Friday, 3 May 2013

Brookhaven Forum 2013 Live Blog: Day 2, Blog III

After that slightly disappointing (from my point of view) plenary session, its time for the parallel sessions.  The ability to select which session to attend should ensure I'm listening to relevant talks, I hope!  In particular, I've decided to hit the Higgs/BSM talks for the first session this afternoon.  Conference Website.

Talk 5: 2pm: Bernard Mistlberger, "Soft Triple-Real Radiation for Higgs Production at N3LO"

Unfortunately, we start with one of the less interesting talks for me, something of a follow-up to the last plenary talk.  Still, with the option to go into more of the details, perhaps things will be more interesting.

Good starting question: why do we need 3N's?
  • Precise prediction for cross-section from theory, given Higgs mass
  • So comparing mass and cross section is another check for BSM
  • Experimental errors rapidly approaching theory errors!
Large changes from LO to NLO, and to a lesser extent from NLO to NNLO, in theory prediction.  Not quite captured by estimates from scale variation

N3LO never done at hadronic machine!

Looks to be addressing one of my questions from earlier, about the gluon fusion production uncertainty.

Ten years from LO to NLO, NLO to NNLO and now from NNLO to N3LO.

  • Purely virtual, known from form factors
  • 2-virtual, 1-real: mostly known from FF
  • Others
  • Focus on 3-real, most complicated challenge; rest easy once done.
200 Integrals!

Unitarity trick is key to method: relate discontinuities in loop integrals to phase space integrals.  Reverse  this, replace phase space cuts as unitary cuts to exploit technology in loop integration calculation (master integrals).

Further approximation: Higgs nearly on-shell (physical approximation).  Leads to Taylor expansion in small parameter, virtuality of Higgs.

Reduce problem to 10 soft Master integrals, all solved analytically.

Talk 6: 2:20pm: Katy Hartling, "Constraining Models with a Large Scalar Multiplet"

Recent consideration has been given to higher Higgs multiplets (above triplet).  What can we say about these?
  • Large multiplet, less than n=8 to avoid violating tree-level unitarity
  • No VEV to help T
  • Lightest is neutral.
If Y = 2T, accidental U(1) symmetry.  As DM, Z boson mediates DD interaction rates completely rules these models out.

n = 5 seems to okay, thanks to non-renormalisable term; even Planck-suppressed, mediates rapid decay rate.

If Y = 1, accidental symmetry reduced to a Z2 so long as n even.  Can now avoid DD constraints ... I missed why.

Non-trivial but loose unitarity constraints on couplings.

Precision constraints push spectrum of multiplet towards degeneracy.

Constraints from Higgs decays.

These constraints essentially rule out scalar as most of DM.  Still possibly detectable at XENON 1Ton.

Talk 7: 2:40pm: Chien-Yi Chen, "Explorting Two Higgs Doublet Models Through Higgs Production"

Looks like a basic 2HDM, i.e. nothing else.

Connection to B physics, this could be interesting.

Higgs diphoton decay rate:
  • Can enhance ratio in some 2HDMs by suppressing coupling to b, tau
  • Also by changing sign of coupling to W, so as to change interference from constructive to destructive.
  • Clearly done with the old excesses in mind
  • Not just, though.  Some interesting prospects for future limits, assuming SM result.
Flavour physics:
  • Constraints from b to s gamma:
  • Can have very light charged Higgs in Type I and Lepton-specific 2HDMs; I guess because different doublets couple to D, L.
  • B to mumu:
  • This seems to capture those very light charged Higgses.
Talk 8: 3pm: Alejandro Celis, "Phenomenology of Two-Higgs-Doublet Models at the LHC"

Looks like the focus is a particular 2HDM, chosen to avoid FCNCs.  This is through something similar (identical) to MFV, taking the Yukawa couplings for the two doublets to be proportional.  It is thus different from the doublets of the previous talk, where each fermion got its mass from only one Higgs doublet.

Non-trivial phases can be included without worry.  Thus we can have CP-violation in the scalar sector, one of the things we do not have enough of in the SM.

This model includes inert doublet, Z2-symmetry protected models as special cases, at least as far as the LHC is concerned.

Not much in the case of results distinct from those limits, though.

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