LHC Report: A smooth recovery and a special physics run?
On Wednesday, 12 September, the third Machine Development (MD) block followed on from the summer luminosity production period. A packed programme of no less than 20 MDs have been successfully performed on various topics. The intensive MD programme finished on Monday, 17 September, signalling the start of a five-day technical stop (TS2), the last long technical stop before Long Shutdown 2 (LS2). There will be a short, three-day technical stop in week 44, prior to the lead-ion run, to prepare the detectors for ion collisions.
In order to allow for an efficient technical stop recovery and return to luminosity production as quickly as possible, all groups working on the machine were requested to perform all necessary maintenance, but to avoid deploying new developments or non-essential upgrades to be tested during the last part of the run and instead to keep them on hold until LS2. In any case, a long commissioning phase will be required following LS2, during which many tests will be carried out.
On Monday, 24 September, the LHC fully recovered from TS2 and completed its “mini” intensity ramp up. The LHC is now back in luminosity production with 2556 bunches per beam, steaming ahead towards the 2018 luminosity goal of 60 fb-1.
In the LHC schedule, there is a seven-day special run block placeholder in week 41, on the subject of which a final decision needs to be made. This run is labelled “low energy and high beta”, which means that collisions are planned at LHC injection energy, 450 GeV per beam, without squeezing the beam to small dimensions in the experiments. The main goal of this low-energy high-beta physics run is to measure the elastic cross-section of protons in a regime where the momentum of the protons only changes by a small amount. This region is interesting since different physical processes contribute to the scattering. At very low energies, electromagnetic scattering dominates, whereas at high energies, nuclear scattering dominates. In the intermediate energy range, these two processes interfere according to the laws of quantum mechanics.
Physicists at the TOTEM and ATLAS/ALFA experiments have already made measurements of the elastic cross-section in this interference region at a centre-of-mass energy of 8 TeV and 13 TeV. However, TOTEM’s published measurements at 13 TeV resulted in values lower than predicted by the known theoretical models. In addition, no measurements between 60 GeV and the TOTEM measurements exist so far. The LHC may offer a unique opportunity to close this gap and to compare the measurements with those made at Fermilab’s Tevatron with proton-antiproton collisions.
In order to be able to detect the elastically scattered protons at very small angles, the TOTEM and ATLAS/ALFA detectors (Roman Pots) need to be moved very close to the beam. This is only possible with specially developed optics involving large ß* values at the interaction points. This and the low centre-of-mass energy lead to large beam sizes. Backgrounds induced by the beam-halo are extremely difficult to control in this configuration and will be the biggest challenge for this measurement.
Today, Tuesday, 2 October, a final beam test has been scheduled for this purpose. The test involves injecting a small number of bunches into the LHC and colliding them without acceleration. The machine settings, mainly relating to the collimators, will then be optimised to reduce the background for the experiments as much as possible. On the basis of this test, the experiments will then need to evaluate if the background is sufficiently suppressed to allow for good quality data-taking. Only then will a final decision be taken on whether the run will take place and when. Stay tuned…