A joint Fermilab/SLAC publication

CERN plans for even more intense year of LHC physics


In a photograph from 2010, physicist Mike Lamont, operations group leader for the LHC and its injectors, shows the LHC's then record energy per beam, almost 3.5 TeV. Operators will run the accelerator at 4 TeV per beam this year.

CERN scientists will begin running the Large Hadron Collider at a higher energy than ever before when this winter’s technical stop comes to a close in mid-March, the laboratory announced in a press release today.

Scientists and CERN management came to the decision to raise the LHC’s energy from 7 to 8 TeV after a week-long meeting in Chamonix, France.

Greater energy leads to a higher rate of collisions between protons in the LHC. The operation crew predicts that, with this increase in energy, the LHC experiments should gather more than twice as much data in 2012 as they did in 2011. This data increase is significant for Higgs searches and should give sufficient data by the end of 2012 to show or exclude the existence of a Higgs.

Raising the collision rate will cause the number of interesting physics events in general to go up, though scientists will have to sift them out from an even larger pile of non-interesting ones than before. Last year, for example, physicists on the ATLAS experiment saw 15 extra events for every one deemed worth studying. That number will likely double this year.

Scientists decided to run the LHC up until now at half the energy the machine was designed to handle. They made the decision to run at a lower energy after an accident that occurred when it first started in 2008. The accelerator’s magnets operate in a superconducting state and are cooled by liquid helium. Problems with an interconnect between magnets caused heating and a rapid expansion of helium, displacing about 50 magnets. To avoid risking another year-long shutdown for repairs, scientists restarted in 2010 at 7 TeV.

Operators are comfortable turning the machine up after running the LHC successfully at 7 TeV in 2011, improving their understanding of the interconnects and completing further testing, said Steve Myers, director of accelerators and technology at CERN. They will not raise the energy to 14 TeV until sometime after a more extensive shutdown of about 20 months at the end of 2012.

For the machine operators, the main challenge of going to 8 TeV and running with higher collision rates will be reducing the size of the particle beams at the collision points inside the detectors, said Mike Lamont, operations group leader for the LHC and its injectors. Squeezing beams at these places takes extra finesse and care.

The plethora of collision events will provide another challenge for the experimentalists, data-hungry as they may be. In order to process everything, the collaborations each use their own elaborate software simulations, called Monte Carlos after the locale famous for its games of chance. The simulations use statistics to predict the types of particles that will be created in collisions with different characteristics in the LHC. The scientists are constantly revising the algorithms to keep up with changes in machine operations.

Although the simulations are necessary, rewriting them and ensuring their accuracy can be a huge challenge for the experiments, said ATLAS physicist Bill Murray. Last September his collaboration began using a new set of Monte Carlos. In three and a half months, they were able to fully simulate a record-breaking 1.5 billion events. But the work was exhausting and just barely finished in time for the December Higgs update.

Now, with the imminent energy increase to 8 TeV, the scientists will have to wrangle with the same issue of adjusting software and finishing analysis for the summer conferences as well. Debugging and simulating events shouldn’t be a problem; it’s a matter of how much time the work will take. With the Higgs race on and thoughts that this could be a Nobel-worthy year, no particle physicist wants to slow down. “It’s going to be extremely painful in May and June,” Murray said. “But it should be entertaining.”

The software used by CMS, the other collaboration contending for a discovery, takes only about half the amount of processing power as the software used by ATLAS, Murray said. It may be easier for CMS to get Higgs results sooner, he said -- not that it will stop his collaboration from trying harder than ever.

“By 2014 the downsides [to having to redo the collaboration’s Monte Carlos] should disappear,” Murray said, “So for me, the downsides are short-term things. In the end we should have more data at higher energy, so a greater physics reach.”

Beams will commence around March 15 and first collisions at full energy should be seen after about three weeks. Then the hunt for Higgs and other physics will shift into the highest gear yet.

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