SciPost: SciPost Physics Proceedings

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.
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.
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.
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.
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.

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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.

2 citations
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.

1 citation
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.

1 citation
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.
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 $\alpha_s^{(n_f=5)}(M_Z^2) = 0.1197\pm 0.0015$.

SciPost Physics Proceedings issue 1
The 15th International Workshop on Tau Lepton Physics (Amsterdam, 2018-09)
Submission deadline: 2018-11-01

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