LHC Report: special run with de-squeezed beams for ATLAS/ALFA and TOTEM

The main high-luminosity proton-proton run of the LHC is complemented by one week per year of special proton-proton runs. The special runs are performed with larger beam sizes at the interaction points to allow the forward physics experiments, TOTEM and ATLAS/ALFA, the chance to make precise measurements of protons as they emerge from collisions at small angles.

In standard high-luminosity operation, the beams are squeezed to give small beam sizes at the interaction points to maximise the collision rates. The “squeeze” takes place at top energy and the beam size at the centre of ATLAS (IP1) and CMS (IP5) is reduced from 66 micrometres at the top of the ramp to 18 micrometres before colliding beams are established.

Protons that avoid the fate of an inelastic collision but yet still interact – in elastic or diffractive events – are scattered and emerge in the forward direction. The reduction in beam size has a side effect of increasing the angle at which protons approach the collision point. This increased angular divergence means that the scattered protons also emerge at larger angles, making life difficult for TOTEM and ATLAS/ALFA.

In the special runs this week, the beams are de-squeezed at IP1 and IP5 to produce bigger beams at the interaction point with the incoming protons approaching the interaction point at smaller angles. Once the de-squeeze is finished and the beams are colliding, the Roman Pot detectors of ATLAS/ALFA in IP1 and of TOTEM in IP5 are moved close to the beam and data-taking starts.

A primary goal for both ATLAS/ALFA and TOTEM is to measure for the first time elastic proton-proton scattering at 13 TeV. This is achieved at low luminosity with a small number of bunches.

In a second phase, the TOTEM and CMS collaborations will coordinate the use of their detectors to perform combined measurements for diffractive physics studies. Of particular interest is the search for new resonance states, glueballs, characterised by the combination of forward scattered protons recorded in the Roman pots and signals at large angles in the central detectors. This phase requires significant amounts of data and operation this week has been with around 700 bunches per beam spaced at 100 ns.

by Helmut Burkhardt for the LHC team