Monday, 9 November 2015

KIAS-CFHEP Workshop Liveblog: Day One Session Two

We return from the coffee break to finish the LHC experimental talks, followed by a discussion of Higgs theory.
11:20 am: BSM searches at the LHC, Tae Jeong Kim

First channel: ttbar resonances.  Requires fat jets in interesting, boosted region.  KK gluon now excluded up to 2.4 (2.8) TeV by ATLAS (CMS).  Comparable searches and limits for a Z'.  Closely-related tb resonances (W') excluded to about 2 TeV.

Next up are several SUSY searches.  Limits continue to improve much as they have throughout this LHC age.  Gluino limits look pretty exhaustive.  Direct stop searches still have degenerate gaps.  Looks like massless neutralino, mstop = mtop still allowed.  I guess I must have seen that before but for some reason I hadn't put things in quite that context.  What are the limits from top cross sections?

Models with light gravitinos searched for using photon final states from NLSP neutralino.  I guess coloured NLSPs would be sensitive to the usual jets + MET searches.  Limits are much like those in other channels.  Interruption: limits assume gravitinos (as clearly stated in talk), but axino allows lighter gluinos.  I don't think that comment was urgent.

Excess! SFOS leptons in GMSB, 3 sigma.  Excess more in electron channe than muon channel, but persists in combined data set.  Excess consistent with leptons coming from Z decay.

Higgs (decay to γγ) now used as tag in SUSY searches.  Expect to use this to exclude 150 GeV Higgsino, but so far fail to do so.  This is important as light Higgsinos are unavoidable if SUSY is natural.

Now on to the diboson excess.  Local significance at the 3 sigma level, global about 2.5, at ATLAS.  I did not follow this section; it seemed to be a lot of unrelated searches with no overall structure.

13 TeV dijets, BH searches; easy results based on early data.  No excesses seen.  Some prospective run-2 limits.

Conclusion slide says that "new surprise is expected soon".  Ignoring the contradiction, does he know something?  Or just clumsy optimism?

What is combined significance for diboson searches across channels?  Has not been done.  Problem because excesses are correlated.
Tagging efficiency of Z bosons? 70%/
Charm tagging?  Already available from ATLAS, not CMS.

12:00 pm: Roles of the perturbative unitarity in extended Higgs scenarios, Masaharu Tanabashi 

Unitarity sum rules imply patterns in the Higgs couplings, susceptible at e.g. ILC.  Consider electroweak chiral Lagrangian and models for the diboson excess.

Chiral Lagrangian describes EWSB without Higgses.  Add arbitrary number of neutral bosons.  This leads to ρ parameter equal to one without custodial symmetry (at tree level).  More interestingly, it also imposes finite oblique corrections at one-loop, without additional assumptions (in particular renormalisability).

2 TeV diboson tension between all-hadronic excess and semi-leptonic constraints.  Another problem comes from comparing branching ratios to WZ to Wh, or WW to Zh.  These decays are to longitudinal polarisations and for a single Higgs doublet, the two channels are equal.  Direct bounds from searches for the Vh final state bound cross sections only one-half of the required hadronic excess cross section.

Non-SM like Higgs allow the breaking of this identity.  This opens up a small region that works consistent with all limits: suppress W' coupling to fermions to one-third, enhance coupling to gauge bosons by 4/5, suppress decay Higgs by ~2.

Seems to need to suppress hWW coupling, though not clear why that is.  And that seems to be in tension with Higgs coupling measurements.

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