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The Sciences

The Dark Side in Aspen

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As Daniel has already mentioned, I recently spent a week in Aspen, at a workshop on Understanding the Dark Sector: Dark Matter and Dark Energy, which I organized in collaboration with Rachel Bean, Carlo Contaldi, Marc Kamionkowski and Jochen Weller. The Aspen Center for Physics is wonderful place. Unlike many physicists, I'm not a big skier, or a hiker, preferring to get my exercise through ball sports and in the gym, and so I don't love Aspen in the way that many people do. But I do really like the Center, and I always have great physics conversations there, learn a lot, and am able to actually get work done. So I had been looking forward to our workshop for a while, and I wasn't disappointed at all. The conference spanned five days. We flew in on Sunday in time to register and have dinner, and then got started properly on the Monday morning. At many conferences, academic and otherwise, the day is full of sessions, and then the evenings are free. At Aspen, in the winter, this is modified somewhat, with the mornings, late afternoons and evenings full of sessions, and the middle part of the day free so that people can ski if they want to. We spend the same amount of time working as at a regular conference, but the time is just distributed differently. This last week, it was good that I don't ski, since I had let a number of important deadlines slip to the point that I had to work each afternoon. I did have a great view while I was doing it though! Our speakers covered a wide range of topics, experimental, observational and theoretical, covering dark matter and cosmic acceleration separately, as well as the possible interactions between them. I can't possibly discuss all of the topic or talks here (but you can take a look at a slightly outdated program), but let me pick out one talk that particularly caught my attention, and one event. The first morning we kicked off with a very nice talk by Doug Finkbeiner (Harvard), discussing his (and others') recent work constructing a particle physics model of dark matter that is designed to account for the PAMELA and ATIC results. This was followed by later theoretical talks on related topics by Dan Hooper and Graham Kribs.The PAMELA results were a recurring theme at the workshop, and there were plenty of discussions in the free time and over dinner about the results themselves, their interpretation, and the models that a number of groups have constructed. One talk that I particularly enjoyed was by Greg Tarle (Michigan), who gave an (originally unplanned) talk on the immense challenge of making the precise measurements that these quite beautiful experiments claim. From my perspective what was wonderful about Greg's talk was that although he presented quite a few experimental details in the latter half, he constructed the first half in such a way as to get across some of the key things for theorists to keep in mind while they are rightfully getting excited by the data. What I took away from it (although any mistakes here are naturally due to me, and not to Greg) is that, as we've discussed before, the big deal for PAMELA (and the earlier HEAT experiment) is an accurate measurement of the flux of cosmic ray positrons. The single largest challenge in measuring these is discriminating against the vast proton background, since protons have the same charge as positrons and are copious in cosmic rays. Ideally one needs to be able to reject the proton signal at the 10^-6 level. If this is done completely reliably and the dramatic rise of the spectrum at high energies persists, then we may indeed be seeing a signature of dark matter annihilation, or something more mundane, like nearby pulsars. On the other hand, it is possible that the signal is due to protons leaking in. Obviously, the key to figuring this out is particle identification. One point of caution Greg pointed out is that on PAMELA, the particle ID is solely dependent on calorimetry. Greg certainly wasn't saying that he believes that the PAMELA signal is contaminated, but he did want to point out how a possible contamination might lead to an erroneous result. In his talk, to demonstrate this, he showed how to reproduce the observed effect (at high energies) with a model of proton contamination at the 3x10^-4 level. Note though, that PAMELA claims proton rejection at the 10^-5 level, but one can at least get a simple idea of what a small error might yield. Again, I as a theorist can't comment on this at all, and my understanding is that the PAMELA experiment is an excellent one. Still, it is nice to have a feeling for the possible issues involved. All should be settled by FERMI (GLAST) hopefully. On the Tuesday evening we had a couple of public events as part of the workshop. In the first, Dan Hooper, particle phenomenologist, dark matter expert and author extraordinaire, teamed up with me to conduct a Physics Cafe in the mezzanine of the Wheeler Opera House. I'm awful at estimating numbers, but I'd say somewhere between fifty and a hundred people, members of the public who either live in or were visiting Aspen, turned up to ask us any kinds of questions they liked about particle physics and cosmology. This was terrific fun, and we got some wonderful questions, my personal favorite of which was "What does entropy have to do with gravity?" Not something to which one can give a full answer (in public or professionally), but a great chance to talk about the general issues that gravity raises for thermodynamics. I participate in a lot of events like these, and one thing that never ceases to surprise me is the general perception that scientists treat theories like general relativity as received wisdom, about which one should not ask questions. Dan and I had to go out of our way to explain that science is, in fact, about challenging ideas, and forcing them to stand up to evidence and scrutiny. This is not the fault of the audience, of course, but reflects the way in which I think science is often taught and portrayed. Directly after this, also at the Wheeler Opera House, my new colleague Bhuvnesh Jain gave a wonderful public lecture on Einstein Rings and Giant Arcs: Mapping Dark Matter with Gravitational Lensing to a large and lively audience. Bhuv has some great animations and movies, plus some fun props - optical lenses that are machined to reproduce the effects of gravitational lenses - and he did an admirable job of the lecture. My own talk at the workshop was as part of a trio of talks, the other two of which were delivered by my collaborator Rachel Bean and her student Istvan Laszlo. I spoke about work with Rachel and Eanna Flanagan on adiabatic instabilities in coupled dark matter-dark energy models, and Istvan and Rachel covered different parts of a longer paper the three of us co-authored with Flanagan a few months ago, mapping out general constraints on such couplings. Speaking of which, I have a seminar at Penn State to give on this topic on Wednesday, and I should probably get to finishing that!

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