Measurement of Neutral and Charged Current Cross Sections in Electron-Proton Collisions at High Q2

The H1 experiment at HERA was designed to measure the details of proton sub-structure at the very highest energies using electrons (e-) and positrons (e+), as probes. Several years of data taking have allowed these measurements to be made revealing how the proton components,  quarks and gluons behave. Current theory, the Standard Model, states that the interaction, or cross section, of electrons scattering off quarks, Deep Inelastic Scattering, (DIS) is different than for positrons. This difference is due to the existence of the weak force which becomes visible when the momentum transfers involved (Q2) are of similar size to the masses of the particles which propagate the force, namely the W and Z bosons. Two reactions have been measured in this paper: The Neutral Current (NC) process where an electron scatters off a quark via exchange of the electrically neutral photon or Z, and the Charged Current (CC) reaction whereby the electron scatters via exchange of a charged W and converts to an unseen neutrino. This paper constitutes the first high statistics measurement of electron induced cross sections at high Q2 using data collected during 1998 and 1999. The measurement spans the range in Q2 from 150 GeV2 up to 30000 GeV2. The cross sections are compared with an earlier measurement by H1 using positrons in the same kinematic range.

The CC cross sections are found to be up to a factor of 10 larger for electron scattering and are shown in the fig. 1 comparing the solid points to the open points. This difference is understood to be due to the fact that the charged W particle picks out only oppositely charged quarks from the proton and since there are more positively charged quarks, the electron cross section is larger. It is enhanced by the fact that  intrinsic angular momentum can be transferred more effectively in e-p scattering. The measurements are found to be in agreement with the Standard Model expectations which take the above effects into account and are indicated by the smooth curves in the figure.

The NC cross sections are up to a factor of four larger than for positron scattering at high Q2 as shown in fig. 2. Although the NC photon exchange process is blind to the sign of the quark charge, the Z exchange is sensitive to the weak charge of the quarks which reflect the coupling to the Z. This is exploited to make a measurement of the xF3 structure function which is derived from the difference between electron and positron NC cross sections. This structure function is mainly sensitive to the valence quarks, those that carry the quantum numbers of the proton, and as such constitutes the first measurement of this structure function at high energy.

Fig. 3a shows the measured e+p and e-p cross sections at three different Q2 values as a function of x, the momentum fraction of the proton. The e+p and e-p cross sections become increasingly asymmetric as Q2 increases, indicating that Z exchange becomes more prevalent. However, as Q2 increases the statistical precision also decreases. The differences are shown in fig. 3b as xF3, which is found to be in agreement with the prediction based on e+p data and thus provides independent confirmation of the flavour decomposition of the proton. Despite the large uncertainties on the data, these new measurements confirm the Standard Model in a new kinematic region. It is clear that continued operation of HERA and the H1 experiment is required to improve the precision of these measurements.

Last Update 11.12.2000