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Ugo Aglietti

Publications and source records attributed to Ugo Aglietti.

At least 19 recordsLinked to original sources
arXiv2021

Operator mixing, UV asymptotics of nonplanar/planar $2$-point correlators, and nonperturbative large-$N$ expansion of QCD-like theories

We work out the interplay between lowest-order perturbative computations in the 't Hooft coupling, $g^2=g^2_{YM} N$, operator mixing, renormalization-group (RG) improved ultraviolet (UV) asymptotics of leading-order (LO) nonplanar/planar contributions to $2$-point correlators, and nonperturbative large-$N$ expansion of perturbatively massless QCD-like theories. As concrete examples, we compute to the lowest perturbative order in $SU(N)$ YM theory the ratios, $r_i$, of LO-nonplanar to planar contributions to the $2$-point correlators in the orthogonal basis in the coordinate representation of the gauge-invariant dimension-$8$ scalar operators and all the twist-$2$ operators. We demonstrate that -- if $\frac{γ_0}{β_0}$ has no LO-nonplanar contribution, with $γ_0$ and $β_0$ the one-loop coefficients of the anomalous-dimension matrix and beta function respectively -- $r_i$ actually coincides with the corresponding ratio in the large-$N$ expansion of the RG-improved UV asymptotics of the $2$-point correlators, provided that a certain canonical nonresonant diagonal renormalization scheme exists for the corresponding operators. Contrary to the aforementioned scalar operators, for the first $10^3$ twist-$2$ operators we actually verify the above conditions, and we get the universal value $r_i=-\frac{1}{N^2}$. Hence, nonperturbatively such $r_i$ must coincide with the UV asymptotics of the ratio of the glueball self-energy loop to the glueball tree contribution to the $2$-point correlators above. As a consequence, the universality of $r_i$ reflects the universality of the effective coupling in the nonperturbative large-$N$ YM theory for the twist-$2$ operators in the coordinate representation.

hep-th↗
arXiv2009

Inclusive Measure of |V_ub| with the Analytic Coupling Model

By analyzing B -> X_u l nu_l spectra with a model based on soft-gluon resummation and an analytic time-like QCD coupling, we obtain |V_ub| = (3.76 +-0.13 +- 0.22)*10^(-3), where the first and the second error refers to experimental and theoretical errors, respectively. The V_ub value is obtained from the available measured semileptonic branching fractions in limited regions of the phase-space. The distributions in the lepton energy E_l, the hadron invariant mass m_X, the light-cone momentum P_+ = E_X - p_X, together with the double distributions in (m_X,q^2) and (E_l,s_h^max), are used to select the phase-space regions. The q^2 is the dilepton squared momentum and s_h^max is the maximal m_X^2 at fixed q^2 and E_l. The V_ub value obtained is in complete agreement with the value coming from exclusive B decays and from an over-all fit to the Standard Model parameters. We show that the slight disagreement (up to +2 sigma) with respect to previous inclusive measurements is not related to different choices for the b (and c) masses but to a different modelling of the threshold (Sudakov) region.

hep-ph↗
arXiv2008

Comment on Resummation of Mass Distribution for Jets Initiated by Massive Quarks

We compute in the heavy quark effective theory the soft coefficient D_2 entering the resummation of next-to-next-to-leading threshold logarithms for jets initiated by a quark with a small mass compared to the hard scale of the process. We find complete agreement with a previous computation in full QCD. Contrary to our previous guess, this coefficient turns out to be different from that one entering heavy flavor decay or heavy flavor fragmentation.

hep-ph↗
arXiv2008

Analytic integration of real-virtual counterterms in NNLO jet cross sections I

We present analytic evaluations of some integrals needed to give explicitly the integrated real-virtual integrated counterterms, based on a recently proposed subtraction scheme for next-to-next-to-leading order (NNLO) jet cross sections. After an algebraic reduction of the integrals, integration-by-parts identities are used for the reduction to master integrals and for the computation of the master integrals themselves by means of differential equations. The results are written in terms of one- and two-dimensional harmonic polylogarithms, once an extension of the standard basis is made. We expect that the techniques described here will be useful in computing other integrals emerging in calculations in perturbative quantum field theories.

hep-ph↗
arXiv2007

Resummed Mass Distribution for Jets Initiated by Massive Quarks

We resum the invariant mass distribution of jets initiated by massive quarks in next-to-leading logarithmic approximation and beyond in heuristic way. We find that the inclusion of mass terms, in the N-moment space, results in the universal factor delta_N(Q^2;m^2), taking into account dead-cone effects and soft radiation characteristic of massive charges. This factor multiplies the massless jet distribution function J_N(Q^2). The variable N is rescaled by the mass correction parameter r = m^2/Q^2 << 1 with respect to the standard massless case, being m the quark mass and Q the hard scale. The functions A(alpha_S) and B(alpha_S), appear with a minus sign suppressing collinear effects at large N, as expected. In the same region, soft radiation not collinearly enhanced, characteristic of massive charges, makes its appearance with the function D(alpha_S). Phenomenological applications, such as the resummation of b --> c l nu decay spectra or the inclusion of beauty mass effects in t --> b W decays, are briefly sketched.

hep-ph↗
arXiv2007

Threshold Resummation in B --> X_c l nu_l Decays

We compute the QCD form factor resumming threshold logarithms in B --> X_c + l + nu_l decays to next-to-leading logarithmic approximation. We present an interpolation formula including soft as well as collinear effects softened by the non-vanishing charm mass.

hep-ph↗
arXiv2007

Modelling non-perturbative corrections to bottom-quark fragmentation

We describe B-hadron production in e+e- annihilation at the Z pole by means of a model including non-perturbative corrections to b-quark fragmentation as originating, via multiple soft emissions, from an effective QCD coupling constant, which does not exhibit the Landau pole any longer and includes absorbitive effects due to parton branching. We work in the framework of perturbative fragmentation functions at NLO, with NLL DGLAP evolution and NNLL large-x resummation in both coefficient function and initial condition of the perturbative fragmentation function. We include hadronization corrections via the effective coupling constant in the NNLO approximation and do not add any further non-perturbative fragmentation function. As part of our model, we perform the Mellin transforms of our resummed expressions exactly. We present results on the energy distribution of b-flavoured hadrons, which we compare with LEP and SLD data, in both x- and N-spaces. We find that, within the theoretical uncertainties on our calculation, our model is able to reasonably reproduce the data at x<0.92 and the first five moments of the B cross section.

hep-ph↗
arXiv2006

Semi-Inclusive B Decays and a Model for Soft-Gluon Effects

We compare experimental spectra of radiative and semileptonic B decays with the predictions of a model based on soft-gluon resummation to next-to-next-to leading order and on a ghost-less time-like coupling. We find a good agreement with photon spectra in the radiative decay and with hadron mass distributions in the semileptonic one: the extracted values for alpha_S(m_Z) are in agreement with the current PDG average within at most two standard deviations. The agreement is instead less good for the electron spectra measured by BaBar and Belle in semileptonic decays for small electron energies (< 2.2 GeV): our spectrum is harder. We also show that, in general, the inclusion of next-to-next-to-leading order effects is crucial for bringing the model closer to the data and that the non-power expansion introduced in the framework of analytic coupling studies does not accurately describe soft-gluon effects.

hep-ph↗
arXiv2006

Threshold resummed spectra in B ->Xu l nu decays in NLO (III)

We resum to next-to-leading order (NLO) the distribution in the light-cone momentum p+ = EX - |pX| and the spectrum in the electron energy, in the semileptonic decays B-> Xu l nu, where EX and pX are the total energy and three-momentum of the final hadron state Xu respectively. By expanding our formulas, we obtain the coefficients of all the infrared logarithms alpha^n L^k at order alpha^2 and at order alpha^3, with the exception of the alpha^3 L coefficient. We explicitly show that the relation between these semileptonic spectra and the hadron mass distribution in the radiative decay B -> Xs gamma is not a purely short distance one. By expanding our result for the p+ distribution and comparing it with the recent O(alpha^2 n_f) computation, we obtain an explicit and non trivial verification of our resummation scheme at the two-loop level. We also discuss the validity of the so-called Brodsky-Lepage-Mackenzie (BLM) scheme in the evaluation of the semileptonic spectra by comparing it with our results, finding that it is a reasonable approximation for the leading terms only. Finally, we show that long-distance phenomena, such as Fermi motion, are expected to have an effect on the electron spectrum smaller than on the hadron mass distribution.

hep-ph↗
arXiv2005

Threshold Resummed Spectra in B -> Xu l nu Decays in NLO (I)

We evaluate thresholds resummed spectra in B -> Xu l nu decays in next-to-leading order. We present results for the distribution in E_X and in m_X^2/E_X^2, for the distribution in E_X and E_l and for the distribution in E_X, where E_X and m_X are the energy and the invariant mass of the final hadronic state Xu respectively and E_l is the energy of the charged lepton. We explicitly show that all these spectra (where there is no integration over the hadronic energy) can be directly related to the photon spectrum in B -> Xs gamma via short-distance coefficient functions.

hep-ph↗
arXiv2005

Threshold resummed spectra in B -> Xu l nu decays in NLO (II)

We resum to next-to-leading order the distribution in the ratio of the invariant hadron mass mX to the total hadron energy EX and the distribution in mX in the semileptonic decays B -> Xu l nu. By expanding our formulas, we obtain the coefficients of all the infrared logarithms at O(alphaS^2) and of the leading ones at O(alphaS^3). We explicitly show that the relation between these semileptonic spectra and the photon spectrum in the radiative decay B -> Xs gamma is not a purely short-distance one. There are long-distance effects in the semileptonic spectra which are not completely factorized by the structure function as measured in the radiative decay and have to be modelled in some way.

hep-ph↗
arXiv2004

A model for next-to-leading order threshold resummed form factors

We present a model for next-to-leading order resummed threshold form factors based on a time-like coupling recently introduced in the framework of small x physics. Improved expressions for the form factors in N-space are obtained which are not plagued by Landau-pole singularities, as the included absorptive effects -- usually neglected -- act as regulators. The physical reason is that, because of faster decay of gluon jets, there is not enough resolution time to observe the Landau pole. Our form factors reduce to the standard ones when the absorptive parts related to the coupling are neglected. The inverse transform from N-space to x-space can be done directly without any prescription and we obtain analytical expressions for the form factors, which are well defined in all x-space.

hep-ph↗
arXiv2002

A new model-independent way of extracting |V_ub/V_cb|

The ratio between the photon spectrum in B -> X_s gamma and the differential semileptonic rate wrt the hadronic variable M_X/E_X is a short-distance quantity calculable in perturbation theory and independent of the Fermi motion of the b quark in the B meson. We present a NLO analysis of this ratio and show how it can be used to determine |V_ub/V_cb| independently of any model for the shape function. We also discuss how this relation can be used to test the validity of the shape-function theory on the data.

hep-ph↗
arXiv2000

The structure function of semi-inclusive heavy flavour decays in field theory

We consider the decay of a heavy flavour into an inclusive hadronic state X of invariant mass m_X small with respect to its energy E_X, m_X << E_X. The electron spectrum and the hadronic mass distribution in semileptonic b -> u decays, or the photon spectrum in b -> s gamma decays, all require, close to their endpoints, a control over this region. This region is affected both by non-perturbative phenomena related to the Fermi motion of the heavy quark and by perturbative soft gluon radiation in the final state (Sudakov form factor).Fermi motion can be described by the shape function f (m*), which represents the distribution of the effective mass m* of the heavy quark at disintegration time. We perform a factorization with a simple technique in order to consistently separate perturbative from non-perturbative effects. We find that the shape function, contrary to naive expectations,is not a physical distribution, as it is affected by substantial regularization scheme effects, controlling even the leading, double-logarithmic term. It factorizes, however, the bulk of non-perturbative effects in lattice-like regularizations. Some non-perturbative effects are present in the coefficient function even at leading twist, but they are expected to be suppressed on physical grounds. Finally, we clarify a controversial factor of 2 in the evolution kernel of the shape function.

hep-ph↗
arXiv1998

Factorization and Effective Theories

We prove factorization in the decay of a B meson into a D* + jet using the Large Energy Effective Theory. The proof is non perturbative, does not require any gauge fixing and is exact in the limit of a very narrow jet. On the other hand, it is shown that this theory is unable to consistently describe completely exclusive processes such as for example a B which decays into a D* + pion, due to an oversimplification of transverse momentum dynamics. Therefore we present a variant of the Large Energy Effective Theory, i.e. a new effective theory for massless particles which properly takes into account transverse degrees of freedom and is the natural framework to study exclusive non-leptonic decays.

hep-ph↗
arXiv1995

Renormalons and confinement

We compute the renormalon ambiguity of the static potential, in the limit of a large number of flavors. An extrapolation of the QED result to QCD implies that the large distance behavior of the quark potential is arbitrary in perturbation theory, as there are an infinite number of prescriptions to assign. The shape of the potential at large distances is not only affected by the renormalon pole closest to the origin of the Borel plane, but a resummation of all renormalon contributions is required. In particular, confinement can be accommodated, but it is not explained. At short distances there is no indication of a linear term in the potential.

hep-ph↗