Following tests in September, a short, dedicated run at the end of October provided “de-squeezed” beams to the ALFA and TOTEM experiments, allowing new measurements of the elastic proton–proton cross-section.
To squeeze the beam and so maximize the number of collisions, LHC beams at full energy typically have a value of β* – the distance to the point where the beam is twice as wide as it is at the interaction point – 0.60 m. However, squeezing to a small beam increases the angular beam divergence such that elastic proton–proton scattering at small angles cannot be observed.
The TOTEM experiment has measured the elastic proton–proton cross-section in previous dedicated runs, resulting in a determination of the total proton–proton cross-section using the optical theorem. To observe the contribution of electromagnetic interaction (Coulomb scattering) and its interference with the nuclear component to the elastic cross-section, scattering angles of the order of 5 μrad have to be reached. Since the Coulomb scattering cross-section is known theoretically, its measurement also gives access to an independent determination of the absolute luminosity of the LHC.
For this recent special run, a new record value of β* = 1000 m was reached, making the beams at interaction points 1 and 5 almost parallel. The angular divergence of the beams at the interaction points was reduced by a factor of 40 compared with low-beta (high-luminosity) operation. These special settings allowed the ALFA and TOTEM experiments – at points 1 and 5, respectively – to measure proton–proton scattering angles down to the microradian level. The experiments’ Roman Pots were moved as close as 0.87 mm to the centre of the beam, which contained three bunches of 1011 protons each. At that distance the beam halo is intense and had to be reduced by an optimized collimation procedure that allowed a reduction of the halo background by a factor of 1000. This configuration enabled data-taking in good conditions for about an hour and, for the first time, ALFA and TOTEM could measure the elastic scattering in the Coulomb-nuclear interference region.
For future runs at 13 TeV, optics with β* of around 2 km will have to be developed. This will require the installation of additional quadrupole power cables in the LHC tunnel.