Thursday, 23 May 2013

Planck 2013 Day Four Live Blog 2

Last parallel session of the conference, and I'm staying with the Higgs talks.  Even if this session will run till 6pm.

4:00pm: Cedric Delaunay, "Probing the flavour structure of the composite Higgs with gg to h and h to γγ"

This talk uses black backgrounds for the slides, which is not a promising sign given the limitations of the projector.  At least I can look at them online now.

A standard composite Higgs model requires that the top and bottom (left) quarks be significantly composite from naturalness.  Consequently there are top partners below the TeV scale.  But the light quarks are largely insensitive to EWSB; so are they composite or elementary, do they have light partners?

This is closely related to the flavour structure of the theory.  A flavour-anarchic scenario a la RS has elementary light quarks, but non-trivial flavour problems with kaon mixing.  A MFV-type scenario instead allows right-handed quarks to be composite, and can have a lower strong scale (check this!)

The idea now is to use the Higgs coupling to gluons and photons to constrain the loop corrections from the quark partners.  Essentially, the deviations from the SM expectations will set a lower limit on the mass.  Interestingly, there's no sensitivity to top partners for accidental reasons (really?).  The light quarks give no contribution (small coupling), so the only new effect is from the light quark partners in the loops.

The size of the effect is not entirely clear to me here, but it looks like it can actually alleviate tensions between the composite Higgs couplings and observations, in particular compensating for the generic suppression of the Higgs-gluon coupling in composite models.

4:20pm: Juan Yepes, "Chiral Approach to a Light Dynamical Scalar Boson"

Start by considering limit where mH is large... but it's not.  I don't see how to relate this NLSM to the model including a physical Higgs.

Ah, I see.  Start with the well-studied NLSM and then ask how it modifies in the presence of h.  Hasn't this been done also?  Perhaps not, to this detail.  Including all possible terms up to some dimension, whereas usually just focus on a small number of terms.  But given that a large number of these terms violate custodial symmetry they must be quite constrained.

Some stuff on flavour and MFV.  Too many slides covered in equations and text, I can't follow this.

4:40pm: Alexandra Oliviera, "Multi Higgs and Vector boson production beyond the Standard Model"

In the absence of a Higgs, the NLSM describing the electroweak sector has production amplitudes for n final state vectors that are not suppressed at high energies.  In particular, if we consider WW scattering, this loses perturbativity at about 1.2 TeV; 2 to 4 processes would lose perturbativity at 2.4 TeV.  The SM Higgs fully linearizes the theory, but if the observed resonance deviates from the SM by even small amounts the theory will remain non-perturbative, its just that that behaviour is delayed to a higher energy scale.

The aim is then to use production of multiple vectors and Higgses to look for this non-perturbativity, as those processes are sensitive at high energies to small deviations.  2 to 2 and 2 to 3 channels are the optimal ones.

The problem is that you really need high centre of mass energies, so there is a pdf suppression.  Effects are small and possibility of observation (comparison to backgrounds) not offered.

5:00pm: Enrico Bertuzzo, "Can vanilla new physics illuminate the Higgs boson?"

Vanilla apparently means vector-like fermions.  No mixing with SM fermions, to simply avoid flavour and precision problems and reduce the number of parameters.

Add minimal content (with renormalisable Higgs couplings) and consider Higgs production, decay and EW precision observables.  Two cases: doublet plus singlet, and triplet plus doublet.

First model is easy to fit for colour singlets, with larger values of the hypercharge.  Hypercharge 1 leads to only a modest enhancement.  Colour triplets and octets lead, of course, to gluon fusion suppression which is problematic.  The second model, triplet plus doublet, gets a larger enhancement to the photon rate based simply on counting.

Direct searches sadly rule out most (all?) of the region where enhancements are possible.

However, these theories cannot be complete; Higgs potential unstable at a relatively low scale.  As such higher dimensional operators can spoil the low energy EFT.

5:20pm: Chan Beom Park, "A light pseudoscalar and two scalar Higgses in the NMSSM"

SUSY search channel: neutralino decays to Higgs + neutralino.  Possible new Higgs production, including of non-standard Higgses.  In the NMSSM, the (mostly-)singlet scalars can be quite light, and in particular can have dominant decays to leptons as a consequence.

5:40pm: Germano Nardini, "Higgs phenomenology in the triplet extension of the MSSM"

Like the NMSSM, the extra triplet can raise the Higgs mass (through extra F-term contributions).  Additionally, the extra charged scalars in the the triple can be relevant to the Higgs decay to diphotons.

Two possible choices for the triplet hypercharge; zero or one.  How does one work for the Higgs coupling?  Triplet must have small vev, which is a bit awkward.  Satisfying that constraint mostly decouples the triplet, but for a new contribution to the Higgs quartic that is highly relevant for the mass. Large Higgs-triplet coupling, useful to raise the mass, also increases the Higgsino-new chargino coupling/mixing.

While we still have a breakdown of perturbativity below the Planck scale, it seems that we can go to higher scales than in the NMSSM.

Ah, I remember reading this paper.  I understood that more than I have this talk, but there's a nifty way to get exactly SM-lik Higgs couplings for any pseudoscalar mass by choosing the triplet couplings appropriately.  The sole exception is the diphoton channel, thanks to extra charged particles in the loops.

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