Hadronic Tau Decays Research Articles

αs(MZ2)from hadronicτdecays

We perform an extraction of alpha_s based on sum rules involving isovector hadronic tau decay data. The particular sum rules employed are constructed specifically to suppress contributions associated with poorly known higher dimension condensates, and hence reduce theoretical systematic uncertainties associated with the treatment of such contributions which are shown to be present in earlier related analyses. Running our results from the n_f=3 to n_f=5 regime we find alpha_s(M_Z^2)=0.1187(16), in excellent agreement with the recently updated global fit to electroweak data at the Z scale, 0.1191(27), and other high-scale direct determinations.

Unraveling duality violations in hadronic tau decays

There are some indications from recent determinations of the strong coupling constant ${\ensuremath{\alpha}}_{s}$ and the gluon condensate that the operator product expansion may not be accurate enough to describe nonperturbative effects in hadronic tau decays. This breakdown of the operator product expansion is usually referred to as being due to ``duality violations.'' With the help of a physically motivated model, we investigate these duality violations. Based on this model, we argue how they may introduce a non-negligible systematic error in the current analysis, which employs finite-energy sum rules with pinched weights. In particular, this systematic effect might affect the precision determination of ${\ensuremath{\alpha}}_{s}$ from tau decays. With a view to a possible future application to real data, we present an alternative method for determining the operator product expansion coefficients that might help with estimating, and possibly even reducing, this systematic error.

Hadronic tau-decays at Belle

Based on a large data sample of τ decays, accumulated with the Belle detector at the KEKB e+e− collider, the branching fraction, B(τ− → KSπ−v) = (0.404 ± 0.002(stat.) ± 0.013(syst.))%, has been obtained with better accuracy than previously. The K*(892)− mass spectrum was also studied, and for the first time the K* (892)− mass and width were obtained in τ decay. The branching fractions of τ-decays involving η were measured with much improved precision: B(τ− → K−ηv) = (1.62 ± 0.10) × 10-4, B(τ− → K−π0ηv) = (0.47 ± 0.11 ± 0.04) × 10-4, B(τ− → K*-ηv) = (1.10 ± 0.19 ± 0.07) × 10-4, and B(τ− → π− π0ηv) = (1.39±0.03±0.07) × 10-3. These results are precise enough to distinguish theoretical calculations in CVC and Chiral models.

Can we probe right-handed charged quark currents?

Different scenarios of new physics beyond the standard model give rise to a direct coupling of right-handed quarks to W bosons. We will discuss how the prediction of the Callan-Treiman low energy theorem for the scalar K π form factor in combination with recent data on K μ 3 L decays can serve as a stringent test of the standard model (SM) and possible extensions, in particular right-handed charged quark currents (RHCs). In addition, we will comment on the impact of hadronic tau decay data on RHCs.

Signature of heavy charged Higgs boson at LHC in the 1 and 3 prong hadronic tau decay channels

We have done a fast simulation analysis of the H± signal at LHC in 1 and 3 prong hadronic τ-jet channels along with a tt background. The τ polarization was effectively used to suppress the background in both channels. Combining this with appropriate cuts on pT of the τ-jet, the missing ET and the azimuthal angle between them reduces the background below the signal level. Consequently, one gets a viable H± signal up to a mass range of 600–700 GeV at moderate to large tanβ.

Formula omitted] and [formula omitted] from hadronic tau decays

5 paginas.-- Comunicacion presentada al 9o International Workshop on Tau Lepton Physics (TAU06) celebrado del 19 al 22 de Septiembre en Pisa (Italia).-- arXiv:hep-ph/0612154v1

The physics of hadronic tau decays

Hadronic $\ensuremath{\tau}$ decays provide a clean laboratory for the precise study of quantum chromodynamics (QCD). Observables based on the spectral functions of hadronic $\ensuremath{\tau}$ decays can be related to QCD quark-level calculations to determine fundamental quantities like the strong-coupling constant, parameters of the chiral Lagrangian $\ensuremath{\mid}{V}_{us}\ensuremath{\mid}$, the mass of the strange quark, and to simultaneously test the concept of quark-hadron duality. Using the best available measurements and a revisited analysis of the theoretical framework, the value ${\ensuremath{\alpha}}_{s}({m}_{\ensuremath{\tau}}^{2})=0.345\ifmmode\pm\else\textpm\fi{}{0.004}_{\mathrm{exp}}\ifmmode\pm\else\textpm\fi{}{0.009}_{\mathrm{th}}$ is obtained. Taken together with the determination of ${\ensuremath{\alpha}}_{s}({M}_{Z}^{2})$ from the global electroweak fit, this result leads to the most accurate test of asymptotic freedom: the value of the logarithmic slope of ${\ensuremath{\alpha}}_{s}^{\ensuremath{-}1}(s)$ is found to agree with QCD at a precision of 4%. The $\ensuremath{\tau}$ spectral functions can also be used to determine hadronic quantities that, due to the nonperturbative nature of long-distance QCD, cannot be computed from first principles. An example for this is the contribution from hadronic vacuum polarization to loop-dominated processes like the anomalous magnetic moment of the muon. This article reviews the measurements of nonstrange and strange $\ensuremath{\tau}$ spectral functions and their phenomenological applications.

Extraction of [formula omitted] and [formula omitted] from Hadronic Tau Decays

We review recent work to determine the strange quark mass m_s as well as the proposal to determine |V_{us}| using hadronic tau decay data. The recent update of the strange spectral function by OPAL and their moments of the invariant mass distribution are employed. Our results are |V_us|=0.2208 +- 0.0034 and m_s (2 GeV)=81 +- 22 MeV. Our result is already competitive to the standard extraction of |V_us| from K_e3 decays and to the new proposals to determine it. The error on V_us is dominated by experiment and will be eventually much improved by the B-factories hadronic tau data. Ultimately, a simultaneous fit of both m_s and |V_us| to a set of moments of the hadronic tau decays invariant mass distribution will provide one of the most accurate determinations of these Standard Model parameters.

Issues in the Extraction of ms and Vus from Hadronic τ Decay Data

Various complications encountered in the process of attempting to extract the basic Standard Model parameters, m_s and V_us, from hadronic tau decay data are discussed. Preliminary results for V_us, based on sum rules displaying good convergence and stability, are also presented.

Disentangling perturbative and power corrections in precision tau decay analysis

Hadronic tau decay precision data are analyzed with account of both perturbative and power corrections of high orders within QCD. It is found that contributions of high order power corrections are essential for extracting a numerical value for the strange quark mass from the data on Cabibbo suppressed tau decays. We show that with inclusion of new five-loop perturbative corrections in the analysis the convergence of perturbation theory remains acceptable only for few low order moments. We obtain ${m}_{s}({M}_{\ensuremath{\tau}})=130\ifmmode\pm\else\textpm\fi{}27\text{ }\text{ }\mathrm{M}\mathrm{e}\mathrm{V}$ in agreement with previous estimates.

V-A sum rules with D = 10 operators

The difference of vector and axial-vector charged current correlators is analyzed by means of QCD sum rules. The contribution of 10-dimensional 4-quark condensates is calculated and its value is estimated within the framework of factorization hypothesis. It is compared to the result, obtained from operator fit of Borel sum rules in the complex q^2-plane, calculated from experimental data on hadronic tau-decays. This fit gives accurate values of the light quark condensate and quark-gluon mixed condensate. The size of the high-order operators and the convergence of operator series are discussed.

Neural network parametrization of spectral functions from hadronic tau decays and determination of QCD vacuum condensates

The spectral function ρV−A(s) is determined from ALEPH and OPAL data on hadronic tau decays using a neural network parametrization trained to retain the full experimental information on errors, their correlations and chiral sum rules: the DMO sum rule, the first and second Weinberg sum rules and the electromagnetic mass splitting of the pion sum rule. Nonperturbative QCD vacuum condensates can then be determined from finite energy sum rules. Our method minimizes all sources of theoretical uncertainty and bias producing an estimate of the condensates which is independent of the specific finite energy sum rule used. The results for the central values of the condensates ⟨6⟩ and ⟨8⟩ are both negative.

Isolated tau leptons in events with large missing transverse momentum at HERA

A search for events containing isolated tau leptons and large missing transverse momentum, not originating from the tau decay, has been performed with the ZEUS detector at the electron–proton collider HERA, using 130 pb−1 of integrated luminosity. A search was made for isolated tracks coming from hadronic tau decays. Observables based on the internal jet structure were exploited to discriminate between tau decays and quark- or gluon-induced jets. Three tau candidates were found, while 0.40+0.12−0.13 were expected from Standard Model processes, such as charged current deep inelastic scattering and single W±-boson production. To search for heavy-particle decays, a more restrictive selection was applied to isolate tau leptons produced together with a hadronic final state with high transverse momentum. Two candidate events survive, while 0.20±0.05 events are expected from Standard Model processes.

QCD condensates for the light quarkV−Acorrelator

We use the procedure of pinched-weight Finite Energy Sum Rules (pFESR) to determine the OPE coefficients a_6, ...,a_16 of the flavor ud V-A correlator in terms of existing hadronic tau decay data. We show by appropriate weight choices that the error on the dominant d=6 contribution, which is known to be related to the K -> Pi Pi matrix elements of the electroweak penguin operator in the chiral limit, may be reduced to below the ~15% level. The values we obtain for the OPE coefficients with d>8 are shown to naturally account for the discrepancies between our results for the d=6 and d=8 terms and those of previous analyses, which were obtained neglecting d>8 contributions.

Hadronicτdecay, the renormalization group, analyticity of the polarization operators, and QCD parameters

The ALEPH data on hadronic tau-decay is throughly analysed in the framework of QCD. The perturbative calculations are performed in 1-4-loop approximation. The analytical properties of the polarization operators are used in the whole complex q^2 plane. It is shown that the QCD prediction for R_{tau} agrees with the measured value R_{tau} not only for conventional Lambda^{conv}_3 = (618+-29) MeV but as well as for Lambda^{new}_3 = (1666+-7) MeV. The polarization operator calculated using the renormgroup has nonphysical cut [-Lambda^2_3, 0]. If Lambda_3 = Lambda^{conv}_3, the contribution of only physical cut is deficient in the explanation of the ALEPH experiment. If Lambda_3 = Lambda^{new}_3 the contribution of nonphysical cut is very small and only the physical cut explains the ALEPH experiment. The new sum rules which follow only from analytical properties of polarization operators are obtained. Basing on the sum rules obtained, it is shown that there is an essential disagreement between QCD perturbation theory and the tau-lepton hadronic decay experiment at conventional value Lambda_3. In the evolution upwards to larger energies the matching of r(q^2) (Eq.(12)) at the masses J/psi, Upsilon and 2m_t was performed. The obtained value alpha_s(-m^2_z) = 0.141+-0.004 (at Lambda_3 = Lambda^{new}_3) differs essentially from conventional value, but the calculation of the values R(s) = sigma(e+e- -> hadrons)/sigma(e+e- -> mu+mu-), R_l = Gamma(Z -> hadrons)/Gamma(Z -> leptons), alpha_s(-3 GeV^2), alpha_s(-2.5 GeV^2) does not contradict the experiments.

Condensates in quantum chromodynamics

The paper presents the short review of our to-day knowledge of vacuum condensates in QCD. The condensates are defined as vacuum averages of the operators which arise due to nonperturbative effects. The important role of condensates in determination of physical properties of hadrons and of their low-energy interactions in QCD is underlined. The special value of quark condensate, connected with the existence of baryon masses is mentioned. Vacuum condensates induced by external fields are discussed. QCD at low energy is checked on the basis of the data on hadronic tau-decay. In the theoretical analysis the terms of perturbation theory (PT) up to alpha^3_s are accounted, in the operator product expansion (OPE) - those up to dimension 8. The total probability of the decay tau->hadrons (with zero strangeness) and of the tau-decay structure functions are best described at alpha_s(m^2_tau) = 0.330+-0.025. It is shown that the Borel sum rules for tau-decay structure functions along the rays in the q^2-complex plane are in agreement with the experiment with the accuracy ~2% at the values of the Borel parameter |M^2|> 0.8 GeV^2. The magnitudes of dimensions 6 and 8 condensates were found and the limitations on gluonic condensates were obtained. The sum rules for the charmed quark vector currents polarization operator was analysed in 3 loops (i.e., in order alpha^2_s). The value of charmed quark mass (in MS-bar regularization scheme) was found to be: m_c(m_c) = 1.275+-0.015 GeV and the value of gluonic condensate was estimated: <(alpha_s/pi)G^2> = 0.009+-0.007 GeV^4. The general conclusion is: QCD described by PT + OPE is in a good agreement with experiment at Q^2 >~ 1 GeV^2.

Resummation of the hadronic tau decay width with the modified Borel transform method

A modified Borel transform of the Adler function is used to resum the hadronic tau decay width ratio. In contrast to the ordinary Borel transform, the integrand of the Borel integral is renormalization--scale invariant. We use an ansatz which explicitly accounts for the structure of the leading infrared renormalon. Further, we use judiciously chosen conformal transformations for the Borel variable, in order to map sufficiently away from the origin the other ultraviolet and infrared renormalon singularities. In addition, we apply Pade approximants for the corresponding truncated perturbation series of the modified Borel transform, in order to further accelerate the convergence. Comparing the results with the presently available experimental data on the tau hadronic decay width ratio, we obtain $\alpha_s(M^z) = 0.1192 +- 0.0007_{exp.} +- 0.0010_{EW+CKM} +- 0.0009_{th.} +- 0.0003_{evol.}$. These predictions virtually agree with those of our previous resummations where we used ordinary Borel transforms instead.

Bilocal expansion of the Borel amplitude and the hadronic tau decay width

The singular part of Borel transform of a QCD amplitude near the infrared renormalon can be expanded in terms of higher order Wilson coefficients of the operators associated with the renormalon. In this paper we observe that this expansion gives nontrivial constraints on the Borel amplitude that can be used to improve the accuracy of the ordinary perturbative expansion of the Borel amplitude. In particular, we consider the Borel transform of the Adler function and its expansion around the first infrared renormalon due to the gluon condensate. Using the next-to-leading order Wilson coefficient of the gluon condensate operator, we obtain an exact constraint on the Borel amplitude at the first IR renormalon. We then extrapolate, using judiciously chosen conformal transformations and Pade approximants, the ordinary perturbative expansion of the Borel amplitude in such a way that this constraint is satisfied. This procedure allows us to predict the $O(\alpha_s^4)$ coefficient of the Adler function, which gives a result consistent with the estimate by Kataev and Starshenko using a completely different method. We then apply this improved Borel amplitude to the tau decay width, and obtain the strong coupling constant $\alpha_s(M_Z) =0.1193 \pm 0.0007_{exp.} \pm 0.0010_{EW+CKM} \pm 0.0009_{meth.} \pm 0.0003_{evol.}$. We then compare this result with those of other resummation methods.

Determining ms from flavor breaking in hadronic τ decays

New finite energy sum rules (FESR's) for extracting m_s from hadronic tau decay data are constructed which (1) significantly reduce potential theoretical uncertainties present in existing sum rule analyses and (2) remove problems associated with both the poor convergence of the OPE representation of the longitudinal part of the $us$ vector and axial vector correlators and the large statistical errors in the $us$ spectral data above the K^* region.

Strange quark mass from flavor breaking in hadronicτdecays

The strange quark mass is extracted from a finite energy sum rule (FESR) analysis of the flavor-breaking difference of light-light and light-strange quark vector-plus-axial-vector correlators, using spectral functions determined from hadronic tau decay data. We point out problems for existing FESR treatments associated with potentially slow convergence of the perturbative series for the mass-dependent terms in the OPE over certain parts of the FESR contour, and show how to construct alternate weight choices which not only cure this problem, but also (1) considerably improve the convergence of the integrated perturbative series, (2) strongly suppress contributions from the region of s values where the errors on the strange current spectral function are still large and (3) essentially completely remove uncertainties associated with the subtraction of longitudinal contributions to the experimental decay distributions. The result is an extraction of m_s with statistical errors comparable to those associated with the current experimental uncertainties in the determination of the CKM angle, V_{us}. We find m_s(1 GeV)=158.6\pm 18.7\pm 16.3\pm 13.3 MeV (where the first error is statistical, the second due to that on V_{us}, and the third theoretical).