Friday, 27 November 2015

Friday wrap-up: XMASS again, Fermi...

Feels like a lot of links and not much thinks this week, but we have been madly working to finish a paper before I go home for Christmas in a couple of weeks. I think we'll get there, but in the mean time...

  • XMASS placed on the arXiv the annual modulation analysis we first saw presented at TAUP a couple of months back, showing some preference for negative annual modulation (opposite to the DAMA/LIBRA claim). They write, "The result of a simple modulation analysis, without assuming any specific dark matter model, showed a slight negative amplitude. As the p-values are 6.1 or 17% in our two independent analyses, these results are consistent with fluctuations." However, staring at their Figure 3, I somehow just can't seem to believe it's consistent with fluctuations...


    Sure, the fits in each bin are within 2σ of zero modulation, but there are a very large number of them below 3 keVee going the same way. If indeed the bins are largely independent (larger than resolution) and there is no correlated (annually modulating?) systematic, then at least naively I would expect that to be very improbable. So, an honest question, why is this not reflected in the p-value? An interesting sentence from the paper is, "Note that the energy bin width in Fig. 3 is one fifth of DAMA/LIBRA’s so that our limits would even get stricter with DAMA/LIBRA’s bin width." Is this suggesting that the result becomes more significant when all of those negative bins are collected into a larger bin? Comments are welcome.

    This is yet another annual modulation measurement with an intriguing result to add to the pile, and perhaps we're seeing a conservative downplaying, especially given the history of such measurements.
  • There's a nice article at Scientific American on Fermi's recent contribution to the galactic centre excess saga. The short story is that they confirm the excess above known backgrounds, which when fit with an diffuse NFW source is in broad agreement with previous works, as shown below (one notes the significant uncertainty in the tail as evidenced by the fits assuming different models of the background).


    We're left now with an official analysis which confirms what we have heard for a while now: that there is definitely something unknown there. So, what is it? The leading standard astrophysical explanation is some population of unresolved point sources (such as millisecond pulsars), which Slatyer now claims are favoured by the data. Still, the dark matter hypothesis is alive, albeit grappling with limits from dwarf spheroidals. And perhaps it can be settled soon; one thing I learned from the SA article is that the pulsar hypothesis might be probed in the near future:

    The good news is that if pulsars are behind the excess, more powerful, telescopes in the future should be able to spot the too-faint spinning stars directly. Pulsars would be prime targets for next-generation radio telescopes... “Should we fail to find them in the next five or ten years, a dark matter explanation becomes more likely again,” Weniger says. “This is pretty much a win–win situation. But we have to be patient.”
  • Tommaso Dorigo has found a publisher for his book on the Tevatron (and in particular CDF), "Anomaly! - Scientific Discoveries and the Quest for the Unknown." Should be out end of 2016; very much looking forward to it.
  • Links without thinks...
    • Starts With a Bang: "Strange But True: Dark Matter Grows 'Hair' Around Stars And Planets."
    • SLAC: "Q&A: SLAC Theorist Lance Dixon Explains Quantum Gravity."
    • New Yorker: "The Space Doctor's Big Idea," the Special Theory of Relativity explained in the 1000 most used words in the English language, by xkcd artist Randall Munroe, who also has a related book out. 
    • New Yorker: "The Doomsday Invention," a long read on artificial intelligence; do yourself a favour and get your hands on Nick Bostrom's very interesting book!
    • Nature: "Einstein was no lone genius."
    • Nature: "The quantum source of space-time."
    • Preposterous Universe: "Thanksgiving [for Riemannian Geometry]"
    • Wired: "Physicists Are Desperate to Be Wrong About the Higgs Boson."

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