Wednesday, 22 May 2013

Planck 2013 Day Three Live Blog 3

We kick off the afternoon talks with a parallel session on BSM physics.  Conference Website.

2:00pm: Michael Schmidt, "The Scale-Invariant NMSSM and the 126 GeV Higgs Boson"

What is the natural region of parameter space in the NMSSM?  Result: coloured sector can all be above a TeV with 5% fine tuning, but electroweak states must be light.

In the region where we can get the Higgs mass, MSSM features large loop corrections to mHu2, that must be cancelled either with mHd2 or μ2 to get to the Z mass; this leads to the usual MSSM fine tuning problem.

Quoting Churchill(!): Never give up!  Look for other tree level contributions to the Higgs mass to alleviate the fine tuning problem.  Hence the NMSSM, with the singlet giving an additional contribution.  But to get the tree level result, the Higgs-singlet coupling must be so large that we have a low-scale Landau pole (non-perturbativity).

Adopt a bottom-up point of view, with the low-scale pole corresponding to an unspecified UV-completion.  Fine tuning measure decomposes into running times weak-scale tuning; former is unchanged(?), latter differs.

Larger values of the Higgs-singlet coupling allows a larger stop mass without issue.

Unexpected problem: for large coupling, the Higgs mass can actually be too heavy!  This requires a cancellation that leads to an additional tuning.  Surely we can use a smaller value of the coupling?  Possibly because that would require lighter stops, coming into tension with direct searches?  Need to consider all tunings, so coupling ~ 1 is optimal point.  Generically do better than MSSM for fine tuning, and a factor of ten better at optimal coupling.

EWPT: tan β less than about 5, so quite small.  No problems with Yukawas, of course.  Higgs searches: done when two-photon signal high, so not included.  LSP is mostly Higgsino in most of parameter space, probably due to small μ for fine tuning reasons.  Also a consequence of scanning linearly in parameters, pushing gaugino masses up.

Typically gluinos just beyond exclusion limits, 300 GeV LSP and several hundred GeV stop.

2:30pm: Akin Wingerter, "SO(10) meets LHC"

SO(10) as a flavour theory (in addition to everything else).

Rare decay Bs to μ+μ: large SUSY contributions for large tan β, which is the generic case in SO(10).  So measurement in excellent agreement with SM is a problem here.

Dermisek-Raby model: uses Frogatt-Nielsen fields at GUT scale to generate the needed Yukawas.  Only 3rd generation has direct coupling to Higgs fields.  Seems to lead to a GUT model for split SUSY.

This wasn't a bad talk--far from it--but I didn't get much from it.

3:00pm: Marcin Badziak, "Light staus and enhanced Higgs to diphoton rate in SO(10) Yukawa Unification"

Still a small enhancement in Higgs to γγ, depending on how you interpret the CMS data.  Really?

Within MSSM, only third generation squarks have large enough couplings to the Higgs to significantly effect the Higgs decays.  Staus have the advantage of not suppressing the coupling to gluons.  However, there is a problem: large stau mixing not only enhances the photon coupling, it gives a negative contribution to the Higgs mass.  Still, an enhancement up to 50% is possible if the lightest stau is right on the LEP bound.

The stau enhancement is well-studied from the low energy perspective, but can it be motivated from a high-scale model?  The purpose of this talk.

SO(10) implies Yukawa unification (at least for the third generation).  This is difficult to achieve, making these models more predictive in terms of the superpartner spectrum.  Also, to achieve radiative EWSB we would need to split the Higgs soft masses, which can only be done through D-term contributions.  This comes from the fact that SO(10) has larger rank than the SM group, but implies shifts to the sfermion masses; in particular pushing the sleptons down.

Additionally, the bottom mass is too large without threshold corrections.  To get the right sign this means that μ is likely negative.

Finally, since we have light staus, we must also have light gaugino masses to avoid a charged LSP.  Combined with limits on the gluino mass, we must abandon universal gaugino masses.  This requires non-trivial SUSY breaking F-term.

Numerical results: can unify Yukawas to the percent level and get a 10% increase in the diphoton rate while meeting usual flavour constraints.  With the reduced diphoton rate, 170 GeV staus are acceptable.  Strongest limits on sparticle masses is not Higgs mass, but flavour in the bottom sector.

No comments:

Post a Comment