Measurement of the Diffractive Structure Function F_2^D(4) at HERA

TL;DR

This paper reports the first measurement of the diffractive structure function F_2^D(4) at HERA, using a leading proton spectrometer to directly detect and measure the proton in diffractive deep inelastic scattering.

Contribution

It provides the first direct measurement of the proton's momentum and the t dependence in diffractive DIS at HERA, extending previous analyses and testing Regge theory predictions.

Findings

The t dependence follows an exponential shape with a specific slope parameter.

The diffractive structure function F_2^D(4) shows a particular dependence on x_P, beta, Q^2, consistent with Regge theory.

The analysis extends the kinematic range of previous measurements, especially to higher x_P and lower beta.

Abstract

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with $\xl>0.97$, where $\xl$ is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of $t$, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section on $t$ is measured in the interval $0.073<|t|<0.4$~$\gevtwo$ and is found to be described by an exponential shape with the slope parameter $b=\tslopeerr$. The diffractive structure function $\ftwodfour$ is presented as a function of $\xpom \simeq 1-\xl$ and $\beta$, the momentum fraction of the struck quark with respect to $\xpom$, and averaged over the $t$ interval $0.073<|t|<\ftwodfourtmax$~$\gevtwo$ and the photon virtuality range $5<Q^2<20~\gevtwo$. In the kinematic range $4 \times 10^{-4} < \xpom < 0.03$ and $0.015<\beta<0.5$, the $\xpom$ dependence of $\ftwodfour$ is fitted with a form $\xpoma$, yielding $a= \ftwodfouraerr$. Upon integration over $t$, the structure function $\ftwod$ is determined in a kinematic range extending to higher $\xpom$ and lower $\beta$ compared to our previous analysis; the results are discussed within the framework of Regge theory.