Wednesday, 1 May 2013

Brookhaven Forum 2013 Live Blog

For the next two-and-a-half days, I'm attending the Brookhaven Forum 2013.  I was here three years ago, and enjoyed it then.  Sadly I had to miss the last meeting, but I'm glad to be back.  I'm not expecting anything dramatic here, but I'm going to live blog the conference in lieu of taking notes.  This way, I might actually go back and look at what I've written before!

Talk 1: 9:15am: Suvadeep Bose, "Recent Physics Results from CMS"

Of course, we're not going to get any really new results at a minor conference like this one, so this will mostly (completely?) be review.
  • Electroweak gauge boson measurements, some interesting results about pdfs; it seems MSTW pdfs give a slightly small cross section.  But the differences are not large, order one sigma.
  • Top mass measurements from CMS alone now comparable to Tevatron combined results.
  • Single top branching widths, most precise measurement of BR(t → Wb)/BR(t → Wq) = 1.02 +- 0.03, percent level agreement with SM.  This is a non-trivial constraint on something I'm currently working on,

  • SUSY Searches: all the usual impressive complexity.  Dividing the signal region into 176 categories, to maximise signal sensitivity.  Not that it helps, of course, SUSY continues to be undiscovered.
  • Even for the more optimistic 3rd generation cases, gluino exclusion limits over a TeV outside of degenerate spectra.
  • All this is not new, of course, just depressing.

  • Mono-X searches: constraints on black holes, minimal dark matter models and so forth.
  • Even looking at the event spectra and really squinting, it doesn't seem possible to trick oneself to seeing something.

  • Higgs Searches: looks like my talk will clash with the main Higgs result talk later this afternoon.
  • As expected, results are those of Moriond.
  • Enough data in the H → ZZ → 4l to compare different leptonic channels; they do seem to agree.
  • Brushes over the (slight) suppression in the two-photon channel.  Of course, it's all consistent with the SM.  I guess the feeling is that this has already been hashed out over the last couple of months.
  • H → bb data still to be updated with the full data set.  With over half the current data, only a 2 sigma excess.
Summary: as noted, the results have all been shown before.  But still worth bringing to mind.

Question: the two-photon channel.  This was high in the original data, is down now; apparently this was simply from the data.  The first half of the data was high, the second half low; most likely, then, it was just a statistical fluctuation.

Talk 2: 9:55am: Elliot Lipeles, "Higgs Studies at ATLAS"

Unlike the CMS talk, this is purely on the Higgs data.  Worth noting that the Higgs program has changed: we're now in the measurement program.  A phrase I've seen thrown around is the "Precision Higgs Era".

Precision measurements must consider ~10-20% theoretical uncertainties in the gluon fusion production channel.  This is the dominant channel.  Possibilities for improvement?
  • ATLAS still has an enhanced two-photon signal.  Obviously they are the better experiment, better defined as giving me what I want'
  • Nice: there are a number of high-purity subregions in signal space, tagging on e.g. WH production with 90% purity.
  • This leads to evidence for enhancement in both the Higgs-gauge boson and Higgs-top couplings, beyond just the overall signal strength measurement.
Like the CMS searches, we have some spin data.  (I didn't go over this above, but it was there.)  The problem is that the main candidates are spin-0 versus spin-2, and while the predictions in the former case are unambiguous, the latter are model-dependent.  There aren't any good theoretical models for the spin-2 case, so these comparisons usually use graviton couplings.  It's probably a necessary check, but I don't find it very exciting.

We also have spin investigations from the ZZ data.  Since a vector can decay to ZZ, spin-1 is considered here, just in case the ZZ and γγ signals come from different particles.

Message from the experimentalists about the two-photon excess is pessimistic: difference between ATLAS and CMS is likely statistical, and went down with more data.  Don't be realistic, damn you!
  • WW searches have expanded to include same-flavour signals (ee and mu-mu, as opposed to just e-mu).
  • Needed some new analysis tools, but I think I zoned out on the details.  That's why I'm not an experimentalist.
  • Yet more spin measurements.  The WW signal criteria include picking leptons somewhat-parallel, which already prefers spin-0; this was pointed out way, way back in December 2011 when the first hints came out.
  • The fits here are by far the most convincing in terms of spin-0 versus spin-1 or 2.  No comparison of scalar vs pseudoscalar shown, though.
Higgs to tau-tau: pointed out to me that the notation is wonderfully confusing.  H means the Higgs, and h means the hadronic stuff the tau decays to.  For theorists, H means the Higgs before symmetry breaking, and h the Higgs after.  So the Higgs decays to the tau which decays to the Higgs!

The ATLAS tau and bb signals are still awaiting updates with the full data set.
  • The Higgs to bb (non-QCD) background was hard to find!  But limits now approaching SM.
  • A Higgs to mumu search!  No hope, of course.
The combined searches now provide 3.1 sigma evidence of VBF production.  This is important, as this production is closely related to the role of the Higgs taming the WW to WW scattering cross section.

In summary, the measurements are very impressive and make the Higgs look more and more like the simple SM one.  Sadly.

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