Friday, 27 March 2015

Friday wrap-up: LHC delay, B to Kμμ, Higgs at N3LO, dark matter self-interaction...

Wherein I list some (mostly) recent happenings, ramble a bit, and provide links, in an order roughly determined by importance and relevance to particle physics. Views are my own. Content very definitely skewed by my own leanings and by papers getting coverage, and it may not even be correct. It is a blog after all...

  • The LHC did not see a circulating beam this week due to a short circuit in one of the magnets, likely due to debris in a diode box. Delay could be days or months. Read the CERN press release here, and there's some more information at Nature News. [Edit: more from CERN here.]
  • The ATLAS/CMS combined Higgs mass measurement is now up on the arXiv (~6000 authors!).


  • There has been a bit more talk on the LHCb $B\to K^*\mu\mu$ excess which was updated from 1/fb to 3/fb of data last week. The Conf Note is now up. There's a short blog post on interpretation from David Straub, and the Straub/Altmannshofer Proceedings paper has quite a nice summary. See also the Resonaances post and the previous-two posts also there.

    Shown below is the old result (blue) and new result (black) against theory (orange) for the angular observable of interest.


    The point is that the 4--8 GeV^2 bins each deviate from the theory prediction, and a naive combination suggests a 3.7σ tension. This happens to be the same tension seen in the analysis with 1/fb, which suggests that the new data have still fluctuated up, but not quite as much. The effect has not gone away. But I am with Tommaso... I would like to know what is the probability of seeing such a deviation or larger in any of the angular observables they looked at, assuming the null. Probably this is difficult to do; likely there are significant correlations between bins of different observables(?)... still, it would be nice to know a number for the global significance as well.

    The question now is whether we are seeing new physics or underestimated theory uncertainty. The data are not suggesting any problem with form factors, but an unexpectedly large charm-loop contribution near $J/\psi$ is a possible explanation. If it is new physics, then the discrepancy can be explained by a single operator$$O_9=(\bar{s}\gamma_\mu P_L b)(\bar{l}\gamma^\mu l)$$quantified by a parameter $C_9^{NP}$. This could also go some way to explaining the 2.6σ deviation from lepton-flavour universality (quantified by $R_K$) also measured by LHCb. According to the Straub/Altmannshofer Proceedings there are sensible ways to proceed if we want to discover the culprit. Here are a few of them:

    1. Keep looking at q^2 dependence of $C_9$; the new physics effect should be q^2 independent.

    2. Measure $B\to K^*\mu\mu$ and $B\to K^*ee$ branching ratios and angular observables. If the same new physics is responsible for this and $R_K$ then you might see "spectacular" deviations.

    3. Search for lepton flavour violations in $B\to K^*\mu e$.

    Certainly these are all measurements to look forward to...
  • If you want more insight into what's going on at Moriond, their twitter stream is excellent.
  • There's another excess that you might see making the rounds soon. They're everywhere! This week it is >2σ in a search for WH resonances in $l\nu b\bar{b}$.


    Of interest are the three events at ~1800 GeV. It is only seen in the electron channel, so if it is new physics, it is not probably not a WH resonance. It is almost not worth mentioning, but CMS also saw an excess in their right-handed W search in $lljj$. There they saw a >2σ discrepancy in the electron channel at around the same mass scale.


    I have not had time to read about these in detail, but if they can be linked, they will be linked. Watch the arXiv...
  • Higgs gluon-fusion production cross-section has been computed at N3LO (and Moriond talk here [pdf]), "the first ever complete computation of a cross-section at N3LO at a hadron collider."
  • Hooper and Linden have weighed in on the Reticulum II gamma-ray excess (seen in one of the newly discovered DES dwarf satellites). From the conclusion: "In order for this excess to be compatible with the lack of significant gamma-ray detections from other dwarf galaxies (most importantly, Segue 1 and Ursa Major II), Reticulum II must contain a high density of dark matter... A measurement of Reticulum II’s J-factor that is much smaller than this value would place serious doubt as to any dark matter interpretation of its excess."
  • There was lot of hype relating to a lazy LHC article in the media this week. On the plus side, Backreaction has an excellent rebuke (on the article and the paper that spawned it), and there you can also learn a bit about rainbow gravity.
  • A paper published in Science used the Chandra and Hubble Space Telescopes to observe 72 galaxy cluster collisions and subsequently constrain dark matter self-interaction. This number now supersedes the constraint from the bullet cluster. You can read the press release or watch the astrophysicists involved (along with others) discussing it on YouTube [1 hour].

    Images of galaxy clusters
  • A paper appeared on the arXiv today, signed by a number of physicists from many different institutions, commenting that the Nature paper on inner galaxy dark matter appearing a month or so back was not even wrong: "Considerable confusion may stem from the use of the term ‘inner’. The Sun’s orbit encompasses roughly 90% of the stellar mass. By this standard, we live in the outskirts of the Galaxy. That some DM is needed interior to the Solar circle is neither surprising nor new."

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