Monday, 9 November 2015

KIAS-CFHEP Workshop Liveblog: Day One Session One

I've been in Korea less than three months and already I'm at my second conference here.  There's another one the first week of December that I could go too, to, though I probably won't; I need some time to do my own research.  This one gave me an excuse for my first visit to Seoul, though I haven't seen much of the city yet.

The full title of this conference is the KIAS-CFHEP Workshop and the 5th KIAS Workshop on Particle Physics and Cosmology, which is a bit too long for a title of this post.  We start, as is conventional, with experimental talks.

9:30 am: Higgs Physics at the LHC, Gouming Chen

This is a combined ATLAS/CMS talk.  Run-1 only, of course.

The discovery of the Higgs marks a new start.  There are two directions forward: to explore the properties of the Higgs and compare to the SM, and to directly search for additional Higgs states.

Individual experiments past 5 sigma for γγ and ZZ, plus WW at ATLAS only.  Combined experiments at ττ also past 5 sigma.

Some minor tension in mass measurements, e.g. ATLAS ZZ low compared to γγ.  Combined ATLAS + CMS ZZ agrees well with ATLAS + CMS γγ, and those with each other.

Direct measurement of Higgs width impossible at LHC.  On-shell/off-shell ratio on ZZ channel imposes bounds of 22 MeV on width.  Direct limits on decay lifetime also exist, but as expected are pretty much irrelevant; nine orders of magnitude worse than SM value.

Signal strength measurements exist in many different forms.  Some minor tension (excess) in ttH production.  All other production channels agree at 1 sigma.  Gluon fusion and vector-boson fusion channels separately confirmed at 5 sigma.  Decay modes no large excesses, but ZZ and bb channels more than 1 sigma from SM.

κ-framework for parameterising Higgs couplings offers another way to present data.  Slight deficit (more than 1 sigma) in coupling to b quarks.  Combined fits on loop couplings support no new particles in the loop; deviations at ATLAS/CMS are in different directions, some best-fit of combined data closer to SM.  Bounds on BSM decays now at 34%.  Interference helps resolve positive and negative couplings, now clear preference for positive couplings as in SM.

Cannot split production and decay in model-independent way.  Use ratios of production channels or decay modes to get most generic parameterisation of deviations in Higgs properties.  Still have large uncertainties, at the level of 10 to 50%.  Generally good agreement, but 2.4 sigma deviation in ratio of branching ratios for bb/ZZ.

All tested spin/CP pure state alternatives to SM excluded at 99% or more.  Tests for mixed CP states reveals no evidence for mixing.

ATLAS sees evidence for enhanced production of boosted Higgs and of Higgs plus jets.  Not supported by CMS.

Both experiments see a small excess in the (SM-forbidden) LFV decay to μτ.  Excess is only a couple of sigma, so far from decisive, needs more study.

ATLAS sees 2.4 sigma excess in diHiggs production at 300-400 GeV, not seen at CMS.

Lots and lots of searches for exotic states.  Generally one slide each, together with the exclusion region.  No excesses.  Only comment is that a light pseudoscalar is hard/impossible to see if it has the same mass as the J/ψ.

Ratio of couplings to up-type/down-type quarks?  bb vs ttH, plus interference in loop decays/
Same for leptons/down-type quarks?  bb vs ττ, but no interference here.
Comparison of κ parameterisations, mostly less that one, vs signal strength preferred bigger than one.  Signal strength computation makes extra assumption, SM decays.

10:10 am: Highlights of the SM Physics at the LHC, Haijun Yang 

Again, this summarises both ATLAS and CMS.  Topics of interest: production of multiple EW bosons, top quark production and some early run-2 results.

ZZ production decaying to 4 leptons.  Very clean, and of course highly relevant to Higgs studies, but extend over a large range of invariant masses.  NNLO QCD and NLO EW theoretical predictions.  Slight excess in region where one Z is off-shell.

Observations of Zγ production.  EW production (two associated jets) evidence at 3 sigma.

Some excess in WW production.  Seen in ATLAS, most recent CMS result no longer includes it.

Evidence for WW scattering (same sign W bosons to suppress gg-initiated background).  First evidence for γγ to WW (missed how).  First evidence for Wγγ production; small excess based on few events.

Lots of results summarised as no deviations.

One-sigma deviations in ttW/ttZ productions in both experiments: excess in ttW.

Run-2 already at 4 inverse fb of luminosity.  Measurement of pp inelastic cross section so far in agreement with expectations.  Charged particle multiplicity measurements important for understanding pile up, in line with expectations.  W and Z cross sections already measured, in excellent agreement with theory.  Same is true for ttbar.

What of the 2 TeV region?  Classify as exotics, not in this talk.  Will results be coming from run 2 soon?  Yes.

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