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Stefano Forte

Publications and source records attributed to Stefano Forte.

At least 145 records · Page 8Linked to original sources
arXiv2002

Renormalization group approach to soft gluon resummation

We present a simple proof of the all-order exponentiation of soft logarithmic corrections to hard processes in perturbative QCD. Our argument is based on proving that all large logs in the soft limit can be expressed in terms of a single dimensionful variable, and then using the renormalization group to resum them. Beyond the next-to-leading log level, our result is somewhat less predictive than previous all-order resummation formulae, but it does not rely on non-standard factorization, and it is thus possibly more general. We use our result to settle issues of convergence of the resummed series, we discuss scheme dependence at the resummed level, and we provide explicit resummed expressions in various factorization schemes.

hep-ph↗
arXiv2002

Neural Network Parametrization of Deep-Inelastic Structure Functions

We construct a parametrization of deep-inelastic structure functions which retains information on experimental errors and correlations, and which does not introduce any theoretical bias while interpolating between existing data points. We generate a Monte Carlo sample of pseudo-data configurations and we train an ensemble of neural networks on them. This effectively provides us with a probability measure in the space of structure functions, within the whole kinematic region where data are available. This measure can then be used to determine the value of the structure function, its error, point-to-point correlations and generally the value and uncertainty of any function of the structure function itself. We apply this technique to the determination of the structure function F_2 of the proton and deuteron, and a precision determination of the isotriplet combination F_2[p-d]. We discuss in detail these results, check their stability and accuracy, and make them available in various formats for applications.

hep-ph↗
arXiv2002

Hadron physics at a neutrino factory

We review the way intense neutrino beams at the front-end of a muon storage ring can be used to probe the structure of hadrons. Specifically, we discuss how the polarized and unpolarized flavor structure of the nucleon can be disentangled in inclusive deep-inelastic scattering, and how less inclusive measurements can shed light on various aspects of hadron structure such as fragmentation functions or generalized parton distributions.

hep-ph↗
arXiv2002

Determination of alpha_s from scaling violations of truncated moments of structure functions

We determine the strong coupling alpha_s(M_Z) from scaling violations of truncated moments of the nonsinglet deep inelastic structure function F_2. Truncated moments are determined from BCDMS and NMC data using a neural network parametrization which retains the full experimental information on errors and correlations. Our method minimizes all sources of theoretical uncertainty and bias which characterize extractions of alpha_s from scaling violations. We obtain alpha_s(M_Z) = 0.124 +0.004-0.007 (exp.) + 0.003- 0.004 (th.).

hep-ph↗
arXiv2001

Theoretical Aspects of HERA Physics

We discuss the theoretical underpinnings for the extraordinary success of perturbative QCD in the description of HERA data. In particular, we examine recent progress in the understanding of perturbative QCD at small x. We explain the relation between evolution equations in Q^2 and x, and how they can be used for simultaneous resummation of the relevant large logs at HERA. We show that while the HERA data can be understood within our current knowledge of the perturbative expansion of QCD, they pose stringent constraints on the perturbatively inaccessible behaviour of QCD in the Regge limit.

hep-ph↗
arXiv2001

Highlights of Short Baseline Physics at a Neutrino Factory

We review the physics potential for experiments with intense neutrino beams at the front-end of a muon storage ring, stressing the way the spin and flavour structure of neutrino interactions with matter can be used to shed light on the structure of the strong and electroweak interactions. Specifically, we discuss precision tests of the standard model, studies of polarized and unpolarized structure functions of the nucleon, and several new and exotic physics items.

hep-ph↗
arXiv2001

Factorization and Resummation of Small x Scaling Violations with Running Coupling

We discuss the inclusion of running coupling effects in perturbative small x evolution equations. We show that a running coupling BFKL-like x-evolution equation is fully compatible, up to higher twist corrections, with the standard factorized perturbative evolution of parton distributions. We then use this result, combined with the well-known Airy asymptotics, to prove that the oscillations which are present in the running-coupling BFKL solution do not affect the associated splitting functions, which instead remain smooth in the small x limit. This allows us to give a prescription to include running-coupling corrections in the small-x resummation of scaling violations. We show that these corrections are small in the HERA region.

hep-ph↗
arXiv2001

Extrapolating Structure Functions to Very Small x

We review small $x$ contributions to perturbative evolution equations for parton distributions, and their resummation. We emphasize in particular the resummation technique recently developed in order to deal with the apparent instability of naive small $x$ evolution kernels and understand the empirical sucess of fixed--order perturbation theory. We give predictions for the gluon distribution and the structure functions F_2(x,Q^2) and F_L(x,Q^2) in an extended kinematic region, such as would be relevant for THERA or LEP+LHC ep colliders.

hep-ph↗
arXiv2001

Small-x Resummation and HERA Structure Function Data

We apply our systematic NLO small x resummation of singlet splitting functions to the scaling violations of structure functions and compare the results with data. We develop various theoretical tools which are needed in order to relate resummed parton distributions to measurable structure functions, and we present results from a variety of fits to HERA data for the structure functions F_2 and F_L using the resummation. The behaviour of the singlet splitting functions at small x and fixed Q^2 is effectively parametrized as x^{-lambda}. We find that, for lambda small or negative, the resummed description of scaling violations may be phenomenologically as good as or even better than the standard next-to-leading order treatment. However, the best fit gluon density and value of alpha_s can be significantly modified by the resummation.

hep-ph↗
arXiv2001

Polarized parton distributions from charged-current deep-inelastic scattering and future neutrino factories

We discuss the determination of polarized parton distributions from charged-current deep-inelastic scattering experiments. We summarize the next-to-leading order treatment of charged-current polarized structure functions, their relation to polarized parton distributions and scale dependence, and discuss their description by means of a next-to-leading order evolution code. We discuss current theoretical expectations and positivity constraints on the unmeasured C-odd combinations Delta q-Delta qbar of polarized quark distributions, and their determination in charged-current deep-inelastic scattering experiments. We give estimates of the expected errors on charged-current structure functions at a future neutrino factory, and perform a study of the accuracy in the determination of polarized parton distributions that would be possible at such a facility. We show that these measurements have the potential to distinguish between different theoretical scenarios for the proton spin structure.

hep-ph↗
arXiv2000

Evolution of truncated moments of singlet parton distributions

We define truncated Mellin moments of parton distributions by restricting the integration range over the Bjorken variable to the experimentally accessible subset x_0 < x < 1 of the allowed kinematic range 0 < x < 1. We derive the evolution equations satisfied by truncated moments in the general (singlet) case in terms of an infinite triangular matrix of anomalous dimensions which couple each truncated moment to all higher moments with orders differing by integers. We show that the evolution of any moment can be determined to arbitrarily good accuracy by truncating the system of coupled moments to a sufficiently large but finite size, and show how the equations can be solved in a way suitable for numerical applications. We discuss in detail the accuracy of the method in view of applications to precision phenomenology.

hep-ph↗
arXiv2000

Resummation of Singlet Parton Evolution at Small x

We propose an improvement of the splitting functions at small x which overcomes the apparent problems encountered by the BFKL approach. We obtain a stable expansion for the x-evolution function chi(M) near M=0 by including in it a sequence of terms derived from the one- and two-loop anomalous dimension gamma. The requirement of momentum conservation is always satisfied. The residual ambiguity on the splitting functions is effectively parameterized in terms of the value of lambda, which fixes the small x asymptotic behaviour x^-lambda of the singlet parton distributions. We derive from this improved evolution function an expansion of the splitting function which leads to good apparent convergence, and to a description of scaling violations valid both at large and small x.

hep-ph↗
arXiv1999

Tests of the Standard Model at HERA

We review how experimental data collected at the HERA lepton-hadron collider have improved our theoretical and phenomenological understanding of the standard model, and specifically of its QCD sector.

hep-ph↗
arXiv1999

Evolution of parton distributions with truncated Mellin moments

Evolution equations for parton distributions can be approximately diagonalized and solved in moment space without assuming any knowledge of the parton distribution in the region of small x. The evolution algorithm for truncated moments is simple and rapidly converging. Examples of applications are outlined.

hep-ph↗
arXiv1999

The Small x Behaviour of Altarelli-Parisi Splitting Functions

We extract the small x asymptotic behaviour of the Altarelli-Parisi splitting functions from their expansion in leading logarithms of 1/x. We show in particular that the nominally next-to-leading correction extracted from the Fadin-Lipatov kernel is enhanced asymptotically by an extra ln 1\x over the leading order. We discuss the origin of this problem, its dependence on the choice of factorization scheme, and its all-order generalization. We derive necessary conditions which must be fulfilled in order to obtain a well behaved perturbative expansion, and show that they may be satisfied by a suitable reorganization of the original series.

hep-ph↗
arXiv1999

Recent developments in Deep-Inelastic Scattering

We review several recent developments in the theory and phenomenology of deep-inelastic scattering, with particular emphasis on precision tests of QCD and progress in the detailed perturbative treatment of structure functions and parton distributions. We discuss specifically determinations of alpha_s; higher twist contributions to structure functions and renormalons; structure functions at small x and resummation of energy logarithms.

hep-ph↗
arXiv1999

Truncated moments of parton distributions

We derive evolution equations satisfied by moments of parton distributions when the integration over the Bjorken variable is restricted to a subset (x_0 <= x <= 1) of the allowed kinematical range 0<= x<= 1. The corresponding anomalous dimensions turn out to be given by a triangular matrix which couples the N--th truncated moment with all (N + K)--th truncated moments with integer K >= 0. We show that the series of couplings to higher moments is convergent and can be truncated to low orders while retaining excellent accuracy. We give an example of application to the determination of alpha_s from scaling violations.

hep-ph↗