arXiv subjects

A. Vogt

Publications and source records attributed to A. Vogt.

At least 91 records · Page 5Linked to original sources

arXiv2005

Parton Distributions: Summary Report

We provide an assessment of the impact of parton distributions on the determination of LHC processes, and of the accuracy with which parton distributions (PDFs) can be extracted from data, in particular from current and forthcoming HERA experiments. We give an overview of reference LHC processes and their associated PDF uncertainties, and study in detail W and Z production at the LHC. We discuss the precision which may be obtained from the analysis of existing HERA data, tests of consistency of HERA data from different experiments, and the combination of these data. We determine further improvements on PDFs which may be obtained from future HERA data (including measurements of $F_L$), and from combining present and future HERA data with present and future hadron collider data. We review the current status of knowledge of higher (NNLO) QCD corrections to perturbative evolution and deep-inelastic scattering, and provide reference results for their impact on parton evolution, and we briefly examine non-perturbative models for parton distributions. We discuss the state-of-the art in global parton fits, we assess the impact on them of various kinds of data and of theoretical corrections, by providing benchmarks of Alekhin and MRST parton distributions and a CTEQ analysis of parton fit stability, and we briefly presents proposals for alternative approaches to parton fitting. We summarize the status of large and small x resummation, by providing estimates of the impact of large x resummation on parton fits, and a comparison of different approaches to small x resummation, for which we also discuss numerical techniques.

hep-ph↗

arXiv2005

Sudakov Resummations at Higher Orders

We summarize our recent results on the resummation of hard-scattering coefficient functions and on-shell form factors in massless perturbative QCD. The threshold resummation has been extended to the fourth logarithmic order for deep-inelastic scattering, Drell-Yan lepton pair production and Higgs production via gluon-gluon fusion. The leading six infrared pole terms have been derived to all orders in the strong coupling constant for the photon-quark-quark and the (heavy-top) Higgs-gluon-gluon form factors. These results have many implications, most notably they lead to a new best estimate for the Higgs production cross section at the LHC.

hep-ph↗

arXiv2005

Three-Loop Results for Quark and Gluon Form Factors

We study the photon-quark-quark and Higgs-gluon-gluon form factors for on-shell massless quarks and gluons in perturbative QCD. Previous third-order results for the quark case are extended by calculating the fermion-loop contributions up to the finite terms in dimensional regularization. For the gluon case the complete set of infrared poles at three loops is derived. Using the exponentiation of the form factor, the latter results are employed to extract a function entering the infrared factorization of general third-order amplitudes. We evaluate the infrared finite absolute ratio of the time-like and space-like gluon form factors up to the fourth order in the strong coupling constant. The result supports previous indications that the perturbative expansion of the Higgs boson production rate at the LHC is under control.

hep-ph↗

arXiv2005

The Quark Form Factor at Higher Orders

We study the electromagnetic on-shell form factor of quarks in massless perturbative QCD. We derive the complete pole part in dimensional regularization at three loops, and extend the resummation of the form factor to the next-to-next-to-leading contributions. These results are employed to evaluate the infrared finite absolute ratio of the time-like and space-like form factors up to the fourth order in the strong coupling constant. Besides for the pole structure of higher-loop QCD amplitudes, our new contributions to the form factor are also relevant for the high-energy limit of massive gauge theories like QED. The highest-transcendentality component of our results confirms a result recently obtained in N=4 Super-Yang-Mills theory.

hep-ph↗

arXiv2005

The third-order QCD corrections to deep-inelastic scattering by photon exchange

We compute the full three-loop coefficient functions for the structure functions F_2 and F_L in massless perturbative QCD. The results for F_L complete the next-to-next-to-leading order description of unpolarized electromagnetic deep-inelastic scattering. The third-order coefficient functions for F_2 form, at not too small values of the Bjorken variable x, the dominant part of the next-to-next-to-next-to-leading order corrections, thus facilitating improved determinations of the strong coupling alpha_s from scaling violations. The three-loop corrections to F_L are larger than those for F_2. Especially for the latter quantity the expansion in powers of alpha_s is very stable, for photon virtualities Q^2 >> 1 GeV^2, over the full x-range accessible to fixed-target and collider measurements.

hep-ph↗

arXiv2004

The Longitudinal Structure Function at the Third Order

We compute the complete third-order contributions to the coefficient functions for the longitudinal structure function F_L, thus completing the next-to-next-to-leading order (NNLO) description of unpolarized electromagnetic deep-inelastic scattering in massless perturbative QCD. Our exact results agree with determinations of low even-integer Mellin moments and of the leading small-x terms in the flavour-singlet sector. In this letter we present compact and accurate parametrizations of the results and illustrate the numerical impact of the NNLO corrections.

hep-ph↗

arXiv2004

Efficient evolution of unpolarized and polarized parton distributions with QCD-PEGASUS

The Fortran package QCD-PEGASUS is presented. This program provides fast, flexible and accurate solutions of the evolution equations for unpolarized and polarized parton distributions of hadrons in perturbative QCD. The evolution is performed using the symbolic moment-space solutions on a one-fits-all Mellin inversion contour. User options include the order of the evolution including the next-to-next-to-leading order in the unpolarized case, the type of the evolution including an emulation of brute-force solutions, the evolution with a fixed number n_f of flavours or in the variable-n_f scheme, and the evolution with a renormalization scale unequal to the factorization scale. The initial distributions are needed in a form facilitating the computation of the complex Mellin moments.

hep-ph↗

arXiv2004

The QCD Splitting Functions at Three Loops: Methods and Results

We have computed the complete next-to-next-to-leading order (NNLO) contributions to the splitting functions governing the evolution of unpolarized parton densities in perturbative QCD. Our results agree with all partial results available in the literature. We illustrate the methods used for this calculation with some examples and display selected results to show the size of the NNLO corrections and their effect on the evolution.

hep-ph↗

arXiv2004

The Three-Loop Splitting Functions in QCD

We have computed the next-to-next-to-leading-order (NNLO) contributions to the evolution of unpolarized parton distributions in perturbative QCD. In this talk, we briefly recall why this huge computation was necessary and outline how it was performed. We then illustrate the structure of the results and discuss their end-point limits which include the three-loop cusp anomalous dimensions of the Wilson lines. Finally the numerical impact of the new contributions is illustrated.

hep-ph↗

arXiv2004

The Three-Loop Splitting Functions in QCD: The Singlet Case

We compute the next-to-next-to-leading order (NNLO) contributions to the splitting functions governing the evolution of the unpolarized flavour-singlet parton densities in perturbative QCD. The exact expressions are presented in both Mellin-N and Bjorken-x space. We also provide accurate parametrizations for practical applications. Our results agree with all partial results available in the literature. As in the non-singlet case, the correct leading logarithmic predictions for small momentum fractions x do not provide good estimates of the respective complete splitting functions. We investigate the size of the corrections and the stability of the NNLO evolution under variation of the renormalization scale. The perturbative expansion appears to converge rapidly at x >~ 10^-3. Relatively large third-order corrections are found at smaller values of x.

hep-ph↗

arXiv2004

The Three-Loop Splitting Functions in QCD: The Non-Singlet Case

We compute the next-to-next-to-leading order (NNLO) contributions to the three splitting functions governing the evolution of unpolarized non-singlet combinations of quark densities in perturbative QCD. Our results agree with all partial results available in the literature. We find that the correct leading logarithmic (LL) predictions for small momentum fractions x do not provide a good estimate of the respective complete results. A new, unpredicted LL contribution is found for the colour factor d^{abc}d_{abc} entering at three loops for the first time. We investigate the size of the corrections and the stability of the NNLO evolution under variation of the renormalization scale. Except for very small x the corrections are found to be rather small even for large values of the strong coupling constant, in principle facilitating a perturbative evolution into the sub-GeV regime.

hep-ph↗

arXiv2003

Three-loop results and soft-gluon effects in DIS

We have calculated the fermionic contributions to the flavour non-singlet structure functions in deep-inelastic scattering (DIS) at third order of massless perturbative QCD. We discuss their implications for the threshold resummation at the next-to-next-to-leading logarithmic accuracy.

hep-ph↗

arXiv2002

First results for three-loop deep-inelastic structure functions in QCD

As a first step towards the complete calculation of deep-inelastic scattering at third order of massless perturbative QCD, we have computed the fermionic (nf) contributions to the flavour non-singlet structure functions in unpolarized electromagnetic scattering. We briefly discuss the approach chosen for this calculation, the problems we encountered and the status of the project. We show the results for the corresponding anomalous dimension in both Mellin-N and Bjorken-x. Together with the nf part of A_3, our calculation facilitates the complete determination of the threshold-resummation coefficients B_2 and D_2^DIS. The latter quantity vanishes in the MSbar scheme.

hep-ph↗

arXiv2002

Non-Singlet Structure Functions at Three Loops: Fermionic Contributions

We compute the fermionic (n_f) contributions to the flavour non-singlet structure functions in unpolarized electromagnetic deep-inelastic scattering at third order of massless perturbative QCD. Complete results are presented for the corresponding nf-parts of the three-loop anomalous dimension and the three-loop coefficient functions for the structure functions F_2 and F_L. Our results agree with all partial and approximate results available in the literature. The present calculation also facilitates a complete determination of the threshold-resummation parameters B_2 and D_2^DIS of which only the sum was known so far, thus completing the information required for the next-to-next-to-leading logarithmic resummation. We find that D_2^DIS vanishes in the MSbar scheme.

hep-ph↗

arXiv2002

Questions on Uncertainties in Parton Distributions

A discussion is presented of the manner in which uncertainties in parton distributions and related quantities are determined. One of the central problems is the criteria used to judge what variation of the parameters describing a set of partons is acceptable within the context of a global fit. Various ways of addressing this question are outlined.

hep-ph↗

arXiv2002

The QCD/SM Working Group: Summary Report

This Report documents the results obtained by the Working Group on Quantum ChromoDynamics and the Standard Model for the Workshop ``Physics at TeV Colliders'', Les Houches, France, 21 May - 1 June 2001. The account of uncertainties in Parton Distribution Functions is reviewed. Progresses in the description of multiparton final states at Next-to-Leading Order and the extension of calculations for precision QCD observables beyond this order are summarized. Various issues concerning the relevance of resummation for observables at TeV colliders is examined. Improvements to algorithms of jet reconstruction are discussed and predictions for diphoton and photon pi-zero production at the LHC are made for kinematic variables of interest regarding searches for a Higgs boson decaying into two photons. Finally, several improvements implemented in Monte-Carlo event generators are documented.

hep-ph↗

arXiv2001

Next-to-next-to-leading order QCD corrections to the photon's parton structure

The next-to-next-to-leading order (NNLO) corrections in massless perturbative QCD are derived for the parton distributions of the photon and the deep inelastic structure functions F_1^gamma and F_2^gamma. We present the full photonic coefficient functions at order alpha alpha_s and calculate the first six even-integer moments of the corresponding O(alpha alpha_s^2) photon-quark and photon-gluon splitting functions together with the moments of the alpha alpha_s^2 coefficient functions which enter only beyond NNLO. These results are employed to construct parametrizations of the splitting functions which prove to be sufficiently accurate at least for momentum fractions x >= 0.05. We also present explicit expressions for the transformation from the MS_bar to the DIS_gamma factorization scheme and write down the solution of the evolution equations. The numerical impact of the NNLO corrections is discussed in both schemes.

hep-ph↗

arXiv2001

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

We summarize recent results on the evolution of unpolarized parton densities and deep-inelastic structure functions in massless perturbative QCD. Due to last year's extension of the integer-moment calculations of the three-loop splitting functions, the NNLO evolution of the parton distributions can now be performed reliably at momentum fractions x >= 10^-4, facilitating a considerably improved theoretical accuracy of their extraction from data on deep-inelastic scattering. The NNLO corrections are not dominated, at relevant values of x, by their leading small-x terms. At large x the splitting-function series converges very rapidly, hence, employing results on the three-loop coefficient functions, the structure functions can be analysed at N^3LO for x > 10^-2. The resulting values for alpha_s do not significantly change beyond NNLO, their renormalization scale dependence reaches about +-1% at N^3LO.

hep-ph↗