First collisions at the LHC with unprecedented Energy! (Ask a particle physisicist)

[Alex K]

How does this compare with the maximum that they were expecting from the revamped collider?  Are there higher energies yet to come?

The original plans for the LHC were that it should have 14 TeV from the start - now it has 13, so this is already pretty close. Maybe they will optimize the magnets over the years and go to 14 and a bit beyond. But radical increases would require a major upgrade and basically rebuilding large parts of the collider. A more realistic goal is the increase of Luminosity, i.e. of the intensity of the beams

http://project-slhc.web.cern.ch/project-slhc/about/

This will allow to collect more statistics at the given energy.

And are there specific questions within the new energy range which they are expecting to answer with it?

Well, that is a very good question. The trouble is that with the Higgs, we knew in what mass range it had to lie in order to work its magic (*roughly* 114...800 GeV), but now that a Higgs boson is found - if it is really the beast everyone expected, we have few clues at what energy scale the next things should be expected, and that's a major source of worry in the community.

There is a whole class of Dark Matter models which work very nicely if the particles in question are near the 1 TeV range. However, Dark Matter could be of a different type which is not observable at the LHC, so this is no safe bet.

The apparent unification of forces works best if Supersymmetric particles exist at masses of 3...10 TeV. However, that is a very indirect argument and you have to interpolate over 13 orders of magnitude in Energy to make it.

There is a general argument from fine tuning of parameters: in the standard model, the overall mass scale of the Higgs and the W and Z bosons etc, -theoretically - tends to be pulled up all the way to the next scale where new phenomena occur, in the worst case the planck scale - and one has to adjust a parameter in the theory to daunting precision in order to pull the masses of all the observed particles back down to where they are (at only a 10^-17 th part of the Planck scale).

Some hypothetical models such as Supersymmetry would remedy this and provide an automatic explanation how such a hierarchy between two scales can exist. The heavier the new particles are, the more fine tuning must be done again in order to keep everything else at light masses where we observe it. This was always one motivation why Supersymmetry was suspected to be "around the corner" for years. Unfortunately, as things are now, the mass of the Higgs already has permille sensitivity to the input parameters (meaning that the Higgs mass varies a thousand times more than the input parameter when you wiggle with its value) and it is bound to get worse. Once you are at this point, no one really thinks 1000 or 100000 would make such a difference, so there is not as much confidence in the fine tuning argument any more that Supersymmetry has to be observable at the LHC. The same problem more or less holds for popular alternatives to Supersymmetry which would explain the hierarchy between the Planck scale and the Mass of the Higgs etc.

Unfortunately, Dark matter is the only concrete problem for which it seems probable that a solution should lie in an energy range accessible to us. Everything else could possibly appear only at much higher masses. But as I said, even Dark Matter could be unobservable at the LHC. But at least one knows exactly how to look for it if it is of the right type, and that's being done.

There are some "anomalies" that have been floating around for years, in particular the magnetic moment of the muon which looks like it shows a deviation. If this deviation is caused by new particles, it is very reasonable to assume that they could be in the observable range - if the anomaly is real.