LHC progress report

Last weekend saw a record physics fill with a tenfold increase in instantaneous luminosity (event rate from collisions), marking an important milestone for the LHC. This physics fill did not only establish luminosities above 1.1 x 10²⁸ cm^-2 s^-1 in all four experiments but was also kept in "stable beam" mode for a new record length of 30 hours. The particle physics experiments were able to more than double the total number of events so far recorded at 3.5 TeV.

 

The very successful weekend had been preceded by hard work on the accelerator side. A factor 5 improvement in luminosity was achieved by "squeezing" (reducing) the beam sizes at all four interaction points. This process, one of the most complex stages in the operation of the accelerator, was finalised the week before. Once the machine is "squeezed", the experimental insertions become aperture bottlenecks for the beam. The squeeze was therefore followed by a number of collimation and beam dump tests to verify sufficient protection of the experiments. The first physics fill with squeezed optics was then prepared early on Saturday morning. A new 3 bunch scheme was used to provide another factor 2 improvement for luminosity. With the increased beam intensity the machine passed for the first time beyond the limit of the set-up beam, at which point all masked interlocks are automatically unmasked. Having smoothly passed this final hurdle, the beams were put into luminosity production on Saturday morning and collisions were optimized. Beams were finally used for end-of-fill studies and then dumped, after 30 hours of unperturbed physics running.

In the first part of this week, the monthly scheduled technical stop has taken place to allow the necessary maintenance of the LHC equipment and in particular of the cryogenics system. This is required in order to maintain the good operation efficiency of the LHC machine. Since Thursday morning the two beams have been circulating again. The programme for the next weeks features the setting-up of the machine and the machine protection for higher bunch intensity. This will lead initially to collisions at 450 GeV with a bunch population close to nominal values and later to 3.5 TeV collisions, with squeezed optics and increasingly higher intensities to deliver more and more luminosity to the experiments.

by CERN Bulletin