Tau Pair Production Research Articles

LHC tau-pair production constraints on $a_\tau$ and $d_\tau$

We point out that relevant constraints on the anomalous magnetic (a_\tauaτ) and electric (d_\taudτ) moment of the tau lepton can be derived from tau-pair production measurements performed at the LHC. Our conclusion is based on the observation that the leading relative deviations from the Standard Model prediction for pp \to \tau^+ \tau^-pp→τ+τ− due to a_\tauaτ and d_\taudτ are enhanced at high energies. Less precise measurements at hadron colliders can therefore offer the same or better sensitivity to new physics with respect to high-precision low-energy measurements performed at lepton machines. We derive bounds on a_\tauaτ and d_\taudτ using the full LHC Run II data set on tau-pair production and compare our findings with the current best limits on the tau anomalous moments.

Tau g{-}2 at e^-e^+ colliders with momentum-dependent form factor

The deviation between the prediction based on the standard model and the measurement of the muon g{-}2 is currently at 3{-}4 sigma . If this discrepancy is attributable to new physics, it is expected that the new contributions to the tau g{-}2 even larger than those of muon due to its large mass. However, it is much more difficult to directly measure the tau g{-}2 because of its short lifetime. In this report, we consider the effect of the tau g{-}2 at e^-e^+ colliders using a model independent approach. Using the tau pair production channel at the Large Electron Position Collider (LEP), we have determined the allowed range for the new physics contribution of the tau g{-}2 assuming a q-square-dependence ansatz for the magnetic form factor. We also investigated the prospect at future e^+e^- colliders, such as International Linear Collider, the Compact Linear Collider, the Future Circular e^+e^- Collider, and Circular Electron Positron Collider, and determined the expected allowed range for the new physics contribution to the tau anomalous magnetic moment. The best limits are about 4{-}5 times more severe than the LEP one due to the beam polarization and the high luminosities at future colliders.

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.

Sneutrino identification in tau pair production at ILC with polarized beams

Many scenarios of new physics can lead to deviations of the observables (cross sections, asymmetries) from the Standard Model predictions in e+e- collision. The possibility of uniquely identifying the indirect effects of s-channel sneutrino exchange, as predicted by supersymmetric theories with R-parity violation, against other new physics scenarios in high-energy electron-positron annihilation into tau pairs at the International Linear Collider has been studied.

TauSpinner: a tool for simulating CP effects in $$ H \rightarrow \tau \tau $$ H → τ τ decays at LHC

In this paper, we discuss application of the TauSpinner package as a simulation tool for measuring the CP state of the newly discovered Higgs boson using the transverse spin correlations in the H to tau tau decay channel. We discuss application for its main background Z/gamma* to tau tau as well. The TauSpinner package allows one to add, with the help of weights, transverse spin correlations corresponding to any mixture of scalar/pseudoscalar state, on already existing events using information from the kinematics of outgoing tau leptons and their decay products only. This procedure can be used when polarimetric vectors of the taus decays and density matrix for tau-pair production are not stored with the event sample. We concentrate on the well-defined effects for the Higgs (or Higgs-like scalar) decays, which are physically separated from the production processes. TauSpinner also allows to reintroduce (or remove) spin correlations to events from Drell-Yan Z/gamma* to tau tau process, the main background for the Higgs parity observables, again with the help of weights only. From the literature, we recall well-established observables, developed for measuring the CP of the Higgs, and use them as benchmarks for illustrating applications of the TauSpinner package. We also include a description of the code and prepared testing examples.

Cross sections and energy loss for lepton pair production in muon transport

We reevaluate electron-positron pair production from electromagnetic interactions of muons in transit through materials. Our approach, through the use of structure functions for inelastic and elastic scattering and including hadronic recoil, make the formalism useful for tau pair production at high energies. Our results for electron-positron pairs agree well with prior evaluations. Tau pair production, however, has significant contributions from inelastic scattering in addition to the usual coherent scattering with the nucleus and scattering with atomic electrons.

Tau Physics Prospects at Super B

The current status of the most significant physics we've learned from tau lepton is summarized. The potential for discovery using the 1011 tau-pair events that can be produced at a e+e− collider facility having a design luminosity of 10 36 cm −2 s −1 is explored. Such a Super Flavour Factory provides a unique window on New Physics associated with tau pair production and decay.

A High Intensity Linear [formula omitted] Collider for a Tau-Charm Factory

A new high intensity e + e − linear collider serving as tau-charm factory is proposed. The design luminosity is 10 35 cm −2 s −1 for center of mass energies of s = 1–5 GeV in the resonance region of Ψ vector mesons and in the region of maximum cross section for tau pair production. This new facility would provide about 1011 tau lepton pairs per year enabling precision studies of τ leptons and searches for rare τ decays with unprecedented sensitivity. Implications on the design of the linear accelerator are discussed together with requirements to achieve luminosities of 10 35 cm −2 s −1 or more. The possibility to exploit energy recovery techniques in order to increase e + beam currents and luminosity is discussed.

Slepton production in polarized hadron collisions

We calculate cross sections and asymmetries for slepton pair production through neutral and charged electroweak currents in polarized hadron collisions for general slepton masses and including mixing of the left- and right-handed interaction eigenstates relevant for third generation sleptons. Our analytical results confirm and extend a previous calculation. Numerically, we show that measurements of the longitudinal single-spin asymmetry at the existing polarized pp collider RHIC and at possible polarization upgrades of the Tevatron or the LHC would allow for a determination of the tau slepton mixing angle and/or the associated supersymmetry breaking parameters Λ for gauge mediation and A0 for minimal supergravity. Furthermore, the Standard Model background from tau pair production can be clearly distinguished due to the opposite sign of the associated asymmetry.

Study of tau-pair production in photon-photon collisions at LEP and limits on the anomalous electromagnetic moments of the tau lepton

Tau-pair production in the process e(+)e(-) --> e(+)e(-) tau(+)tau(-) was studied using data collected by the DELPHI experiment at LEP2 during the years 1997 - 2000. The corresponding integrated luminosity is 650 pb(-1). The values of the cross-section obtained are found to be in agreement with QED predictions. Limits on the anomalous magnetic and electric dipole moments of the tau lepton are deduced.

Muon-pair and tau-pair production in two-photon collisions at LEP

The QED processes e+e−→e+e−μ+μ− and e+e−→e+e−τ+τ− are studied with the L3 detector at LEP using an untagged data sample collected at centre-of-mass energies 161 GeV⩽s⩽209 GeV. The τ-pairs are observed through the associated decay of one τ into eνν and the other into ππν. The cross sections are measured as a function of s. For muon pairs, the cross section of the γγ→μ+μ− process is also measured as a function of the two-photon centre-of-mass energy for 3 GeV⩽Wγγ⩽40 GeV. Good agreement is found between these measurements and the O(α4) QED expectations. In addition, limits on the anomalous magnetic and electric dipole moments of the τ lepton are extracted.

τ EDM at low energies

Low energy tau pair production, at B factories and on top of the T resonances, allows for a detailed investigation on the CP violation at the electromagnetic tau pair production vertex. High statistic available at low energies offers the opportunity for an independent analysis of CP-violation in the τ lepton physics. We show that stringent and independent bounds on the T electric dipole moment, competitive with the high energy measurements, can be established in low energies experiments.

CLEO contributions to tau physics

We review many of the contributions of the CLEO experiment to tau physics. Topics discussed are: branching fractions for major decay modes and tests of lepton universality; rare decays; forbidden decays; Michel parameters and spin physics; hadronic sub-structure and resonance parameters; the tau mass, tau lifetime, and tau neutrino mass; searches for CP violation in tau decay; tau pair production, dipole moments, and CP violating EDM; and tau physics at CLEO-III and at OLEO-c.

IMPROVED BOUNDS ON THE TAU MAGNETIC MOMENTS

New limits on the tau magnetic couplings to the photon, the Z and the W bosons are obtained from the most general effective Lagrangian. Model independent bounds are set using data for tau pair production and W decay into tau leptons at LEP1, LEP2, SLD and hadron colliders. For the anomalous magnetic moment the bounds are, for the first time, of the order of magnitude of the standard model prediction.

Signals forCPViolation in Scalar Tau Pair Production at Muon Colliders

We discuss signals for CP-violation in $\mu^+ \mu^- \to \tilde \tau_i^- \tilde \tau_j^+$, where $i,j = 1,2$ label the two scalar $\tau$ mass eigenstates. We assume that these reactions can proceed through the production and decay of the heavy neutral Higgs bosons present in supersymmetric models. CP-violation in the Higgs sector can be probed through a rate asymmetry even with unpolarized beams, while the CP-odd phase associated with the $\tilde \tau$ mass matrix can be probed only if the polarization of at least one beam can be varied. These asymmetries might be ${\cal O}(1)$.

CP violation beyond the standard model and tau pair production in e+e− collisions

We show that the CP-violating dipole form factors of the tau lepton can be of the order of α π in units of the length scale set by the inverse Z boson mass. We propose a few observables which are sensitive to these form factors at LEP2 and higher e +e − collision energies.

Non-standard tau pair production in two photon collisions at LEP II and beyond

We study the sensitivity of LEP II and NLC, via two photon collisions, to the effects of anomalous τ τγ couplings. We also discuss some CP-odd observables that can be useful to disentangle the contributions from the anomalous couplings.

A study of muon pair production and evidence for tau pair production in photon-photon collisions at LEP

We have studied the reactionsγγ→μ + μ − andγγ→τ + τ − by measuring the properties of events of the typese + e −→e + e − μ + μ − ande + e −→e + e − τ + τ − ate + e − centre-of-mass energies between 88 and 94 GeV. The data sample corresponds to an integratede + e − luminosity of 40.4 pb−1 collected by the OPAL detector at LEP. The QED structure functionF 2 is extracted from a sample of 1462γγ→μ + μ − events in which one photon is off the mass shell (single-tagged). TheQ 2 range for these massive photons is 4–400 GeV2, with an averageQ 2 of 8 GeV2. We have observed 48e + e −→e + e − μ + μ − events in which both final state electrons are detected. In the sample of single-tag events we have identified 34.9±6.7 events as due to the reactionγ;→τ + τ −, on an estimated background of 5.1±2.7 events. In all cases the measured event distributions agree with QED calculations.

Final results on muon and tau pair production by the JADE Collaboration at PETRA

The cross-sections and the forward-backward charge asymmetries of muon and tau pairs produced ine + e − collisions at $$\sqrt s = 35 GeV$$ have been measured by the JADE Collaboration. The cross-sections, $$\sigma _\mu (\sqrt s = GeV) = 69.79 \pm 1.35 \pm 1.40 pb$$ and $$\sigma _\mu (\sqrt s = GeV) = 71.72 \pm 1.48 \pm 1.61 pb$$ , are in agreement with the QED α3 prediction. The charge asymmetries areA μ=−(9.9±1.5±0.5)% andA τ=−(8.1±2.0±0.6)% in agreement with the value −9.2% predicted by the standard model, usingM Z=91.0 GeV and sin2θ W =0.230.

Measurement of the decay tau --> rho nu.

The decay tau..-->..rho..nu.. is studied in tau pair production by e/sup +/e/sup -/ annihilation at ..sqrt..s = 3.77 GeV, 0.20 GeV above tautau threshold. The branching fraction is measured to be B(tau..-->..rho..nu..) = (23.0 +- 1.3 +- 1.7)%. The measured distribution of the helicity angle of the charged pion from the rho decay agrees with the theoretical prediction.