The LHC and a milestone

Particle tracks as protons collide in the LHC.

On December 17, 2012, the European Organisation for Nuclear Research (CERN) completed the first proton run with the Large Hadron Collider (LHC). The remarkable first three-year run of the world’s most powerful particle accelerator, during which it has most likely seen the much-sought-after Higgs Boson, was crowned by a new performance milestone. The space between proton bunches in the beams was halved to further increase beam intensity. “This new achievement augurs well for the next LHC run, starting in 2015,” said Steve Myers, CERN’s Director for Accelerators and Technology, adding: “More intense beams mean more collisions and a better chance of observing rare phenomena.”

To put this in perspective, of the six million billion proton-proton collisions generated by the LHC, the ATLAS and CMS experiments have each recorded around five billion collisions of interest over the past three years. Of these, only around 400 produced results compatible with the Higgs-like particle, whose discovery was announced in July. A beam in the LHC is not a continuous string of particles but is divided into hundreds of bunches, each a few tens of centimetres long. Each bunch contains more than a hundred billion protons. Recently, the space between two bunches was successfully halved, achieving the design specification of 25 nanoseconds rather than the 50 ns used so far. A record number of 2,748 bunches were recorded in each beam, almost twice as many as the maximum reached in 2012.

“The LHC’s performance has exceeded all expectations over the last three years,” said Myers. “The accelerator delivered more than six million billion collisions and the luminosity has continuously increased.” The luminosity, a crucial parameter measuring the rate of collisions of an accelerator, has reached a value that is more than twice the maximum value obtained in 2011. The collision energy was increased from 7 TeV in 2011 to 8 TeV in 2012. This performance has allowed the LHC experiments to obtain important results quicker than expected. At the beginning of 2013, the LHC will collide protons with lead ions before going into a maintenance stop until the end of 2014. Running will resume in 2015 with an increased collision energy of 13 TeV.