As hinted in the last email, I can now blog about a conference that should hold my attention better. Planck 2013, which opens with a solid day of plenary talks, ten in total. The morning session involves two.
9:30 am: The Stability of the Electroweak Vacuum, Jose Espinosa
We start with a fairly clear talk, and follow with one that I have no idea what it will be about. Unexpectedly he gets to start the conference as a birthday present :) The precis: we have found the Higgs but no BSM, at a mass where the EW vacuum is unstable. The end (and meat of the talk) is about the implications of this.
Long known that fermions heavier than scalars can make vacuum unstable. Obvious implications given heavy top.
Interesting point: Higgs mass is perhaps worst value theoretically: compatible with SM, MSSM and composite Higgs, if marginally.
Let us assume that naturalness is wrong and the scale of new physics is the Planck scale. SM is weakly coupled to that scale. Instability (negative quartic coupling) is dominantly due to one-loop contribution from top. As known, decay nonetheless acceptable as very, very slow.
This talk has taken care to show how a well-known plot arises, that shows the stability in the Higgs mass-top mass plane.
Why are we close to the critical boundary for stability? Is it related to being close to the boundary between breaking/not breaking EWS? Is EW scale determined by BSM, or is it just coincidence?
Need BSM for e.g. DM, ν masses. Can make stability worse, better or have no effect; easy to find examples; e.g. See-saw neutrino masses fits in all three categories, depending on parameters. Heavy νR make worse, as large Yukawa coupling (same effect as top), while light irrelevant and other models with extra scalars cure problem.
Thermal decay during early universe? Not a problem. During inflation? If BICEP true, a problem, though this is contested in literature.
Notable open question if vacuum is unstable is initial conditions, i.e. landing in the correct vacuum.
10:00 am: Agravity, Alessandro Strumia
As the previous talk, assume that naturalness is a lie. Offer an alternative theory valid to arbitrary energies with a light Higgs.
Starting with "finite naturalness", but I have to say it's a good thing I read his paper else I wouldn't be able to follow this. He does make the connection to conformal theories explicit this time. I don't see that it follows that this is okay with it being an accidental symmetry, though...
EW scale generated dynamically. Specific example based on extra scalar with no Yukawas having a self-coupling that runs negative.
Agravity: how to include gravity in this idea. Start with dimensionless gravity action: no Einstein term. Planck scale dynamically generated from scalar vev with minimal coupling. Get ghost-like terms; argue fine as in Lee-Wick theories.
Scalar that sets Planck scale needs zero self-coupling and beta-function at Planck scale to get scale and zero cosmological constant. Add mirror of SM with mirror Higgs gaining vev at Planck scale.
Landau poles: unavoidable for hypercharge in such a theory. Can only be avoided with a GUT-like theory, but for finite naturalness this cannot be a simple GUT. It mush be a Pati-Salaam-like theory near the weak scale.
This idea is interesting if very dubious and I don't believe it. But it might be worth looking at in more detail at some point.