A Determination of Electroweak Parameters at HERA

At HERA, a unique machine in which electrons (or its anti-particle, positrons) of 27.6 GeV colliding with protons of up to 920 GeV, has pushed the study of deep inelastic scattering into the highest energy frontier.

One of the studies made is the measurement of inclusive cross sections of neutral current (NC) and charged current (CC) interactions covering a large kinematic region. The inclusive NC and CC cross sections are primary source of information for probing the inner structure of the proton in terms of the momentum distributions of quarks (PDFs). The inclusive cross sections are also sensitive to electroweak (EW) parameters involved in the interactions. One example for the CC interaction is the propagator mass of the W boson exchanged in the ep collisions. In the NC case, one is sensitive to the weak neutral couplings of light quarks to the Z boson.

The W propagator mass has been determined previously by H1 and ZEUS experiments based on the CC cross sections. An improved measurement is obtained here by using the full H1 data taken in the first phase of the HERA operation (HERA-I) and for the first time by taking into account of the small yet non-negligible correlation between the W mass and the PDFs. This is performed in a combined EW-PDF analysis using all NC and CC cross sections measured by H1 at HERA-I. The HERA measurement is made in a space-like regime and thus complements those obtained from the Tevatron and LEP experiments by studying the decays of the W boson production.

In a similar EW-PDF analysis, a first determination of weak couplings of light quarks (u and d) to the Z boson is achieved (see figures below). The precision of the H1 determination is comparable with those from the Tevatron and from a combined LEP/SLC analysis. The H1 and Tevatron determinations are sensitive to u and d quarks separately, contrary to other measurements of the light quark-Z couplings in nuN scattering and atomic parity violation on heavy nuclei. They also resolve any sign ambiguities between vector and axial-vector couplings based on observables measured at the Z resonance.

Results for the vector and axialvector couplings of u and d quarks.

Last Update 02.01.2006