ALFA in ATLAS measures pp cross-section with high precision

Data from a special run of the LHC using dedicated beam optics at 7 TeV have been analysed to measure the total cross-section of proton–proton collisions in ATLAS. Using the Absolute Luminosity For ATLAS (ALFA) Roman Pot sub-detector system located 240 m from the interaction point, ATLAS has determined the cross-section with unprecedented precision to be σ_tot (pp → X) = 95.4±1.4 mb.

The total cross-section is a fundamental parameter of the strong interactions, setting the scale of the size of the interaction region at a given energy. To measure the total cross-section, the optical theorem is used, which states that the total cross-section is proportional to the imaginary part of the forward elastic-scattering amplitude, extrapolated to momentum transfer, t = 0. From a measurement of the elastic-scattering cross-section differential in t, the value of the total cross-section is inferred, and is found to increase logarithmically with the centre-of-mass energy (see figure).

Measuring elastic scattering is a challenge because elastically scattered protons escape the interaction at very small angles of tens of micro-radians or less. To detect these protons, dedicated detectors are installed, such as ALFA. To achieve the required focusing properties, the LHC was operated with special beam optics of β^* = 90 m. The detectors can then be moved as close as a few millimetres from the LHC beam, to access the smallest scattering angles.