Friday, 8 July 2016

SUSY 2016 Liveblog: Day Five Session One

One thing that's clear in the negative category for this conference is the lack of power outlets.  I'll discuss the talks I had to see yesterday without power in the next few days.  Instead, we move on to the last day of talks, which is another half-day of plenaries.  Last night was the conference banquet, and I didn't get back to my hotel room till after 1am, so I'm a little surprised I've only missed one talk.

9:00 am: Scattering Amplitudes, Yu-tin Huang

This is the talk I (mostly) missed.  Which is a bit of a shame, as these ideas about more efficient methods of calculation are interesting, even if they can't be applied to any real theory yet.  Apparently (from the conclusion), what makes the theories where these ideas can be applied is their soft properties: universal behaviour as momenta go to zero.

What does perturbative completion with gravity say about UV gravity?  Perturbative completeness means string theory is the only current working possibility.

9:30 am: Baryogenesis, M.J. Ramsey-Musolf

The baryogenesis problem: one number we cannot explain in the SM (baryon asymmetry).  But, the number of BSM explanations is almost unbound. If we're going to look to experiment to resolve which idea is correct, it helps to know the energy scale associated with each mechanism.  High-scale ideas like leptogenesis are obviously harder to test, but not entirely impossible.

However, the focus in this talk is on a low-scale model: EW baryogenesis, where ΔB is generated at the same time as EWSB.  This demands a first-order phase transition, which is not the case for the SM Higgs parameters.  New scalars are thus a necessary condition.

The classical example in recent years for these scalars are light stops.  When they are below about 160 GeV, a first order PT occurs.  However, for baryogenesis we need even lighter stops, around 100 GeV, for the transition to be sufficiently strong.  These are in severe tension with the LHC null searches.  There are also constraints from Higgs measurements.

So naturally people consider different alternatives.  A simple but interesting possibility is to consider a standard scalar Higgs portal.  These show up in many theories but a single real scalar is a computable toy model.  Several feasible scenarios (pre-LHC).  When the singlet is heavy, the search channel is resonant diHiggs production; this might be discoverable with 100 fb-1.  However, to fully probe this parameter space would need a 100 TeV machine.

When the singlet cannot decay to the Higgs, then the search strategy is based on precision measurements of the Higgs couplings.  The states are similar in mass and so mix fairly strongly.  Most of the parameter space can be tested with a linear Higgs factory.  When the Higgs decays to the singlet, there's some question what can or cannot be probed.

Moving from a singlet to an EW multiplet as the new scalar, you get the interesting possibility of two-step transitions.  First the new scalar gets a VEV, then you transition to a phase where only the Higgs VEV is non-zero.  The presence of charged partners in the new scalar multiplet means that the Higgs diphoton width is a natural place to look.

A strong 1st order PT is not the only requirement, of course.  Sufficient CPV is required, and this runs into problems from EDMs.  A rough bound, if EDMs are generated at one loop, is that the new states are either multi-TeV; or, if they are at the EW scale, the CP phase is small (0.01).  Though (no details due to time) apparently this does not prevent EWBG.

What of Affleck-Dine baryogenesis in SUSY? Possible, to be sure; just wanted to focus on rich EWBG pheno.
Flavoured BG (from skipped slide)? Generation through flavour-violating lepton couplings.  Suppresses EDMs.  Probed by Higgs couplings to τs.
Two-step scenario excluded for same reasons as SM EWBG?  No, due to lower temperature of second transition; never reactivate sphalerons.
What about computation of baryon asymmetry?  Some details.

10:00 am: Exotic Searches at ATLAS and CMS, Shahram Rahatlou

Another speaker who feels no need to upload their slides.  Given that this is an experimental summary talk, I'm not sure if I should even bother annotating this talk; it's just going to be a lot of null searches that I won't be able to get all the details for.

General search strategy: do the easy stuff first.  Hence, look for resonances first.  Do the easy resonances (dileptons, diphotons, dijets) before harder ones (ditops, dibosons, mixed final states).  New result: different flavour dileptons (eμ).  No signal, of course.

Top partner searches recently added single partner production.  Enhance mass reach at cost of some model-dependence in production.

Discussion on diphotons.  (We have a whole talk on this later from Strumia.)  Not too much new to say.  He's being less pessimistic than the experimental speakers from the parallel sessions on Tuesday were, but I'll talk about that in a later plot.  Main conclusion seems to be: despite the absurd amount of theoretical work, no one has suggested something the experimentalists should be doing that they aren't.  Note that , which has to exist as well, shows no excess at 750 GeV.

Mono-X and similar DM searches.  Greatly expand the final states searched for compared to run-I.

Long-lived objects difficult, so only one run-II result so far (heavy stable charged particles).

Conclusions: extensive search program, but only the most basic and simple theories have been probede so far.  The next two years are critical; the happy ending is finding a new particle, which tells us what to do and might even justify an upgrade in the LHC energy.  The sad direction is nothing and we have to focus on Higgs couplings.

Resonances searches should really include examinations of subtle interference effects.  True, but this is not really a day-one strategy.

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