Measurement of the inelastic proton-proton cross-section at √s=7 TeV with the ATLAS detector

Abstract

The dependence of the rate of proton-proton interactions on the centre-of-mass collision energy, √s, is of fundamental importance for both hadron collider physics and particle astrophysics. The dependence cannot yet be calculated from first principles; therefore, experimental measurements are needed. Here we present the first measurement of the inelastic proton-proton interaction cross-section at a centre-of-mass energy, √s, of 7 TeV using the ATLAS detector at the Large Hadron Collider. Events are selected by requiring hits on scintillation counters mounted in the forward region of the detector. An inelastic cross-section of 60.3 ± 2.1 mb is measured for ξ > 5×10⁻⁶, where ξ is calculated from the invariant mass, M(X), of hadrons selected using the largest rapidity gap in the event. For diffractive events, this corresponds to requiring at least one of the dissociation masses to be larger than 15.7 GeV.

Figure 1. Dependence of the fraction of single-sided events (R_ss) on the relative diffractive contribution (f_D).

The ratio of the single-sided to inclusive event sample R_ss as a function of the fractional contribution of diffractive events, f_D, to the inelastic cross-section. The data value for R_ss is shown as the horizontal line with its systematic uncertainties (grey band). Also shown are predictions of several models as a function of an assumed value of f_D. For all three DL predictions, the α′ value is 0.25 GeV⁻². The default f_D values are indicated by the markers; they are 32.2% for all models except Phojet that uses 20.2%.

Figure 2. MBTS multiplicity distributions.

The MBTS multiplicity distribution in the data (filled circles) compared with MC expectations for the inclusive (a) and single-sided (b) samples for several MC models (histograms), using the fitted f_D values. The band around the data indicates the systematic uncertainty due to the MBTS detector response and the amount of material in front of the MBTS detector. For all three DL predictions, the α′ value is 0.25 GeV⁻².

Figure 3. The inelastic cross-section versus √s.

The ATLAS measurement for ξ>5×10⁻⁶ is shown as the red-filled circle and compared with the predictions of Schuler and Sjöstrand and Phojet for the same phase space. Data (filled circles for pp data and unfilled circles for pp̂ data) from several experiments are compared with the predictions of the pp inelastic cross-section from Schuler and Sjöstrand (as used by Pythia), by Block and Halzen and by Achilli et al. An extrapolation from the measured range of ξ>5×10⁻⁶ to the full inelastic cross-section using the acceptance of 87±10% is also shown (blue-filled triangle) based on the model by Donnachie and Landshoff for ɛ=0.085 for dσ/dξ. The experimental uncertainty is indicated by the error bar whereas the total (including the extrapolation uncertainty) is represented by the blue-shaded area.