Usually, the motto of the LHC operations team is “maximum luminosity”. For a few days per year, however, this motto is put aside to run the machine at very low luminosity. The aim is to provide data for the broad physics programme of the LHC’s “forward physics” experiments – TOTEM and ATLAS/ALFA. By running the LHC with larger beam sizes at the interaction points, corresponding to a lower luminosity, the dedicated TOTEM and ATLAS/ALFA detectors can probe the proton–proton elastic-scattering regime at small angles.

In elastic scattering, two protons survive their encounter intact and only change direction by exchanging momentum. TOTEM, which is located in the straight sections of the LHC on either side of CMS at Point 5, and ATLAS/ALFA at Point 1, are not able to study this process during normal operation. To facilitate the special run, which took place in the third week of September, the LHC team has developed a special machine configuration that delivers exceptionally large beams at the interaction points (IP) of ATLAS and CMS. The focusing at the IP is normally parameterised by β*: the higher the value of β*, the bigger the beams and, importantly, the lower the angular divergence. For this year’s high-β* run, its value had to be raised to 2.5 km compared with around 1 km during LHC Run 1 at an energy of 8 TeV, because the higher energy of LHC Run 2 causes the two incoming protons to scatter at smaller angles. The measurements were carried out with very low-intensity beams, allowing  TOTEM and ALFA to bring their “Roman Pot” detectors remarkably close to the beam.

In addition to the precise determination of the total proton–proton interaction probability at 13 TeV, TOTEM will focus on a detailed study of elastic scattering in the low-transferred momentum regime. The experiment will investigate how Coulomb scattering interferes with the nuclear component of the elastic interaction, which can shed light on the internal structure of the protons. TOTEM will also search for special states formed by three gluons.

ATLAS/ALFA also intends to carry out a precision measurement of the proton–proton total cross-section, and will use this to determine the absolute LHC luminosity at Point 1. For ATLAS/ALFA, the interesting part of the spectrum is at low values of transferred momentum, where Coulomb scattering is dominant. Since the Coulomb scattering cross-section is theoretically known, its measurement provides an independent estimate of the absolute luminosity of the LHC. This would provide an important cross-check of the luminosity calibration measurements performed via van der Meer scans during dedicated LHC fills.