Friday, 11 December 2015

Friday wrap-up: diphoton excess?...

Things feel like they're starting to wrap up for the year down here, but the holiday season might not be so laid back for us theorists...

  • ATLAS and CMS are giving a joint presentation on 15th December: "ATLAS and CMS physics results from Run 2". The discipline is abuzz-with-rumours-circling online and in particle physics offices around the world suggesting there will be something very interesting presented indeed. The suggestion is a ~750 GeV diphoton excess at >3σ, seen by both ATLAS and CMS. If true, it seems very hard to accommodate with the absence of such a bump in Run 1, which makes it all the more interesting...

    I was going to make some speculations here, but let us just wait until next week... I'm sure by then there'll even be a bunch of papers on the arXiv which already have the answer.

    We've seen anomalies come (and some go) during Run 1 (Higgs diphoton, Higgs LFV, CMS kinematic edge, on-Z, diboson, B to KμμWH, etc.), but the buzz around this one seems different somehow. Perhaps because it is such a clean channel, because it is hard to explain on its own, because it is in a quite unexpected place for a first signature of genuinely new physics, and because it has shown up so damn early in the new high energy data. There seems to be a strong hope that this is only part of the story, and we're excited to wake up to a reality which might be rich and complex and full of new puzzles, when, in all honesty, many were expecting a desert (I know I was). Maybe this is just the tip of the TeV-scale.

    Certainly the presentation is one to watch. To be continued...
  • First new physics searches from Run 2 have hit the arXiv: dijet searches from ATLAS and CMS.
  • There was a workshop this week called, "Why Trust a Theory? Reconsidering Scientific Methodology in Light of Modern Physics." The workshop aims to tackle questions such as: "Can a high degree of trust in an empirically unconfirmed or inconclusively confirmed theory be scientifically justified? Does the extent to which empirically unconfirmed theories are trusted today constitute a substantial change of the character of scientific reasoning? Might some important theories of contemporary fundamental physics be empirically untestable in principle?"

    Sounds very interesting, and has speakers such as Gross, Dawid, Kane, Silk, Polchinski, Dvali, et al. Some notes were also taken at this blog. Unfortunately I have not had time yet to peruse the talks, but hope to get some time this week.
  • Some evidence against the 3.5 keV line appeared on the arXiv.
  • Sean Carroll has finished preparation of a new book, "The Big Picture: On the Origins of Life, Meaning, and the Universe Itself." He published the Contents on his blog.
  • Links without thinks:
    • Quanta: "Math Quartet Joins Forces on Unified Theory."
    • Paul Davies via Guardian: "100 years on, is this Einstein’s greatest gift to human understanding?"
    • Nature: "How to build a better PhD."
    • Guardian: "Chris Hadfield meets Randall Munroe: Are we alone in the universe?"
    • Quantum Frontiers: "BTZ Black Holes."
    • Information Processing: "The cult of genius?"


  1. Could this be due to an improved trigger?

    1. Unlikely, I think. The analyses will be using some diphoton trigger(s). These machines were specifically designed to be very efficient for diphotons, in order to discover the Higgs. So the trigger efficiency was very high in Run 1 and will remain very high in Run 2. There is not much to be improved. That's one reason why this excess will be interesting: it is an extremely clean channel with a smooth irreducible background component. It reduces to a simple counting experiment on top of a well modelled background - a bump hunt!

  2. Dear Jackson, I have largely switched from 700 to your number 750 GeV as well, mostly because there are two 8 TeV CMS papers that suggest a new Higgs near 750 GeV. One on diphoton decays of the Higgs; one on lepton-nu-q-qbar. It's getting pretty thick.

  3. Could be easily explained by a higher mass particle (above 8 TeV) being produced, which decays to a ~750 GeV state, which then goes to diphotons. This could not happen in run 1.

    Diphotons are of course a "classic" graviton signal.

    1. Yes, this idea is definitely being discussed. Another thing that makes it interesting -- if indeed at the local significance that is rumoured it might be hard to accommodate as a single lonely resonance. Is it hinting at more? Interesting question is whether the events consistent with the excess are associated with any extra activity: jets and/or MET? Probably too few statistics to tell yet. Details in a few hours...!

    2. The events in the peak region don't show more jets, MET or anything else that would differ from the events around them.