The Sciences

Decision for the LHC: 1 Inverse fb at 7 TeV or Bust!

Cosmic VarianceBy John ConwayJan 29, 2010 3:34 PM


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Last week in Aspen we learned that this week would be when a major decision was reached by CERN at the annual Chamonix meeting as to how to operate the LHC at high energy. Following the magnet quench incident in September 2008, a year-long shutdown ensued for repairs to the magnets, and retrofitting of the rest of the machine for better quench protection circuitry and helium pressure release valves. Not all sectors were warmed up to room temperature for the retrofit last year, but all magnets were trained to go as high as beam energies of 5 TeV (design energy is 7 TeV per beam). In November and December the LHC commissioning resumed, and it became the world's highest energy collider on December 8, eventually delivering about 50,000 collisions at 2.36 TeV to CMS and ATLAS before shutting down for Christmas. But the question facing the LHC managers this week was whether attempting to operate the LHC at 5 TeV on 5 TeV in 2010 was worth the risk to the machine itself. Clearly another disaster of the scale of the one in 2008 would cripple the program for a long time. In the end the decision is to operate the LHC at 3.5 TeV on 3.5 TeV (7 TeV collision energy, 3.5 times that of the Tevatron) and accumulate a substantial amount of physics-quality data: 1 inverse femtobarn, or stop by the end f 2011, whichever comes first. This corresponds to something like ten trillion proton-proton collisions, of which only a small fraction will yield events interesting enough to record for later analysis by the experiments, and of these, only a tiny fraction yielding data relevant for physics. After a one to one-and-a-half year shutdown in 2012 to retrofit the rest of the machine and make other preparations, the LHC will attempt to double the energy, to 14 TeV in the center of mass, in 2013 and accumulate substantial physics data. My best guess is that if the Higgs boson is to be discovered, it will be at high energy with this large sample of 14 TeV data. We might be able to rule it out at 95% confidence in certain mass ranges if it's not there, but we ought not be able to do that if it is, right? Patience, patience! Nevertheless, there is no question that in a few weeks, when operated at 7 TeV collision energy, the LHC will become an awesome discovery machine. There are many new physics scenarios in which we will be able to see new phenomena with just a fraction of the full 1 fb^-1 sample. Will nature give up her secrets so readily though? She may not - we may spend this year and the next rediscovering the Standard Model, building up understanding of the detector, and sharpening our analysis tools in order to discover quite subtle effects. No matter what happens, this is the most exciting time in particle physics in decades.

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