Thursday, 23 May 2013

Planck 2013 Day Four Live Blog 1

So, last night was the conference banquet and this morning we had string theory talks.  A combination of events that gave me little motivation to attend and less to pay any attention, hence no blogging from me.  After lunch, we have the last set of parallel talks, and I think I'll be attending the Higgs talks.  My own talk here was on a similar subject, albeit off in one of the BSM sessions, and it is an obviously exciting area right now.

Another useful thing is that they have finally gotten around to putting the parallel session talks on the conference website.  Better late than never, I guess.

2:00pm: Jesus Moreno, "Double Higgs production in a singlet-extended Higgs model"

So it looks like the model we are dealing with involves the SM plus a real scalar singlet that mixes with the Higgs.  I wait to see what the motivation for that is.  Looks like the biggest motivation is minimality, which is fair enough, and a number of claims about what can be achieved to which I am a bit skeptical. The most plausible is stability; as is well-known by now, the 125 GeV Higgs is on the edge of the stability region, but probably only metastable.  An extra scalar would give a positive contribution to the running of the Higgs quartic, and that contribution need not be large to make the potential absolutely stable.

Of course, there are some interesting consequences of a metastable potential, but that's not this talk.

Most general renormalisable potential in the proper EFT approach.  Assume new scalar not forbidden to gain a VEV.  Higgs measurements give us a constraint on the mixing, which suppresses all Higgs couplings.

Looks like we have a number of missing plots in the slides, which is unfortunate.  I'm guessing this was a keynote file exported to PDF.  But it's going to make following the talk tricky.  Ooh, plots in chalk on a blackboard!

Precision constraints, also well-known that these prefer a lighter Higgs than actually observed.  The discrepancy is not large, maybe one sigma.  The singlet-Higgs mixing modifies the loop corrections, equivalent to an effective contribution to the S/T/U parameters.  This prefers a singlet in the 300 to 600 GeV range.

Direct production of the new scalar is most interesting when it decays to two Higgs; consequent four-bottom resonance signal.  Of course, that's not easy to find.  The subleading decays to leptonic Ws are easy to find but the loss in mass resolution is important.  At this point we have a fairly standard analysis looking for this signal, everything looks fine that I can see.  Cuts applied both to the two-bottom resonance and the leptonic side of the decay.  However, it looks like we need hundreds of inverse femtobarns, to get only a moderate two-sigma significance.  In short, little hope of discovering this at the LHC; evidence at best.

Finally, this singlet can serve as a portal to some fermionic dark matter.  Getting a thermal relic density requires being "close" to a Higgs or new-scalar resonance.

2:30pm: Rick Gupta, "BSM theories face Higgs coupling data"

Opening question: how precisely to we need to measure the Higgs couplings?  What is the largest deviation in Higgs couplings possible even if no other EWSB state is seen?  If we discover a new state, we already know that the Higgs sector is exotic.  If we don't, precision measurements are the only tool we have; how high a scale can we probe?

Example 1: mixing with complex scalar singlet.  Mixing reduces all couplings uniformly.  Maximal allowed deviation for undetectable scalar is 6%.

Example 2: composite Higgs, SILH: deviations by ratio of scales, v to strong scale f.  Maximal deviation tens of percent.

The speaker has a microphone, he does not need to shout.  Yeah, I stopped listening because it was giving me a headache.  He's talking quite normally now in the Q&A, so he can.  If he'd talked like that through the whole thing I'd have been able to pay more attention.

3:00pm: Felix Bruemmer, "Engineering a 125 GeV Higgs in the MSSM"

Browsing the slides, it looks like this is an attempt to do things in a top-down way.

Start with the well-known requirements for minimal fine-tuning in the MSSM: moderate tan β, higgsinos below about 500 GeV, stops below a TeV and gluinos below 2 TeV.  Finally, we need maximal stop mixing, the real kicker.

A key part of the problem is that there is a low-energy attractor for the stop trilinear term and the stop mass, thanks to the large effect of the gluino mass on the running.  This leads to At ~ - mstop.  To avoid this we need fairly extreme conditions at the high (mediation) scale, either very large At or very small mstop, so that we do not manage to reach the attractor by the time we reach the electroweak scale.

We start by considering models that can generate large trilinear terms.  Failed examples: gauge/gaugino mediation, radion mediation.  Successes: Kahler moduli-dominated mediation (from string theory); extreme hierarchies between the first two and the third generation squark masses, from negative contributions to stop masses; any model where trilinear is a free parameter, such as gaugino-Higgs mediation.

A slide of plots on the last example; it can satisfy the Higgs mass, but I don't see a whole lot here.  The main point as far as predictions go is a suppression of the order of 10% to the diphoton signal, so consistent with CMS.

In the latter bloc of the talk we are looking at gauge mediation, with the goal of using a solution to the μ problem to lower the stop mass.  The method is to gauge a flavour group; this violates flavour universality, but that is now a good thing as it will lower the stop masses relative to the over squarks.  In particular, break an SU(3)F symmetry to SU(2)F, and the latter will protect us from FCNC; and if some of the VEVs that break the symmetry also break SUSY, giving mass splittings to the gauge messenger fields.  These give tachyonic contributions to the stop mass.

I've gotten a bit lost on the details here, but it looks like a very interesting model-building idea.

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