Thursday, 29 August 2013

SUSY 2013 Live Bog: Day Four, Session Three

I've decided to spend the afternoon sessions attending the non-SUSY talks.  Obviously, that's the place to be at a conference called Supersymmetry.

2:30pm: Alessio Bonato, "Searches for resonances decaying to SM dibosons"

We start with an experimental talk, from the CMS collaboration.  WW, ZZ resonances are an obvious place to look for new physics as they are related to EWSB and possible modifications of it.  Four main search channels; zero, one, two and three leptons.  (Four leptons is really a heavy Higgs search.)  Work with benchmarks for practical reasons, but try to be as general as possible in analysis so as to maximise applicability.

Heavy resonances leads to boosted topologies, with the W/Z decaying to overlapping jets.  Thus this search makes use of substructure techniques.  Use jet mass on groomed (pruned) jets, plus N-subjettiness.

V-tags work well but there is still some disagreement with Monte Carlos; this suggests a problem with hadronisation algorithms.  (Main systematic error).

One-lepton channel: limits of a few to a few tens of femtobarns for resonances around a TeV.

Zero-lepton (fully hadronic): No excesses, limits around a few TeV.

Two leptons (Z + V): Involves collimated leptons, so standard isolation cuts run into problems.  Limits again tens of fb but goes down to 600 GeV.

Three leptons (fully leptonic): SSM W' limits now up to 1.45 TeV.

Future work includes combined limits, though more model-dependent (branching ratios to W/Z) plus searches including Higgses (!) that require b tags.

2:55pm: Anna Kaminska, "Spin-1 resonances as a signature of composite Higgs at the LHC"

Spin-1 resonances here can be thought of as analogues to the ρ of QCD.  Look to create an EFT to describe them.  They live in the unbroken global strong group and mix with W, Z; from the gauge (fermion) kinetic terms we get couplings to gauge bosons (fermions).  Couplings to fermions can be enhances with fermion mixing (partial compositeness).

Use hidden local symmetry method, extend gauge group so vector resonances can be gauge bosons of the new, fictional group.  Two expansion parameters: v/f and g/gρ.

Minimal case is SO(5)/SO(4).  LHC limits at 1.5-2 TeV.

3:15pm: Satoshi Mishima, "Electroweak precision fit and model independent constraints on new physics"

Historically, EWPFits have played an important role in constraining new physics.  The Higgs mass plus new top, W mass measurements make returning to the topic relevant.

Fifteen precision observables; W boson mass and width, plus thirteen Z pole observables, parameterised using effective couplings.

Most theoretical calculations at two loops.  There do seem to be some outstanding corrections to the Z couplings to fermions.  Recent two-loop calculations had larger subleading contributions than expected, suggesting relatively large corrections to experimental observables.

Fit done using Bayesian analysis.  Particularly useful since nuisance parameters included to cover expected uncertainties mentioned above.  Their presence seems to worsen the fit compared to the old choice, which confuses me.

Interestingly coupling h-V-V fit to slightly over 1, while most composite models prefer it to be slightly below 1.

3:35pm: Hsin-Chia Cheng, "A light Higgs boson from a composite Higgs theory"

Composite Higgs masses model-dependent; is the quartic coupling a tree-level or radiative parameter?

Top mass a well-known problem, that can be solved using partial compositeness.

Top condensation a different approach.  Problem is that wants a heavy top (600 GeV) and a heavier Higgs (~2mt).  One approach is a top-seesaw using vector-like singlet quarks that mix with the top.  Alternative used here is breaking of a strong U(3) down to U(2). so Higgs is pNGB.  Symmetry breaking seems to be explicit?

Upper limit on the Higgs mass around mt.  But this can be lowered by EW explicit breaking of the U(3), among other possibilities.

Lack of custodial symmetry means T parameter can be a problem.  Needs some fine tuning, since leads to f > 6 TeV.

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