Monday, 9 November 2015

KIAS-CFHEP Workship Liveblog: Day One Session Three

We return after lunch for more plenary talks (only two days this week have parallel sessions).  Our focus has moved to theory.
2:00 pm: The Quantum Critical Higgs, Seung J. Lee

Will NP keep hiding or finally show up at run 2?  Consider here a kind of generalised composite Higgs.  Consider the Ising model.  At high temperatures, have randomness in spins; but as temperature rolls down to critical value, correlation length diverges and the model goes through a phase transition.  Critical Ising model is scale invariant.

Higgs mass in SM can be thought of as a temperature-like parameter.  The quantum critical point is the boundary between the vacuum being the broken or unbroken phase.  Higgs field should be characterised by scaling dimension, which lies in [1, 2).  Work with CFT scalar two-point function, invert to simplest Lagrangian, and then add mass term to regularise theory.  Interpretation as a partial embedding of Higgs into strongly coupled sector.

Strong sector modifies n-point functions with form factors.  In particular, a non-trivial four-point function with momentum dependence.  Note that strongly coupled dynamics in the UV need not result in strongly-coupled IR physics; counter examples include Sieberg  duality, AdS/CFT and the rho meson at large N.

Some work with modified propagators, including derivations from RS models and SUSY QCD.

Is unitarity violated here? No
Does this affect gauge boson couplings?  If so, bounds from anomalous gauge couplings? Yes and yes, but not constraining.
Similarity to unparticles?  Yes, but that put continuum before pole, now ruled out by discovery of Higgs.

2:30 pm: SIMP and naturalness from dark QCD, Hyun Min Lee

Various small scale problems as motivation for going beyond WIMP paradigm.

Dark flavour symmetry is broken by QCD-like condensation, leaving five-point WZW term.  Residual flavour symmetry stabilises DM.  Temperature suppression of WZW derivative interaction is not severe at freeze out, can be compensated by group factors e.g. large number of colours.

Narrow range of masses consistent with perturbativity and bullet cluster constraints; 0.3 ~ 0.9 GeV in least constrained case.

Kinetic equilibrium between SM and dark sectors requires some coupling.  Kinetic mixing portal is obvious choice given low temperatures at freeze out.  Cancellation of anomalies requires dark quarks have particular dark charges, which lead to particular charges for mesons.  This imposes a further constraint, as different charges in the multiplet will be split by loop effects.  This splitting must be small enough to not interfere with freeze-out.  A high cut-off scale is also needed to protect from higher-dimensional operators.

Very brief discussion of twin Higgs variant.  Instead of using a Z2 to exactly cancel quadratic divergences, use larger symmetries to parametrically suppress them.  (All these theories need low-scale UV completions so that is sufficient.)  Claim is that this can connect with the SIMP framework but this was skinned over very lightly.

3:00 pm: Energy Peak: Back to Cosmic-Ray Peaks, Jong-Chul Park

Looks like I already attended this talk at CosPA.  Of course, this talk is longer so maybe there'll be more information.

The key idea is that peaks in the lab frame energy distributions corespond to peaks in the parent rest frame.  This can be easily proved using a Lesbegue-type construction when the parent decays to two particles.

Idea can be applied to photons (easy to measure, preserve spectral information from source to detection).  But thermal broadening in galaxy is small (v ~ 0.001c).  So have successive annihilation/decay channels.  2 -> 2 gives rest-frame delta function; 2 -> 2 -> 3 gives top hat function; 2 -> 2- > 3 -> 4 gives a peak function; and 2 -> 2 -> 3 -> 4 -> 5 gives a bump.

This is all pretty similar to what happens in the QCD cascade that gives conventional explanations for the GCE (for example).  It just uses a dark sector cascade instead.  By keeping all decays to 2-body, preserve the position of energy peaks.  This requires a whole GeV-scale hidden sector.

What cross sections do you need? Larger than thermal.

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