Saturday, 18 April 2015

Friday wrap-up: 6.5 TeV, AMS, young pulsars, dark matter...

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...

  • On this day last week we had the first 6.5 TeV beam fly around the LHC, and a day later we had both beams at once. You can read a little more at symmetry magazine.

  • AMS have presented their results on the cosmic ray proton/helium/lithium flux and the antiproton-proton ratio at a CERN mini-conference. Recordings of the talks are here and slides here. The press release is here. The plot they are pushing is the following one:

    Figure 1. Antiproton to proton ratio measured by AMS. As seen, the measured ratio cannot be explained by existing models of secondary production.

    The claim in the press release is, "This behavior cannot be explained by secondary production of antiprotons from ordinary cosmic ray collisions," with the suggestion that a new primary source(s) may be needed. But Sam Ting made sure during his talk to emphasise that it cannot be explained by existing secondary production models, and when presenting this figure he noted that there are many secondary production models, but "this is the one we choose" -- I wonder why... (an aside: he also in the same talk referred to his $J/\psi$ particle first as $J$ and then as $\psi$ on the same slide!). The point is that these models are very uncertain and the claim in the press release is unfounded. Indeed, there was a preprint on hep-ph yesterday which had the following to say: "Our first and main result is that there is no unambiguous antiproton excess that can be identified in the first place, and thus, at this stage, no real need for primary sources of antiprotons. Within errors, secondary astrophysical production alone can account for the data." Their Figure 2:

    So, don't believe the hype.
  • There was an arXiv preprint on Wednesday suggesting that young pulsars can explain the galactic centre excess. Their money plot is the following comparison of the expected dark matter spectrum with that from a prototypical young pulsar (Geminga).

  • Spectroscopic measurements of Reticulum II (here, here, and here) confirm it is an ultra-faint dwarf galaxy. Recall from the Hooper paper on the observed gamma ray excess, "In order for this excess to be compatible with the lack of significant gamma-ray detections from other dwarf galaxies... Reticulum II must contain a high density of dark matter, corresponding to $J \gtrsim 10^{19.6}\text{ GeV}^2/\text{cm}^{-5}$." One the new preprints has the J-factor measured at $10^{18.8\pm0.6}\text{ GeV}^2/\text{cm}^{-5}$ within 0.2 degs, and $10^{18.9\pm0.6}\text{ GeV}^2/\text{cm}^{-5}$ within 0.5 degs, and another has it at $10^{19.5^{+1.0(+1.6)}_{−0.6(−1.3)}}\text{ GeV}^2/\text{cm}^{-5}$ within 0.5 degs. So measurements seem to be disfavouring a dark matter interpretation. Also, it is of note that there is a radio source (likely a blazar) located 0.1 degs from the Ret II location which could be responsible for excess gamma rays...
  • An arXiv preprint (press release here) has observed that, in a system of four colliding elliptical galaxies, "each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars." The abstract adds, "With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments – but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section $\sigma_{DM}/m \sim (1.7\pm0.7)\times10^{−4} \text{ cm}^2/\text{g}\times(t_{infall}/10^9\text{yrs})^{−2}$, where $t_{infall}$ is the infall duration."

    We should keep in mind that these limits are based on the assumption that the interaction is velocity-independent, which is not true of a low-mass mediator. I don't have anything illuminating to add, but I do find it interesting that the previous study of colliding galaxy clusters set an upper limit of $\sigma_{DM}/m < 0.47 \text{ cm}^2/\text{g}$, and this measurement is almost four orders of magnitude smaller! Are measurements of these kinds of systems that much more sensitive? Why doesn't this system set an even stronger upper limit?
  • The "Evidence for dark matter in the inner Milky Way" saga appears to have reached an end. Both the comment and the reply to comment were updated this week, with the former reproducing a plot from a 1988 paper which is "in essence, identical to that of Iocco et al," and the latter writing, "In our letter we made a claim based on a specific technical point: that current data are constraining enough to make the claim robust against statistical and systematic errors. We believe we have made this point clear with our letter and two replies, and we shall not continue the discussion on the arXiv."
  • The Dark Energy Survey has produced a dark matter map (arXiv here, nature article here) of part of our sky using graviational lensing. Below is a heat map showing the mass density along with locations of galaxy clusters superimposed as grey dots. The map supports the standard picture that dark matter drives large-scale structure formation.

  • The EPS HEPP prizes were given out this week; the main prize went to James D. Bjorken “for his prediction of scaling behaviour in the structure of the proton that led to a new understanding of the strong interaction”, and to Guido Altarelli, Yuri L. Dokshitzer, Lev Lipatov, and Giorgio Parisi “for developing a probabilistic field theory framework for the dynamics of quarks and gluons, enabling a quantitative understanding of high-energy collisions involving hadrons”.
  • Strassler has been tackling the issue of dark matter searches at the LHC this week. He has a blog post and a new article for the layman.
  • Paul Jackson has written a blog post for the ATLAS blog on CoEPP and our conference in February.
  • Our understanding of the particle zoo as a function of time at Scientific American.
  • Frank Wilczek has a new book coming out in July, "A Beautiful Question: Finding Nature’s Deep Design."
  • Lastly, space images...
    • The first colour image of Pluto (and Charon) from New Horizons:

    • And here is our first view of Ceres from Dawn's new address:

      Ceres' North Pole

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