Diffractive Deep-Inelastic Scattering with a Leading Proton at HERA

TL;DR

This paper reports a leading-proton diffractive deep-inelastic scattering measurement at HERA using the H1 Forward Proton Spectrometer. It presents the t, x_IP, β, and Q^2 dependences of the diffractive cross sections, extracting a pomeron intercept α_IP(0) ≈ 1.114 and showing a small α_IP' in the low-x_IP regime, consistent with a hard pomeron picture. The data support factorisation between the proton-vertex and the hard interaction for both the pomeron and sub-leading exchanges, and demonstrate compatibility with H1 LRG and ZEUS LPS measurements after accounting for proton dissociation. The results provide constraints on diffractive parton distributions and the role of sub-leading exchanges in diffractive DIS. Overall, the FPS measurement validates the compatibility of different diffraction measurement techniques and reinforces the Regge-based description of diffraction in DIS.

Abstract

The cross section for the diffractive deep-inelastic scattering process $ep \to e X p$ is measured, with the leading final state proton detected in the H1 Forward Proton Spectrometer. The data analysed cover the range \xpom <0.1 in fractional proton longitudinal momentum loss, 0.08 < |t| < 0.5 GeV^{-2} in squared four-momentum transfer at the proton vertex, 2 < Q^2 < 50 GeV^2 in photon virtuality and 0.004 < β= x / \xpom < 1, where x is the Bjorken scaling variable. For $\xpom \lapprox 10^{-2}$, the differential cross section has a dependence of approximately ${\rm d} σ/ {\rm d} t \propto e^{6 t}$, independently of \xpom, βand Q^2 within uncertainties. The cross section is also measured triple differentially in \xpom, βand Q^2. The \xpom dependence is interpreted in terms of an effective pomeron trajectory with intercept $α_{\pom}(0)=1.114 \pm 0.018 ({\rm stat.}) \pm 0.012 ({\rm syst.}) ^{+0.040}_{-0.020} ({\rm model})$ and a sub-leading exchange. The data are in good agreement with an H1 measurement for which the event selection is based on a large gap in the rapidity distribution of the final state hadrons, after accounting for proton dissociation contributions in the latter. Within uncertainties, the dependence of the cross section on x and Q^2 can thus be factorised from the dependences on all studied variables which characterise the proton vertex, for both the pomeron and the sub-leading exchange.

Figures (9)

• Figure 1: Schematic illustration of the diffractive DIS process $ep \rightarrow eXp$ and the kinematic variables used for its description in a model in which the pomeron (${I\!\!P}$) and a sub-leading (${I\!\!R}$) trajectory are exchanged.
• Figure 2: (a) The differential cross section $x_{I\!\!P} \, {\rm d}^2 \sigma / {\rm d}x_{I\!\!P} {\rm d}t$ measured in the kinematic range $2<Q^2<50~{\rm GeV}^2, 0.02<y<0.6$ for different $x_{I\!\!P}$ intervals. The results of fits of the form $x_{I\!\!P} {\rm d}^2 \sigma /{\rm d} x_{I\!\!P} {\rm d}t \propto e^{Bt}$ are also shown. (b) The slope parameter $B$ obtained from these fits, shown as a function of $x_{I\!\!P}$. The results obtained with the ZEUS LPS ZEUSLPS and the parameterisation of the H1 data described in section \ref{['f2d4sec']} are also shown. The inner error bars represent the statistical errors and the outer error bars indicate the statistical and systematic errors added in quadrature.
• Figure 3: (a,b) The differential cross section $x_{I\!\!P} \, {\rm d}^2 \sigma / {\rm d}x_{I\!\!P} {\rm d}t$ measured in different regions of (a) $Q^2$ and $x_{I\!\!P}$ and (b) $\beta$ and $x_{I\!\!P}$. The results of fits of the form $x_{I\!\!P} {\rm d}^2 \sigma /{\rm d} x_{I\!\!P} {\rm d}t \propto e^{Bt}$ are shown. (c,d) The slope parameter $B$ obtained from these fits, shown as a function of (c) $Q^2$ and (d) $\beta$ for two $x_{I\!\!P}$ intervals. The inner error bars represent the statistical errors and the outer error bars indicate the statistical and systematic errors added in quadrature.
• Figure 4: The diffractive reduced cross section $x_{I\!\!P} \, \sigma_r^{D(4)}(\beta,Q^2,x_{I\!\!P},t)$, shown as a function of $x_{I\!\!P}$ for $|t|=0.25~\rm GeV^2$ at different values of $\beta$ and $Q^2$. The inner error bars represent the statistical errors. The outer error bars indicate the statistical and systematic errors added in quadrature. An overall normalisation uncertainty of 10.1% is not shown. The solid curves represent the results of the phenomenological 'Regge' fit to the data, including both pomeron (${I\!\!P}$) and sub-leading (${I\!\!R}$) trajectory exchange, as described in section \ref{['f2d4sec']}. The dashed curves represent the contribution from pomeron exchange alone according to the fit.
• Figure 5: The diffractive reduced cross section $x_{I\!\!P} \, \sigma_r^{D(3)}(\beta,Q^2,x_{I\!\!P})$ for $|t| < 1 \ {\rm GeV^2}$, shown as a function of $Q^2$ for different values of $x_{I\!\!P}$ and $\beta$. The inner error bars represent the statistical errors. The outer error bars indicate the statistical and systematic errors added in quadrature. An overall normalisation uncertainty of 10.1% is not shown. The solid curves represent the results of the 'H1 2006 DPDF Fit A' to LRG data H1LRG, modified as described in section \ref{['lrgcomp']}. The dashed curves represent the extrapolation of this prediction beyond the $Q^2$ range which is included in the fit. The dotted curves indicate the contribution of pomeron exchange alone in this model.