One of the most basic quantities in particle physics, the rate at which protons scatter off of one another (the cross section), cannot be calculated from the theory of strong interactions, quantum chromodynamics. It must instead be measured, and those measurements can then be used to tune the numerical models of LHC proton–proton collisions.
ATLAS has measured the inelastic proton–proton cross section – the cross section of collisions in which at least one of the protons is broken apart – which accounts for about 80% of the total interaction cross section. The measurement makes use of two sets of plastic scintillator counters mounted on either end of the ATLAS tracking detector. These counters were newly installed for Run 2, and they had to be carefully calibrated in order to complete the measurement.
Using the first few days of data at 13 TeV, the measured cross section of 73 mb is slightly lower than most theoretical models anticipated. Thanks to excellent machine performance and the high quality of these detectors, the experimental uncertainty of the measurement is only 0.8%. The total uncertainty of about 10% is driven by our understanding of how many collisions are actually occurring in the detector (the luminosity), which is expected to improve significantly in the next few months.
This measurement adds a new energy regime to the dozens of other measurements of this quantity made over more than 50 years. Read about previous results from ATLAS and CMS in the papers linked below.