As you can read about in this article in the New York Times, the decision about the plan for starting up the LHC is to be decided later this week. The first collisions will not be at the design energy of the LHC (center-of-mass energy of 14 TeV), but at something lower. This has been known for a while, but now we are close to finding out, given the results of tests that have been made during the last few months, what energy the accelerator experts think is achievable.
We hoped to get 10 TeV after finding out that 14 TeV was not achievable. And in fact many physics simulations have been carried out in the last months in order to get ready for collisions at 10 TeV. But now we know that 10 TeV is also not going to happen, and we have to prepare for something lower.
Why does the center-of-mass energy matter? Well, according to e=mc^2, the energy in the collision is converted into the mass of new particles that we are trying to create. So the more energy we have in the collisions, the more massive particles we could potentially discover. For example, to discover a dark matter particle, the energy of the collision is converted into the mass of the new particle. Right now, we don’t know exactly what mass the dark matter particle has, so the higher the collision energy, the more massive particle we could potentially make. Our potential to discover something new depends on the energy of the collisions.
The other factor that matters is called luminosity, which is basically how many collisions we have. This is something else that will be decided this week. How long, probably some number of months, will we run at whatever energy we have? This is a difficult question because a shorter amount of running means starting the repairs of the LHC sooner, and getting to 14 TeV sooner. But a longer amount of running now would give us more data to analyze in the meantime, even though it is at a lower energy, and that would be very useful to have right now to calibrate the detector and find any problems.
My opinion is that the LHC should run at the highest energy it can as soon as possible, and run until we have a reasonable amount of data collected, enough to do some calibration and maybe even some first studies. Then shut the machine down, and we will do those studies while the repairs are done. Repairs often take much longer than expected, so getting data now, even if it is not the data we need to find the Higgs boson, is the way to go.
