Measurement of the Proton Structure Function F_2 at low Q^2 in QED Compton Scattering at HERA

TL;DR

This work reports the first measurement of the proton structure function $F_2(x,Q^2)$ using QED Compton scattering (QEDC) at HERA, extending the low-$Q^2$ reach to $0.5$ GeV$^2$ and $x$ up to ~0.06. The analysis relies on a detailed QEDC event simulation, meticulous background treatment, and a hadronic final-state-based Sigma-method to reconstruct kinematics in a region where standard DIS methods struggle. The extracted $F_2$ values are consistent with fixed-target measurements and the ALLM97 parameterisation, thereby bridging the DIS and photoproduction regimes and providing a complementary cross-check of proton structure in the non-perturbative transition region. Overall, the study demonstrates the viability of QEDC as a probe of proton structure at low $Q^2$ and enhances the HERA mapping of $F_2$ in a previously sparse kinematic domain.

Abstract

The proton structure function F_2(x,Q^2) is measured in inelastic QED Compton scattering using data collected with the H1 detector at HERA. QED Compton events are used to access the kinematic range of very low virtualities of the exchanged photon, Q^2, down to 0.5 GeV^2, and Bjorken x up to \sim 0.06, a region which has not been covered previously by inclusive measurements at HERA. The results are in agreement with the measurements from fixed target lepton-nucleon scattering experiments.

Figures (4)

• Figure 1: Lowest order Feynman diagrams for the radiative process $ep \rightarrow e \gamma X$ with photon emission from the electron line. Here $l$ and $P$ represent the four-momenta of the incoming electron and the incoming proton, while $l^{\prime}$, $k$ and $X$ are the four-momenta of the scattered electron, the radiated photon and the hadronic final state, respectively.
• Figure 2: a) ratio of the total measured transverse momentum of hadrons to the total transverse momentum of the $e\gamma$ system; b) uncorrected $y_\Sigma$ distribution after applying all selection cuts. The data are depicted by the closed circles. The solid histogram represents the sum of COMPTON MC events and all background contributions. The hatched histogram denotes the sum of all background processes.
• Figure 3: Control distributions for the measured electron and photon in events used for the measurement: a) energy of the particle with the higher energy; b) energy of the particle with the lower energy; c) sum of both energies; d) $e\gamma$ acoplanarity; e) polar angle of the particle with the higher energy and f) polar angle of the particle with the lower energy. The data are depicted by the closed circles. The solid histogram represents the sum of COMPTON MC events and all background contributions. The hatched histogram denotes the sum of all background processes.
• Figure 4: $F_2$ measurements from QED Compton scattering by H1 (closed circles), compared with other measurements at HERA (closed squares Breitweg:2000yn, open triangles Adloff:2000qk and open circles Adloff:1997mf) and fixed target experiments (open squares Whitlow:1992uw, open stars Arneodo:1997qe and open crosses Adams:1996gu). The inner error bars for the QEDC data represent the statistical errors and the total error bars the statistical and systematic errors added in quadrature. The solid line depicts the ALLM97 parameterisation Abramowicz:1997ms. The data of the other measurements are shifted to the $Q^2$ values of the present measurement using the ALLM97 parameterisation.