Measurement of the Proton Structure Function F₂ at low Q² in QED Compton Scattering at HERA

High energy electron-proton collisions offer the best possibilities for detailed studies of the structure of the proton. The electron probes the proton constituents via the exchange of virtual photons, much in the spirit of the famous Rutherford experiment which established the modern picture of the atom. The present-day studies play a decisive role in the development of Quantum Chromodynamics (QCD), the theory of the strong force which binds protons and neutrons together in atomic nuclei and confines quarks in protons and other hadrons.

By using large scale detectors, the H1 and ZEUS collaborations at the HERA ep-collider measure the inclusive ep-scattering cross section, which depends on the exchanged photon virtuality Q², corresponding to the resolution at which the proton structure is probed, and on the fraction of proton momentum x carried by the struck quark. Due to the colliding beam kinematics, HERA has extended the x and Q² range explored in ep-interactions by orders of magnitude as compared to the previous fixed target experiments. The unknown proton structure is expressed in terms of several structure functions. In most of the measured kinematic region the cross section is simply related to the structure function F₂(x,Q²). The H1 and ZEUS experiments have shown that the Q² evolution of F₂ is well described by perturbative QCD throughout a wide range in x and Q². However, at small Q² a transition takes place into a region in which non-perturbative effects dominate. The data in this region are best described by phenomenological models such as those derived from the Regge approach.

Measurements at small Q² require special experimental techniques since the outgoing electron is scattered under very low angles and often escapes the main detector, disappearing unseen into the beam pipe. In this paper a new measurement of F₂ in this low Q² kinematic domain is presented, which uses events in which a high energy real photon is emitted at a wide angle from the electron, so called QED Compton (QEDC) events. The processes under study are depicted in the Feynman diagrams of Fig. 1. The experimental signature is an approximately back-to-back azimuthal configuration of the outgoing electron and photon. Most importantly, due to the wide angle of the photon emission, both the electron and the photon are detected in the main experimental apparatus, even at Q² values where the electron would be unseen in a standard event. Low values of Q² thus become experimentally accessible.

Figure 1: Lowest order Feynman diagrams for the QED Compton process ep ---> e gamma X with photon emission from the electron line.

The F₂ values measured using the QED Compton technique are shown in Fig. 2 as a function of x at fixed Q², together with other HERA and fixed target data. Not only do the data extend to low Q² values, but the x-range is also significantly extended towards larger values than have previously been explored at HERA. This extension is achieved through an improved treatment of the hadronic final state at low invariant masses. The region covered overlaps with the domain of fixed target experiments and agrees well with them. The data are well described by the ALLM97 parameterisation, based on a Regge approach.

Figure 2: F2 measurements from QED Compton scattering and other methods