Luminosity-independent measurement of the proton–proton total cross-section at 8 TeV

The TOTEM collaboration has published the first luminosity-independent measurement at the LHC of the total proton–proton cross-section at a centre-of-mass energy of 8 TeV. This follows the collaboration’s published measurement of the same cross-section at 7 TeV, which demonstrated the reliability of the luminosity-independent method by comparing several approaches.

The method requires the simultaneous measurements of the inelastic and elastic rates, as well as the extrapolation of the latter down to a four-momentum transfer squared, |t| = 0. This is achieved with the experimental set-up consisting of two telescopes, T1 and T2, to detect charged particles produced in inelastic proton–proton collisions, and Roman Pot stations to detect elastically scattered protons at very small angles.

The analysis at 8 TeV was performed on two data samples recorded in July 2012 during special fills of the LHC with the magnets set to give the parameter β^* = 90 m. During these fills, the Roman Pots were inserted close to the beam, allowing the detection of around 90% of the nuclear elastic-scattering events. Simultaneously, the inelastic scattering rate was measured by the T1 and T2 telescopes.

By applying the optical theorem, the collaboration determined a total proton–proton cross-section of 101.7 ± 2.9 mb, which is in good agreement with the extrapolation from lower energies. The method also allows the derivation of the luminosity-independent elastic and inelastic cross-sections: σ_el = 27.1 ± 1.4 mb and σ_inel = 74.7±1.7 mb. The two measurements are consistent in terms of detector performance, showing comparable systematic uncertainties, and they are both in good agreement with the extrapolation of the lower-energy measurements.