ZEUS NLOQCD fits

NLO QCD fits using the DGLAP formalism have been made to the high precision ZEUS ep reduced cross-section data and to fixed target structure function data, in order to determine parton distribution functions and the value of αs(M 2 Z), taking full account of correlated systematic errors. The new high precision ZEUS data on the neutral current e+p reduced cross section [1] are fit to the predictions of NLO QCD using the DGLAP equations, in order to determine parton distribution functions and the value of αs(M 2 Z). The increased range (6.3 × 10−5 < x < 0.65, 2.7 < Q2 < 30000GeV 2) and precision (systematic errors ∼ 3%, statistical errors < ∼ 1%, for 2 < Q2 < 800GeV 2) of the ZEUS data allow a much improved determination of the gluon and sea distributions compared to our previous work [2]. In recent years more emphasis has been placed on estimating errors on extracted parton distributions. We present parton distributions including full accounting for uncertainties from experimental correlated systematic errors. We are also able to measure the value of αs(M 2 Z) taking full account of the correlations between the shape of the parton distribution functions and αS . Full details of the analysis are given in [3]. This analysis is performed within the conventional paradigm of leading twist, NLO QCD, with the renormalisation and factorization scales chosen to be Q2. The heavy quark production scheme is the general mass variable flavour number scheme of Roberts and Thorne [4] The kinematics of lepton hadron scattering is described in terms of the variables Q2, the invariant mass of the exchanged vector boson, Bjorken x, the fraction of the momentum of the incoming nucleon taken by the struck quark (in the quark-parton model), and y which measures the energy transfer between the lepton and hadron systems. The differential cross-section for the process is given in terms of the structure functions by d2σ dxdQ2 = 2πα2 Q4x [ Y+ F2(x,Q 2)− y FL(x,Q)− Y− xF3(x,Q) ] , where Y± = 1 ± (1 − y). The structure functions F2 and xF3 are directly related to non-singlet and singlet quark distributions, and their Q2 dependence, or scaling violation, is predicted by pQCD. At Q2 < ∼ 1000GeV2 F2 dominates the charged lepton-hadron cross-section and for x < ∼ 10−2 the gluon contribution dominates the Q2 evolution of F2, such that ZEUS data provide crucial information on quark and gluon distributions. (Schematically, F2 ∼ xq, dF2/dlnQ ∼ αsPqgxg). ∗Speaker. P r H E P h e p 2 0 0 1 International Europhysics Conference on HEP Amanda Cooper-Sarkar We have performed a global fit of ZEUS and fixed target DIS data. The fixed target data sets used are those with precision data for which full information on the correlated systematic errors is available (NMC, E665, BCDMS muon induced F2 data on proton and deuterium targets and CCFR ν, ν̄ xF3 data on an Fe target [5]). These data are used to gain information on the valence quark distributions and the flavour composition of the sea, and to constrain the fits at high x. However, our focus is on the additional information to be gained from the new precision ZEUS data, particularly on the gluon and quark densities at low x and on the value of αs(M 2 Z). In the standard fit the following cuts are made on the ZEUS and the fixed target data: (i) W 2 > 20 GeV2 to reduce the sensitivity to target mass and higher twist contributions which become important at high x and low Q2; (ii) Q2 > 2.5 GeV2 to remain in the kinematic region where perturbative QCD should be applicable. The QCD predictions for the structure functions needed to construct the reduced cross-section are obtained by solving the DGLAP evolution equations at NLO in the MS scheme. These equations yield the quark and gluon momentum distributions (and thus the structure functions) at all values of Q2 provided they are input as functions of x at some input scale Q0. The parton distribution functions (PDFs) for u valence, d valence, total sea, gluon and the difference between the d and u contributions to the sea, are each parametrized by the form p1x p2(1− x)3(1 + p5x) at Q0 = 7GeV 2. Thus the flavour structure of the light quark sea allows for the violation of the Gottfried sum rule. We also impose a suppression of the strange sea of a factor of 2 at Q0, consistent with neutrino induced dimuon data from CCFR. The parameters p1− p5 are constrained to impose the momentum sum-rule and the number sum-rules on the valence distributions. The gluon distribution has p5 = 0, because non zero values have minimal effect on the χ2 of the fit, and because this choice constrains the high x gluon to be positive without the need for penalty χ2 terms. There are 11 free parameters in the standard fit when the strong coupling constant is fixed to αs(M 2 Z) = 0.118 [6], and 12 free parameters when αs(M 2 Z) is determined by the fit. Full account has been taken of correlated experimental systematic errors as follows. The definition of the χ2 is χ = ∑ i [Fi(p, s)− Fi(meas))] (σ2 stat + σ 2 unc) + ∑