LHC Restart

By John Conway | April 30, 2009 11:39 am

CERN announced today that the final replacement LHC magnet was lowered into the tunnel, and is making its way to Sector 3-4 (between collision points 3 and 4). Last September’s incident led to 53 magnets – about half a kilometer of the 27-km ring – having to be removed, repaired or cleaned, and replaced. Check out the video of the final dipole going in on April 16.


New systems are being installed to better detect incipient magnet quenches, and the helium pressure relief systems are being upgraded. My understanding is that this will be done on the greater part of the LHC magnets this year, but not all of them.

The plan is to complete the installation of the replacement magnets, and then cool down all sectors. At present, half the machine is being held at liquid nitrogen temperatures, and the other half is at room temperature. It will take a couple months, once they start, to cool the whole machine down to superconducting temperature, about 2 degrees K.

So, by late summer the LHC commissioning can begin where it left off last September. Assuming all goes well, physics collisions are foreseen at 10 TeV by late fall. The machine will not run at the design energy of 14 TeV at least until the entire machine is retrofit with the new quench detection and pressure relief systems. And, the beam intensity (and hence collision luminosity) will not be anywhere near the ultimate goal during the first physics run.

The usual running pattern at CERN is to start in the spring, and collide beams through the late fall, and do machine maintenance, etc. during the winter when electric power is more expensive in Europe (they heat with nukes, basically; we burn fossil fuel in the US in winter).

But a decision was reached earlier this year to run the LHC through next winter, with only a brief two-week shutdown for Christmas and New Year.

What we can reasonably expect is that if all goes well, we can accumulate something like 100 inverse picobarns of collisions by spring 2010, and perhaps 200 pb-1 by the end of the run in fall, 2010. Now, pb-1 this is a strange unit – it has dimensions of inverse area. Formally we call it integrated luminosity. Basically it tells you how many collisions you’ve had, in essence. To get the number of some type of interesting events, you need to know the cross section – which has units of area – for producing that type of event. Then you simply multiply the cross section times the integrated luminosity.

Once the machine shuts down in late 2010, and if we do have a sample of about 200 pb-1, there will ensue a long shut down in 2010-2011 to complete the magnet retrofit. The LHC will then not run until late 2011.

This means that the lower-energy, relatively small sample of physics data is all we will have to analyze until 2012, three years from now! The experiments have already been simulating collisions at the lower energy and retuning analyses.

Though everyone is waiting breathlessly for the LHC to discover the Higgs boson, with lower energy and a smaller sample, I would not bet on the LHC finding it any time before 2012. In fact, a full analysis of the Higgs sensitivity at 10 TeV is yet to be done in ATLAS and CMS. This is a huge task, and will take months, but there is no question that it is more difficult at the lower energy, and it’s already very hard for the LHC to see, say, a 120 GeV Higgs boson. As I wrote in my post in March, this is also very hard for the Tevatron in the same time period. Those of us looking for a standard model Higgs boson have to exercise a bit of patience while working very hard toward the ultimate goal!

Neverheless, there is a ton of new physics that *could* emerge from even the first LHC physics sample from the 2009/10 run. If nature has new high-mass particles giving observable pair-production resonances at energies not accessible at the Tevatron, they could stand out in sharp relief above the standard model. Similarly if there are extra dimensions of space time, we may see excess pair production of standard model particles. If supersymmetry exists, and the experiments manage to understand well the apparent missing momentum transverse to the beam direction (a big challenge) then a first observation of the presence of supersymmetric particles might be possible.

At this point, all you can do is admire the wisdom of the great Zen master Yogi Berra, who said “If this was easy it wouldn’t be so hard!” But then, maybe we’ll get lucky.

CATEGORIZED UNDER: News, Science, Technology

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

Cosmic Variance

Random samplings from a universe of ideas.

See More

Collapse bottom bar