_{\mu}\to\nu_{\tau}$ oscillations in appearance mode. It was based at the INFN Gran Sasso laboratory (LNGS) and took data from 2008 to 2012 with the CNGS neutrino beam from CERN. After the discovery of $\nu_\tau$ appearance in 2015, with $5.1\sigma$ significance, the criteria to select $\nu_\tau$ candidates have been extended and a multivariate approach has been used for events identification. In this way the statistical uncertainty in the measurement of the oscillation parameters and of $\nu_\tau$ properties has been improved. Results are reported.
Recent results from the long-baseline (LBL) neutrino oscillation experiments

Pablo Fernández

SciPost Phys. Proc. 1, 029 (2019) · published 20 February 2019 | · pdf

The results from the current long-baseline neutrino oscillation experiments are reviewed. These experiments are designed to measure the remaining unknown neutrino oscillation parameters via the oscillation of accelerator neutrinos over long distances. The performance of these experiments and their results are described and discussed, giving a precise picture of the current status of the measurements for the neutrino mass ordering, the value of ∆m 232 , the θ 23 and the lepton CP-violating phase, δ CP .
Tau neutrinos in IceCube, KM3NeT and the Pierre Auger Observatory

Daan van Eijk

SciPost Phys. Proc. 1, 030 (2019) · published 20 February 2019 | · pdf

In 2018, the IceCube collaboration reported evidence for the identification of a blazar as an astrophysical neutrino source. That evidence is briefly summarised here before focusing on the prospects of tau neutrino physics in IceCube, both at high energies (astrophysical neutrinos) and at lower energies (atmospheric neutrino oscillations). In addition, future neutrino detectors such as KM3NeT and the IceCube Upgrade and their tau neutrino physics potential are discussed. Finally, the detection mechanism for high-energy (tau) neutrinos in the Pierre Auger Observatory and the resulting flux upper limits are presented.
Hadronic light-by-light scattering in the anomalous magnetic moment of the muon

Nils Asmussen, Antoine Gerardin, Andreas Nyffeler, Harvey B. Meyer

SciPost Phys. Proc. 1, 031 (2019) · published 20 February 2019 | · pdf

Hadronic light-by-light scattering in the anomalous magnetic moment of the muon $a_\mu$ is one of two hadronic effects limiting the precision of the Standard Model prediction for this precision observable, and hence the new-physics discovery potential of direct experimental determinations of $a_\mu$. In this contribution, I report on recent progress in the calculation of this effect achieved both via dispersive and lattice QCD methods.
The history of the muon (g-2) experiments

B. Lee Roberts

SciPost Phys. Proc. 1, 032 (2019) · published 20 February 2019 | · pdf

I discuss the history of the muon $(g-2)$ measurements, beginning with the Columbia-Nevis measurement that observed parity violation in muon decay, and also measured the muon $g$-factor for the first time, finding $g_\mu=2$. The theoretical (Standard Model) value contains contributions from quantum electrodynamics, the strong interaction through hadronic vacuum polarization and hadronic light-by-light loops, as well as the electroweak contributions from the $W$, $Z$ and Higgs bosons. The subsequent experiments, first at Nevis and then with increasing precision at CERN, measured the muon anomaly $a_\mu = (g_\mu-2)/2$ down to a precision of 7.3 parts per million (ppm) The Brookhaven National Laboratory experiment E821 increased the precision to 0.54 ppm, and observed for the first time the electroweak contributions. Interestingly, the value of $a_\mu$ measured at Brookhaven appears to be larger than the Standard Model value by greater than three standard deviations. A new experiment, Fermilab E989, aims to improve on the precision by a factor of four, to clarify whether this result is a harbinger of new physics entering through loops, or from some experimental, statistical or systematic issue.
Status of the Muon g-2 experiment at Fermilab

A. Driutti for the Muon g-2 Collaboration

SciPost Phys. Proc. 1, 033 (2019) · published 21 February 2019 | · pdf

The aim of the Muon g − 2 Experiment at Fermilab (E989) is to measure the muon anomalous magnetic moment (aμ) with a relative precision of 140 parts- per-billion (ppb). This precision, which is a factor of four improvement from the current experimental result, will allow for a much more stringent test of the Standard Model. This paper present the current status of the experimental measurement of aμ after the first physics run.
Experimental input from $e^+e^-$ machines to hadronic contribution to muon (g-2)

M. N. Achasov, R. R. Akhmetshin, A. N. Amirkhanov, A. V. Anisenkov, V. M. Aulchenko, V. Sh. Banzarov, A. Yu. Barnyakov, N. S. Bashtovoy, K. I. Beloborodov, A. V. Berdyugin, D. E. Berkaev, A. G. Bogdanchikov, A. E. Bondar, A. A. Botov, A. V. Bragin, T. V. Dimova, V. P. Druzhinin, S. I. Eidelman, D. A. Epifanov, L. B. Epshteyn, A. L. Erofeev, G. V. Fedotovich, S. E. Gayazov, V. B. Golubev, A. A. Grebenuk, S. S. Gribanov, D. N. Grigoriev, F. V. Ignatov, V. L. Ivanov, L. V. Kardapoltsev, S. V. Karpov, V. F. Kazanin, A. G. Kharlamov, A. N. Kirpotin, I. A. Koop, A. A. Korol, A. A. Korobov, S. V. Koshuba, A. N. Kozyrev, E. A. Kozyrev, D. P. Kovrizhin, P. P. Krokovny, A. S. Kupich, A. E. Kuzmenko, A. S. Kuzmin, R. A. Litvinov, I. B. Logashenko, P. A. Lukin, K. A. Martin, N. A. Melnikova, K. Yu. Mikhailov, A. E. Obrazovsky, V. S. Okhapkin, A. V. Otboev, E. V. Pakhtusova, Yu. N. Pestov, A. S. Popov, K. V. Pugachev, G. P. Razuvaev, A. A. Ruban, N. M. Ryskulov, A. E. Ryzhenenkov, A. I. Senchenko, S. I. Serednyakov, Yu. M. Shatunov, P. Yu. Shatunov, V. E. Shebalin, D. N. Shemyakin, D. A. Shtol, B. A. Shwartz, D. B. Shwartz, A. L. Sibidanov, Z. K. Silagadze, E. P. Solodov, I. K. Surin, A. A. Talyshev, V. M. Titov, S. S. Tolmachev, A. I. Vorobiov, I. M. Zemlyansky, Yu. V. Yudin

SciPost Phys. Proc. 1, 034 (2019) · published 21 February 2019 | · pdf

Precise study of the hadrons production in $e^+e^-$ annihilation at low energies provides important information about interactions of light quarks and spectroscopy of their bound states. These studies are especially important for theoretical calculations of the muon $(g-2)$ via the Standard Model because of the present difference between theoretical and experimental values exceeds three standard deviations. In this report we will discuss current status and recent results obtained by the CMD-3 experiment and will give a brief look to status of the BaBar and Belle II experiments.
The strong coupling from $e^+e^-\to$~hadrons

Diogo Boito, Maarten Golterman, Alex Keshavarzi, Kim Maltman, Daiskuke Nomura, Santiago Peris, Thomas Teubner

SciPost Phys. Proc. 1, 035 (2019) · published 21 February 2019 | · pdf

We use a new compilation of the hadronic $R$-ratio from available data for the process $e^+e^-\to$ hadrons below the charm mass to determine the strong coupling $\alpha_s$, using finite-energy sum rules. Quoting our results at the $\tau$ mass to facilitate comparison to the results obtained from similar analyses of hadronic $\tau$-decay data, we find $\alpha_s(m_\tau^2)=0.298\pm 0.016\pm 0.006$ in fixed-order perturbation theory, and $\alpha_s(m_\tau^2)=0.304\pm 0.018\pm 0.006$ in contour-improved perturbation theory, where the first error is statistical, and the second error combines various systematic effects. These values are in good agreement with a recent determination from the OPAL and ALEPH data for hadronic $\tau$ decays. We briefly compare the $R(s)$-based analysis with the $\tau$-based analysis.
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 $\alpha_s^{(n_f=5)}(M_Z^2) = 0.1197\pm 0.0015$.
The Mu3e experiment at PSI

Alessandro Bravar on behalf of the Mu3e Collaboration

SciPost Phys. Proc. 1, 037 (2019) · published 21 February 2019 | · pdf

Mu3e will search for charged Lepton Flavor Violation in the neutrinoless muon decay $\mu^+ \rightarrow e^+ e^- e^+$ with a sensitivity down to $10^{-16}$ (90\% C.L.) using the world most intense continuous muon beam at PSI. This search requires a large acceptance detector capable of coping with rates of up to $2 \times 10^9$ stopped muons per second with excellent momentum, spatial, and time resolution. The Mu3e detector is based on thin monolithic active silicon pixel sensors for tracking in conjunction with scintillating fibers and tiles for timing measurements. The Mu3e apparatus is under constructions and first data is expected in 2020.
Searching for muon to electron conversion: The Mu2e experiment at Fermilab

Richard Bonventre on behalf of the Mu2e Collaboration

SciPost Phys. Proc. 1, 038 (2019) · published 21 February 2019 | · pdf

The Mu2e experiment will measure the charged-lepton flavor violating (CLFV) neutrino-less conversion of a negative muon into an electron in the field of a nucleus. Mu2e will improve the previous measurement by four orders of magnitude, reaching a 90% C.L. limit of $8\times10^{-17}$ on the conversion rate. The experiment will reach mass scales of nearly $10^4$ TeV, far beyond the direct reach of colliders. The experiment is sensitive to a wide range of new physics, complementing and extending other CLFV searches. Mu2e is under design and construction at the Muon Campus of Fermilab; we expect to start taking physics data in 2022 with 3 years of running to achieve our target sensitivity.
$μ$-$e$ conversion experiments at J-PARC

Natsuki Teshima

SciPost Phys. Proc. 1, 039 (2019) · published 21 February 2019 | · pdf

There are two plans of experiments to search for muon to electron ($\mu$-$e$) conversion at J-PARC, which are called DeeMe and COMET. The $\mu$-$e$ conversion is one of the charged lepton flavor violation processes. The processes are forbidden in the Standard Model, but some theories beyond the Standard Model predict relatively large branching ratios at orders of $10^{-12}$ to $10^{-17}$. DeeMe will be conducted with a sensitivity of $1\times10^{-13}$ using a carbon target for 1 year, and COMET will be done with that of $3\times10^{-15}$ in Phase-I. In this article, the current status of these two experiments will be presented.
MEG final results and progress towards MEG II

Toshiyuki Iwamoto on behalf of the MEG II collaboration

SciPost Phys. Proc. 1, 040 (2019) · published 21 February 2019 | · pdf

The MEG experiment, which is to search for lepton flavor violating muon decay, had been successfully finished in 2013. The final sensitivity of the experiment was 5.3 × 10−13, and since the experiment did not find any signal, the upper limit of the branching ratio of the μ+ → e+γ was set to be 4.2 × 10−13 at 90% CL. The MEG II experiment will improve the sensitivity by an order of magnitude with three years data taking, and the target sensitivity is 6×10−14. In 2018 after the detector integration, the muon beam data taking will be planned with limited number of readout channels. In 2019, the engineering run and the physics run will be started.
Tau-lepton Physics at the FCC-ee circular e$^+$e$^-$ Collider

Mogens Dam

SciPost Phys. Proc. 1, 041 (2019) · published 21 February 2019 | · pdf

The future FCC-ee collider is designed to deliver $\mathrm{e^+e^-}$ collisions to study with ultimate precision the Z, W, and Higgs bosons, and the top quark. In a high-statistics scan around the Z pole, $1.3\times 10^{11}$ events $\mathrm{Z}\to\tau\tau$ will be produced, the largest sample of $\tau\tau$ events foreseen at any lepton collider. With their large boost, $\tau$ leptons from Z decays are particularly well suited for precision measurements. The focus of this report is on tests of lepton universality from precision measurement of $\boldsymbol{\tau}$ properties and on tests of charged lepton flavour violation in Z decays and in $\tau$ decays. In both of these areas, FCC-ee promises sensitivities well beyond present experimental limits.
GRAND: A Giant Radio Array for Neutrino Detection

Charles Timmermans for the GRAND collaboration

SciPost Phys. Proc. 1, 042 (2019) · published 21 February 2019 | · pdf

GRAND will be a distributed set of about 20 arrays of radio antennas, totalling 200,000 km2. We aim to study the ultra high energy universe through the detection of neutrinos, photons and cosmic rays at energies at the EeV scale. In addition GRAND will be used to study astronomical objects like FRBs and GRBs. We will describe the physics motivation, the detection strategy, the first stages of GRAND and the path to creating the complete setup.
Status of DUNE

Alessandra Tonazzo on behalf of the DUNE Collaboration

SciPost Phys. Proc. 1, 043 (2019) · published 21 February 2019 | · pdf

The Deep Underground Neutrino Experiment (DUNE) is a next-generation underground observatory, to be located in the USA, aiming at precise measurements of long-baseline neutrino oscillations over a 1300 km baseline, detection of supernova neutrinos and search for nucleon decay and other physics beyond the Standard Model. The far detector, a very large liquid argon time projection chamber, requires a dedicated prototyping effort (ProtoDUNE), currently ongoing at CERN.
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.
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.
Measurement of the spectral function for the $τ^-\to K^-K_Sν_τ$ decay in BABAR experiment

Sergey I. Serednyakov on behalf of the BABAR collaboration

SciPost Phys. Proc. 1, 046 (2019) · published 22 February 2019 | · pdf

The decay $\tau^{-}\to K^{-}K_S\nu_{\tau}$ has been studied using $430\times10^6$ $e^+e^-\to \tau^+\tau^-$ events produced at a center-of-mass energy around 10.6 GeV at the PEP-II collider and studied with the BABAR detector. The mass spectrum of the $K^{-}K_S$ system has been measured and the spectral function has been obtained. The measured branching fraction ${\cal B}(\tau^{-}\to K^{-}K_S\nu_{\tau}) = (0.739\pm 0.011(\rm stat.)\pm 0.020(\rm syst.))\times 10^{-3}$ is found to be in agreement with earlier measurements.
Ultra-high energy neutrinos searches with the Pierre Auger Observatory

Marta Trini for the Pierre Auger Collaboration

SciPost Phys. Proc. 1, 047 (2019) · published 22 February 2019 | · pdf

In the EeV range, neutrinos are expected to be produced by ultra-high energy cosmic rays interactions with the Cosmic Microwave Background during propagation in the Universe. We report on the search for ultra-high energy neutrinos in data collected with the Surface Detector of the Pierre Auger Observatory. The searches are most efficient in the zenith angle range from 60 degrees to 95 degrees with tau neutrinos skimming in the Earth playingdominant role. The present non-detection of UHE neutrinos in the Pierre Auger Observatory excludes the most optimistic scenarios of neutrino production in terms of UHE cosmic rays chemical composition and cosmological evolution of the acceleration sites. We also report on the searches for neutrinos in coincidence with the recent Gravitational Wave events detected by LIGO/Virgo.
Search for lepton-flavour-violating decays of the $Z$ boson into a $\tau$-lepton and a light lepton with the ATLAS detector

Wing Sheung Chan on behalf of the ATLAS collaboration

SciPost Phys. Proc. 1, 048 (2019) · published 22 February 2019 | · pdf

Lepton flavour violation in the charged lepton sector is an unambiguous signal of physics beyond the Standard Model. Searches for lepton flavour violation in decays of the $Z$ boson with the ATLAS detector is reported, focusing on decays into an electron or muon and a hadronically decaying $\tau$-lepton, using $pp$ collisions data with a centre-of-mass energy of 13 TeV. Upper limits on the branching ratios of lepton-flavor-violating decays are set at the 95\% confidence level: $\mathcal{B}(Z\to e\tau) < 5.8\times10^{-5}$ and $\mathcal{B}(Z\to \mu\tau) < 2.4\times10^{-5}$. When combined with a previous ATLAS result based on $pp$ collisions data with a centre-of-mass energy of 8 TeV, an upper limit of $\mathcal{B}(Z\to \mu\tau) < 1.3\times10^{-5}$ is obtained.
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.
Tau reconstruction at CMS with a focus on high $ p_{T} $ taus

Soham Bhattacharya on behalf of The CMS Collaboration

SciPost Phys. Proc. 1, 050 (2019) · published 22 February 2019 | · pdf

We present the algorithm and performance of tau reconstruction at the CMS experiment, while highlighting a dedicated reconstruction algorithm that uses calorimeter hits instead of tracks to reconstruct taus with high transverse momentum. Describing the standard Hadrons-Plus-Strips (HPS) algorithm and its dependence on track reconstruction and shower modelling, we present the calorimetric tau (calo-tau) reconstruction that uses minimal track information for high $ p_{T} $ taus. The pros and cons of these algorithms are discussed along with their performance and potential uses. It is found that the calo-tau algorithm outperforms the HPS algorithm in the high efficiency region. This study is work in progress, and is an attempt to tune the reconstruction for high $ p_{T} $ taus. The calo-tau algorithm is not yet an official tau reconstruction algorithm for CMS.
Development of the detectors for the DeeMe experiment

Natsuki Teshima on behalf of the DeeMe Collaboration

SciPost Phys. Proc. 1, 051 (2019) · published 22 February 2019 | · pdf

In the DeeMe experiment, approximately $70\ \mathrm{GHz/mm^{2}}$ prompt-charged particles will hit the multi-wire proportional chambers (MWPCs) before signal electrons come. To avoid the space charge effect, we developed fast HV-switching MWPCs in order to control the gas gain dynamically. All four of the MWPCs were manufactured. Last year, the circuit of readout amplifiers is slightly modified to further improve the efficiency of the detector, and we also started to investigate other possible choices of the gas mixture. In this article, the development of the detectors and results of performance tests will be presented.
The rare and forbidden: Testing physics beyond the standard model with Mu3e

Ann-Kathrin Perrevoort on behalf of the Mu3e Collaboration

SciPost Phys. Proc. 1, 052 (2019) · published 22 February 2019 | · pdf

The upcoming Mu3e experiment aims to search for the lepton flavour violating decay $\mu^+ \to e^+ e^- e^+$ with an unprecedented final sensitivity of one signal decay in $10^{16}$ observed muon decays by making use of an innovative experimental design based on novel ultra-thin silicon pixel sensors. In a first phase, the experiment is operated at an existing muon beam line with rates of up to $10^8$ muons per second. Detailed simulation studies confirm the feasibility of background-free operation and project single event sensitivities in the order of $10^{-15}$ for signal decays modelled in an effective field theory approach. The precise tracking of the decay electrons and large geometric and momentum acceptance of Mu3e enable searches for physics beyond the Standard Model in further signatures. Examples of which are searches for lepton flavour violating two-body decays of the muon into an electron and an undetected boson as well as for electron-positron resonances in $\mu^+ \to e^+ e^- e^+ \bar{\nu}_\mu \nu_e$ which could result for instance from a dark photon decay. The Mu3e experiment is expected to be competitive in all of these channels already in phase I.
Determining $\alpha_s$ from hadronic $\tau$ decay: the pitfalls of truncating the OPE

Diogo Boito, Maarten Golterman, Kim Maltman, Santiago Peris

SciPost Phys. Proc. 1, 053 (2019) · published 22 February 2019 | · pdf

We discuss sum-rule determinations of $\alpha_s$ from non-strange hadronic $\tau$-decay data. We investigate, in particular, the reliability of the assumptions underlying the ``truncated OPE strategy,'' which specifies a certain treatment of non-perturbative contributions, and which was employed in Refs.~\cite{Pich,Aleph, Opal}. Here, we test this strategy by applying the strategy to the $R$-ratio obtained from $e^+e^-$ data, which extend beyond the $\tau$ mass, and demonstrate that the assumptions underlying this strategy are not, in general, valid. We then present a brief overview of new results on the form of duality-violating non-perturbative contributions, which are conspicuously present in the experimentally determined spectral functions. As we show, with the current precision claimed for the extraction of $\alpha_s$, including a representation of duality violations is unavoidable if one wishes to avoid uncontrolled theoretical errors.
Experimental input from $e^+e^-$ for $a_{\mu}$ light-by-light

Yuping Guo on behalf of BESIII Collaboration

SciPost Phys. Proc. 1, 054 (2019) · published 9 May 2019 | · pdf

The anomalous magnetic momentum of the muon, $a_{\mu}$, has been measured and calculated with a precision up to $~0.5$ ppm, but there is a 3 to 4 standard deviations between these two values. The uncertainty in the calculation is dominated by the hadronic part, including the hadronic vacuum polarization and the hadronic light-by-light. The meson transition form factors and the helicity amplitudes can be used as input or constraint to the calculation of the hadronic light-by-light contribution. Latest experimental studies of the transition form factors of $\pi^0$, $\eta$, and $\eta^{*}$ and the cross-section of $\gamma\gamma^*\to\pi^+\pi-$from $e^+e^-$ collider are presented.