Inelastic cross section measurements at LHC

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 measurements of the inelastic proton–proton interaction cross-section at a centre-of-mass energy, √s, of 7 TeV using the ATLAS and CMS detectors at the Large Hadron Collider. For ATLAS the 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, MX, 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. For CMS a new method to measure the inelastic pp cross section has been developed based on the assumption that pile-up events are randomly distributed according to a Poisson probability. This method relies only on the accurate knowledge of the CMS tracking system and it does not depend upon a specific Monte Carlo simulation. The value of the cross section for events with at least 2 charged particles with |eta|<2.4$ and p_T > 200 MeV is determined to be sigma = 58.7 ± 2.0 (Syst) ± 2.4 (Lum) mb. Using model-dependent extrapolations, the value for the total inelastic p-p cross section is determined to be sigma_inel(pp) = 68.0 ± 2.0(Syst) ± 2.4 (Lum) ± 4 (Ext.) mb. The results of the two experiments are then compared with several MC models and with theoretical analythical calculations.