Friday, 3 May 2013

Brookhaven Forum Day 3

Unfortunately, I missed the first talk this morning on direct detection of dark matter.  I'm really not much of a morning person in general, and the last day of a conference is always a bit tricky because you start to flag.

Still, ready for the second talk of the day.

Talk 2: 9:40 am: Tracy Slater, "Dark Matter Phenomenology and Indirect Detection"

Looks like a talk on the various anomalies that have been kicking around.  In particular, we have a review of the gamma ray line, as well as the recent AMS-02 results and what they say about the long-standing positron excesses.

Dark Matter equals WIMPs for the purposes of this talk, which is fine by me.

The Fermi Line:
  • About a year old now.
  • Original claim was 3.3 sigma after accounting for the look-elsewhere effect.
  • The big question: is it instrumental systematics?  On the one hand, Finkbeiner & Su showed that the signal is, to good approximation, only at the galactic centre and in particular not associated with any particular incidence angle.
  • Bump also present from Earth limb, and near to Sun.  These are troublesome.
The line is also much stronger than would be expected for WIMPs.  In particular, limits from low-energy continuum photons severely restrict standard DM candidates, such as the neutralino.  I've talked here about a few ways to resolve this inconsistency, and we have a slide full of papers as examples.

Future tests of the line:
  • HESS II online now, expected to present results by 2014.
  • GAMMA-400, satellite to launch by 2018 ... maybe.

The GeV Excess: I don't think I know this one.
  • In Fermi public data, claim of extended region localised around the galactic centre peaking at a few GeV.  Ah, this is the legacy of the Goodenough & Hooper paper from a few years ago.
  • Near galactic centre, so has been problematic for some time.
  • Shown here that spectrum clearly shows the bump only close to the galactic centre, but out to a distance of 5 to 10 degrees from it.
  • Seems to fit well with expectations from NFW profile.
  • I'm going to have to dig out the relevant paper here, aren't I?
  • Comparison to pulsars; this is the usual thing.  I've seen similar plots from the pulsar advocates, they always seem a bit better at fitting the data.
I don't know how I feel about this claim.  I've generally been skeptical of these whole series of claims, but I don't want to dismiss things out of hand.

Finally we come to the positron fraction excess, and the recent AMS-02 results.  New results clearly confirm the PAMELA rise above 10 GeV, contrary to the natural expectation.  Of course, a lot of work has already been done back when the PAMELA results first came out.  Focus on models with a light dark vector that gives us the Sommerfeld enhancement, and ensures leptonic decays.
  • AMS has confirmed PAMELA; it's not a systematic effect or other nastiness.
  • The spectrum is slightly softer, which IIRC would weaken some of the constraints on possible DM models.
  • Still unable to rule out the pulsar explanation on the basis of the data.
  • CMB constraints on late-time annihilations to leptons also in tension with the data.
I think the main point is perhaps the last line on the last slide, the various signals lead us beyond the simple WIMP paradigm.  I've been saying for too long that I want to do something with dark matter, I should stop saying it and start doing it.

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