The LHC, de-squeezed
Rare processes like the Higgs production require maximizing the number of proton collisions. This is done by squeezing the beams to very small sizes. However, interesting physics processes also happen when beams are not squeezed at interaction points. Last week, a dedicated run showed that the LHC is a record-breaking machine also with de-squeezed beams.
This figure shows an online hit map of one of the ATLAS/ALFA detectors. The narrow elliptical shape is the typical signal produced by elastically scattered protons. The removal of the background (central bulge) is a challenge for both experiments.
The beam squeezing parameter is known by experts as beta-star (ß*): the smaller the ß*, the stronger the squeezing. To obtain as many collisions as possible in the heart of the experiments, the ß* at full energy is 0.60 m – that is, beams are squeezed to very small beam sizes. This maximizes the rate of proton collisions as required for rare processes like Higgs production. However, squeezing to a small beam increases the angular beam divergence such that elastic proton-proton scattering at small angles cannot be observed.
The elastic proton-proton cross-section had been measured in previous dedicated runs at the LHC, resulting in a determination of the total proton-proton cross-section using the optical theorem. To observe the contribution of electromagnetic interaction (also known as “Coulomb scattering”) and its interference with the nuclear component to the elastic proton-proton cross-section, scattering angles of the order of 5 microradians have to be reached. Since the Coulomb scattering cross-section is theoretically known, its measurement also gives access to an independent determination of the absolute luminosity of the LHC.
In this TOTEM plot, the correlation between the reconstructed scattering angles of the two outgoing protons demonstrates a dominance of elastic events.
For this recent special run, a new record value of ß*= 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 to low-beta (high-luminosity) operation. These special settings allowed the ATLAS/ALFA and TOTEM experiments to measure proton-proton scattering angles down to the microradiant level. The experiments’ Roman Pots were moved as close as 0.87 mm to the centre of the beam, which contained 3 bunches of 1011 protons each. At that distance the beam halo is intense and has to be reduced by an optimized collimation procedure that allows a reduction of the halo background by a factor of 1000. This configuration enabled data-taking in good conditions for about one hour and, for the first time, ATLAS/ALFA and TOTEM were able to measure the elastic scattering in the Coulomb-Nuclear Interference region.
For future runs at 13 TeV, optics with beta values around 2 km will have to be developed. This will require the installation of additional quadrupole power cables in the LHC tunnel.
