In semi-leptonic $B$ decays with a tau lepton, features of the production process are imprinted on the tau helicity states. Since the tau momentum cannot be fully reconstructed experimentally, the available information on the tau properties is encoded in its visible decay products. Focusing on the process $B\to Dτν$, we find explicit relations between the tau properties and the kinematics of the charged particles in the decays $τ\toπν$, $τ\toρν$, and $τ\to\ellν\barν$. In particular, we show that the perpendicular polarization, $P_\perp$, and the forward-backward asymmetry, $A_τ$, of the tau lepton can simultaneously be extracted from an angular asymmetry of the charged particle against the $D$ meson. For the most sensitive decay channel, $τ\toπν$, we expect a relative statistical precision of about $10\%$ for $P_\perp$ and $A_τ$ in a measurement based on $50\,$ab$^{-1}$ of data at BELLE II.
Searches for heavy neutrinos often rely on the possibility that the heavy neutrinos will decay to detectable particles. Interpreting the results of such searches requires a particular model for the heavy-neutrino decay. We present a method for placing limits on the probability that a tau can couple to a heavy neutrino, |U_tau4|^2, using only the kinematics of semi-leptonic tau decays, instead of a specific model. Our study suggests that B factories with large datasets, such a Belle and BaBar, may be able to place stringent limits on |U_tau4|^2 as low as O(10^-7 - 10^-3) when 100 MeV < m_4 < 1.…
This article summarizes recent developments in $B\to D^{(\ast)}τν$ 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.
Since the $ν_τ(\barν_τ) A_Z \to τ^\mp X$ reaction is notoriously difficult to be directly measured, the information on the dynamics of this nuclear process should be extracted from the analysis of the energy and angular distributions of the tau decay visible products. These distributions depend on the components of the tau-polarization vector. We give, for the first time, the general expression for the outgoing hadron (pion or rho meson) energy and angular differential cross section for the sequential $ν_τA_Z \to τ^-(π^- ν_τ, ρ^-ν_τ) X$ and $\barν_τA_Z \to τ^+(π^+ \barν_τ, ρ^+ \barν_τ) X$ reac…
A novel method for the direct measurement of the elusive magnetic and electric dipole moments of the tau lepton is presented. The experimental approach relies on the production of tau+ leptons from Ds+ -> tau+ nu_tau decays, originated in fixed-target collisions at the LHC. A sample of polarized tau+ leptons is kinematically selected and subsequently channeled in a bent crystal. The magnetic and electric dipole moments of the tau+ lepton are measured by determining the rotation of the spin-polarization vector induced by the intense electromagnetic field between crystal atomic planes. The exper…
The invariant mass of tau lepton pairs turns out to be smaller than the resonant mass of their mother particle and the invariant mass distribution is stretched wider than the width of the resonant mass as significant fraction of tau lepton momenta are carried away by neutrinos escaping undetected at collider experiments. This paper describes a new approach to reconstruct resonant masses of heavy particles decaying to tau leptons at such experiments. A typical example is a Z or Higgs boson decaying to a tau pair. Although the new technique can be used for each tau lepton separately, I combine t…
Measuring tau-polarisation in Neutralino2 decays at the LHC
我们展示了如何在LHC上测量具有保守R宇称的最小超对称性的衰变链chi 20-> stau 1 tau -> tau tau chi 10中的两个平均tau偏振之和。这是通过利用可见的di-tau质谱的偏振依赖性来实现的。这样的测量提供了有关所涉及的超对称性粒子的耦合的信息，并允许更精确地确定di-tau质量终点。如果可以区分不同的τ衰变模式，则可以进一步改进偏振和终点测量。
We derive compact expressions for the helicity amplitudes of the many-body $B \to D^{(*)}(\to DY)τ(\to Xν)ν$ decays, specifically for $X = \ell ν$ or $π$ and $Y = π$ or $γ$. We include contributions from all ten possible new physics four-Fermi operators with arbitrary couplings. Our results capture interference effects in the full phase space of the visible $τ$ and $D^*$ decay products which are missed in analyses that treat the $τ$ or $D^*$ or both as stable. The $τ$ interference effects are sizable, formally of order $m_τ/m_B$ for the standard model, and may be of order unity in the presence…
The very high statistics, low backgrounds and clean back-to-back kinematics of a TeraZ facility would provide an optimal laboratory for precision measurements of the $τ$ properties. A few important topics in $τ$ physics where very relevant contributions could be made are highlighted.
We study the $d^2 Γ_d /(dωd\cosθ_d) $, $dΓ_d /d\cosθ_d$ and $ dΓ_d /dE_d $ distributions, which are defined in terms of the visible energy and polar angle of the charged particle from the $τ-$decay in $b\to c τ\, (μ\bar ν_μν_τ,πν_τ,ρν_τ) \barν_τ$ reactions. The first two contain information on the transverse tau-spin, tau-angular and tau-angular-spin asymmetries of the $H_b\to H_cτ\barν_τ$ parent decay and, from a dynamical point of view, they are richer than the commonly used one, $d^2 Γ_d /(dωdE_d) $, since the latter only depends on the tau longitudinal polarization. We pay attention to the…
We investigate potential for measuring CP state of the Higgs boson in the H to tau tau$ decay with consecutive tau-lepton decays in channels: tau^+- to rho^+- nu_tau and tau^+- to a1^+- nu_tau combined. Subsequent decays rho^+- to pi^+- pi^0, a1^+- to rho^0 pi^+- and rho^0 to pi^+ pi^- are taken into account. We will explore extensions of the method, where acoplanarity angle for the planes build on the visible decay products, pi^+- pi^0 of tau^+- to pi^pm pi^0 nu_tau, was used. The angle is sensitive to transverse spin correlations, thus to parity. We show, that in the case of the cascade deca…
In the framework of precision experiments, the search for electric dipole moments and the precise determination of magnetic dipole moments (g-2) have since long been of prime interest. Hadronic decays offer the best accuracy, since only the kinematic information carried by a single neutrino per decay is lost. Thus, they reveal more easily precious information on the helicity of the initial tau lepton. However, in contrast to one- or two-body hadronic final states, the description of hadronic multi-body final states depends on the model for the hadronic current. In this work, we determine how t…
The tau-lepton plays an important role in the physics program at the Large Hadron Collider (LHC). It offers a probe in searches for New Physics and to measure parameters of the Standard Model. Spin of tau lepton is an interesting phenomenological quantity for separation of signal from background or in measuring properties of particles. A proper treatment of tau spin effects in the Monte Carlo simulations is important for understanding the detector acceptance as well as for the measurements/use of tau polarization and tau spin correlations. The TauSpinner is a tool to modify tau$spin effects in…
Lepton flavour universality (LFU) in B-decays is revisited by considering a class of semileptonic operators defined at a scale Lambda above the electroweak scale v. The importance of quantum effects, so far neglected in the literature, is emphasised. We construct the low-energy effective Lagrangian taking into account the running effects from Lambda down to v through the one-loop renormalization group equations (RGE) in the limit of exact electroweak symmetry and QED RGEs from v down to the 1 GeV scale. The most important quantum effects turn out to be the modification of the leptonic coupling…
Revealing the Two-Fold Ambiguity: Tau Momentum Reconstruction and Its Impact on Entanglement Observables
Shaanxi Normal University（陕西师范大学）；Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；University of Chinese Academy of Science（中国科学院大学）
Anomalies from the LHCb lepton flavour universality and Fermilab muon anomalous magnetic momentum, show tantalizing hints of possible new physics from the lepton sectors. Due to its large mass and shorter lifetime than muon, the tau lepton is believed to couple more to possible new physics beyond the standard model. Traditionally, tau leptons are probed through the decay products due to tau's short life time. On the other hand, at a high energy scale, a large fraction of tau leptons could be boosted to a much longer life time and fly a visible distance from several centimetres up to kilometer …
TauDecay is a library of helicity amplitudes to simulate polarized tau decays, constructed in the FeynRules and MadGraph5 framework. Together with the leptonic mode, the decay library includes the main hadronic modes, τ\to ν_τ+π, 2π, and 3π, which are introduced as effective vertices by using FeynRules. The model file allows us to simulate tau decays when the on-shell tau production is kinematically forbidden. We also demonstrate that all possible correlations among the decay products of pair-produced taus through a Z boson and a scalar/pseudoscalar Higgs boson are produced automatically. The …
The meson decays $B\to Dτν$ and $B\to D^* τν$ are sensitive probes of the $b\to cτν$ transition. In this work we present a complete framework to obtain the maximum information on the physics of $B\to D^{(*)}τν$ with polarized $τ$ leptons and unpolarized $D^{(*)}$ mesons. Focusing on the hadronic decays $τ\to πν$ and $τ\toρν$, we show how to extract seven $τ$ asymmetries from a fully differential analysis of the final-state kinematics. At Belle II with $50~\text{ab}^{-1}$ of data, these asymmetries could potentially be measured with percent level statistical uncertainty. This would open a new w…
Entanglement measures and Bell-type spin-correlation observables in tau-lepton pairs at the Super Tau-Charm Facility
Hefei University of Technology（合肥工业大学）；University of Science and Technology of China（中国科学技术大学）；State Key Laboratory of Particle Detection and Electronics（粒子探测与电子学国家重点实验室）
The dispersive approach to QCD, which properly embodies the intrinsically nonperturbative constraints originating in the kinematic restrictions on relevant physical processes and extends the applicability range of perturbation theory towards the infrared domain, is briefly overviewed. The study of OPAL (update 2012) and ALEPH (update 2014) experimental data on inclusive tau lepton hadronic decay in vector and axial-vector channels within dispersive approach is presented.
Hunting the Unseen: Deep Learning Analysis for Semi-Visible Jet Tagging
University of Southampton（南安普顿大学）；Rutherford Appleton Laboratory（卢瑟福·阿普尔顿实验室）；KIAS（韩国高等科学研究院）；Uppsala University（乌普萨拉大学）；KEK（高能加速器研究机构）；The Graduate University of Advanced Studies (Sokendai)（综合研究大学院大学（索肯代））；Kavli IPMU (WPI), University of Tokyo（东京大学木户宇宙物质研究所（世界顶级研究中心））；University of Manchester（曼彻斯特大学）；CERN（欧洲核子研究组织）
Motivated by models proposed to explain the Standard Model anomalies, and the unprecedented $τ^{+}τ^{-}$ data to be collected by the Belle~II experiment during the next years, we study the kinematics of tau pair decays and propose a new method to search for lepton flavor violating processes in tau lepton decays to invisible beyond Standard Model particles, such as $τ\to \ell α$, where $\ell$ is either an electron or a muon, and $α$ is a massive particle that escapes undetected. The new method improves by one order of magnitude the expected upper limit on the $τ\to \ell α$ production in 3x1 pro…
The classic method for mass determination in a SUSY-like cascade decay chain relies on measurements of the kinematic endpoints in the invariant mass distributions of suitable collections of visible decay products. However, the procedure is complicated by combinatorial ambiguities: e.g., the visible final state particles may be indistinguishable (as in the case of QCD jets), or one may not know the exact order in which they are emitted along the decay chain. In order to avoid such combinatorial ambiguities, we propose to treat the final state particles fully democratically and consider the sort…
The dispersive approach to QCD, which extends the applicability range of perturbation theory towards the infrared domain, is developed. This approach properly accounts for the intrinsically nonperturbative constraints, which originate in the low-energy kinematic restrictions on pertinent strong interaction processes. The dispersive approach proves to be capable of describing OPAL (update 2012) and ALEPH (update 2014) experimental data on inclusive tau lepton hadronic decay in vector and axial-vector channels in a self-consistent way.
In the context of lepton flavor universality violation (LFUV) studies, we fully derive a general tensor formalism to investigate the role that left- and right-handed neutrino new-physics (NP) terms may have in $b\to c τ\barν_τ$ transitions. We present, for several extensions of the Standard Model (SM), numerical results for the $Λ_b\toΛ_cτ\barν_τ$ semileptonic decay, which is expected to be measured with precision at the LHCb. This reaction can be a new source of experimental information that can help to confirm, or maybe rule out, LFUV presently seen in $\bar B$ meson decays. The present stud…
The vector and axial-vector ALEPH hadronic spectral functions from $τ$-decay are used to probe potential quark-hadron duality violations (DV). This is done in the framework of finite energy QCD sum rules (FESR). A pinched integration kernel is introduced in the FESR in order to (a) quench potential duality violations on the real axis in the complex squared energy $s$-plane, and (b) effectively extend the analysis well beyond the kinematical $τ$-decay end-point where there is no longer data, i.e.\ in the range $s = 3 - 10 \,{\mbox{GeV}}^2$. In the vector channel this procedure is supplemented w…
The $μ-τ$ reflection symmetry of the lepton mixing matrix accommodates maximal atmospheric mixing ($θ_{23}=π/4$) as well as maximal Dirac CP phase ($δ=\pm π/2$) for the Dirac case. In the standard parametrization of the PMNS matrix the reflection symmetric nature is not directly visible while substituting the maximal values of the mixing parameters. This issue has been addressed in this paper. It is found that the reflection symmetry in the 'standard' PMNS matrix can be restored by allowing maximal values of the Majorana CP phases ($α$, $β$) as well, along with maximal $δ$. To accommodate non-…
We consider measurements of exclusive rare semi-tauonic $b$-hadron decays, mediated by the $b \rightarrow s τ^+ τ^-$ 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 $τ$ decay modes (up to discrete ambiguities). This, together with the potentially large statistics of $Z\to b\bar b$, opens the door for the experimental determination of $τ$ polarizations in these rare $b$-hadron decays. In light of the curre…
