Wednesday, 25 May 2016

Planck 2016 Liveblog: Day Two Session Four

For me, the last session of the day is BSM 3, AKA the 750 GeV session.  I'm hoping this will be more interesting for me.

5:10 pm: A toolbox for diphoton model building, Manuel Krauss

There are a lot of things a model builder must consider when trying to fit the excess.  If we want to address all of them, automised tools make our jobs a lot easier.  This talk is to advertise SARAH, a Mathematica package that will analytically generate things like masses, vertices and RGEs; then put all of that into SPheno for numerical study.

In particular, new inclusion of calculating the gg/γγ decay rates up to NNNLO/NLO.  This is all that's needed to get the associated cross sections.  Radiative corrections cannot be ignored; in particular, factor of 2 in gg partial width.

Applied to SUSY model with U(1)X and pseudoscalar.  Most interesting point: easy to see that large width preferred by ATLAS excluded by vacuum stability constraints.

5:30 pm: Radion/Higgs phenomenology and the diphoton excess at the LHC, Anibal Medina

I'm not taking notes here because I know this paper quite well.

5:50 pm: Drell-Yan Constraints on New Electroweak States and the Di-photon Anomaly, Christian Gross

The archetypal model involves new charged states, not too heavy.  How can these be probed?
  • Direct collider searches are model dependent.  Limits range from 100 GeV for lepton-like states with SM charges, up to TeV for coloured states with exotic charges.
  • EW precision observables are not very constraining.
  • Modification of gauge beta functions is generally quite significant.
The modifications to the beta functions can show up in differential distributions of dimuon cross sections, measured into multi-TeV range.  Can also alter forward-backward asymmetries, but this is currently less constraining.

The resultant bounds are significant, but they don't quite exclude the relevant parameter space.  However, they are not too far from it.  Given that we are considering model-independent bounds, this is pretty good and raises the hope of a definitive test before too long.

This is all one-loop; surely the necessary large Yukawas would have two-loop effects?  Perhaps.  Imposed Yukawa perturbativity to 3 TeV.

6:10 pm: Characterising the 750 GeV diphoton excess, Dipan Sengupta

A number of things we'd like to know about the putative resonance: questions like spin, width, decay mode etc.  Trying to resolve these using various kinematic distributions.

6:30 pm: A closer look at the sgoldstino interpretation of the diphoton excess, Pietro Baratella

Non-zero gaugino masses imply sgoldstino-gauge boson coupling.  This means that the resonance coupling to gg/γγ is related to the gaugino masses, a non-trivial constraint on the model.  Leads to a low SUSY breaking scale, order several TeV.  Suggests GMSB for flavour.  Messenger mass can not then be too heavy; leads to inconsistency between gluino bounds and signal cross section.

Need to move to a (fine-tuned) limit with an anomalously light messenger, due to an accidental cancellation.  This enhances the effective coupling, though it pushes things towards the strong coupling limit.

6:50 pm: Di-photon excess illuminates Dark Matter, Alberto Mariotti

Take the ATLAS width seriously.  Must decay to something other than dijets/digamma.  Choose DM to avoid constraints.  DM parameters (assuming Dirac fermion) are then approximately fixed by demanding signal cross section, total width and relic density: mass ~ 300 GeV and resonance-DM coupling ~ 2.

Then we can ask if this is allowed.  LHC searches in dibosons, monojets as well as DD limits.  Taking a grand total of four benchmark points, 3 survive.  All are close to DD bounds, so could be found in near future.

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