Measurement and QCD Analysis of Neutral and Charged Current Cross Sections at HERA
The NC and CC cross section data in e+p and e-p scattering are complementary in probing different types of quark inside the proton. In this paper, the H1 experiment has been used to explore this complementarity. The huge kinematic coverage of the high Q2 data, together with equally precise recent low Q2 data have allowed a determination of the behavior of quark and gluon parton distribution functions (PDF), using H1 data alone. Five components of proton structure are extracted simultaneously. The first is the up-type quark density, U, corresponding to up quarks together with their heavier cousins, the charm quarks. Secondly, there is the down-type quark density, D, corresponding to the sum of down and strange quarks. The anti-up-type (Ubar) and anti-down-type (Dbar) quarks make up the third and fourth components. The fifth type is the gluons that bind the proton together. The resulting PDFs are shown in the figure. They are compared with those obtained by the MRST and CTEQ groups, who perform global fits to deep-inelastic scattering data together with various other results. The comparison is remarkably good, given the many differences in terms of the data sets used, the theoretical input and the assumptions made.
The NC photon exchange process measures the sum of all quark densities weighted by their squared charges. On the other hand, Z exchange and its quantum-mechanical interference with the photon measures the difference between quarks and anti-quarks. Furthermore, the dominant components of the Z exchange and interfernce contributions act with opposite sign for positron and electron scattering and can therefore be separated from photon exchange by comparing data from the two beam charges. These properties were exploited in an earlier H1 publication to make a first measurement of the xF3 structure function, which basically measures the valence quarks, carrying the quantum numbers of the proton. Using the combined e+p 94-00 NC cross section, an improved measurment is obtained in this paper, superseding the earlier measurement.
The NC analysis analysis is extended to lower energies of the scattered
electron than has previously been possible. This has allowed a determination
of the longitudinal structure function, FL, for the first time in
this large momentum transfer range. The results are fully consistent
with the prediction for FL obtained from the PDFs extracted in
the fit to the full data.
Last Update 04.04.2003