Friday, 25 March 2016

Friday wrap-up: Moriond...

The 50th anniversary Rencontres de Moriond (electroweak indicotwitterhashtag) was on over the past few weeks. Here's the updated logo [credit Strumia]:


Not sure how the 7 got in there... probably insignificant.
  • The most anticipated results were updates on the 750 GeV diphoton saga. Slides from ATLAS and CMS are here and here.

    There are excellent detailed write-ups at Résonaances and PhysicsMatt already, and I don't have much to add to these (for pop-sci articles see e.g. Guardian, symmetry, Scientific American). You should read them, if you haven't. In short, with the addition of new "B=0" data from CMS and an updated analysis from ATLAS, the excess is not going away. Below I reproduce one of the third-party combination plots published on PhysicsMatt which tells some of the story. On the left is the combination of previous data, and on the right after the Moriond update, assuming the Volksmodel $gg\to S\to \gamma\gamma$ and a narrow width:


    One can see by eye that the reanalysis of the ATLAS 8 TeV data shows it is more consistent with a $gg\to S$ 13 TeV excess than previously believed, and there's an excess in the new CMS 13 TeV data in the right ballpark. Taken together this adds a little fuel to the fire.

    As well, there are strong rumours that ATLAS are sitting on an analysis in which they relax some of their cuts (increasing acceptance of events), and that this alone bumps up the local (global) significance of the excess to ~4.7σ (>3σ) [see e.g. Résonaances and comment section]. If this is true then hep-ph might as well become hep-γγ...

    For your interest see below some (obviously biased) surveys in the twittersphere. Clearly people are taking this seriously. If the rumoured ATLAS analysis is true I would give the 750 GeV excess a dice throw at sticking around.

  • If you're out of ideas for how to explain the excess, then maybe you can find inspiration at snarXiv.
  • About a month ago D0 announced observation of a tetraquark $X(5568)\to B_s^0\pi^\pm$ state. It received quite a bit of press. Here's the plot from the D0 preprint:


    At Moriond LHCb announced that they see no evidence for such a tetraquark state (slides 22-24 here). A few days ago there was an LHC Seminar on the analysis. From what I can gather, there is some talk of bias introduced by a "cone cut" in the D0 analysis. In the Conf Note LHCb write:

    In the D0 analysis, a requirement is imposed on the opening angle between the $B^0_s$ candidate and the companion pion in the plane of pseudorapidity and azimuthal angle [$\Delta R$]... No such requirement is imposed here, as $\Delta R$ is strongly correlated with $Q$ value and, when combined with kinematic requirements imposed by the LHCb detector acceptance, a cut on this variable can cause broad peaking structures.

    There is speculation that this might have introduced some spurious shape or impacted the statistical interpretation somehow for D0. I find the following slide from the seminar rather telling.


    Here $\rho_X$ is the fraction of $B_s^0$ coming from tetraquark decays. Could there be some major difference in the production of $X$ or of $B^0_s$ in a $p\bar{p}$ (as in D0) versus a $pp$ (as in LHCb) collider? Would love to hear from an expert. In the mean time we wait for results from ATLAS/CMS, and in particular CDF (the partner experiment to D0 at Tevatron) to tell us more. There's a pop-sci article at Scientific American here.
  • LHC beam splashes tonight!
  • Links without thinks:
  • In audio/video media:

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