I tell everyone I meet that we are at the dawn of the Dark Matter Decade. Usually they slowly back away, but I'm pretty persistent. Our technology has reached the point that we have an excellent chance of actually detecting most of the matter in the universe for the first time.
We're very happy to have a guest post from Neal Weiner, one of the leading theorists working in the fast-moving area. (Don't forget our previous guest post from one of the leading experimentalists.) Neal is responsible for some of the most imaginative models for what's going on in the dark sector, and is excited about the upcoming experimental prospects. If you want to know what particle physicists are thinking about dark matter these days, you've come to the right place. For anyone in the New York area, Neal is giving a public lecture on dark matter at AMNH on Friday the 4th (tomorrow). If you have a chance to go, I'd recommend not missing it. ----------------------------------------------------------------- The Era of Dark Matter Direct Detection Commonly, when I speak to my friends who don’t spend their time obsessing about the prospects for dark matter discovery, I am confronted by indifference, or worse, pessimism, when I mention the next few years of dark matter experiment. The history of dark matter direct detection has largely been a string of experiments, increasingly able to better find nothing, interrupted by occasional unverified claims, they point out. Why should this era be any different? In contrast, I remain incredibly optimistic about the coming era. I feel this level of sensitivity is special, and that if we are to discover WIMP scattering, it should be in the next few years. Why am I so optimistic?
1)This level of sensitivity is special
When we talk about discovering dark matter through direct detection, we are typically referring to discovering WIMPs, or Weakly Interacting Massive Particles (although a variety of searches for axions are ongoing). These are particles with masses ranging from roughly the proton mass, to 1000 x the proton mass. The hope is that by putting large (~100 kg or larger) experiments underground, where cosmic rays are shielded, experiments can detect the rare scattering of one of these WIMPs as they pass through the detector. (Estimates of the local density suggest that for WIMPs 300 x the proton mass, there should be about 1000 of them in a cubic meter of space near Earth.) For dark matter to scatter off of the nucleus, it must interact with it. In the standard model, there are only a limited number of possibilities, and for “renormalizable” interactions, there are only two. It can scatter by exchanging a Z-boson, or by exchanging a Higgs boson.
If the interaction is through a Z-boson, the strength is completely calculable. While a “weak” interaction, the Z-boson provides a relatively strong interaction as far as weak interactions go. Indeed, a WIMP exchanging a Z-boson to elastically scatter off a nucleus would have been seen already about a decade ago, and is excluded by about four orders of magnitude by present experiments (i.e., current experiments would have seen roughly 10^4 events, instead of few or none).
However there is a second possibility - that the WIMP interacts through a Higgs boson. The coupling of the Higgs to ordinary matter is orders of magnitude weaker, with a strength 10 - 100 times weaker than the current generation of experiments, but within reach of the next decade’s experiments. This is not something just pointed out now - Burgess, Pospelov and ter Veldhuis pointed this out a decade ago. While other force carriers appear in new physics models, such as supersymmetry, even there, the Higgs is often the dominant one. Thus, if you had asked me twenty years ago* what the most interesting levels of sensitivity to think about were, I’d have told you to look for the Z and the Higgs exchange. We know it’s not the Z, and we’re about to know about the Higgs. *OK, twenty years ago I’d actually have said “huh?”, but that misses the point.2) If anomalies mean anything, we should find out soon A great deal of thinking and excitement on the theoretical side has come from considering dark matter anomalies. The DAMA collaboration has reported an annual modulation in the flashes of light in a NaI(Tl) experiment for a decade. This modulation signature was pointed out by Drukier, Freese and Spergel in 1986. When the Earth orbits the sun, sometimes we move with the galactic rotation and sometimes we move against it, consequently the flux of WIMPs should change seasonally, and events in the detector should as well. This is precisely what the DAMA collaboration has observed.
Competing experiments, such as XENON, CDMS, Edelweiss, ZEPLIN and others have seen no such evidence, however, excluding the most conventional scenarios. This has prompted a variety of new ideas: light dark matter, inelastic dark matter, resonant dark matter, luminous dark matter... All of these allow a signal at DAMA consistent with other searches. When compelled by a novel result, theorists begin to see a wider range of possibilities. But even these possibilities make predictions.
More recently, the CoGeNT experiment has seen event rates in their detector above what is expected from background. While no claim has been made of discovery, it is in a range where light dark matter should be expected to be found. XENON and CDMS (and in particular a recent low-energy analysis of the CDMS data, who use the same target) do not see what would have been expected, but a clear background explanation is lacking. These may be signs of dark matter, and they may not be. If they are, we may already have guessed the correct model, or we may not have, but enough upcoming experiments have sensitivity that almost any scenario should be tested. What should we be looking for this year?
CoGeNT will update its data: with more exposure time, their radioactive backgrounds should decay, allowing the signal to be extracted more clearly. Does it modulate as expected? If so, theorists will have to go back to the drawing board.
KIMS should report soon: the KIMS experiment (Korea Invisible Mass Search) is a CsI(Tl) experiment, with a 100kg target. DAMA began as a 100kg, and grew to 250 kg target of NaI(Tl). KIMS will not test WIMP-sodium scattering explanations of DAMA, but will test WIMP-iodine explanations, and even scenarios where the tiny amount of thallium is what the dark matter interacts with.
COUPP: the Chicagoland Observatory for Underground Particle Physics is now operating a 4kg target of CF3I at SNOLAB in Canada. With both fluorine (which is light) and iodine (which is heavy and present in DAMA), it should have the ability to test most interpretations of DAMA as well as CoGeNT.
XENON100: the gorilla in the room is the XENON100 experiment. With already a large exposure on a 30kg target of XENON recorded, the community is eagerly awaiting their results. They could come early in 2011 and may shake up the field.
Going forward, improvements to established detector technologies (such as CDMS) and the maturation of the liquid nobles (such as XENON, but also LUX, DEAP/CLEAN, WARP, DarkSide and more) promise an era of rapid progress, with sensitivity improving by orders of magnitude over the next decade. If WIMPs are there, this coming era is our best opportunity to see them. When coupled with the LHC and new data from astrophysics experiments (Fermi, and PLANCK among others), our attitudes of what dark matter is - or at least what it is not - will soon be entirely different.