A new measurement of the structure functions $P_{LL}-P_{TT}/epsilon$ and $P_{LT}$ in virtual Compton scattering at $Q^2=$ 0.33 (GeV/c)$^2$

TL;DR

This paper reports a new measurement of proton structure functions in virtual Compton scattering at low momentum transfer, using refined data analysis that suggests a non-trivial Q^2 evolution of generalized polarizabilities.

Contribution

The study provides the first measurement of specific structure functions at Q^2=0.33 GeV^2 using an improved analysis method, confirming complex Q^2 dependence of proton polarizabilities.

Findings

Larger structure function values than previous measurements.

Results support non-trivial Q^2 evolution of generalized polarizabilities.

Highlights need for further low-Q^2 measurements.

Abstract

The cross section of the $ep \to e' p' \gamma$ reaction has been measured at $Q^2 = 0.33$ (GeV/c)$^2$. The experiment was performed using the electron beam of the MAMI accelerator and the standard detector setup of the A1 Collaboration. The cross section is analyzed using the low-energy theorem for virtual Compton scattering, yielding a new determination of the two structure functions $P_LL}-P_{TT}/epsilon$ and $P_{LT}$ which are linear combinations of the generalized polarizabilities of the proton. We find somewhat larger values than in the previous investigation at the same $Q^2$. This difference, however, is purely due to our more refined analysis of the data. The results tend to confirm the non-trivial $Q^2$-evolution of the generalized polarizabilities and call for more measurements in the low-$Q^2$ region ($\le$ 1 (GeV/c)$^2$).