Study of deep inelastic inclusive and diffractive scattering with the ZEUS forward plug calorimeter

TL;DR

This ZEUS study shows that diffractive DIS in ep collisions at HERA is a major component of the total cross section and cannot be described by a single Regge-Pomeron exchange with Regge factorisation. By extending the forward coverage with the FPC, the analysis measures $F_2$, the diffractive cross section, and the diffractive structure function $F^{D(3)}_2$ over $2.2\le Q^2\le80\ { m GeV}^2$, $37\le W\le245\ { m GeV}$, and $0.28\le M_X\le35\ { m GeV}$. The data reveal a $W$-dependent rise of the diffractive cross section, a $Q^2$-dependent Pomeron intercept, and positive scaling violations in $F^{D(3)}_2$ suggesting substantial perturbative effects; BEKW(mod) provides a successful description of $F^{D(3)}_2$ via dipole-type photon fluctuations and gluon-dominated diffraction. The diffractive-to-total cross-section ratio remains sizable and almost constant in $W$, while Regge-factorisation is broken, indicating that diffraction in DIS probes diffractive parton densities and perturbative dynamics, with significant implications for the interpretation of DIS structure functions and DGLAP-based analyses.

Abstract

Deep inelastic scattering and its diffractive component, ep -> e'gamma*p ->e'XN, have been studied at HERA with the ZEUS detector using an integrated luminosity of 4.2 pb-1. The measurement covers a wide range in the gamma*p c.m. energy W (37 - 245 GeV), photon virtuality Q2 (2.2 - 80 GeV2) and mass Mx. The diffractive cross section for Mx > 2 GeV rises strongly with W; the rise is steeper with increasing Q2. The latter observation excludes the description of diffractive deep inelastic scattering in terms of the exchange of a single Pomeron. The ratio of diffractive to total cross section is constant as a function of W, in contradiction to the expectation of Regge phenomenology combined with a naive extension of the optical theorem to gamma*p scattering. Above Mx of 8 GeV, the ratio is flat with Q2, indicating a leading-twist behaviour of the diffractive cross section. The data are also presented in terms of the diffractive structure function, F2D(3)(beta,xpom,Q2), of the proton. For fixed beta, the Q2 dependence of xpom F2D(3) changes with xpom in violation of Regge factorisation. For fixed xpom, xpom F2D(3) rises as beta -> 0, the rise accelerating with increasing Q2. These positive scaling violations suggest substantial contributions of perturbative effects in the diffractive DIS cross section.

Figures (25)

• Figure 1: Non-peripheral deep inelastic scattering.
• Figure 2: Diffractive deep inelastic scattering, $e p \to e X N$.
• Figure 3: The measured versus the generated $\ln M^2_X$ values for the lowest and highest $W$ bins at low and high $Q^2$ as determined by Monte Carlo simulation. The horizontal lines give the maximum values of $\ln M^2_X$ for which the diffractive contribution was determined. Lines are drawn at $\ln (M^{\rm meas}_X)^2 = \ln (M^{\rm true}_X)^2$ to guide the eye.
• Figure 4: Distributions of $\ln M^2_X$ ($M_X$ in units of GeV) at the detector level for different ($W$, $Q^2$) bins. The points with error bars show the data. The shaded areas show the non-peripheral contributions as predicted by DJANGOH. The diffractive contributions from $\gamma^* p \to Xp$ ($\gamma^* p \to XN$, $M_N < 2.3$ GeV) as predicted by SATRAP+ZEUSVM (SANG) are shown as hatched (cross-hatched) areas. The dash-dotted lines show the results for the non-diffractive contribution from fitting the data in the $\ln M^2 _X$ range delimited by the two vertical dashed lines.
• Figure 5: The reaction $\gamma^* p \to Xp$ proceeding via the exchange of a Reggeon $\alpha_j$ in the $t$-channel. The system $X$ is produced by the scattering of the virtual photon on the Reggeon via the exchange of the pole $\alpha_k$.