Tuesday, 27 August 2013

SUSY 2013 Live Blog: Day Two

The second day of SUSY starts with a pair of experimental talks on the Higgs results.

9:00am: Sridhara Dasu, "Recent results on Higgs physics from CMS"

Subtitle: Understanding the New Boson

125 GeV is an excellent Higgs mass for experimentalists; "by induction" this means it's good for theorists(!)

4-lepton channel; single and double Z production serve as a nice calibration on mass.  Full reconstruction of event gives significant angular information (spin, CP).  Main problem is lack of data (only 18 events so far).  Multivariate techniques gives some additional discriminating power (and, in particular, bring this channel alone to better than 5 sigma).  Data favours scalar over pseudoscalar at CLs 0.16%; scalar over tensor at 1.5%.

WW to lepton modes slightly below expectations; excess over no-Higgs at 4 sigma, expected 5.  Probes within VBF and associated production consistent with SM but not yet sensitive.

Diphotons; signal strength depends on analysis method.  Simpler tools give stronger signal, more advanced slightly below the SM rate.  This channel alone now approaching 4 sigma significance.

What of Higgs-fermion couplings?  Search for Higgs to b-bbar in associated production.  MVA tools needed because signal is so small compared to backgrounds.  Now have a 2 sigma excess, about what was expected.  Consider VBF to get more signal; backgrounds are four jets, so horrendous QCD.  Need to use quark vs gluon jet tagging.  Also at about two sigma.

Higgs to ditau alternative search.  Multiple subchannels; five tau pair decay modes, plus VBF/associated production etc.  This is a bit below three sigma in the end, slightly above expectations. Now have evidence for couplings to fermions in Higgs decays.  Fermiphobic Higgs now reasonably excluded, though best fit coupling a bit (1 sigma) below SM.

BSM channels; invisible Higgs decays searched for.  Associated production gives direct upper limit on branching ratio of 75%, better than expectations.  Focusing on VBF improves limit to 69%, but worse than expectations.

SUSY (MSSM) Higgs searches: look for neutral scalars.  Exclude tan β above 8 for mA below 200 GeV, and weaker limits at higher masses.

Question: fit to top coupling slightly suppressed; origin no photons in gluon fusion.  Why?  Don't know, ATLAS sees it.

9:35am: Joost Vossebeld, "Overview of Higgs results from the ATLAS experiment"

Main results; the Higgs to dibosons and mass, coupling, spin and parity implications.  Also a number of rare channels, to some extent preliminary.

Diphotons at ATLAS benefit from granularity of LAr calorimeter, help discriminate from neutral pions.  This channel now over seven sigma, and signal strength 50%±30% higher than SM.  New measurements based on differential cross sections; photon pT, number of jets etc.  Errors large so still consistent but some potentially interesting hints, e.g. photon pT harder than expected.  Definitely something to keep an eye on.

Four lepton channel is also in excess of five sigma and of SM expectations.  WW to leptons slightly below four sigma significance, in line with expectations.

Mass fits for diphoton and ZZ differ at 2.4 sigma, or 1.5% probability under the null hypothesis.  Energy scale systematics could well be the explanation here.

Combined signal strength over all bosonic channels still above 1 by slightly more than one sigma.

Interestingly, ratio of couplings to W and Z also differs from one by slightly over one sigma, which helps remind us how much weight we should put on these small deviations.  WW channels at SM expectations, ZZ slightly high.

Spin/CP slightly different to CMS.  Strongly prefer scalar over vector (0.3%) and tensor (0.1%), but preference for scalar over pseudoscalar not as good as CMS (2.2%).  Combining both experiments seems to give clear preference for the 0+ scalar.

Fermionic channels most interesting for ditau; still consistent with zero, but 1 sigma excess and signal strength 0.7±0.7.  Not all 2012 data included yet.  In contrast, b-bbar seems to use all data and no evidence for it not being zero.

Rare channels all give the same result: no evidence yet, nor any expected.  BSM searches similarly not exciting.  Invisible decays limited from associated production of 65%.

Ooh, one new result: charged Higgs decaying to τ-ν.  Nothing seen, of course.  Limits on signal strength set up to 600 GeV.

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