Friday, 13 November 2015

KIAS-CFHEP Workshop Liveblog: Day Five Session Three

Our third session of the day is another plenary, three talks with titles that don't make the connection immediately obvious.

2:00 pm: Higgscision and Higgs Boson Pair Production, Kingman Cheung

Higgscision means "Higgs precision".  Which I would totally not have guessed; I thought it referred to some cuts, experimental or theoretical, or perhaps odd types of Higgs decays.  All Higgs couplings so far well consistent with SM, as is well known.  The SM Higgs potential is the simplest possibility.  In the future, we will get more and more precise measurements of the Higgs couplings.  We will need to use that to constrain extended Higgs sectors.  Use a variant of the standard Higgs effective coupling formalism.  In particular, include possible contributions to CP-violating Higgs couplings to fermions, and do not assume custodial SU(2) couplings.  So very general.

Okay, assume custodial SU(2) and generation-blind deviations to reduce parameters to only 15.  Then within this system, consider only certain selections of parameters to be non-zero.  Key point: all fits considered worse than SM!

Notable fit: invisible width only, bound < 19%.  Stronger than other claims.

Notable fit: vary leptonic couplings only.  Degeneracy to sign flips.  Negative top Yukawa still allowed at 95% confidence level, though positive is preferred.

Also, pseudoscalar Higgs couplings still consistent with data.  This can be constrained by using electric dipole moments.  Wasn't clear to me, sounds like constraints are very stringent.

Use single top + Higgs production to resolve the sign of the top Yukawa.  Interference between radiation of Higgs of top quark with gluon-fusion like production.  SM cross sections of 1pb, but can be large enhancement for negative coupling so should be easy to resolve.

Higgs boson pair production is, as mentioned a few times already at this conference, very important (connection to Higgs self coupling).  But also sensitive to non-standard Higgs couplings, especially triangle diagram with four-point HHtt coupling.  (Relevant to some composite Higgs extensions.)  To focus on Higgs self-coupling diagram, focus on region where intermediate Higgs is as close to on-shell as possible; corresponds to softer Higgses, decay products have larger angular separations.

The labels on his plots are too small to read, even on my laptop.  This makes it harder to interpret his results.  Conclusion seems to be that measuring sign of trilinear coupling is hard, even with large data set.  But can, by combining measurements from photonic and bbbar decays, distinguish the coupling from zero.

Question about where the couplings come from.  This talk is about constraining the couplings, not about a specific Higgs model.

Lots of work still to do in fitting the Higgs parameters.

Limits from dipole moments?  Current data, but allow for possible cancellations.
Bounds on Higgs invisible width?  Based on key assumption that no other contributions.
DiHiggs production in presence of heavy Higgs?  Kind of like contribution to ttHH coupling.  Should be focused out by selection cuts.
Electroweak diHiggs production? Probably suppressed, might be different.

2:30 pm: New resonances at the LHC, Seongchan Park

First (non-title) slide gets to the heart of the matter.  Notes no BSM discovered yet, and writes "Why?" in big, red letters!  3 ways out: heavy, non-coloured or non-standard interactions.  Within the last category, consider two new resonances: topphilic and disbosons.

Topphilic resonance means a state which only couples to tops (at least among SM fermions).  Consider an RS-type model for concreteness.  Couplings to light quarks suppressed by small wavefunction overlaps.  But having done that, move instead to an effective coupling description.

Decay is simple: ttbar.  Production is through gluon loops.  Except that this is forbidden for everything on-shell by the Landau-Yang theorem for vector-like coupling.  Axial vector coupling okay.  Regions in parameter space where no production as a consequence.

Also have associated production channels.  These can probe vector-like couplings.  Lead to final states with 3 or 4 tops.  Finally have production with gluon jet.  All can be probed.

Diboson resonance is obviously relevant in view of the ATLAS/CMS excesses.  Consider WZ as faked, despite it being the largext anomaly.  Then explaining signal as a single heavy neutral state.  Must be leptophobic and photophobic due to existing constraints. Couples to gg (probably loop level) and WW/ZZ.  Work in EFT: use minimal (not complete) set of operators to generate signal.  Coupling to gauge bosons gives irreducible coupling to photons; choose couplings to turn off the decay to diphotons.

Considered different spins/CP.  Brief discussion of using kinematics to resolve.  Fully hadronic final state, so in principle can fully reconstruct event.

Top-philic model is incomplete, surely? I don't think RS exactly reproduces the model (coupling to b quarks).
Pair production cross section constraints?  Box diagram specifically?  Not considered, I'd guess too small.

3:00 pm: Top-tagging at the Energy Frontier, Minho Son

Unfortunately, this talk is missing from online.  Also unfortunately, technical problems with getting this talk on the projector.  But we're underway now.

Large mass gap between EW scale and NP scale will lead to a lot of boosted objects.  This is key to the whole field of jet substructure.  Many top-taggers exist on the market.  How will their efficiency evolve as the top jet pT increases?

First problem is instability from soft radiation.  A 6 TeV jet with a soft, 5 GeV radiated jet at R ~ 1 would have a jet mass of the top mass.  Use some kind of variable jet cone size to fix this.  I think the idea was that the size varies with pT?

Instrumental problems as detector granularity becomes a limiting factor. Highly booted tops live within single calorimeter cell, can never resolve.  This will be a definite challenge at 100 TeV.

Trying to understand LHC detector better to understand future detectors better.  Outline slide halfway in, so I'm guessing this was multiple talks combined together without too much editing.  Run single top tagger over different models for detector, see how the optimal parameters vary.

Use ECAL and trackers, with better resolution, to track the HCAL energy deposits. Various ways to do this.

Oh dear,  I rather dozed off there.

Conclusion is that trackers and ECAL data can allow necessary tagging resolution at 100 TeV; indeed, they are essential for that role.

Pile up?  Not considered yet.
Neutral states in jet? Limits in what can be done.  Typically only 10% of jet.
Additional jet radiation? I didn't quite get the answer.

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