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.
The QCD analysis of hadronic tau decays is reviewed and a summary of the present phenomenological status is presented. The following topics are discussed: the determination of alpha_s(m_tau) = 0.338 +- 0.012 from the inclusive tau hadronic width, the measurement of |V_us| through the Cabibbo-suppressed decays of the tau, and the extraction of chiral-perturbation-theory couplings from the spectral tau data.
The new parameterization of form factors developed for 4pi channels of the tau lepton decay and based on Novosibirsk data on e^+e^- -> 4pi has been coded in a form suitable for the TAUOLA Monte Carlo package. Comparison with results from TAUOLA using another parameterization, i.e. the CLEO version of 1998 is also included.
TAUOLA is a Monte Carlo generator dedicated to generating tau-lepton decays and it is used in the analysis of experimental data both at B-factories and LHC. TAUOLA is a long-term project that started in the 90's and has been under development up to now. In this note we discuss the status of the predominant hadronic tau-lepton decays into two ($Br \simeq 25.52\%$) and three pions ($Br \simeq 18.67\%$).
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.
Until recently, determinations of alpha_s(M_Z) from hadronic tau decays and the analysis of short-distance-sensitive lattice observables yielded results which, though precise, were not in good agreement. I review new analyses that bring these into good agreement and provide some details on the source of the main changes in the tau decay analysis.
In the two body hadronic tau decays, such as tau->to K pi (eta)nu, vector mesons play important role. Belle and Babar measured hadronic invariant mass spectrum of tau -> K pi nu decay. To compare the spectrum with theoretical prediction, we develop the chiral Lagrangian with vector mesons in Kimura:2012nx. We compute the form factors of the hadronic tau decay taking account of the quantum corrections of Nambu Goldstone bosons. We also show how to renormalizethe divergence of the Feynman diagrams with arbitrary number of loops and determine the counterterms within one loop using background fiel…
We present an update of the Monte Carlo event generator TAUOLA for tau lepton decays, with substantially increased list of decay channels and new initialization options. The core of the program remains written in FORTRAN but necessary arrangements have been made to allow handling of the user-provided hadronic currents and matrix elements at the execution time. Such solution may simplify preparation of new hadronic currents and may be useful for fitting to the experimental data as well. We have implemented as default for TAUOLA a set of hadronic currents, which is compatible with the default in…
Comparing the results of finite energy and Borel transformed sum rules for the difference of vector and axial vector current correlators we evaluate the instanton contributions to the $τ$ hadronic width. In contrast to an explicit theoretical calculation we find that the instanton contributions to the $τ$ hadronic width are much smaller than the standard nonperturbative corrections.
It has been pointed out recently that current low-energy constraints still allow for sizable flavor-changing decay rates of the 125 GeV boson into leptons, h -> tau ell (ell= e, mu). In this work we discuss the role of hadronic tau-lepton decays in probing lepton flavor violating couplings in the Higgs sector. At low energy, the effective Higgs coupling to gluons induced by heavy quarks contributes to hadronic tau decays, establishing a direct connection with the relevant process at the LHC, pp (gg)-> h -> tau ell. Semileptonic transitions like tau -> ell pi pi are sensitive to flavor-changing…
We examine CP violation in the Delta S=0 decays tau -> omega pi nu_tau and tau -> a_1 pi nu_tau and the Delta S=1 decay tau -> K pi pi nu_tau. We assume that the new physics is a charged Higgs. We show that sizeable CP-violating effects are possible in tau -> a_1 pi nu_tau (polarization-dependent rate asymmetry) and tau -> omega pi nu_ tau (triple-product asymmetry). The Delta S=1 decay tau -> K pi pi nu_tau can proceed via several resonances. We construct two modified rate asymmetries and a triple product asymmetry for this decay and discuss the potential sensitivities of these asymmetries.
The hadronic tau decay channel offers by far the best signature for heavy charged Higgs boson search at the LHC in the large $\tanβ$ 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 perturbative quark-mass corrections to the $τ$ hadronic width are analysed to $O(α_s^3 m_q^2)$, using the presently available theoretical information. The behaviour of the perturbative series is investigated in order to assess the associated uncertainties. The implications for the determination of the strange quark mass from $τ$ decay data are discussed.
We update the hadronic tau determination of |V_{us}|, showing that current strange branching fractions produce results 2-3 sigma lower than 3-family unitarity expectations. Issues related to the size of theoretical uncertainties and results from an alternate, mixed tau-electroproduction sum rule determination (which, by construction, produces a much smaller theoretical uncertainty), are also considered.
$τ\toππν$, $τ\toπKν$, $τ\to Kην$, $τ\to 3πν$ and $τ\to ππKν$ have been investigated using chiral symmetry with dispersion relation in agreement with the unitarity condition.
Tau decays into hadrons foresee the study of the hadronization of vector and axial-vector QCD currents, yielding relevant information on the dynamics of the resonances entering into the processes. We analyse tau -> (3 pion) nu_tau decays within the framework of the Resonance Chiral Theory, comparing this theoretical scheme with the experimental data, namely ALEPH spectral function and branching ratio. Hence we get values for the mass and on-shell width of the a_1(1260) resonance, and provide the structure functions that have been measured by OPAL and CLEO-II.
We report on some technical aspects of our calculation of alpha_s^4 corrections to R(s) and the semi-leptonic tau decay width [1-3]. We discuss the inner structure of the result as well as the issue of its correctness. We demonstrate recently appeared independent evidence positively testing one of two components of the full result.
The perturbative quark mass corrections to the tau hadronic width are studied to O(alpha_s^3 m_q^2). Including up to dimension four corrections, we get m_s(4 GeV^2)=(143 \pm 42) MeV [m_s(1 GeV^2)=(193 \pm 59) MeV ]. Possible improvements to reduce the theoretical uncertainty are pointed out.
We re-examine CP violation in the Delta S = 0 decays tau -> N pi nu_tau (N=2,3,4). We assume that the new physics (NP) is a charged Higgs. We show that there is no NP contribution to tau -> pi pi nu_tau, which means that no CP violation is expected in this decay. On the other hand, NP can contribute to tau -> N pi nu_tau (N=3,4). These are dominated by the intermediate resonant decays tau -> omega pi nu_tau, tau -> rho pi nu_tau and tau -> a_1 pi nu_tau. We show that the only sizeable CP-violating effects which are possible are in tau -> a_1 pi nu_tau -> 4 pi nu_tau (polarization-dependent rat…
Hadronic tau decays belong to the processes that show a resonance-like structure in the axial vector current in the $1-2$ GeV range. This structure, often denoted as the $a_1$ meson, seems to show different properties in different processes. The process $τ\rightarrow 3πν_τ$ allows for a clean separation of weak and strong effects and a clear production mechanism. We examine how this structure can be related to interactions between the three pions that emerge in the final state. In particular we start from the interactions between all two body combinations.
The results of calculation of light gluino contributions to Gamma(Z--->hadrons) and Gamma(tau--->hadrons) to O(alpha_s^3) are presented. The net effect in the case of Z decay is noticeable. For the tau width the effect is very large and, if a light gluino exists, suggests that alpha_s increases by more than 15% relative to the Standard Model analysis.
I report on some recent work done in collaboration with Santi Peris and Boris Phily (hep-ph/0007338) where, using the Aleph data on vector and axial-vector spectral functions, we test simple duality properties of QCD in the large-Nc limit which emerge in the approximation of a {\it minimal hadronic ansatz} of a spectrum of narrow states. These duality properties relate the short- and long-distance behaviours of specific correlation functions, which are order parameters of spontaneous chiral symmetry breaking, in a way that we find well supported by the data.
The high-precision data from hadronic tau decays allows one to extract information on QCD condensates. Using the finalized ALEPH data, we obtain a more rigorous determination of the dimension 6 and 8 condensates for the V-A correlator. In particular, we find that the recent data fix a certain linear combination of these QCD condensates to a precision at the level of O(2%). Our approach relies on more general assumptions than alternative approaches based on finite energy sum rules.
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 s…
The dispersive approach to QCD is applied to the study of the inclusive tau lepton hadronic decay. This approach provides the unified integral representations for the hadronic vacuum polarization function, related R function, and Adler function. These representations account for the intrinsically nonperturbative constraints, which originate in the kinematic restrictions on the functions on hand, and retain the effects due to hadronization, which play a valuable role in the analysis of the strong interaction processes at low energies. The dispersive approach proves to be capable of describing r…
We discuss a preliminary study of the impact of duality violations on extractions from tau decay data of the D=6 VEVs which determine chiral limit Standard Model K-->pi pi matrix elements of the electroweak penguin operators.
We have reevaluated the hadronic contribution to the anomalous magnetic moment of the muon (g-2) and to the running of the QED fine structure constant alpha(s) at s=M_Z**2. We incorporated new data from hadronic tau decays, recently published by the ALEPH Collaboration. In addition, compared to previous analyses, we use more extensive e+e- annihilation data sets. The integration over the total hadronic cross section is performed using experimental data up to 40 GeV and results from perturbative QCD above 40 GeV. The improvement from tau data concerns mainly the pion form factor, where the unce…
Recent experimental data on tau decays are used to reconstruct the difference in hadronic spectral densities with vector and axial-vector quantum numbers. The saturation of Das-Mathur-Okubo and Weinberg sum rules is studied. Two methods of improving convergence and decreasing errors are applied, and good agreement with the predictions of current algebra and chiral perturbation theory is observed. The resulting value of the pion polarisability is (2.64 +/- 0.36) 10^-4 fm^3.
The total tau hadronic width can be accurately calculated using analyticity and the operator product expansion. The result turns out to be very sensitive to the value of alpha_s(m_tau^2), providing a precise determination of the strong coupling constant. The theoretical description of this observable is updated, including the recently computed O(alpha_s^4) contributions. The experimental determination of alpha_s(m_tau^2) and its actual uncertainties are discussed.
We study the LHC sensitivity to the charged Higgs discovery in the channel H^- -> tau_L^- nu in models with a singlet neutrino in large extra dimensions. The observation of such a signal would provide a distinctive evidence for these models since in the standard two Higgs doublet model type II, H^- -> tau_L^- nu is completely suppressed.
The total $τ$ hadronic width can be accurately calculated using analyticity and the operator product expansion. The theoretical analysis of this observable is updated to include all available perturbative and non-perturbative corrections. The experimental determination of $α_s(M_τ^2)$ and its actual uncertainties are discussed.
The recent update of the strange spectral function and the moments of the invariant mass distribution by the OPAL collaboration from hadronic tau decay data are employed to determine |V_us| as well as m_s. Our result, |V_us|=0.2208\pm0.0034, is competitive to the standard extraction of |V_us| from K_e3 decays and to the new proposals to determine it. Furthermore, the error associated to our determination of |V_us| can be reduced in the future since it is dominated by the experimental uncertainty that will be eventually much improved by the B-factories hadronic tau data. Another improvement tha…
We present an update of the measurement of alpha_s(m_tau) from ALEPH tau hadronic spectral functions. We report a study of the perturbative prediction(s) showing that the fixed-order perturbation theory manifests convergence problems not presented in the contour-improved calculation. Potential systematic effects from quark-hadron duality violations are estimated to be within the quoted systematic errors. The fit result is alpha_s(m_tau) = 0.344 +- 0.005 +- 0.007, where the first error is experimental and the second theoretical. After evolution, the alpha_s(M_Z) determined from tau data is the …
Hadronic decays rates of the $τ$ lepton into multi meson final states are presented. The structure of the hadronic matrix elements for various decay modes is discussed. The formalism of structure functions allows for a detailed test of these matrix elements. Various correlations are discussed which are sensitive to possible CP violation and new physics effects in the decay modes.
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.
Hadronic tau decays represent a clean laboratory for the precise study of quantum chromodynamics (QCD). Observables (sum rules) based on the spectral functions of hadronic tau decays can be related to QCD quark-level calculations to determine fundamental quantities like the strong coupling constant, parameters of the chiral Lagrangian, |V_us|, 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 alpha_s(m_tau) = 0.345 +- 0.004[exp] +- 0.009[theo] is o…
We describe how complications encountered in the extraction of V_us from flavor-breaking hadronic tau decay sum rules, in particular those associated with the slower than previously anticipated convergence of the relevant D=2 OPE series, can be successfully dealt with through judicious choices of sum rule weight. Problems with the previously proposed ``(0,0) spectral weight'' version of the analysis, and the much improved situation produced by the use of a number of alternate, non-spectral-weights, are demonstrated. The method is shown to promise a sub-0.0010-accuracy determination of V_us onc…
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.
We use the Aleph data on vector and axial-vector spectral functions to test simple duality properties of QCD in the large Nc limit, which emerge in the approximation of a minimal hadronic ansatz of a spectrum of narrow states. These duality properties relate the short- and long-distance behaviours of specific correlation functions, which are order parameters of spontaneous chiral symmetry breaking, in a way that we find well supported by the data.
An updated measurement of alpha_s(m_tau) from ALEPH tau hadronic spectral functions is presented. We report a study of the perturbative prediction(s) showing that the fixed-order perturbation theory manifests convergence or principle problems not presented in the contour-improved calculation. Potential systematic effects from quark-hadron duality violations are estimated to be within the quoted systematic errors. The fit result is alpha_s(m_tau) = 0.344 +- 0.005 +- 0.007, where the first error is experimental and the second theoretical. After evolution, the alpha_s(M_Z) determined from tau dat…
We present a brief (mainly bibliographical) report on recently performed calculations of terms of order O(α_s^4 n_f^2) and O(α_s^4 n_f^2 m_q^2) for hadronic Z and τdecay rates. A few details about the analytical evaluation of the masters integrals appearing in the course of calculations are presented.
We study extractions of |V_us| based on finite energy sum rule (FESR) analyses of hadronic tau decay data. We show (i) that the ``(0,0) spectral weight'' implementation (proposed previously in the literature as a favorable version of this analysis) suffers from significant convergence problems, but (ii) that alternate implementations exist which bring these problems under control. Results based on present spectral data are shown to be in agreement with those of other approaches, though with, at present, somewhat larger experimental errors. Sub-1% determinations of |V_us| are also shown to be e…
We discuss the extraction of alpha_s using isovector hadronic tau decay data and sum rules constructed specifically to suppress contributions associated with poorly known higher dimension condensates. We show, first, that problems with the treatment of such contributions affect earlier related analyses and, second, that these problems can be brought under good theoretical control through the use of an alternate analysis strategy. Our results, run up to the n_f=5 regime, correspond to alpha_s(M_Z^2)=0.1187(16), in excellent agreement with the recently updated global fit to electroweak data at t…
Recent progress in the determination of |V_us| employing strange hadronic tau-decay data are reported. This includes using the recent OPAL update of the strange spectral function, as well as augmenting the dimension-two perturbative contribution with the recently calculated order alpha_s^3 term on the theory side. These updates result in |V_us| = 0.2220 +- 0.0033, with the uncertainty presently being dominated by experiment, and already being competitive with the standard extraction from K_e3 decays and other new proposals to determine |V_us|. In view of the ongoing work to analyse tau-decay d…
Various moments of the hadronic spectral functions have been employed in the determination of the strong coupling alpha_s from tau decays. In this work we study the behaviour of their perturbative series under different assumptions for the large-order behaviour of the Adler function, extending previous work on the tau hadronic width. We find that the moments can be divided into a small number of classes, whose characteristics depend only on generic features of the moment weight function and Adler function series. Some moments that are commonly employed in alpha_s analyses from tau decays shoul…
Using recent precise hadronic tau-decay data on the V-A spectral function and general properties of QCD such as analyticity, the operator product expansion and chiral perturbation theory, we get accurate values for the QCD chiral order parameters L_10 and C_87. At order p^4 we obtain L_10^r(M_rho)=-(5.22+-0.06)10^-3, whereas at order p^6 we get L_10^r(M_rho)=-(4.06+-0.39)10^-3 and C_87^r(M_rho) = (4.89+-0.19)10^-3 GeV^-2.
It is well known that in every experiment involving charged particles there are individual photons emitted with small energy /omega and those with /omega < /omega_{0} will not be detected but are present in the data. The typical procedure to estimate this effect consists in taking the leading logarithm from the soft photon approximation and to make a simulation through a Monte Carlo Algorithm (PHOTOS). The aim of this work is to compute the model independent QED correction to tau--> K /pi /nu and to estimate its effect in the decay width in the particular process tau^{+}-->K^{0} /pi^{+}/nu.
I review sum rule determinations of |V_us| employing hadronic tau decay data, taking into account recent HFAG updates of exclusive tau branching fractions and paying special attention to the impact of the slow convergence of the relevant integrated D=2 OPE series and the potential role of contributions of as-yet-unmeasured higher multiplicity modes to the strange inclusive spectral distribution. In addition to conventional flavor-breaking sum rule determinations, information obtainable from mixed tau-electroproduction sum rules having much reduced OPE uncertainties, and from sum rules based on…
We analyze the lowest spectral moments of the left-right two-point correlation function, using all known short-distance constraints and the recently updated ALEPH V-A spectral function from tau decays. This information is used to determine the low-energy couplings L10 and C87 of chiral perturbation theory and the lowest-dimensional contributions to the Operator Product Expansion of the left-right correlator. A detailed statistical analysis is implemented to assess the theoretical uncertainties, including violations of quark-hadron duality.
The discrepancy between the FOPT and CIPT approaches for hadronic $τ$ spectral function moments constitutes the major theoretical uncertainty for strong coupling determinations from tau decay data. We show the discrepancy can be analytically understood since the Borel representations -- which have been assumed to be identical for both approaches previously -- differ in the presence of IR renormalons. This implies that the OPE condensate corrections are different for both approaches and that the discrepancy may eventually be reconciled. In the talk we explain the difference and some mathematica…
We present the electroweak one-loop corrections to the processes e+e- -> f bar-f, f = tau, c, b, t, at energies relevant for a future linear collider. The results of two independent calculations are compared and agreement is found at a technical-precision level of ten to twelve digits.
Recent evaluations of the hadronic vacuum polarization contributions to the effective fine-structure constant alpha_{em}(M_Z) are summarized and commented. A new update based on corrected CMD-2 data is presented. My new estimates are Delta alpha_{had}^{(5)}(M_Z) = 0.027773 +/- 0.000354 (e^+e^- - data based) and Delta alpha_{had}^{(5)}(M_Z) = 0.027664 +/- 0.000173 (via the Adler-function and extended use of pQCD). Prospects of further possible progress is discussed.
A sum rule analysis of the hadronic tau-decay data can be used to determine the low-energy constants L10(mu) and C87(mu). These constants are QCD chiral-order parameters, which appear at order p^4 and p^6, respectively, in the chiral perturbation theory expansion of the V-A correlator. At order p^4 we obtain L10(M_rho) = -(5.22\pm 0.06)10^-3. Including in the analysis the order p^6 contributions, we get L10(M_rho) = -(4.06\pm 0.39)10^-3 and C87(M_rho) = (4.89\pm 0.19)10^-3 GeV^-2.
The spectral function $ρ_{V-A}(s)$ 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 fi…
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.
I review recent evaluations of the hadronic contributions to the muon anomalous magnetic moment (g-2)_mu and to the effective fine structure constant alpha (M_Z). A new estimate for the hadronic shift delta_alpha hadronic = 0.027782 +/- 0.000254 is presented which implies 1/alpha (M_Z) = 128.913 +/- 0.035. It is based on a recent perturbative calculation of the Adler function which includes mass effects up to three-loops in a MOM scheme and requires little ad hoc assumptions. I then discuss perspectives for possible improvements in estimations of g-2 which we expect from the Phi--factory DAPHN…
We analyze the sensitivity of hadronic tau decays to non-standard interactions within the model-independent framework of the Standard Model Effective Field Theory (SMEFT). Both exclusive and inclusive decays are studied, using the latest lattice data and QCD dispersion relations. We show that there are enough theoretically clean channels to disentangle all the effective couplings contributing to these decays, with the $τ\to ππν_τ$ channel representing an unexpected powerful New Physics probe. We find that the ratios of non-standard couplings to the Fermi constant are bound at the sub-percent l…
The QCD corrections to the R_τ^{12} moment of the invariant mass distribution in hadronic decays of the tau lepton are discussed. The next-to-next-to-leading order prediction is shown to be stable with respect to change of the renormalization scheme, provided that the contour integral expression is used. The optimized predictions are obtained using the principle of minimal sensitivity to select the preferred renormalization scheme. The optimized predictions for R_τ^{12} and R_τ are used in a simplified fit to the experimental data to determine the strong coupling constant and the parameter cha…
The two highest precision determinations of alpha_s(M_Z^2), that based on the analysis of short-distance-sensitive lattice observables, and that based on an analysis of hadronic tau decay data, have, until very recently, given results which are not in good agreement. I review new versions of these analyses which bring the two determinations into excellent agreement, and discuss prospects for additional future improvements.
