Hadronic Tau Decays Research Articles

The hadronic tau decay signature of a heavy charged Higgs boson at LHC

The hadronic tau decay channel offers by far the best signature for heavy charged Higgs boson search at the LHC in the large tan β region. By exploiting the distinct polarization of the tau and its large transverse mass, along with the accompanying missing– p T , one can probe for a charged Higgs boson up to a mass of about 600 GeV in an essentially background-free environment. The transverse mass distribution of the tau jet also provides a fairly unambiguous estimate of the charged Higgs boson mass.

The uncertainty in αs( MZ) from hadronic tau decay measurements

The uncertainty in α s ( M Z ) extracted from hadronic tau decay measurements is assessed by performing various renormalon-inspired RS-invariant resummations of the QCD perturbation series. We estimate a rather small uncertainty δα s ( M Z ) = 0.003.

Measurement of the strong coupling constant $\alpha_{\rm s}$ and the vector and axial-vector spectral functions in hadronic tau decays

The spectral functions of the vector current and the axial-vector current have been measured in hadronic $\tau$ decays using the OPAL detector at LEP. Within the framework of the Operator Product Expansion a simultaneous determination of the strong coupling constant $\alpha_{\rm s}$ , the non-perturbative operators of dimension 6 and 8 and of the gluon condensate has been performed. Different perturbative descriptions have been compared to the data. The Contour Improved Fixed Order Perturbation Theory gives $\alpha_{\rm s}(m_\tau^2) = 0.348 \pm 0.009_{\rm exp} \pm 0.019_{\rm theo}$ at the $\tau$ -mass scale and $\alpha_{\rm s}(m^2_{\rm Z}) = 0.1219 \pm 0.0010_{\rm exp} \pm 0.0017_{\rm theo}$ at the ${\rm Z}^0$ -mass scale. The values obtained for $\alpha_{\rm s}(m^2_{\rm Z})$ using Fixed Order Perturbation Theory or Renormalon Chain Resummation are 2.3% and 4.1% smaller, respectively. The ‘running’ of the strong coupling between $s_0 \simeq 1.3 {\rm GeV}^2$ and $s_0 = m_\tau^2$ has been tested from direct fits to the integrated differential hadronic decay rate $R_\tau(s_0)$ . A test of the saturation of QCD sum rules at the $\tau$ -mass scale has been performed.

The uncertainty in αs ( MZ2) determined from hadronic tau decay measurements

We show that QCD Minkowski observables such as the e + e − R-ratio and the hadronic tau decay R τ are completely determined by the effective charge (EC) beta-function, ϱ( x), corresponding to the Euclidean QCD vacuum polarization Adler D-function, together with the next-to-leading order (NLO) perturbative coefficient of D. An efficient numerical algorithm is given for evaluating R, R τ from a weighted contour integration of D ( se iΘ around a circle in the complex squared energy s-plane, with ϱ( x) used to evolve in s around the contour. The EC beta-function can be truncated at next-to-NLO (NNLO) using the known exact perturbative calculation or the uncalculated N 3 LO and higher terms can be approximated by the portion containing the highest power of b, the first QCD beta-function coefficient. The difference between the R, R τ constructed using the NNLO and “leading- b” resummed versions of ϱ( x) provides an estimate of the uncertainty due to the uncalculated higher order corrections. Simple numerical parametrizations are given to facilitate these fits. For R τ we estimate an uncertainty Δα s ( m τ 2) ≅ 0.01, corresponding to Δα s ( M z 2) ≅ 0.002. Thus there is no necessary implication that the uncalculated higher order corrections contribute a large uncertainty in α s ( M Z 2) as had been claimed by other authors based on analogous all-orders resummations. We show that these earlier estimates are dependent on the renormalization scheme chosen, a difficulty which is avoided in our formulation.

The uncertainty in αs( MZ2) from hadronic tau decay measurements

The uncertainty in α s ( M Z 2) extracted from hadronic tau decay measurements is assessed by performing a renormalon-inspired RS-invariant resummation of the QCD perturbation series. We estimate a rather small uncertainty δα s ( M Z 2) = 0.002.

Determining theβFunction of the Strong Interaction Coupling and Closing the Light Gluino Window

We discuss the perturbative running of alpha_s in a model independent way. Our analysis contains data on the hadronic tau decay and hadronic cross sections in e+e- annihilation between 5 GeV and M_Z. We determine the color coefficients and the perturbative beta-function of the strong interaction. The results are in agreement with QCD and rule out the QCD+light gluino scenarios on the 70.8 - 93.0% CL. We combine our method with that of the multi-jet analysis at LEP. The combined result rules out light gluinos at least on the 99.76% CL, provided that nonperturbative effects are not large.

QCD tests in τ decays with optimized perturbation expansion

The next-to-next-to-leading order perturbative QCD corrections to R_tau and the higher moments of the invariant mass distribution in the hadronic tau decays are considered. The renormalization scheme dependence of these corrections is discussed. The optimized predictions are obtained, using the principle of minimal sensitivity as a guide to select the preferred renormalization scheme. A simplified fit is performed, using R_tau and R^12_tau, to see how the use of the optimized expansion may affect the determination of the alpha_s and the dimension six condensates from the experimental data.

Tau lepton physics: theory overview

The pure leptonic or semileptonic character of tau decays makes them a good laboratory to test the structure of the weak currents and the universality of their couplings to the gauge bosons. The hadronic tau decay modes constitute an ideal tool for studying low-energy effects of the strong interactions in very clean conditions; a well-known example is the precise determination of the QCD coupling from tau-decay data. New physics phenomena, such as a non-zero tau-neutrino mass or violations of (flavour / CP) conservation laws can also be searched for with tau decays.

Measurement of the spectral functions of vector current hadronic tau decays

A measurement of the spectral functions of non-strange tau vector current final states is presented, using 124 358 tau pairs recorded by the ALEPH detector at LEP during the years 1991 to 1994. The spectral functions of the dominant two-and four-pion tau decay channels are compared to published results of ece-annihilation experiments via isospin rotation. A combined fit of the pion form factor from tau decays and e(+)e(-)data is performed using different parameaizations. The mass and the width of the rho(+/-)(770) and the p(0)(770) are separately determined in order to extract possible isospin violating effects. The mass and width differences are measured to be M<rho+/-(770) - M-rho 0(770), = (0.0 +/- 1.0) MeV/c(2) and Gamma(rho+/-(770)) - Gamma(rho 0(770)) = (0.1 +/- 1.9) MeV/c(2).

Tau hadronic branching ratios

From 64492 selected tau-pair events, produced at the Zeta(o) resonance, the measurement of the tau decays into hadrons from a global analysis using 1991, 1992 and 1993 ALEPH data is presented. Special emphasis is given to the reconstruction of photons and pi(o)'s, and the removal of fake photons. A detailed study of the systematics entering the pi(o) reconstruction is also given. A complete and consistent set of tau hadronic branching ratios is presented for 18 exclusive modes. Most measurements are more precise than the present world average. The new level of precision reached allows a stringent test of tau - mu universality in hadronic decays, g(tau)/g(mu) = 1.0013 +/- 0.0095, and the first measurement of the vector and axial-vector contributions to the non-strange hadronic tau decay width: R(tau,V) = 1.788 +/- 0.025 and R(tau,A) = 1.694 +/- 0.027. The ratio (R(tau,V) - R(tau,A))/(R(tau,V) + R(tau,A)), equal to (2.7 +/- 1.3) %, is a measure of the importance of QCD non-perturbative contributions to the hadronic tau decay width.