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A. Vogt

Publications and source records attributed to A. Vogt.

At least 109 records · Page 6Linked to original sources
arXiv2001

Non-singlet structure functions beyond the next-to-next-to leading order

We study the evolution of the flavour non-singlet deep-inelastic structure functions F_{2,NS} and F_3 at the next-to-next-to-next-to-leading order (N^3LO) of massless perturbative QCD. The present information on the corresponding three-loop coefficient functions is used to derive approximate expressions of these quantities which prove completely sufficient for values x > 10^{-2} of the Bjorken variable. The inclusion of the N^3LO corrections reduces the theoretical uncertainty of alpha_s determinations from non-singlet scaling violations arising from the truncation of the perturbation series to less than 1%. We also study the predictions of the soft-gluon resummation, of renormalization-scheme optimizations by the principle of minimal sensitivity (PMS) and the effective charge (ECH) method, and of the Pade' summation for the structure-function evolution kernels. The PMS, ECH and Pade' approaches are found to facilitate a reliable estimate of the corrections beyond N^3LO.

hep-ph↗
arXiv2000

Next-to-next-to-leading logarithmic threshold resummation for deep-inelastic scattering and the Drell-Yan process

The soft-gluon resummation exponents G^N in moment space are investigated for the quark coefficient functions in deep-inelastic structure functions and the quark-antiquark contribution to the Drell-Yan cross section dsigma/dM. Employing results from two- and three-loop calculations we obtain the next-to-next-to-leading logarithmic terms alpha_s (alpha_s ln N)^n of G^N to all orders in the strong coupling constant alpha_s. These new contributions facilitate a reliable assessment of the numerical effect and the stability of the large-N expansion.

hep-ph↗
arXiv2000

On soft gluon effects in deep-inelastic structure functions

The behaviour of the quark coefficient functions in deep-inelastic scattering is investigated for large values of the Bjorken variable x. By combining results of soft-gluon resummation and fixed-order calculations, we determine the coefficients of the four leading large-x logarithms, alpha_s^k [{ln(1-x)}^{2k-l}/(1-x)]_+, l = 1, ...4, to all orders in the strong coupling constant alpha_s. This result includes two more terms for the three-loop coefficient functions than previously specified in the literature. The effect of the fifth logarithmic contribution is approximately evaluated. The terms derived here are required, but also seem to be sufficient, for a reliable representation of the coefficient functions at large x.

hep-ph↗
arXiv2000

Improved Approximations for the Three-Loop Splitting Functions in QCD

We update our approximate parametrizations of the three-loop splitting functions for the evolution of unpolarized parton densities in perturbative QCD. The new information taken into account is given by the additional Mellin moments recently calculated by Retey and Vermaseren. The inclusion of these constraints reduces the uncertainties of our approximations considerably and extends their region of applicability by about one order of magnitude to lower momentum fractions x .

hep-ph↗
arXiv2000

Parton densities and structure functions beyond the next-to-leading order

We briefly discuss recent results on the evolution of unpolarized parton densities and structure functions in massless perturbative QCD. Present partial results on the next-to-next-to-leading order (NNLO) evolution kernels prove sufficient for reliable calculations at not too small values of the Bjorken variable, x > 10^-3. One order more can be taken into account at x >= 0.2. Inclusion of these terms considerably reduces the main theoretical uncertainties of determinations of alpha_s (to about 1% at the Z-mass) and the parton densities from structure functions.

hep-ph↗
arXiv2000

NNLO evolution of deep-inelastic structure functions: the singlet case

We study the next-to-next-to-leading order (NNLO) evolution of flavour singlet parton densities and structure functions in massless perturbative QCD. Present information on the corresponding three-loop splitting functions is used to derive parametrizations of these quantities, including Bjorken-x dependent estimates of their residual uncertainties. Compact expressions are also provided for the exactly known, but in part rather lengthy two-loop singlet coefficient functions. The size of the NNLO corrections and their effect on the stability under variations of the renormalization and mass-factorizations scales are investigated. Except for rather low values of the scales, the residual uncertainty of the three-loop splitting functions does not lead to relevant effects for x > 10^-3. Inclusion of the NNLO contributions considerably reduces the theoretical uncertainty of determinations of the quark and gluon densities from deep-inelastic structure functions.

hep-ph↗
arXiv2000

QCD

We discuss issues of QCD at the LHC including parton distributions, Monte Carlo event generators, the available next-to-leading order calculations, resummation, photon production, small x physics, double parton scattering, and backgrounds to Higgs production.

hep-ph↗
arXiv1999

NNLO evolution of deep-inelastic structure functions: the non-singlet case

We study the next-to-next-to-leading order (NNLO) evolution of flavour non-singlet quark densities and structure functions in massless perturbative QCD. Present information on the corresponding three-loop splitting functions is used to derive parametrizations of these quantities, including Bjorken-x dependent estimates of their residual uncertainties. Compact expressions are also provided for the exactly known, but rather involved two-loop coefficient functions. The size of the NNLO corrections and their effect on the stability under variations of the renormalization scale are investigated. The residual uncertainty of the three-loop splitting functions does not lead to appreciable effects for x > 10^-2. Inclusion of the NNLO contributions reduces the main theoretical uncertainty of alpha_s determinations from non-singlet scaling violations by more than a factor of two.

hep-ph↗
arXiv1998

Dynamical Parton Distributions Revisited

Dynamical parton densities, generated radiatively from valence-like inputs at some low resolution scale, are confronted with recent small-x data on deep inelastic and other hard scattering processes. It is shown that within theoretical uncertainties our previous (1994) dynamical/radiative parton distributions are compatible with most recent data and still applicable within the restricted accuracy margins of the presently available next-to-leading order calculations. Due to recent high precision measurements we also present an updated, more accurate, version of our (valence-like) dynamical input distributions. Furthermore, our perturbatively stable parameter-free dynamical predictions are extended to the extremely small-x region, 10^-8 ~< x ~< 10^-5, relevant to questions concerning ultra-high-energy cosmic ray and neutrino astronomy.

hep-ph↗
arXiv1998

The Unpolarized Gluon Anomalous Dimension at Small x

We discuss the quantitative consequences of the resummation of the small-x contributions to the anomalous dimensions beyond next-to-leading order in alpha_s and up to next order in ln(1/x) (NLx) in a framework based on the renormalization group equations. We find large and negative effects leading to negative values for the {\sf total} splitting function P_{gg}(x,alpha_s) already for x \lsim 0.01 at Q^2 \simeq 20 GeV^2. Terms less singular than those under consideration turn out to be quantitatively as important and need to be included. We derive the effects of the conformal part of the NLx contributions to the anomalous dimensions and discuss the exponent omega describing the s^omega behavior of inclusive cross sections.

hep-ph↗
arXiv1998

The Resummed Gluon Anomalous Dimension and Structure Functions at Small x

The impact of the resummed next-to-leading logarithmic small-$x$ contributions to the anomalous dimension $γ_{gg}$ is evaluated for the unpolarized parton densities and structure functions of the nucleon. These new terms diminish the gluon distribution and overcompensate the contribution caused by the resummed leading logarithmic small-$x$ anomalous dimension and the quarkonic anomalous dimensions beyond next-to-leading order.

hep-ph↗
arXiv1998

The Evolution of Unpolarized Singlet Structure Functions at Small x

A systematic study is performed of the impact of the various resummed small-$x$ contributions to the anomalous dimensions and coefficient functions on the evolution of unpolarized structure functions in deep-inelastic scattering. The proton structure functions $F_2^p$ and $F_L^p$ as well as the photon structure function $F_2^γ$ are considered together with the corresponding parton densities. The general analytic solution of the evolution equations in Mellin-$N$ space is derived, and different approximate solutions are compared. Potential effects of less singular small-$x$ terms in the anomalous dimension and coefficient functions are discussed.

hep-ph↗
arXiv1997

Small-x Resummation for the Structure Functions $F_2^p$, $F_L^p$ and $F_2^γ$

The numerical effects of the known all-order leading and next-to-leading logarithmic small-$x$ contributions to the anomalous dimensions and coefficient functions of the unpolarized singlet evolution are discussed for the structure functions $F_2^{ ep}(x,Q^2)$, $F_L^{ ep}(x,Q^2)$, and $F_2^{ eγ}(x,Q^2)$.

hep-ph↗
arXiv1996

The Small-x Evolution of Unpolarized and Polarized Structure Functions

A brief overview is presented of recent developments concerning resummed small-x evolution, based upon the renormalization group equation. The non-singlet and singlet structure functions are discussed for both polarized and unpolarized deep-inelastic scattering. Quantitative results are displayed and uncertainties from uncalculated subleading terms are discussed.

hep-ph↗
arXiv1996

The Extraction of the Gluon Density from Jet Production in Deeply Inelastic Scattering

The prospects of a direct extraction of the proton's gluon density in next-to-leading order via jet rates in deeply inelastic scattering are studied. The employed method is based on the Mellin transform, and can be applied, in principle, to all infra-red-safe observables of hadronic final states. We investigate the dependence of the error band on the extracted gluon distribution on the statistical and systematic error of the data.

hep-ph↗
arXiv1996

Structure Functions in Deep Inelastic Scattering at HERA

An introduction and summary is given of the main results achieved by working group 1: Structure Functions in Deep Inelastic Scattering at HERA. The prospects were discussed of future measurements of the structure functions $F_{2}, F_{L}, xG_{3}, F_{2}^{Q\overline{Q}}$ and $F_π$ at HERA. The results represent a long term programme of experimentation with high luminosity, different lepton beam charges, proton and deuteron beams, allowing for precision measurements. The theoretical investigations focussed on QED corrections, higher order QCD corrections for different observables, resummation of small $x$ contributions, and the detailed understanding of NLO QCD evolution codes, to allow for a high precision analysis of the forthcoming deep inelastic data.

hep-ph↗