Wednesday, 22 May 2013

Planck 2013 Day Three Live Blog 2

Second morning session, after a much-needed coffee break.  We continue the study of flavour physics.  Conference website.

11:00am: Gino Isidori, "Adding Flavour to Higgs Studies"

People need to shut up when the speaker is talking.

Note that most Higgs couplings are flavour(-violating?); these are also the least known couplings.  We are in the early stages of exploring Higgs, so many interesting things have not been done, including:
  • Fully exploiting 3-body amplitudes
  • Rare semi-hadronic channels
  • Possible non-Yukawa couplings.
The last of these can be naturally included in a somewhat model-independent way as a SM non-renormalisable operator, for example f f H3.

Imposing constraints from FCNC gives very stringent constraints in the quark sector, ~ 10-10.  In the lepton sector for μ-τ couplings this is not the case.  That is, Higgs decays to different flavour leptons can have a branching ratio of 10% consistent with current constraints.  Higgs measurements can certainly constrain this!

Moving to three-body decays, note that Higgs to WW is strictly not measured.  Since on of the W (or Z) is off-shell, we measure Higgs to Vff for V = W, Z.  In BSM physics this need not equal the two gauge boson coupling.  We can use distributions to probe the difference.  The most general coupling is parameterised by four form factors; this is more general than the current experimental analysis.

Proposal to measure the differential rate (as a function of lepton invariant mass) for Higgs to four leptons.  That might be a worthy approach, but right now the number of events is too small for me to believe any results can be extracted.  Plus, backgrounds?

Finally, there is semi-hadronic channels as a handle on the Higgs coupling to quarks.  This is working in the Higgs to Vff case for f a quark.  To control hadronic uncertainties, the proposal is to look for a single hadronic state, a (quasi-)stable pseudoscalar.  That immediately hits my suspicion sense.  Backgrounds and pile-up?  The branching ratios are all smaller than Higgs to two photons, and the signals apparently are "clean" in that they involve displaced vertices.

11:30am: Ulrich Nieste, "Flavour Physics, Supersymmetry, and GUTs"

"ATLAS and CMS are working hard on solving the SUSY flavour problem by not finding supersymmetry."  Common approach to SUSY flavour problem is MFV, but then flavour effects unobservable.  Goal is to construct plausible scenarios with observable effects that meet the constrains.  GUTs offer a framework to do this by imposing some flavour structure above the GUT scale.

For example, the large atmospheric neutrino mixing angle could come from mixing of 5s of SU(5), leading to large b-s mixing.   Down-Yukawa matrix diagonalised by PMNS matrix (on right); leads to PMNS mixing in squark mass matrix.  Soft masses are universal at GUT scale, though running will modify third generation terms at low scales.

Generic result of numerical study is largest effect in Bs mixing and τ to μ γ.  No!  Work done in 2011, results since then cause problems.  Also not surprising.  In particular, θ13 means generate mixing in first two generations, relevant for μ to e γ.  So, combined with the Higgs mass, squarks and gluinos get pushed up to around 10 TeV.  Why is this model interesting again?

12:00pm: Jose Valle, "Neutrino Mass: Status and Implications"

This might be difficult; the slides are running into projector problems again, namely that it's hard to read text on a dark background.

Big problem with understanding the lepton mixing sector is the question of how many sterile neutrinos exist.  Still, in practice we integrate them out to get the 3 by 3 PMNS matrix.

Hints that atmospheric mixing angle not maximum?  Plot unclear if this is 1/2 sigma.  Ah, later plot shows consistent at three sigma, not at two.  But the preferred octant depends on what data sets are included, showing just how weak that statement is.

Measurement of θ13 brings the new phase: search for CP violation in leptonic physics.

Neutrino mass generation; beyond the usual see-saw, there seems to be a "VEV seesaw", but I don't see what this offers or even how it works.

SUSY generation of neutrino mass; arising from sneutrino VEVs (is that allowed?) leading to induced RPV.  RPV is then correlated to the neutrino mixing matrix, and the solar neutrino angle is radiatively generated to be small compared to the atmospheric angle.  Neutralino decays to leptons, so I doubt that this is all that safe from constraints.  Apparently this is in analysis now.

Flavour symmetry approaches:

  • Get structure from first principles; end of TBM does eliminate nicest models, try to work as small deviation from there.
  • Anarchy; there is not structure, it's all random.
  • Structure based on some non-TBM framework.
Last idea: one approach based on noting that Cabbibo angle is very similar to θ13.  "Numerology", putting everything in terms of this number but looks like theory is underdeveloped at this point in time.

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