Measurement of the inelastic proton-proton cross section at $\sqrt{s}$ = 13 TeV

This Letter presents a measurement of the inelastic proton-proton cross section using 60 μb−1 of pp collisions at a center-of-mass energy √ s of 13 TeV with the ATLAS detector at the LHC. Inelastic interactions are selected using highly efficient plastic scintillators in the forward region of the detector. A cross section of 68.2±1.5 mb is measured in the fiducial region ξ = M2 X/s > 10 −6, where MX is the largest invariant mass of the two hadronic systems separated by the largest rapidity gap in the event. For diffractive events this corresponds to cases where at least one proton dissociates to a system of mass larger than 13 GeV. The measurement agrees with a range of theoretical predictions. When extrapolated to the full phase space, the result is consistent with an inelastic cross section increasing with center-of-mass energy, as observed at lower energies. c © 2016 CERN for the benefit of the ATLAS Collaboration. Reproduction of this article or parts of it is allowed as specified in the CC-BY-3.0 license. The rise of the total proton-proton (pp) cross section with center-of-mass energy √ s, first predicted by Heisenberg [1] and observed by experiments at the CERN Intersecting Storage Rings [2], provides a probe of the non-perturbative regime of quantum chromodynamics (QCD). General arguments based on unitarity, analyticity, and factorization imply an upper bound (the Froissart bound [3–5]) on the highenergy behavior of total hadronic cross sections that prevents them from rising more rapidly than ln2(s). This Letter presents a measurement of the inelastic cross section σinel using pp collisions at √ s = 13 TeV with the ATLAS detector at the Large Hadron Collider (LHC). In inelastic interactions, one or both protons dissociate as a result of colored (non-diffractive) or colorless (diffractive) exchange. The fiducial region of this measurement excludes elastic pp scattering and diffractive dissociation processes in which neither proton is dissociated into a system of mass MX > 13 GeV, or equivalently, ξ = M2 X/s > 10 −6. The measurement is reported for this fiducial region and after extrapolation to the total inelastic cross section using models of inelastic interactions. It is performed using two sets of scintillation counters in a dataset corresponding to an integrated luminosity of 60.1 ± 1.1 μb−1 collected in June 2015. During this data-taking period, the mean number of pp collisions in the same LHC bunch crossing was 2.3 × 10−3, and thus the contribution from multiple collisions is negligible. Many experiments have measured σinel and found an increase with √ s [6]. The previous measurement of σinel with the ATLAS detector at √ s = 7 TeV [7] used the method employed in this letter in a fiducial region defined by ξ > 5 × 10−6. The TOTEM and ATLAS collaborations also determined σinel at √ s = 7 TeV using the optical theorem and a measurement of the elastic cross section with Roman pot detectors [8, 9]. The TOTEM Collaboration performed similar measurements at √ s = 8 TeV [10, 11]. Using a variety of different techniques, the CMS, ALICE, and LHCb experiments have made measurements of σinel at √ s = 7 TeV [12–14]. The ALICE Collaboration additionally measured it at √ s = 2.76 TeV [13]. The Pierre Auger Collaboration measured the inelastic p-air cross section at √ s = 57 TeV and extracted σinel using the Glauber model [15]. The ATLAS detector is a cylindrical particle detector1 composed of several subdetector layers [16]. The inner tracking detector (ID) comprises silicon pixel and strip detectors and a straw-tube tracker immersed in a 2 T magnetic field provided by a superconducting solenoid. Around the tracker there is a system of electromagnetic and hadronic calorimeters, which use liquid argon and lead, copper, or tungsten absorber for the electromagnetic and forward (|η| > 1.7) hadronic components of the detector, and scintillator-tile active material and steel absorber for the central (|η| < 1.7) hadronic component. At z = ±3.6 m, thin plastic scintillation counters, named the minimum-bias trigger scintillators (MBTS), are installed on the front face of each endcap calorimeter. These detectors cover the region 2.07 < |η| < 3.86. They are similar to those described in Ref. [16] but were rebuilt during 2014, when the coverage was slightly extended from 2.08 < |η| < 3.75 after the previous run at √s = 7 TeV. The MBTS are divided into inner (149 < r < 445 mm) and outer (444.5 < r < 895 mm) octagonal rings, with 8 counters in the inner ring and 4 counters in the outer ring. The ATLAS experiment uses a hardware-based first-level trigger to select events at about 75 kHz and a software-based high-level trigger to record events at about 1 kHz for offline analysis. Three trigger configurations were used to collect data for this analysis. The primary triggers use the MBTS detector and constant-fraction discriminators to select events when two proton bunches collide in the detector. To 1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the center of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2).