Physical Gluons and high - E T Jets

Abstract

We propose a more physical parameterization of the gluon distribution for global parton analyses of deep inelastic and related hard scattering data. In the new parameterization the gluon distribution at large x in the MS-scheme is driven by the valence quarks, which naturally produces a shoulder-like form at high x, and hence produces a better description of the Tevatron inclusive jet data. We perform the new analysis at both NLO and NNLO. The improvement is found to be even better at NNLO than at NLO. We make available the new sets of NLO and NNLO partons, which we denote by MRST2004. A detailed knowledge of the partonic structure of the proton is an essential ingredient in the analysis of hard scattering data from pp or pp̄ or ep high energy collisions. The parton distributions are determined by a global analysis of a wide range of deep inelastic and related hard scattering data. The Bjorken x dependence of the distributions is parameterized at some low scale, and a fixed order (either LO or NLO or NNLO) DGLAP evolution performed to specify the distributions at the higher scales where data exist. A global fit to the data then determines the parameters of the input distributions, see, for example, Refs. [1, 2]. The uncertainties in the resulting distributions have been the subject of much detailed study; see, for example, Refs. [3, 4, 5]. The gluon distribution at high x, x ∼ 0.3 is particularly ill-determined. Indeed, in the past, this ambiguity has been exploited to describe ‘anomalous’ behaviour of the inclusive jet distribution observed at high ET at the Tevatron. Royal Society University Research Fellow. Figure 1: The x behaviour of CTEQ6.1M parton distributions, xf(x,Q), at Q = 5 GeV. It is informative to illustrate the present situation for high x gluons and the Tevatron jet data in both the CTEQ and MRST global analyses. First, we note that the simple spectator counting rules [6] predict the following behaviour at high x qval ∼ (1− x) , g(x) ∼ (1− x), (1) for valence quarks and the gluon respectively. ¿From Fig. 1 we see that this behaviour is not true for CTEQ6.1M (NLO) partons [2]. The gluon is harder than both the up and the down quark distributions as x → 1, which results in a good fit to the Tevatron jet data. On the other hand, the MRST parameterizations do not naturally allow such a hard gluon and, as a consequence the description of the jet data is not quite so good, the χ being about 30 units higher. In fact we have noticed that the problem is worse in the NNLO fit, than in the NLO analysis. The NNLO coefficient functions are positive for F2 at the largest x, leading to smaller quarks and a larger gluon is consequently needed for a good fit. Sometime ago Klasen and Kramer [10] noticed that the description of the jet data was better in the DIS factorization scheme than in the MS scheme. This is for reasons which we will discuss in a moment. Note that the latter scheme is the default adopted in the global analyses. Of Such plots can be readily obtained from http://durpdg.dur.ac.uk/hepdata/pdf3.html

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Cite this paper

@inproceedings{Martin2004PhysicalGA, title={Physical Gluons and high - E T Jets}, author={Angela D . Martin and Rodney G. Roberts and W . James Stirling and Robert S. Thorne}, year={2004} }