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W. L. van Neerven

Publications and source records attributed to W. L. van Neerven.

At least 73 records · Page 4Linked to original sources
arXiv1997

Comparison between the various descriptions for charm electroproduction and the HERA-data

We examine the charm component F_{2,c}(x,Q^2,m^2) of the proton structure function F_2(x,Q^2) in three different schemes and compare the results with the data in the x and Q^2 region explored by the HERA experiments. Studied are (1) the three flavour number scheme (TFNS) where the production mechanisms are given by the photon-gluon fusion process and the higher order reactions with three light-flavour parton densities as input (2) the four flavour number scheme (FFNS) where F_{2,c} is expressed in four light flavour densities including one for the charm quark and (3) a variable-flavour number scheme (VFNS) which interpolates between the latter two. Both the VFNS and the TFNS give good descriptions of the experimental data. However one cannot use the FFNS for the description of the data at small Q^2.

hep-ph↗
arXiv1997

Order α_s^2 Contributions to charm production in charged-current deep-inelastic lepton-hadron scattering

The most important part of the order $α_s^2$ corrections to the charm component of the charged-current structure functions $F_2(x,Q^2)$ and $F_3(x,Q^2)$ have been calculated. This calculation is based on the asymptotic form of the heavy-quark coefficient functions corresponding to the higher order corrections to the W-boson-gluon fusion process. These coefficient functions which are in principle only valid for $Q^2 \gg m^2$ can be also used to estimate the order $α_s^2$ contributions at lower $Q^2$ values provided $x < 0.1$. It turns out that the above corrections are appreciable in the large $Q^2$-region and they explain the discrepancy found for the structure functions between the fixed-flavour scheme (FFS) and the variable-flavour-number scheme (VFNS). These corrections also hamper the extraction of the strange-quark density from the data obtained for the charged-current and the electromagnetic-current processes.

hep-ph↗
arXiv1996

Charm electroproduction viewed in the variable-flavour number scheme versus fixed-order perturbation theory

Starting from fixed-order perturbation theory (FOPT) we derive expressions for the heavy-flavour components of the deep-inelastic structure functions FL and F2 in the variable-flavour number scheme (VFNS). These expressions are valid in all orders of perturbation theory. This derivation establishes a relation between the parton densities parametrized at N and N light flavours. The consequences for the existing parametrizations of the parton densities are discussed. Further we show that in charm electroproduction the exact and asymptotic expressions for the heavy-quark coefficient functions yield identical results for F2 when Q^2>20 (GeV/c)^2. We also study the differences between the FOPT and the VFNS descriptions for F2. It turns out that the charm structure function in the VFNS is larger than the one obtained in FOPT over the whole Q^2-range. Furthermore inspection of the perturbation series reveals that the higher order corrections in the VFNS are smaller than those present in FOPT for Q^2>10 (GeV/c)^2. Therefore the VFNS gives a better prediction for the charm structure function at large Q^2-values than FOPT.

hep-ph↗
arXiv1996

A Detailed Comparison of NLO QCD Evolution Codes

Seven next-to-leading order QCD evolution programs are compared. The deviations of the results due to different theoretical prescriptions for truncating the perturbative series are clarified, and a numerical agreement between five codes of better than 0.1% is achieved. Reference results for further comparison are provided.

hep-ph↗
arXiv1996

$O(α_s^2)$ Contributions to the asymmetric fragmentation function in $e^+e^-$ annihilation

The order α_s^2 contributions to the coefficient functions corresponding to the asymmetric fragmentation function $F_A(x,Q^2)$ in $e^+e^-$ annihilation are calculated. From this calculation we infer that the order $(α_s/4π)^2$ correction to the flavour asymmetry sum rule is non vanishing and amounts to $-12β_0C_Fζ(3)$. We also study the effect of the higher order QCD corrections on $F_A(x,Q^2)$ and compare them with the OPAL data. The latter put a strong constraint on the valence part of the fragmentation densities $D_q^H(x,μ^2)$.

hep-ph↗
arXiv1996

Higher order QCD corrections to the transverse and longitudinal fragmentation functions in electron-positron annihilation

We present the calculation of the order $α_s^2$ corrections to the coefficient functions contributing to the longitudinal ($F_L(x,Q^2)$) and transverse fragmentation functions ($F_T(x,Q^2)$) measured in electron-positron annihilation. The effect of these higher order QCD corrections on the behaviour of the fragmentation functions and the corresponding longitudinal ($dσ_L(x,Q^2)/dx$) and transverse cross sections ($dσ_T(x,Q^2)/dx$) are studied. In particular we investigate the dependence of the above quantities on the mass factorization scale ($M$) and the various parameterizations chosen for the parton fragmentation densities $D_p^H(x,M^2)$ ($p=q,g$; $H=π^\pm, K^\pm, P, \bar{P}$). Our analysis reveals that the order α_s^2 contributions to $F_L(x,Q^2)$ are large whereas these contributions to $F_T(x,Q^2)$ are small. From the above fragmentation functions one can also compute the integrated cross sections $σ_L$ and $σ_T$ in an independent way. The sum $σ_{tot} = σ_L + σ_T$, corrected up to order \alphastwo, agrees with the well known result in the literature providing us with an independent check an our calculations.

hep-ph↗
arXiv1996

Deep-inelastic production of heavy quarks

Deep-inelastic production of heavy quarks at HERA, especially charm, is an excellent signal to measure the gluon distribution in the proton at small $x$ values. By measuring various differential distributions of the heavy quarks this reaction permits additional more incisive QCD analyses due to the many scales present. Furthermore, the relatively small mass of the charm quark, compared to the typical momentum transfer $Q$, allows one to study whether and when to treat this quark as a parton. This reaction therefore sheds light on some of the most fundamental aspects of perturbative QCD. We discuss the above issues and review the feasibility of their experimental investigation in the light of a large integrated luminosity.

hep-ph↗
arXiv1996

Theoretical Uncertainties in the QCD Evolution of Structure Functions and their Impact on $α_s(M_Z^2)$

The differences are discussed between various next-to-leading order prescriptions for the QCD evolution of parton densities and structure functions. Their quantitative impact is understood to an accuracy of 0.02\%. The uncertainties due to the freedom to choose the renormalization and factorization scales are studied. The quantitative consequences of the different uncertainties on the extraction of the strong coupling constant $α_s$ from scaling violations in deep--inelastic scattering are estimated for the kinematic regime accessible at HERA.

hep-ph↗
arXiv1996

$O(α_s^2)$ corrections to polarized heavy flavour production at $Q^2\gg m^2$

We present analytic formulae for the heavy flavour coefficient functions for polarized deep inelastic lepton-hadron scattering. The expressions are valid in the kinematical regime $Q^2\gg m^2$ where $Q^2$ and $m^2$ stand for the masses squared of the virtual photon and heavy quark respectively. Using these coefficient functions we have computed the next-to-leading order $α_s$ corrections to polarized charm production at HERA collider energies, where both the electron and proton beams are polarized. We also give an estimate of these corrections at fixed target experiments where the typical $Q^2$ values are much smaller than at HERA.

hep-ph↗
arXiv1996

Determination of the asymptotic behaviour of the heavy flavour coefficient functions in deep inelastic scattering

Using renormalization group techniques we have derived analytic formulae for the next-to-leading order heavy-quark coefficient functions in deep inelastic lepton hadron scattering. These formulae are only valid in the kinematic regime Q^2 >> m^2, where Q^2 and m^2 stand for the masses squared of the virtual photon and heavy quark respectively. Some of the applications of these asymptotic formulae will be discussed.

hep-ph↗
arXiv1996

$O(α_s^2)$ Contributions to the longitudinal fragmentation function in $e^+\,e^-$ annihilation

We present the order $α_s^2$ contributions to the coefficient functions corresponding to the longitudinal fragmentation function $F_L(x,Q^2)$. A comparison with the leading order $α_s$ result for $F_L(x,Q^2)$ shows that the corrections are large and vary from 44\% to 67\% in the region $0.01 < x < 0.9$ at $Q^2=M_Z^2$. Our calculations also reveal that the ratio of the longitudinal and total cross section $σ_L/σ_{\rm tot}$ amounts to 0.054. This number is very close to the most recent value obtained by the OPAL collaboration which obtained $0.057\pm 0.005$.

hep-ph↗
arXiv1996

Heavy quark coefficient functions at asymptotic values $Q^2 \gg m^2$

In this paper we present the analytic form of the heavy-quark coefficient functions for deep-inelastic lepton-hadron scattering in the kinematical regime $Q^2 \gg m^2$ . Here $Q^2$ and $m^2$ stand for the masses squared of the virtual photon and heavy quark respectively. The calculations have been performed up to next-to-leading order in the strong coupling constant $α_s$ using operator product expansion techniques. Apart from a check on earlier calculations, which however are only accessible via large computer programs, the asymptotic forms of the coefficient functions are useful for charm production at HERA when the condition $Q^2 \gg m_c^2$ is satisfied. Furthermore the analytical expressions can also be used when one applies the variable heavy flavour scheme up to next-to-leading order in $α_s$.

hep-ph↗
arXiv1995

Heavy flavor contributions to the Drell-Yan cross section

We investigate the effect of heavy flavor contributions to vector boson ($V$ = $γ$, $Z$, $W$) production which is described by the Drell-Yan mechanism. All reactions with bottom and top quarks ($Q_i$ = $b$, $t$) in the final state, like $q_1 + \bar{q}_2\rightarrow V + Q_1 + \bar{Q}_2$ and $g + g\rightarrow V + Q_1 + \bar{Q}_2$, are considered. This study also includes the virtual contributions containing heavy flavor loops which were not taken into account earlier in the literature. Our analysis reveals that the above corrections to the Drell-Yan cross section are very small. Only at energies characteristic for the LHC they are of the same order of magnitude as the order \alphastwo~QCD contributions due to light quark and gluon subprocesses calculated earlier in the literature.

hep-ph↗
arXiv1994

Rates for Inclusive Deep-Inelastic Electroproduction of Charm Quarks at HERA

The coefficient functions for heavy-flavour production in deeply inelastic electron-hadron scattering have been calculated previously. Analytic expressions are impossible to publish due to their length. Therefore we have tabulated them as two-dimensional arrays as is often done for the scale-dependent parton densities. Using this computer program we present event rates for charm production at HERA in bins of $x$ and $Q^2$. These rates are insensitive to variations in the factorization and renormalization scale $μ$.

hep-ph↗
arXiv1994

Order $α_s^{2}$ contributions to the Drell-Yan cross section at fixed target energies

{}From the literature one infers that the bulk of the order $α_{s}$ corrections to the Drell-Yan cross sections $dσ/dm$ and $m^{3}d^{2}σ/dmdx_{F}$ is constituted by the soft plus virtual gluon part of the coefficient function. In the case of $dσ/dm$ it can be shown that at fixed target energies the effect of the exact order $α_{s}^{2}$ corrected coefficient function is very well approximated by its soft plus virtual gluon part. Since the complete order $α_{s}^{2}$ contribution to the coefficient function is missing we have to assume that the same approximation also holds for $m^{3}d^{2}σ/dmdx_{F}$. It appears that the discrepancy between the exact order $α_{s}$ corrected cross section and the massive lepton pair data taken at fixed target experiments can be partially explained by including the order $α_{s}^{2}$ soft plus virtual gluon part of the coefficient function.

hep-ph↗
arXiv1993

Top Quark Production Cross Section

The production rate for top quarks at the Fermilab Tevatron is presented using the exact order $α_s^3$ corrected cross section and the resummation of the leading soft gluon corrections in all orders of perturbation theory.

hep-ph↗