Measurement and QCD Analysis of Neutral and Charged Current Cross Sections at HERA

The H1 experiment at HERA was designed to measure the details of proton sub-structure at the very highest energies using positrons (e+) and electrons (e-), as probes. During almost one decade of data taking of the first phase of HERA operation (HERA-I) from 1992 to 2000, several important data samples were collected by the H1 experiment at centre-of-mass energies of 319 GeV and 301 GeV resulting from collisions of positrons/electrons of 27.6 GeV with protons of 920 GeV and 820 GeV. Two reactions have been measured in this paper: The Neutral Current (NC) process where a positron scatters off a quark via the exchange of the electrically neutral photon or Z boson and the Charged Current (CC) reaction where the positron scatters via the exchange of a charged W boson and converts into an unseen neutrino. This paper constitutes the highest statistics measurement of positron induced cross sections at high photon virtuality (Q2) using data collected during 1999 and 2000. The measurement spans the range in Q2 from 100 GeV2 up to 30000 GeV2. The cross sections are compared with an earlier measurement based on the data taken from 1994 to 1997 at 301 GeV by H1 using positrons in a similar kinematic range. These results, together with those based on the e-p data , complete the H1 measurements of the inclusive cross sections at high Q2 from HERA-I.

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