arXiv subjects

Vicent Mateu

Publications and source records attributed to Vicent Mateu.

49 records · Page 3Linked to original sources

arXiv2014

Charm and Bottom Masses from Sum Rules with a Convergence Test

In this talk we discuss results of a new extraction of the MS-bar charm quark mass using relativistic QCD sum rules at O(as**3) based on moments of the vector and the pseudoscalar current correlators and using the available experimental measurements from e+e- collisions and lattice results, respectively. The analysis of the perturbative uncertainties is based on different implementations of the perturbative series and on independent variations of the renormalization scales for the mass and the strong coupling following a work we carried out earlier. Accounting for the perturbative series that result from this double scale variation is crucial since some of the series can exhibit extraordinarily small scale dependence, if the two scales are set equal. The new aspect of the work reported here adresses the problem that double scale variation might also lead to an overestimate of the perturbative uncertainties. We supplement the analysis by a convergence test that allows to quantify the overall convergence of QCD perturbation theory for each moment and to discard series that are artificially spoiled by specific choices of the renormalization scales. We also apply the new method to an extraction of the MS-bar bottom quark mass using experimental moments that account for a modeling uncertainty associated to the continuum region where no experimental data is available. We obtain m_c(m_c) = 1.287 +- 0.020 GeV and m_b(m_b) = 4.167 +- 0.023 GeV.

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arXiv2013

Oriented Event Shapes at N3LL + O(as^2)

We analyze oriented event-shapes in the context of Soft-Collinear Effective Theory (SCET) and in Fixed-Order perturbation theory. Oriented event-shapes are distributions of event-shape variables which are differential on the angle theta_T that the thrust axis forms with the electron-positron beam. We show that at any order in perturbation theory and for any event shape, only two angular structures can appear: F_0 = 3/8(1 + cos^2 theta_T) and F_1 = (1 - 3cos^2 theta_T). When integrating over theta_T to recover the more familiar event-shape distributions, only F_0 survives. The validity of our proof goes beyond perturbation theory, and hence only these two structures are present at the hadron level. The proof also carries over massive particles. Using SCET techniques we show that singular terms can only arise in the F_0 term. Since only the hard function is sensitive to the orientation of the thrust axis, this statement applies also for recoil-sensitive variables such as Jet Broadening. We carry out resummation at N3LL of singular terms for Thrust, Heavy-Jet Mass, the sum of the Hemisphere Masses and C-parameter. We also compute the Fixed-Order distributions for these event-shapes at O(as) analytically and at O(as^2) with the program Event2.

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arXiv2013

Charm Mass Determination from QCD Charmonium Sum Rules at Order alpha_s^3

We determine the MS-bar charm quark mass from a charmonium QCD sum rules analysis. On the theoretical side we use input from perturbation theory at O(alpha_s^3). Improvements with respect to previous O(alpha_s^3) analyses include (1) an account of all available e+e- hadronic cross section data and (2) a thorough analysis of perturbative uncertainties. Using a data clustering method to combine hadronic cross section data sets from different measurements we demonstrate that using all available experimental data up to c.m. energies of 10.538 GeV allows for determinations of experimental moments and their correlations with small errors and that there is no need to rely on theoretical input above the charmonium resonances. We also show that good convergence properties of the perturbative series for the theoretical sum rule moments need to be considered with some care when extracting the charm mass and demonstrate how to set up a suitable set of scale variations to obtain a proper estimate of the perturbative uncertainty. As the final outcome of our analysis we obtain m_c(m_c) = 1.282 \pm 0.006_stat \pm 0.009_syst \pm 0.019)_pert \pm 0.010_alpha \pm 0.002_GG GeV. The perturbative error is an order of magnitude larger than the one obtained in previous O(alpha_s^3) sum rule analyses.

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arXiv2013

Power Corrections to Event Shapes with Mass-Dependent Operators

We introduce an operator depending on the "transverse velocity" r that describes the effect of hadron masses on the leading 1/Q power correction to event-shape observables. Here, Q is the scale of the hard collision. This work builds on earlier studies of mass effects by Salam and Wicke and of operators by Lee and Sterman. Despite the fact that different event shapes have different hadron mass dependence, we provide a simple method to identify universality classes of event shapes whose power corrections depend on a common nonperturbative parameter. We also develop an operator basis to show that at a fixed value of Q, the power corrections for many classic observables can be determined by two independent nonperturbative matrix elements at the 10% level. We compute the anomalous dimension of the transverse velocity operator, which is multiplicative in r and causes the power correction to exhibit non-trivial dependence on Q. The existence of universality classes and the relevance of anomalous dimensions are reproduced by the hadronization models in Pythia 8 and Herwig++, though the two programs differ in the values of their low-energy matrix elements.

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arXiv2013

Hadron Mass Effects in Power Corrections to Event Shapes

We study the effect of hadron masses on the leading power correction of dijet event-shape distributions. We define the transverse velocity operator, that describes the effects of hadron masses. It depends on the "transverse velocity" r, which is different from one only for non-vanishing hadron masses. We find that hadron-mass effects in general break universality. However we provide a simple method to identify universality classes of event shapes with a common power correction. We also compute the anomalous dimension of the power correction and the structure of the corresponding Wilson coefficient, finding a nontrivial result.

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arXiv2012

Precision Thrust Cumulant Moments at N^3LL

We consider cumulant moments (cumulants) of the thrust distribution using predictions of the full spectrum for thrust including O(alpha_s^3) fixed order results, resummation of singular N^3LL logarithmic contributions, and a class of leading power corrections in a renormalon-free scheme. From a global fit to the first thrust moment we extract the strong coupling and the leading power correction matrix element Omega_1. We obtain alpha_s(m_Z) = 0.1141 \pm (0.0004)_exp \pm (0.0014)_hadr \pm (0.0007)_pert, where the 1-sigma uncertainties are experimental, from hadronization (related to Omega_1) and perturbative, respectively, and Omega_1 = 0.372 \pm (0.044)_exp \pm (0.039)_pert GeV. The n-th thrust cumulants for n > 1 are completely insensitive to Omega_1, and therefore a good instrument for extracting information on higher order power corrections, Omega'_n/Q^n, from moment data. We find (\tilde Omega'_2)^(1/2) = 0.74 \pm (0.11)_exp \pm (0.09)_pert GeV.

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arXiv2010

Thrust at N^3LL with Power Corrections and a Precision Global Fit for alphas(mZ)

We give a factorization formula for the e+e- thrust distribution dsigma/dtau with tau=1-T based on soft-collinear effective theory. The result is applicable for all tau, i.e. in the peak, tail, and far-tail regions. The formula includes O(alphas^3) fixed-order QCD results, resummation of singular partonic alphas^j ln^k(tau)/tau terms with N^3LL accuracy, hadronization effects from fitting a universal nonperturbative soft function defined in field theory, bottom quark mass effects, QED corrections, and the dominant top mass dependent terms from the axial anomaly. We do not rely on Monte Carlo generators to determine nonperturbative effects since they are not compatible with higher order perturbative analyses. Instead our treatment is based on fitting nonperturbative matrix elements in field theory, which are moments Omega_i of a nonperturbative soft function. We present a global analysis of all available thrust data measured at center-of-mass energies Q=35 to 207 GeV in the tail region, where a two parameter fit to $α_s(m_Z)$ and the first moment Omega_1 suffices. We use a short distance scheme to define Omega_1, called the R-gap scheme, thus ensuring that the perturbative dsigma/dtau does not suffer from an O(Lambda_QCD) renormalon ambiguity. We find alphas(mZ)=0.1135 \pm (0.0002)_{expt} \pm (0.0005)_{hadr} \pm (0.0009)_{pert}, with chi^2/dof=0.91, where the displayed 1-sigma errors are the total experimental error, the hadronization uncertainty, and the perturbative theory uncertainty, respectively. The hadronization uncertainty in alphas is significantly decreased compared to earlier analyses by our two parameter fit, which determines Omega_1=0.323 GeV with 16% uncertainty.

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arXiv2010

Global Fit of alpha_s(m_Z) to Thrust at NNNLL Order with Power Corrections

From soft-collinear effective theory one can derive a factorization formula for the e+e- thrust distribution dsigma/dtau with tau = 1-T that is applicable for all tau. The formula accommodates available O(alpha_s^3) fixed-order QCD results, resummation of logarithms at NNNLL order, a universal nonperturbative soft function for hadronization effects, factorization of nonperturbative effects in subleading power contributions, bottom mass effects and QED corrections. We emphasize that the use of Monte Carlos to estimate hadronization effects is not compatible with high-precision, high-order analyses. We present a global analysis of all available e+e- thrust data measured at Q = 35 to 207 GeV in the tail region, where a two-parameter fit can be carried out for alpha_s(m_Z) and Omega_1, the first moment of the soft function. To obtain small theoretical errors it is essential to define Omega_1 in a short-distance scheme, free of an O(Lambda_QCD) renormalon ambiguity. We find alpha_s(m_Z) = 0.1135 +- (0.0002)_expt +- (0.0005)_Omega_1 +- (0.0009)_pert with chi^2/dof = 0.9.

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arXiv2009

Chiral corrections to the f_V^T/f_V ratio for vector mesons

In this letter we compute the leading chiral corrections to the ratio between the tensor and the vector decay couplings for the lowest lying vector meson multiplet (rho, K^*, phi). We show that the leading chiral logarithms arise from tadpole contributions and are therefore entirely fixed by chiral symmetry, while the next to leading corrections are purely analytic. Interestingly, the flavour structure of the chiral logarithms implies that only the rho meson is sensitive to pion logarithms. By examining SU(2)_L\times SU(2)_R\times U(1)_S theories we show that this result can be easily understood and that it holds to all orders in the m_s expansion. Recent lattice data seem to comply with our results.

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arXiv2008

Heavy Quark Vacuum Polarization Function at O(alpha_s^2) and O(alpha_s^3)

We determine the full mass and $q^2$ dependence of the heavy quark vacuum polarization function $Π(q^2)$ and its contribution to the total $e^+e^-$ cross section at ${\cal O}(α_s^2)$ and ${\cal O}(α_s^3)$ in perturbative QCD. We use known results for the expansions of $Π(q^2)$ at high energies, in the threshold region and around $q^2=0$, conformal mapping and the Padé approximation method. From our results for $Π(q^2)$ we determine numerically at ${\cal O}(α_s^3)$ the previously unknown non-logarithmic contributions in the high-energy expansion at order $(m^2/q^2)^i$ for $i=0,1$ and the coefficients in the expansion around $q^2=0$ at order $q^{2n}$ with $n\ge 2$. We also determine at ${\cal O}(α_s^2)$ the previously unknown ${\cal O}(v^0)$ constant term in the expansion of $Π(q^2)$ in the threshold region, where $v$ is the quark velocity. Our method allows for a quantitative estimate of uncertainties and can be systematically improved once more information in the three kinematic regions becomes available by future multi-loop computations. For the contributions to the total $e^+e^-$ cross section at ${\cal O}(α_s^2)$ we confirm results obtained earlier by Chetyrkin, Kühn and Steinhauser.

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arXiv2008

OPE-RchiT matching at order alpha_s: hard gluonic corrections to three-point Green functions

In this work we push the matching between the QCD operator product expansion (OPE) and resonance chiral theory (RchiT) to order alpha_s. To this end we compute two- and three-point QCD Green functions (GFs) in both theories and compare the results. GFs which are order parameters of chiral symmetry breaking make this matching more transparent and thus we concentrate on those. On the OPE side one needs to calculate the hard-gluon virtual corrections to the quark condensate, and in particular for three-point GFs this computation was hitherto missing. We also discuss the need for including the infinite tower of hadronic states in the hadronic representation of the GF when non-analytic terms such as logarithms are present in the OPE Wilson coefficients.

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arXiv2007

Chiral Perturbation Theory with tensor sources

We construct the most general chirally-invariant Lagrangian for mesons in the presence of external sources coupled to the tensor current \barψσ_{μν}ψ. In order to have only even terms in the chiral expansion, we consider the new source of O(p^2). With this choice, we build the even-parity effective Lagrangian up to the p^6-order (NLO). While there are only 4 new terms at the p^4-order, at p^6-order we find 78 terms for n_f=2 and 113 terms for n_f=3. We provide a detailed discussion on the different mechanisms that ensure that our final set of operators is complete and non-redundant. We also examine the odd-parity sector, to conclude that the first operators appear at the p^8-order (NNLO).

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arXiv2007

Chiral Lagrangians with tensor sources

The implementation of tensor sources in Chiral Lagrangians allows the computation of Green functions and form factors involving tensor currents, that is, quark bilinears of the form \bar{q}_iσ^{μν}q_j. Whereas only four new terms show up at O(p^4), we find around a hundred of them at O(p^6). So it becomes essential to ensure that this set o operators is indeed minimal and non-redundant (i.e., it is a basis). We discuss two phenomenological applications in the context of vector meson resonances and the radiative pion decay.

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