Hadronic Tau Decays Research Articles

Electromagnetic corrections in hadronic tau decays

We revisit the isospin-breaking and electromagnetic corrections of some hadronic tau decays, which can also be employed to extract the Cabibbo-Kobayashi-Maskawa (CKM) matrix element Vus. We extend former analyses by Antonelli et al. working with ChPT with resonances. We find that going beyond the Low approximation, these corrections play an important role between the π− K0 and K− π0 modes. The K− K0 channel is also discussed.

Mathematical aspects of the asymptotic expansion in contour improved perturbation theory for hadronic tau decays

Mathematical aspects of the asymptotic expansion in contour improved perturbation theory for hadronic tau decays

Cross-section measurements of the Higgs boson decaying into a pair of τ -leptons in proton-proton collisions at s=13 TeV with the ATLAS detector

A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the $H\rightarrow\tau\tau$ decay channel. The analysis is performed using $36.1\mathrm{fb}^{-1}$ of data recorded by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of $\sqrt{s}=13$ TeV. All combinations of leptonic ($\tau \rightarrow l \nu \bar{\nu}$ with $l = e$, $\mu$) and hadronic ($\tau \to \mathrm{hadrons} \nu$) tau decays are considered. The $H\rightarrow\tau\tau$ signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of $H\rightarrow\tau\tau$ decays. Using the data taken at $\sqrt{s}=13$ TeV, the total cross section in the $H\rightarrow\tau\tau$ decay channel is measured to be $3.77^{+0.60}_{-0.59}\mathrm{(stat.)}^{+0.87}_{-0.74}\mathrm{(syst.)}\mathrm{pb}$, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the $H\rightarrow\tau\tau$ decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be $0.28 \pm 0.09\mathrm{(stat.)}^{+0.11}_{-0.09}\mathrm{(syst.)}\mathrm{pb}$ and $3.1 \pm 1.0\mathrm{(stat.)}^{+1.6}_{-1.3}\mathrm{(syst.)}\mathrm{pb}$, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.

Tau polarimetry in B meson decays

This article summarizes recent developments in B\to D^{(\ast)}\tau\nuB→D(*)τν 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.

Study of the Drell-Yan process with the pair-production of polarized tau leptons in collisions of polarized proton beams

The new method for studying polarized parton distribution functions via the Drell-Yan process with tau production is proposed. The hadronic decay of tau to single charged pi-meson and neutrino is utilised for determination of the polarization state of the tau-lepton. The key feature of our approach is that we do not sum over the polarizations states of tau, but preserve this information through energies ofpions produced in corresponding decays. The new method in addition to existing ones should improve the accuracy of measurements of polarized structure functions of hadrons. In the current work, the feasibility of such an approach is assessed through numerical simulations.

Measurement of tau polarisation in Z/gamma ^{*}rightarrow tau tau decays in proton\u2013proton collisions at sqrt{s}=8 TeV with the ATLAS detector

This paper presents a measurement of the polarisation of tau leptons produced in Z/gamma ^{*}rightarrow tau tau decays which is performed with a dataset of proton—proton collisions at sqrt{s}=8 TeV, corresponding to an integrated luminosity of 20.2 fb^{-1} recorded with the ATLAS detector at the LHC in 2012. The Z/gamma ^{*}rightarrow tau tau decays are reconstructed from a hadronically decaying tau lepton with a single charged particle in the final state, accompanied by a tau lepton that decays leptonically. The tau polarisation is inferred from the relative fraction of energy carried by charged and neutral hadrons in the hadronic tau decays. The polarisation is measured in a fiducial region that corresponds to the kinematic region accessible to this analysis. The tau polarisation extracted over the full phase space within the Z/gamma ^{*} mass range of 66 < m_{Z/gamma ^{*}} < 116 GeV is found to be P_{tau } =-0.14 pm 0.02 (text {stat}) pm 0.04 (text {syst}). It is in agreement with the Standard Model prediction of P_{tau } =-0.1517 pm 0.0019, which is obtained from the ALPGEN event generator interfaced with the PYTHIA 6 parton shower modelling and the TAUOLA tau decay library.

|Vus| determination from inclusive strange tau decay and lattice HVP

We propose and apply a novel approach to determining |Vus| which uses inclusive strange hadronic tau decay data and hadronic vacuum polarization functions (HVPs) computed on the lattice. The experimental and lattice data are related through dispersion relations which employ a class of weight functions having poles at space-like momentum. Implementing this approach using lattice data generated by the RBC/UKQCD collaboration, we show examples of weight functions which strongly suppress spectral integral contributions from the region where experimental data either have large uncertainties or do not exist while at the same time allowing accurate determinations of relevant lattice HVPs. Our result for |Vus| is in good agreement with determinations from K physics and 3-family CKM unitarity. The advantages of the new approach over the conventional sum rule analysis will be discussed.

Mass restricting variables in semi-invisible production at the LHC

We outline some of the popular mass restricting variables for the semi-invisible productions at the Large Hadron Collider. In this context, heavy resonating mass, if produced through antler decay topology may already be detectable. New mass variables constructed by applying this mass constraint proved to have an array of interesting properties, including a new kink solution at the true masses of the produced particles. This enables one to measure the mass of the invisible particle and the parent particle simultaneously. This variable in turn can also be applied in reconstructing such events with the momenta of invisible particles. This feature is further demonstrated with the Higgs boson decaying into a pair of third-generation tau-lepton ( $$\tau $$ ) and thus exploring direct Higgs coupling with the leptonic sector. Dominant discovery signatures rely upon the hadronic decay of tau which is associated with a pair of invisible neutrinos. Exploiting the already measured Higgs mass bound, present technique is capable of providing unique event reconstruction. Moreover, a significant efficiency enhancement is demonstrated in comparison with the existing methods.

Lepton polarization asymmetries in rare semi-tauonic b rightarrow s exclusive decays at FCC-ee

We consider measurements of exclusive rare semi-tauonic b-hadron decays, mediated by the b rightarrow s tau ^+ tau ^- transition, at a future high-energy circular electron–positron collider (FCC-ee). We argue that the high boosts of b-hadrons originating from on-shell Z boson decays allow for a full reconstruction of the decay kinematics in hadronic tau decay modes (up to discrete ambiguities). This, together with the potentially large statistics of Zrightarrow bbar{b}, opens the door for the experimental determination of tau polarizations in these rare b-hadron decays. In the light of the current experimental situation on lepton flavor universality in rare semileptonic B decays, we discuss the complementary short-distance physics information carried by the tau polarizations and suggest suitable theoretically clean observables in the form of single- and double-tau polarization asymmetries.

Leptophilic neutral Higgs bosons in two Higgs doublet model at a linear collider

This paper addresses the question of the observability of neutral Higgs bosons through the leptonic decay in a two Higgs doublet model (2HDM). Both scalar and pseudo-scalar Higgs bosons (H, A) are considered. The model is set to type IV to enhance the leptonic decay. In such a scenario, a signal production process like e^+e^- rightarrow A^0H^0 rightarrow tau tau mu mu or mu mu tau tau would provide a clear signal on top of the background in a di-muon invariant mass distribution far from the Z boson pole mass. The analysis is based on a tau -id algorithm which preselects events if they have two tau jets by requiring a hadronic tau decay. Several benchmark points are defined for the search, requiring a linear collider operating at sqrt{s} = 0.5 and 1 TeV. It is shown that the signal can be observed on top of the background in all benchmark points at an integrated luminosity of 1000 mathrm{fb}^{-1}.

Study of the semileptonic τ -→π- π0ντ and τ- → π- π0ℓ+ℓ-ντ (ℓ = e, μ) decays.

We analyze the decay modes τ -→π- π0ντ and τ - π- π0ℓ+ℓ-ντ, ℓ=e, μ. The first one is studied in order to show that the three resonances p(770), p(1450) and p(1700) are necessary to fix the model vector dominance parameters on the two pion system dynamics. We have obtained BR(τ -→π- π0vτ) = (25.40±0.22)% which is in good agreement with the world average reported in the PDG. The second decay, which is the target of our study, is important because it can participate as a background in processes of lepton flavour and lepton number violation and thus its accurate description is essential in searches for such processes. It is also important to test the model dependence of the most important (ππ) contribution to the hadron vacuum polarization (HVP) entering the muon anomalous magnetic moment, if evaluated using hadronic tau decay data. In these preliminary results, we have only taken into account the total Inner Bremmsstrahlung (IB) contribution to the τ -→π- π0ℓ+ℓ-ντ decays obtaining BR(τ- → π- π0e+e-ντ) = (8.09 ±0.26) x 10-5 and BR(τ- → π- π0μ+μ-ντ-) = (2.094 ±0.004) x 10-6. These results have been obtained using the VEGAS routine to integrate numerically the phase space.

αs analyses from hadronic tau decays with OPAL and ALEPH data

Recently, we extracted the strong coupling, [Formula: see text], from the revised ALEPH data for non-strange hadronic tau decays. Our analysis is based on a method previously used for the determination of the strong coupling from OPAL data. In our strategy, we employ different moments of the spectral functions both with and without pinching, including duality violations, in order to obtain fully self-consistent analyses that do not rely on untested assumptions (such as the smallness of higher dimension contributions in the operation product expansion (OPE)). Here we discuss the [Formula: see text] values obtained from the ALEPH and the OPAL data, the robustness of the analysis, as well as non-perturbative contributions from DVs and the OPE. We show that, although the [Formula: see text] determination is sound, non-perturbative effects limit the accuracy with which one can extract the strong coupling from tau decay data. Finally, we discuss the compatibility of the data sets and the possibility of a combined analysis.

Identification of hadronic tau decays in CMS

The algorithm used for reconstruction and identification of hadronic tau decays by the CMS experiment at the LHC will be presented. The tau reconstruction in CMS takes advantage of the particle-flow algorithm which allows to reconstruct individual hadronic decay modes. The performance of the algorithm in terms of tau identification efficiency and in terms of the rates with which jets, electrons and muons are misidentified as hadronic tau decays, is measured in pp collision data recorded in 2012 at a center–of–mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb−1.

Reconstructing semi-invisible events in resonant tau pair production from Higgs

We study the possibility of utilising the constrained mass variable, $M_{2Cons}$, in reconstructing the semi-invisible events originated from a resonant production at the LHC. While this proposal is effective for any similar antler type production mechanism, here we demonstrate with potentially interesting scenario, when the Higgs boson decays into a pair of third generation $\tau$ leptons. Buoyed with a relatively large Yukawa coupling, the LHC has already started exploring this pair production to investigate the properties of Higgs in the leptonic sector. Dominant signatures through hadronic decay of tau, associated with invisible neutrinos compound the difficulty in the reconstruction of such events. Exploiting the already existing Higgs mass bound, this new method provides a unique event reconstruction, together with a significant enhancement in terms of efficiency over the existing methods.

Results of Beyond-the-Standard Model 2HDM Higgs Searches using the ATLAS detector in Run-I

One of the most alluring models used to probe Beyond the Standard Model physics in run-I at ATLAS stems from a simple extension of the Higgs sector. Models where the Higgs sector is assumed to have two Higgs doublets, known as two-Higgs-Doublet-Models (2HDMs), give rise to five Higgs bosons: two neutral CP-even (h,H) bosons, two charged (H±) bosons, and one neutral CP-odd (A) boson. Direct searches for the additional Higgs bosons became an ever more promising prospect after the Higgs boson discovery, and various such searches have already been conducted in Run-I ATLAS data. The most recent 2HDM search is for the A boson in the sensitive Zh decay, where h is assumed to be the LHC discovered Higgs boson, within the mass range of 220 — 1000 GeV. In such a search, upper limits on the cross section times branching ratio of the A boson decaying to Zh are set for various 2HDM scenarios. Where no excess is observed, exclusion limits are set on ranges of the 2HDM phase-space. In this note the search for A → Zh → ℓℓττ with fully hadronic tau decay is presented in detail, along with summaries from two of the most recent 2HDM searches in ATLAS.

Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

Results of a search for $H \to \tau \tau$ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 $\rm{fb}^{-1}$ and 20.3 $\rm{fb}^{-1}$ at centre-of-mass energies of $\sqrt{s}$ = 7 TeV and $\sqrt{s}$ = 8 TeV respectively. All combinations of leptonic ($\tau \to \ell \nu \bar \nu$ with $\ell = e, \mu$) and hadronic ($\tau \to \rm{hadrons} \nu$) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of $\mu = 1.43 ^{+0.43}_{-0.37}$ is consistent with the predicted Yukawa coupling strength in the Standard Model.

Study of an anomalous tau lepton decay using a chiral Lagrangian with vector mesons

The hadronic tau decay τ−→ντηπ−π0 occurs through V-A weak current. In this decay mode, the vector current contribution is intrinsic parity violating and the axial current contribution is G parity violating. The latter contribution is suppressed due to tiny isospin breaking. We have computed both vector and axial vector form factors using a chiral Lagrangian with vector mesons including the effect of isospin breaking and intrinsic parity violation. A numerical result of the invariant mass distribution is shown and the structure of ρ resonance can be seen in the distribution with respect to Mπ−π0.

Impact of Majorana Neutrinos to Hadronic Tau Decays

Extending the Standard Model with only one or two gauge singlet fermions and a second Higgs doublet allows the radiative generation of neutrino masses. Parameters of this model are varied to find light neutrino masses that are compatible with the experimental data of Δmsolar2 and Δmatm2 and the neutrino oscillation angles. We look to further restrict the parameter space and discuss simple possibilities of using measurements in the charged lepton sector to achieve this reduction.

Resonance Chiral Lagrangians and alternative approaches to hadronic tau decays

Exclusive semi-leptonic decays of the tau lepton offer a clean probe to study the hadronization of QCD currents in its non-perturbative regime and learn about resonance dynamics, which drives strong interactions in these processes. In this theory outlook, I will use the simplest non-trivial di-pion tau decays to illustrate briefly recent theoretical progress on these analyses and their comparison to data.

Leptons and QCD

Three important QCD-related aspects of the τ and μ dynamics are reviewed: the determination of the strong coupling from the hadronic tau decay width, leading to the updated value αs(mτ2)=0.331±0.013; the measurement of |Vus| through the Cabibbo-suppressed decays of the τ, and the Standard Model prediction for the muon anomalous magnetic moment.