If I understood correctly, the LHC will be shut down at the end of 2012 to prepare for the full-power, 14 TeV collisions in 2014. I also remember reading about a proposed luminosity upgrade some time in 2019 and also about a preliminary plan for VLHC, which should, when fully operational, achieve about 175-200 TeV.

My question basically is, what are the goals after these upgrades? I know there's SUSY, but that's supposedly in reach of those 14 TeV collisions, so what experiments are set around the 200 TeV mark? Are we already reaching into the proposed "desert" between the TeV scale and GUT scale?

  • $\begingroup$ For what it's worth, in a recent Higgs-announcement seminar at Imperial, Tejinder Virdee stated that it's probably a bit early to know exactly what to look for in whatever accelerator replaces the LHC. $\endgroup$ Jul 9 '12 at 10:17
  • $\begingroup$ 175-200 TeV will certainly not happen in 2019. The energy limitation of the current LHC comes from the maximum field the dipole (bending) magnets can produce. To go to higher energies, one needs to either dig a longer tunnel or do extensive research & development to build large scale magnets achieving higher magnetic fields or both (this is called FCC-hh, there is also a similar Chinese project). Given the large cost of such a project, studies are currently going on to reduce cost and a decision whether or not to build such a machine will not be taken until we have more results from LHC. $\endgroup$ Jul 24 '14 at 15:51

In short, as with any exploration, you are doing it because you have no idea what you might find. I am sure with many frontiers in the past there were people skeptical about what would be found. Take the ocean for example - I sure there people that didn't think anything would be found at the extreme depths and pressures that they did end up finding living things. As for physics, the possible desert between TeV and GUT scale is a worst case scenario. Besides there is a ton of motivation for new physics beyond the Standard Model other than just GUT unification. For one, you mention SUSY - even if we see some SUSY particles at the LHC before or after the upgrade, most likely some superpartners of SM particles will be out of reach of even the LHC. Among others motivations are: dark matter, strong CP problem, the fine tuning of the Higgs mass ("the hierarchy problem") and any of the numerous mass hierarchies in the Standard model amongst the fermions.


Even if there is no new physics between 200 GeV and the GUT scale (and I would take a 100:1 bet against this), you can still potentially see very interesting physics in the standard model itself at these high energies:

  1. Baryon number violation: instanton mediated proton decay is known to occur in the standard model, but at ridiculously small rates. In collision with much higher energy than the Higgs scale, nontrivial topological configurations of the SU(2) field could be excited which give rise to violations of baryon number. We could get parameters for leptogenesis, which uses the instanton mediated decay to populate the early universe. There is controversy now in the literature whether you can excite instantons efficiently by collisions, or whether you need a thermal background. My feeling is that it should happen, but I have seen arguments against. At 200 GeV collisions, maybe we will find out.
  2. Thermalization of high-energy collisions, and plasma generation by collisions: one of the interesting things that distinguish a thermalizing RHIC collision from a LHC collision is jet quenching. Maybe at higher energies you will be able to link the two types of things, point collision and nuclear collisions, in some regimes, so that you might see jet quenching in certain types of partonic collisions at 200 GeV, and not in others (depending on the interaction of the products).
  3. Regge physics: QCD is at least approximately equivalent to some sort of not precisely known string theory. The information from very high energy collisions near the beam line tells you the full trajectory structure when all 6 quarks can lie on the string ends. I like this stuff, although it is extremely old fasioned.
  4. Axions: maybe we'll learn about these, although I am not sure how exactly.

These are just random thoughts--- there might be good arduments against 2, and 3 might be understood well enough from LHC and the previous generation of experiments, I don't know (these are things nobody talks about). In this answer, I am ignoring the great likelihood that we will find a whole Higgs sector, with SUSY and a whole world that has been dormant since the big bang, waiting for us to build a machine to excite it.


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