Tuesday, 27 August 2013

SUSY 2013 Live Blog: Day 1 Session 4

For the final SUSY 2013 session of the day I'm at the Higgs session.  To noone's surprise, this is by far the most over-subscribed group of sessions.

4:30pm: Satyanarayan Mukhopadhyay, "Jet angular correlations in VBF topology: top and bottom quark processes as a testing groung"

This is about looking for correlations in the Higgs decay products as a handle on the Higgs CP.  The idea seems to be to look at the experimental feasibility in QCD background processes as a stepping stone to applying it to the Higgs.

The specific correlations are azimuthal in VBF-type topologies (two additional forward jets).  Being azimuthal, they must arise from interference effects.  Special cases of velocity (of ?) goes to zero or one, only one type of correlation remains.  In soft t-channel limit, correlations approach maximal.

Reliance on interference means cannot simply use parton shower.  Matching of exact matrix elements to shower necessary, ongoing.

4:45pm: Kentarou Mawatari, "Higgs characteristation -- Beyond Leading Order"

Characterising the Higgs is no more (or less) than determining the Higgs Lagrangian.  The most natural way to do this from a phenomenological way is, of course, using an effective theory.  Implemented possible bosonic EFTs in FeynRules, assuming a single new state up to a cut-off Λ.

Spin-zero case is usual familiar one.  Include possible mixed CP state.  Include dimension-5 operators, necessary as no renormalisable coupling to W, Z for pseudoscalar.

Spin-one case included; what of the photon observations?

Spin-two, included though if I recall correctly the experimentalists claim to be ruling this out now.

With this systematic approach, can now implement NLO calculations of matrix element.  This is just QCD so is now automated.

5:00pm: Ben O'Leary, "Checking for Undesired Vacua Quickly"

As is well known, even a single scalar with tree-level potentials have the potential for multiple minima. This gets absurd once we start adding more scalars, e.g. SUSY.

While this has of course been looked at, so far it has been implemented only on a model-by-model basis and, in most cases, only at tree level.  Hence new software package vevacious.  This works on (SUSY) models, finds minima quickly enough for scans, and calculates tunnelling time for undesired local minima.

Applications: excluding some stau NLSP models.

The essence of this talk is simple, but if the software works as well as advertised it can become a standard tool in explorations of SUSY models.  I will likely download it soon and play around with it.

5:15pm: Vincenzo Branxhina, "New Physics and Electroweak Vacuum Stability"

Well-known issue in SM that, with current measured Higgs and top masses, we are in a metastable but long-lived vacuum.  However, we are close to the stable/metastable boundary, compared to the experimental errors.  This would say that it is consistent to have the SM valid to the Planck scale.

For the observed Higgs and top masses, the second minimum is at a very super-Planck VEV.  However, Planck-suppressed operators are ignored in their contributions to the tunnelling time.  This does not look consistent.

An explicit way to consider things is to add terms in the Lagrangian with six and eight powers of the Higgs mass.  These terms indeed stabilise the Higgs potential now near the Planck scale.  We can also compute the tunnelling time in this case.  In particular, we obtain a new bounce solution that lowers the classical action.  Since that appears exponentially it has a large effect, it exponentially dominates over the usual case.

In particular, with order-1 coefficients of the Planck-suppressed operators, with the Higgs-to-the-sixth coupling negative, then the tunnelling time is an absurdly small fraction of a second.

Planck-scale physics can be important in some cases, though not all; if the first non-renormalisable term has positive coefficient then the standard results are acceptable.

5:30pm: Alberto Parolini, "UV Completions for Composite Higgs"

I think this talk is a bit too technical for me at this point of the day.  UV completion is a SUSY model, with Seiberg duality being used to compute things.  SUSY also leads to partial cancellations in computing the effective potential of the Higgs pNGB, allowing it to be much lighter than the compositeness scale.

No comments:

Post a Comment