Measurement of the Diffractive Longitudinal Structure Function F_L^D at HERA

TL;DR

This paper reports the first measurements of the diffractive longitudinal structure function F_L^D at HERA, using data from 2006-2007, and compares results with QCD predictions and dipole models to enhance understanding of diffraction in deep-inelastic scattering.

Contribution

It provides the first measurements of F_L^D at multiple energies and compares them with theoretical models including leading twist and higher twist contributions.

Findings

F_L^D measured across a range of Q^2 and x_{IP} values.

Comparison shows consistency with leading twist predictions.

Evidence of higher twist effects from dipole model comparison.

Abstract

First measurements are presented of the diffractive cross section $\sigma_{ep \rightarrow eXY}$ at centre-of-mass energies $\sqrt{s}$ of 225 and 252 GeV, together with a precise new measurement at $\sqrt{s}$ of 319 GeV, using data taken with the H1 detector in the years 2006 and 2007. Together with previous H1 data at $\sqrt{s}$ of 301 GeV, the measurements are used to extract the diffractive longitudinal structure function F_L^D in the range of photon virtualities 4.0 <= Q^2 <= 44.0 GeV^2 and fractional proton longitudinal momentum loss 5 10^{-4} <= x_{IP} <= 3 10^{-3}. The measured F_L^D is compared with leading twist predictions based on diffractive parton densities extracted in NLO QCD fits to previous measurements of diffractive Deep-Inelastic Scattering and with a model which additionally includes a higher twist contribution derived from a colour dipole approach. The ratio of the diffractive cross section induced by longitudinally polarised photons to that for transversely polarised photons is extracted and compared with the analogous quantity for inclusive Deep-Inelastic Scattering.