The past two weeks have been busy! I was on the road, consulting with and learning from particle experimenters and theorists at Caltech and the University of California at Irvine. And I’ve been giving talks: at the University of California Santa Barbara (for the Joe Polchinski Fest conference), at the University of California at Irvine, and yesterday in Boston at M.I.T. The Santa Barbara talk was only semi-technical, and is on-line. The latter two, much more technical, focused on the two big projects that I completed this fall (one on whether searches for supersymmetry have been comprehensive, one on looking for unusual things the Higgs particle might do.)
While this has all been going on, there have been two big stories developing in dark matter searches, and those of you who already have heard about them will have noticed I have not written much about them yet. (In fact I only wrote about one of them, and very partially.) These stories are important, and also have some subtleties, which I want to make sure I understand fully before I try to explain them. After consultations with some of the experts (including Kev Abazajian of U.C. Irvine and Tracey Slatyer of M.I.T) I’m a lot closer to that point, so an explanation will come soon, after I’ve done a bit more reading and learning.
For the moment let me just note that there are two completely different excesses —
- one in X-ray photons (specifically photons with energies of about 3500 eV) noticed by two groups of scientists in a number of different galaxies, and
- one in gamma ray photons (specifically photons with energies of 1 – 10 GeV [GeV = 1,000,000,000 eV]), extracted with care by one group of scientists from a complex set of astrophysical gamma ray sources, coming from a spherical region around, and extending well beyond, the center of our own galaxy.
These seem to the experts I’ve spoken with to be real excesses, signs of real phenomena — that is, they do not appear to be artifacts of measurement problems or to be pure statistical flukes. This is in contrast to yet another bright hint of dark matter — an excess of photons with energy of about 130 GeV measured by the Fermi satellite — which currently is suspected by some experts, though not all, to be due to a measurement problem.
But even if the experts are right about that, it still leaves the big question: are these excesses signals of previously unknown astrophysical phenomena, or are they signals of decaying or annihilating dark matter particles? New astrophysics would be interesting too, but probably not Nobel-worthy, as dark matter would be. There are arguments against astrophysical explanations in both cases, but they don’t seem by any means airtight yet.
Since the two excesses are completely different, it is highly likely that at least one of them is due to astrophysics. [You can invent types of dark matter that would give you both signals -- but it would take a small miracle for two signals of the same dark matter particles to show up in the same year.] In fact, it is quite likely, in my mind, that they’re both due to astrophysics, not particle physics. But dark matter might show up in this way, so these excesses have to be explored fully. It could be that this is the moment when dark matter is finally revealed. If so — would the real dark matter excess please stand up?