At 12.38 a.m. on 5 April, the LHC shift crew declared “stable beams” as two 4 TeV proton beams were brought into collision at the LHC’s four interaction points. This signalled the start of physics data-taking by the LHC experiments for 2012. The collision energy of 8 TeV is a new world record. By 11 April the LHC had already delivered a total integrated luminosity of 0.2 fb–1 to the experiments. Last year, it took six weeks achieve the same number.
Although the increase in collision energy is relatively modest, it translates to an increased discovery potential that can be several times higher for certain hypothetical particles. Some, such as those predicted by supersymmetry, would be produced much more copiously at 8 TeV than the 7 TeV of 2011. Larger numbers of Standard Model Higgs bosons, if they exist, will also be produced at 8 TeV but background processes that mimic the Higgs signal will also increase. That means that the full year’s running will still be necessary to convert the tantalizing hints seen in 2011 into a discovery – or to rule out the Standard Model Higgs particle altogether.
Protons were accelerated to 4 TeV for the first time on the evening 16 March just two days after beam returned to the machine for 2012. A period of beam commissioning followed, during which the teams checked that the various systems are working flawlessly with beam. The optics measurements included setting the β* of the squeezed beam at the interaction regions. The aim this year is to have a smaller β* of 60 cm for the ATLAS and CMS experiments. The smaller β* is then the thinner and more squeezed the beams are at the collision points, but it also requires that the collimators are positioned closer to the beam. The collimation system is therefore carefully set up in different machine modes: injection energy; full energy; full energy with squeezed bunches; and full energy with collisions. By provoking beam losses and making “loss maps”, the operators verify that the beam is lost in the collimation region and not in places where it can cause damage. All of these checks take place with a few, often low-intensity, bunches.
The LHC is now scheduled to run until the end of 2012, when it will go into its first long shutdown in preparation for running at an energy of 6.5 TeV per beam in late 2014, with the ultimate goal of ramping up to the full design energy of 7 TeV per beam.
• To keep up to date on news from the LHC, see The Bulletin, http://cern.ch/bulletin.