High energy deep-inelastic electron proton collisions are the cleanest way to study the inner structure of the proton. The electron can probe the proton and its constituents via the electromagnetic interaction, much in the spirit of the famous Rutherford experiment. The electrons thereby send virtual photons towards the proton scanning its internal structure like an electron microscope renders the image of an object. By using a large scale detector, the H1 collaboration measures the inclusive deep-inelastic scattering cross section amounting to a measurement of the rate at which the beam electrons recoil from the quarks inside the protons. The precise measurement of this scattering cross section is confronted with the expectation from Quantum Chromodynamics (QCD) a modern field theory, which describes strong interactions as the exchange of coloured gluons between quarks inside the proton. In a large part of the measured kinematic region the deep-inelastic scattering cross section is proportional to a linear combination of quark distribution functions.
The cross section of the electron/positron - proton scattering as measured by H1 experiment (coloured points) and previous experiments BCDMS and NMC. The data exhibit so called scaling violations (experimental points at the same x depend on Q2), the manifestation of the gluon radiation from quarks inside the proton.
The photons which mediate the electromagnetic interaction do not couple directly to electrically neutral gluons but to charged quarks. However, the presence of gluons inside the proton can be felt by the struck quark. At large quark momenta x, quarks can have lost sizable momentum by radiating gluons prior to the interaction with the virtual photon. Gluons can produce pairs of sea quarks which enhances the amount of quarks available for interaction with the photon at low momenta x. These processes can be resolved if the resolution of the photon probe, determined by its virtuality Q2 is sufficiently large. The amount of quark scattering partners is thus expected to increase with Q2, i.e. at low x the cross section σ(x,Q2) should rise with Q2, and this rise is determined by the gluon momentum distribution. As can be seen in the above figure, these so-called scaling violations are indeed observed and they are well described by the theoretical calculation using QCD (coloured curves). The cross section difference between electron (blue) and positron (red) data at high Q2 is due to the different strength of the weak interaction of both particles.
The sets of measured cross sections were used for the extraction of parton densities in the proton for four different groups of constituents: uv and dv valence quarks, so called sea quarks S and gluons g. Their properties as a function of x are shown in the next figure. From the figure we can clearly see the dominant role of gluons in the proton at low x (low parton momenta in the proton).
Parton distribution functions in the proton extracted from the H1 data. The gluon g and sea S distributions are scaled by a factor 0.01. The uncertainties include experimental (inner) and model (middle) uncertainties and the parametrization variations (outer).
For the kinematical region in question the data the following subdetectors were of particular importance - BST (Backward Silicon Tracker), SPACAL (Spagheti Calorimeter) and liquid argon calorimeter. Physicists and technicians from FZÚ were active in the construction, production, operation and calibration of the subdetectors, during the data taking and also in the analysis of the physical results.
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