Rapidity gap cross sections measured with the ATLAS detector in pp collisions at sqrt(s) = 7 TeV

TL;DR

The paper measures forward rapidity-gap cross sections in 7 TeV pp collisions, using ATLAS minimum-bias data to probe diffractive processes via the Δη^F observable across a wide pT^cut range. By combining calorimeter and tracking information and applying Bayesian unfolding, it analyzes ND, SD, and DD contributions, constraining diffractive dynamics and extracting the Pomeron intercept α_IP(0) around 1.058. The results reveal a plateau in large-gap cross sections dominated by diffraction and quantify a non-negligible low-ξ contribution to the total inelastic cross section, with implications for soft diffraction modelling and Regge-based approaches. The study also benchmarks MC models (Pythia6/8, Phojet, RMK, HERWIG++) and provides a data-driven foundation for tuning diffraction in event generators.

Abstract

Pseudorapidity gap distributions in proton-proton collisions at sqrt(s) = 7 TeV are studied using a minimum bias data sample with an integrated luminosity of 7.1 inverse microbarns. Cross sections are measured differentially in terms of Delta eta F, the larger of the pseudorapidity regions extending to the limits of the ATLAS sensitivity, at eta = +/- 4.9, in which no final state particles are produced above a transverse momentum threshold p_T Cut. The measurements span the region 0 < Delta eta F < 8 for 200 < p_T Cut < 800 MeV. At small Delta eta F, the data test the reliability of hadronisation models in describing rapidity and transverse momentum fluctuations in final state particle production. The measurements at larger gap sizes are dominated by contributions from the single diffractive dissociation process (pp -> Xp), enhanced by double dissociation (pp -> XY) where the invariant mass of the lighter of the two dissociation systems satisfies M_Y <~ 7 GeV. The resulting cross section is d sigma / d Delta eta F ~ 1 mb for Delta eta F >~ 3. The large rapidity gap data are used to constrain the value of the pomeron intercept appropriate to triple Regge models of soft diffraction. The cross section integrated over all gap sizes is compared with other LHC inelastic cross section measurements.

Figures (10)

• Figure 1: Schematic illustrations of the single-diffractive dissociation (a), double-diffractive dissociation (b) and central diffractive (c) processes and the kinematic variables used to describe them. By convention, the mass $M_{Y}$ is always smaller than $M_{X}$ in the double dissociation case and $M_{Y}\xspace = M_{p}\xspace$ in the single dissociation case, $M_{p}$ being the proton mass.
• Figure 2: Cell energy significance, $S\xspace = E/\sigma_\mathrm{noise}\xspace$, distributions for the EM (a), HEC (b) and FCal (c) calorimeters. Each cell used in the analysis is included for every event, with the normalisation set to a single event. MBTS-triggered minimum bias data (points) are compared with events randomly triggered on empty bunch crossings (histograms) and with a Monte Carlo simulation (shaded areas).
• Figure 3: Comparisons of uncorrected distributions between data and MC models. (a)-(d) Total calorimeter cluster multiplicities $N_C$ for events reconstructed with (a) $0 < \Delta\eta^{F}\xspace < 2$, (b) $2 < \Delta\eta^{F}\xspace < 4$, (c) $4 < \Delta\eta^{F}\xspace < 6$ and (d) $6 < \Delta\eta^{F}\xspace < 8$. (e) Probability of detecting significant calorimeter energy in the most central region $|\eta| < 0.1$ as a function of the highest transverse momentum ${\rm max} (p_\mathrm{T}^{\rm Track}\xspace)$ of the tracks reconstructed in the inner detector in the same $|\eta|$ range. The bin at zero corresponds to events where no charged track with $p_{\mathrm{T}} > 160 \mathrm{\ Me V} \textrm{Me V}$ is reconstructed. (f) Forward rapidity gap distribution for $p_\mathrm{T}^{\rm cut}\xspace = 200 \mathrm{\ Me V} \textrm{Me V}$. The final bin at $\Delta\eta^{F}$ = 10 corresponds to cases where no reconstructed particles have $p_{\mathrm{T}} > p_\mathrm{T}^{\rm cut}\xspace$.
• Figure 4: Migration matrix between the reconstructed and hadron level values of $\Delta\eta^{F}\xspace$ for $p_\mathrm{T}^{\rm cut}\xspace = 200 \mathrm{\ Me V} \textrm{Me V}$, according to pythia8. The distribution is normalised to unity in columns and is shown to beyond the limit of the measurement at $\Delta\eta^{F}\xspace = 8$.
• Figure 5: Inelastic cross section differential in forward gap size $\Delta\eta^{F}\xspace$ for particles with $p_{\mathrm{T}} > 200 \mathrm{\ Me V} \textrm{Me V}$. The shaded bands represent the total uncertainties. The full lines show the predictions of phojet and the default versions of pythia6 and pythia8Ṫhe dashed lines in (b-d) represent the contributions of the ND, SD and DD components according to the models. The CD contribution according to phojet is also shown in (d).