Heavy Flavour Production
We review the status of heavy flavour production in QCD. Comparison of experimental and theoretical results for top and bottom production are given. Selected topics in charm production are also discussed.
hep-ph↗
arXiv subjects
P. Nason
Publications and source records attributed to P. Nason.
Infrared Renormalons and Power Suppressed Effects in $e^+e^-$ Jet Events
We study the effect of infrared renormalons upon shape variables that are commonly used to determine the strong coupling constant in $e^+e^-$ annihilation into hadronic jets. We consider the model of QCD in the limit of large $n_f$. We find a wide variety of different behaviours of shape variables with respect to power suppressed effects induced by infrared renormalons. In particular, we find that oblateness is affected by $1/Q$ non--perturbative effects even away from the two jet region, and the energy--energy correlation is affected by $1/Q$ non--perturbative effects for all values of the angle. On the contrary, variables like thrust, the $c$ parameter, the heavy jet mass, and others, do not develop any $1/Q$ correction away from the two jet region at the leading $n_f$ level. We argue that $1/Q$ corrections will eventually arise at subleading $n_f$ level, but that they could maintain an extra $\as(Q)$ suppression. We conjecture therefore that the leading power correction to shape variables will have in general the form $α^n_{\rm S}(Q)/Q$, and it may therefore be possible to classify shape variables according to the value of $n$.
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Differential Distributions for Heavy Flavour Production at HERA
We compute pseudorapidity and transverse momentum distributions for charm and bottom production at HERA. We examine the effect of next-to-leading order QCD corrections, the effect of possible intrinsic transverse momenta of the incoming partons, and of fragmentation. We compare our results with full Monte Carlo simulation using HERWIG. The importance of the hadronic component of the photon is also studied. We examine the possibility to distinguish among different parametrizations of the photon parton densities using charm production data, and the possibilty to extract information about the small-$x$ behaviour of the gluon density of the proton. We also give a prediction for the transverse momentum and pseudorapidity distributions for bottom production at HERA.
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Top Quark Distributions in Hadronic Collisions
We present kinematical distributions for top quark pairs produced at the Tevatron \ppbar\ Collider, as predicted within Next-to-Leading-Order QCD. We consider single and double-inclusive distributions, and compare our results to those obtained with the shower Monte Carlo HERWIG. We discuss the implications of our findings for experimental issues such as the measurement of the top quark mass.
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A Study of Ultraviolet Renormalon Ambiguities in the Determination of $\as$ from $τ$ Decay
The divergent large-order behaviour of the perturbative series relevant for the determination of $\as$ from $τ$ decay is controlled by the leading ultraviolet (UV) renormalon. Even in the absence of the first infrared (IR) renormalon, an ambiguity of order $Λ^2/m_τ^2$ is introduced. We make a quantitative study of the practical implications of this ambiguity. We discuss the magnitude of UV renormalon corrections obtained in the large-$N_f$ limit, which, although unrealistic, is nevertheless interesting to some extent. We then study a number of improved approximants for the perturbative series, based on a change of variable in the Borel representation, such as to displace the leading UV renormalon singularity at a larger distance from the origin than the first IR renormalon. The spread of the resulting values of $\as(m^2_τ)$ obtained by different approximants, at different renormalization scales, is exhibited as a measure of the underlying ambiguities. Finally, on the basis of mathematical models, we discuss the prospects of an actual improvement, given the signs and magnitudes of the computed coefficients, the size of $\as(m^2_τ)$ and what is known of the asymptotic properties of the series. Our conclusion is that a realistic estimate of the theoretical error cannot go below $δ\as(m^2_τ) \sim \pm 0.060$, or $δ\as(m^2_{\sss Z}) \sim \pm 0.006$.
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Total Cross Sections for Heavy Flavour Production at HERA
We compute total cross sections for charm and bottom photoproduction at HERA energies, and discuss the relevant theoretical uncertainties. In particular we discuss the problems arising from the small-$x$ region, the uncertainties in the gluon parton density, and the uncertainties in the hadronic component of the cross section. Total electroproduction cross sections, calculated in the Weizsäcker-Williams approximation, are also given.
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Fragmentation Function Method for Charge Asymmetry Measurements in $\e^+e^-$ Collisions
We propose a method for measuring the hadron charge asymmetry in $\ee$ collisions which is based upon the fragmentation function formalism, and is largely independent of modelling of fragmentation effects. Furthermore, in this method, QCD radiative corrections can be accounted for in a systematic way.
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On the Q^2 Dependence of Measured Polarized Structure Funtions
We analyse the available data on the polarized asymmetries $A_1$ for proton, neutron and deuteron targets. We use a homogeneous and updated set of unpolarized structure functions to derive $g_1$ from $A_1$, and we accurately correct for the scaling violations in order to obtain $g_1(x,Q^2)$ with the same $Q^2$ for all $x$ values. The contribution to the $Q^2$ evolution of a possible large gluon polarized density is also considered. The implications for the Ellis-Jaffe and for the Bjorken sum rules are discussed.
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Improving the Weizsäcker-Williams Approximation in Electron-Proton Collisions
We critically examine the validity of the Weizsäcker-Williams approximation in electron-hadron collisions. We show that in its commonly used form it can lead to large errors, and we show how to improve it in order to get accurate results. In particular, we present an improved form that is valid beyond the leading logarithmic approximation in the case when a small-angle cut is applied to the scattered electron. Furthermore we include comparisons of the approximate expressions with the exact electroproduction calculation in the case of heavy-quark production.
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Heavy Quark Correlations in Photon-Hadron Collisions
We describe a next-to-leading-order calculation of the fully exclusive parton cross section for the photoproduction of heavy quarks. We use our result to compute quantities of interest for current fixed-target experiments. We discuss heavy-quark total cross sections, distributions and correlations.
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Small-Size Instanton Corrections to the $τ$ Hadronic Width
We compute the effect of small-size instanton corrections to current-current correlators, for all combinations of axial, vector, and possibly flavour non-diagonal currents. We apply our result to the hadronic decays of the $τ$ lepton, in order to assess the reliability of the determination of $\as$ from the $τ$ hadronic width.
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