Standard Model Physics at HERA

TL;DR

This paper reports comprehensive tests of the Standard Model at HERA, measuring various cross sections, structure functions, and fundamental parameters like alpha_s and W boson mass, confirming SM predictions with high precision.

Contribution

It provides the first detailed measurements of structure functions and fundamental parameters at HERA, including the strong coupling constant and gluon distribution, using a wide range of Q2 and x data.

Findings

Precise measurement of F2 and FL at low x with 1-3% uncertainties.

Determination of alpha_s(MZ)=0.1150±0.0017 with combined experimental and model uncertainties.

Observation of e+p and e-p asymmetry and extraction of the W boson mass.

Abstract

Both components of the Standard Model, QCD and the electroweak sector, are tested in the H1 and ZEUS experiments at HERA. The inclusive ep single and double differential cross sections for neutral and charged current processes are measured in the range of four-momentum transfer squared, Q2, between 0.045 and 30000 GeV2, and Bjorken x between 10^{-6} and 0.65. The neutral current double differential cross section, from which the proton structure function F2 and the longitudinal structure function FL are extracted, is measured at low x with typically 1% statistical and 2-3% systematic uncertainties. In a next-to- leading order (NLO) DGLAP QCD analysis using the H1 measurements and the mu p data of the BCDMS collaboration the strong coupling constant alpha_s and the gluon momentum distribution are simultaneously determined. A value of alpha_s (MZ)=0.1150+-0.0017(exp){+0.0009}{-0.0005}(model) is obtained, with an additional theoretical uncertainty of about +-0.005, mainly due to the uncertainty of the renormalisation scale. F2c, the charm contribution to F2, is measured and well described by the boson gluon fusion production mechanism using the gluon distribution from the NLO QCD analysis. The gluon distribution and the strong coupling constant alpha_s are also determined from jet production. For Q2>1000 GeV2, an asymmetry between e+p and e-p neutral current scattering is observed and the structure function xF3 is extracted. A fit to the charged current data is used to determine a value for the W boson propagator mass. All data are found to be in good agreement with Standard Model predictions.