CERN is reporting that the Large Hadron Collider is ready to resume running. There was a minor problem with an electrical connection that has been fixed, so hopefully everything is ready for collisions at a center of mass energy of 13 TeV, just below the design energy of 14 TeV.
The beam is expected to start this weekend, as long as no other problems arise. Physics beam won’t happen quite yet though. It sounds like there is going to be an extended period of beam tuning, with actual collisions at full beam energy starting in June.
Higher energy allows for a great deal of new work to be done. In some important analyses, such as Higgs searches, the background can fall faster with energy than the signal, boosting the overall statistical power of results. Furthermore, higher energy increases the possible reach of searches for new physics. Many hope for new physics at the TeV scale in order to avoid fine-tuning in the electroweak sector of the Standard Model (such as from heavy quark loop corrections to the Higgs mass). In collisions at these energies, the protons can’t be treated as individual particles, but rather than collections of quarks and gluons (collectively called partons in this context). While the center of mass energy of a proton-proton collision will be 13 TeV, the actual amount of energy accessible in a collision will be smaller. If, for example, we take two typical quarks with about 30% of the total momentum, the center of mass energy would instead be around 4 TeV. Gluons typically carry even less energy (the actual momentum distributions of quarks and gluons in a proton are called “parton distribution functions”). So, in order to access TeV-scale physics, it would be useful to have an overall energy around an order of magnitude higher.