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