Your Week in Seminars Dark Edition

Good afternoon everyone, and welcome to another week of seminars here in the physics department. Our theme of the week is dark matter - where does it come from, how do we see it, and why is there so much of it. Along with that we have a little more AdS/CFT, seemingly continuing last week’s subjects theme. All in all, it looks like seminars on similar subjects tend to condense here in the department. We start with the Monday Colloquium, where Richard Schnee from Syracuse University told us about What’s the Matter in the Universe? Direct Searches for WIMP Dark Matter. Dark matter, we’ll recall, is the astrophysics name for any kind of matter that doesn’t emit light - one that is not inside stars. Our knowledge of our own solar system, which has its mass concentrated almost entirely inside the sun, led us to expect that mass in the universe in general would behave similarly. It turns out that the motion of observed galaxies is not consistent with the mass we measure them to have, and so we hypothesize the existence of non-luminary matter around us. These days dark matter is an object of interest not only for astrophysicists but for particle theorists as well. With our variety of beyond-the-standard models of the universe we try to account for dark matter, guess at its properties and explains why it is dark. That last quality is rather easy to explain, in fact. Our expectation of dark matter is simply that it does not interact electromagnetically, and so does not emit photons. If we also posit that it does not interact strongly, we are left with a particle that can only decay weakly, and so we might expect a lot of it to stick around. Of course, the particle must also have a mass, which is the original property we postulated for it, and so we are looking for the WIMP, the Weakly Interacting Massive Particle. Schnee talked about the various ways we hope to see WIMPs in the coming decade, focusing on two avenues, the LHC and passive detectors trying to pick up cosmic particles passing through the Earth. WIMPs are by definitions hard to detect, because they interact only Weakly and thus only weakly. This means that we can’t actually see them directly in our detectors, and we have to look for either missing energy in accelerator results, which we deduce has gone to them, or their effects on detectable particles in large particle reservoir, much as we would detect neutrinos. Of course, when we have such weak signals the art is in reducing the background noise, by putting them underground, using the least radioactive materials we can find, and so on. The last part of the talk revolved around two events in his own detector that seemed to be far enough above background level to be WIMPs . Schnee then explained how statistical analysis proved in fact these many statistical outliers were, well, statistically expected, which I found very interesting. There was also a mention at the very end, of another experiment called DAMA, which is looking for a “WIMP wind”, checking for signals as the Earth moves through space in opposite directions, in a kind of modern parallel of the Michelson-Morley experiment. This one has actually shown a positive signal, though this is of course still controversial. On Wednesday we had a local postdoc, Enrico Pajer, talk about Striped holographic superonductor. I mentioned AdS/CFT last week, and how it’s induced some crossover between the condensed matter study of high-temperature superconductors and particle physics. This was one of those crossover seminars, with a few CM people in the audience. Enrico spent about half of the talk introducing the audience to the basics of superconductors - there were a lot of discontent as people asked questions or had objections to statements which I imagine would have gone over more smoothly with a condensed matter crowd . The bottom line of the first half were three important attributes of high-temperature superconductors, being a strong coupling between the electrons, the existence of a quantum critical point and an inhomogeneity of the material. There was then another general introduction of the AdS/CFT duality, and I’ll send you to last week’s summary (or the rest of the internet) if you want to hear more about that. Enrico was working on a field theory in AdS space and trying to apply the results to superconductors through the duality. In particular, strong coupling and quantum criticalities are known features of the AdS theory, and the addition here was of striped inhomogeneity, where things change alone one axis only. This is incorporated into the AdS space by applying boundary conditions, in particularl to the gauge field in the relevant field theory, and reading the results by applying Einstein’s, Maxwells and the Klein-Gordon equations, in the bulk of the theory. One interesting feature that was reproduced from other, non-AdS theories, was the dependence of the critical temperature Tc on the inhomogeneity. This is the temperature where superconductors turn into normal conductors, and previous work had shown that it would to drop as the scale of the inhomogeneity grows either very large or very small, and have some maximum point for a finite scale of inhomogeneity. Enrico’s work showed a dropoff in Tc for inhomogeneity on very small scales, giving the same qualitative behavior albeit with an exponential rather than logarithmic dropoff. There were more details and math then, as they studied these inhomogeneities in the AdS model, trying to determine the conductivity along the stripes and perpendicular to them and so on, producing some promising results and promising to produce some more. Finally on Friday we had Kuver Sinha of Texas A&M talk about The Cosmological Moduli Problem and Non-thermal Histories of the Universe. As mentioned above, this talk revolved around dark matter as well, though from a particle theory perspective. The trick in particle physics is always making sure that your solution to one problem, in this case dark matter, does not interfere with our solution to another problem. The other problem here was the baryon asymmetry, or the overabundance of matter compared with antimatter in the universe. In particle physics the two are generally sides of the same coin and we have no reason to prefer one or the other, and so we must have some reason that we only observe regular matter in the universe. There is a well-established model for this, called nucleosynthesis, and now when we explain the amount of dark matter in the universe we have to keep from interfering with it. And while we’re at it, we might solve a third problem - why is the density of dark matter and regular matter in the universe on the same order of magnitude? Sinha went through this, presenting two models of baryon genesis, occurring at different times in the history of the universe, each with their own features problems. Finally, he suggested one solution to the coincidence problem that is non-thermal - that is, rather than configuring the equilibrium point of matter and dark matter to be similar separately, we have them coming from the same source with similar decay rate. And that was that for last week, as dark matter obscures the better-lit one. This week have a sweeping overview of particle physics, toric singularities on branes, and possibly some news from the LHC. See you in seven.

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By Yariv