Tuesday, 10 November 2015

KIAS-CFHEP Workshop Liveblog: Day Two Session Three

This is the first of two afternoons of parallel sessions.  I've decided to stay on the fifth floor, and attend the session on composite Higgs followed by supersymmetry.

2:00 pm: Composite Scalars at the LHC: the Higgs, the Sextet and the Octet, Thomas Flacke

Work based on UV embedding of composite Higgs model that lead to additional scalar fields.  Sextet and octet refer to colour charges, and expected to be light.  Specifically, 2014 classification of possible UV completions based on simple hypercolour groups gave minimal model as based on global symmetry breaking SU(4)/Sp(4).

The microscopic fields are fundamental, colour-neutral fields (that produce the Higgs) and tensor fields (that allow SM quarks to couple to the composite sector).  Mesons formed from the latter are Goldstones of the breaking of a global symmetry (that is broken by gauging SM colour).  They should then be light, and are coloured.  These are the particles of the title.

We have some new particles, the obvious question is what the limits on them are.  This in turn depends on their couplings.  Both can be pair produced via QCD.  Sextet has gauge-preserving coupling to tR, and can be singly produced.  Preferential decays to tops (strongest coupling to composite sector).  So four-top or two-top final states, bound at 800 GeV to 1 TeV.  Projected limits at run 2 difficult; main backgrounds from misidentification of charges, hard to simulate.  But if discovery made, can resolve the sextet and the octet based on the lepton opening angle.

Precision EW measurements? Custodial symmetry okay for T; S parameter constrains f, also top partner masses.  Generally ok if top partner masses above a TeV.
Gluon fusion single production (top loop)?  Good question, considered possibilities, don't remember exactly.

2:20 pm: Anarchic Yukawas and top partial compositeness: the flavour of a successful marriage, Alberto Parolini

Interested in relation and tension between flavour and naturalness.  In particular, the idea is to use partial compositeness for the top (maybe also bottom) and technicolour-like (bilinear) mixing for the other quarks.  I think I remember reading this paper earlier in the year. Bilinear couplings okay for light quarks; large scales okay for flavour, and compatible with small masses.  This allows us to not have to include partners for the light quarks.

What about CP violation? Was included in analysis.
New CP phases for baryogensis? Will exist, did not consider though.

2:40 pm: Searching for Top Partners at LHC Run II, Jeong Han Kim

Bounds on top and light quark partners from run 1 already quite important.  Limits from light quarks set using Higgs diphoton decay.  Not set by experimental collaborations.  Bounds of 300 GeV from pair production; stronger single production limits but depend on (unknown) effective coupling.  However, it is only at run II that direct searches for top partners will surpass EWPT constraints.

Bounds depend on boosted objexts, so must use substructure techniques.  e.g. look for Higgs decaying to bbbar as a fat jet; specific tool is template overlap method.  TeV partner (pair) produces order 10 events with 5 sigma significance in first year.

Single production becomes important for top partners.  Forward jet tagging important to reduce background.  Surprisingly, in invisible decays of Z looks to win over leptonic final states (similar to monotop plus MET).  1 TeV top partner easiest to find if decays to bW; 1.5 TeV easiest in Zt final state thanks to MET signal.

3:00 pm: Holographic Models with a Small Cosmological Constant at Finite T, Bithika Jain

Standard RS involves fine-tuning of bulk and brane cosmological constants.  Goldberger-Wise theory only offers partial resolution: need small back reaction.  Problems related to hard IR-brane cut-off.  Thus use instead soft-wall, bulk scalar profile that grows in the IR effectively preventing fields entering that region of space.  Most soft-wall models suffer a run-away potential; this needs some IR brane tension to forbid this.  Something of a hybrid model.

Finite temperature physics involves a horizon a finite distance from UV brane.  Hawking radiation leads to thermal equilibrium.  IR brane now has been replaced by horizon.  New minima with back reaction important.  Critical temperature with 1st order phase transition and bubble nucleation.  This happens sufficiently promptly to forbid  eternal inflation.

Reheating?  Not examined yet.

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