The state of play in dark matter searches just refuses to settle down. Just a few weeks ago, the XENON100 experiment released the best-yet limits on WIMP dark matter (a two-dimensional parameter space, “mass of the dark matter particle” and “cross section with ordinary matter”). These limits seemed to firmly exclude the hints of a signal that had been trickling in from other experiments. But… the story isn’t over yet.
Remember that XENON, like CDMS and other experiments, tries to find dark matter by making a very quiet experiment and picking out individual events where a dark matter particle bumps into a nucleus inside the detector. There is a complementary strategy, looking for annual modulations in the dark matter signal: rather than being very picky about what event is and is not a DM interaction, just take lots of events and look for tiny changes in the rate as the Earth moves around the Sun. Dark matter is like an atmosphere through which we are moving; when we’re moving into a headwind, the rate of interactions should be slightly higher than when our relative speed through the ambient dark matter is smaller. The DAMA experiment was designed to look for such a modulation, and it certainly sees one. The problem is that lots of things modulate on a one-year timescale; as Juan Collar explained in a guest post here, there were many questions about whether what DAMA is detecting is really dark matter.
Now one of Juan’s own experiments, CoGeNT, has seen (very tentative) hints of an annual modulation itself! CoGeNT had already teased us with a hint of a dark matter signal, which (like DAMA) seemed to imply lower masses (about 10 GeV, where 1 GeV is the mass of a proton) rather than the usual masses for weakly-interacting dark matter favored by theorists (hundreds of GeV). But the competitor experiment CDMS, and later of course XENON, seemed to put the kabosh on those claims. The CDMS result was especially hurtful to CoGeNT’s claims, as both experiments use germanium as their detector material. Theorists are very clever at inventing models in which dark matter interacts with one substance but not some other substance (see e.g.), but it’s harder to invent models where dark matter interacts with one substance in one experiment but not the same substance in some other experiment.
Yesterday Juan Collar gave a talk at the April Meeting of the APS, where he revealed something about CoGeNT’s latest findings. (I don’t think there’s a paper yet, but it’s supposed to come very soon, and they are promising to share their data with anyone who asks.) Now, unlike for their earlier results, they are explicitly looking for annual modulation. And … they see it. Maybe. Well, not really enough to take it seriously, but enough to be intrigued. Or, in science-speak: it’s a 2.8 sigma result. It doesn’t seem to have hit the news very hard, but there are writeups by Valerie Jamieson and David Harris. The CoGeNT folks have 442 days of data, with a rate of about three events per day.
Ordinarily, a tasteful physicist would claim that a 2.8 sigma result doesn’t even rise to the level of “intriguing”; you need three sigma to count as “evidence,” and five sigma for “discovery,” by the accepted standards of the field. The reason this is even blogworthy (a low bar indeed) is that it’s the first attempt to check DAMA by looking for an annual modulation signal, and the result matches the phase of DAMA’s oscillation, and is claimed to be consistent with its amplitude (the experiments use different materials, so it’s hard to do a direct comparison). Also, of course, because the team was looking to bury DAMA, not to praise it: “We tried like everyone else to shut down DAMA, but what happened was slightly different.” On the other hand, what you would need to explain this purported signal is at first glance still very much incompatible with XENON’s limits.
In the end: probably still nothing to get too excited about. But at least it will keep the pot boiling a while longer. Don’t fear; the experiments are getting better and better, and temporary confusions eventually evaporate. Or are swept away by the dark matter wind.
If they were trying to verify (or de-verifty, anyways) the signal from DAMA, and they knew the parameters of DAMA’s signal, wouldn’t they build the experiment so that the detection of such a signal would give a better confidence then 2.8 sigma, or if better confidence was impossible, then not bother to build it at all?
There is a vacuum background active toward massed fermions (matter) but not toward massless bosons (photons, arxiv:0912.5057, 0905.1929, 0706.2031). It would be better to discover what it is rather than to use it to curve-fit physically defective, politically facile prior theory.
Two much larger detectors are required: More of the same to confirm, and a much bigger XENON to deny. The truth bridges the gap. BTW, are the phase angles of the annual sinusoidal signals identical?
“Just messing with us”? No, I don’t think that’s quite right.
The designers of the universe set the galaxies’ rotation, but they didn’t do it right. Then some people noticed that. Fortunately the designers had time to fix the bug, so they invented dark matter. But now it seems they didn’t do it quite right, and some people have noticed.
You don’t think that’s a reasonable description of things? Then what about the neutrino fiasco? The designers didn’t get the mass of the neutrino right, and when people noticed, they had to invent that ridiculous neutrino which changes flavours as it flies. And from what I see lately they didn’t quite get that right either.
And then there’s the proton which doesn’t decay like it was supposed to. That appears to be still waiting for a bug fix from the designers.
I’m sure you could go on and produce more examples of sloppy design. (The “flatness” problem?)
It appears that humans’ ability to find these bugs is speeding up. If the designers don’t fix them fast enough then things could really start to unravel. Especially if they put in fixes which are worse than the original bugs. However the designers could deal with this problem by causing the funding for the bug-finders to be reduced; perhaps this strategy is already underway.
(Anyone know how to put smilies in comments?)
Where’s the Southern Hemisphere verification?
“Dark matter is like an atmosphere through which we are moving;…”
did you mean aether?
If they could build more sensitive detectors (bigger lump of Ge?), would they then be able to see a daily variation, and then confirm the effect by the phase difference of detectors on opposite sides of the planet..?
I’ve heard 4.8 and 2.8 sigma now, from two different places, neither with a citation; anyone know for sure what the number is?
If there’s 5 times as much dark matter as visible matter that means the Milky Way has 5 times more mass in dark matter. That also means the solar system should have 5 times more mass than we can detect. Why don’t we see the gravitational effects of dark matter on the solar system? Is all the dark matter concentrated at the galaxies core so that it only affects the rotation of the galaxy but not small scale objects like the Solar System?
The curve describing the orbital velocity of stars in galaxies shows that orbital velocities are constant as you move out from the core of the galaxy. This implies that the dark matter isn’t concentrated in the core but is evenly distributed throughout the whole galaxy. Again, if this is the case then why don’t we see the effect of dark matter on the orbital velocities of the planets?
Hmm…’annual modulation’…*chuckle* I can guess that this is triggering some scientist’s “NOT ASTROLOGY!!!” gag reflex 🙂
(similar to the finding that it seems that some radioactive decay changes over time as well…)
# 3: Hmm, Douglas Adam’s as soon as we understand universe, it will disappear and come back as something more inexplicable 🙂
You asked how do you put smiles in comments? Just do it the regular ascii way, do it in text, and it will, after you save the message, appear as a graphical smiley. So colon shift-zero will produce a 🙂 smiley and so forth.
It’s “kibosh”, not “kabosh”.
@Paul Clapham, to see what to type to make a particular smily, just hover your pointer over it for a couple seconds. 🙄
Sure, because the aether was a completely unexpected phenomenon that was only postulated after observations suggested it existed. *rolls eyes*
“That also means the solar system should have 5 times more mass than we can detect.”
CDM models generally assume a dark matter halo that is distributed within a galaxy with a distribution different than that of the ordinary matter in the galaxy. Thus, a given proportion of dark matter does not imply a given proportion of dark matter in a particular part of a galaxy.
Also, the 5-1 ratio (which is probably off, as it was based on an underestimate of the amount of ordinary matter in ellipical galaxies and is probably closer to 1-1), isn’t necessarily the same for all types of galaxies (and indeed probably isn’t the same). The ratio at the galaxy level may very well be different in ellipical and spiral galaxies of particular sizes.
One of the notable annual cycle events that ultimately turned out to be nothing was an apparent change in beta decay rate of some radioactive substance that some initial studies had seemed to support at levels far greater than GR time dialation effects could explain.
# 12: “One of the notable annual cycle events that ultimately turned out to be nothing was an apparent change in beta decay rate of some radioactive substance that some initial studies had seemed to support at levels far greater than GR time dialation effects could explain.”
Nifty, got a link? 🙂
To me, this says DAMA and CoGeNT are seeing annually-varying backgrounds. When experiments that can distinguish between electronic and nuclear recoils see no signal, but background-dominated experiments see something, then it smells to me like a background issue.
not sure of your point
both of these things are true.
aether was a completely unexpected phenomenon that was only postulated after observations suggested it existed.
dark matter is a completely unexpected phenomenon that was only postulated after observations suggested it existed.
*rolls eyes*
“If there’s 5 times as much dark matter as visible matter that means the Milky Way has 5 times more mass in dark matter. That also means the solar system should have 5 times more mass than we can detect. ”
I don’t know much about Dark Matter physics but this is an error in logic. It’s similar to saying that if 70% of the earth’s surface is ocean, 70% of my back garden must be ocean.
# 16: Yes, it could mean too that 70% of Earth is dark matter, 70% of all elections are dark matter (well, we already knew that, right?) and 70% of our bodies are dark matter 😉
# 13: arXiv: 0808.3283 and 1004.1761 (season variations of
beta-decays) (zhogin.narod.ru — am I black-listed? fie)
Perhaps they’re seeing a GeV-scale sterile neutrino signal? A low mass would seem to imply something that should have been seen at LEP?
Sean is right, this is very confusing!
yeah, that’s what Michelson-Morley dod, too. after not seeing a straight signal they were looking for anual modulations. and then starlight.
Could dark matter be not a separate entity but rather a “tail” of normal matter?
Right now the mass of an elementary particle is thought to be completely localized within a small distance from the particle. But imagine instead that the distribution of mass is not of a Dirac delta sort, but rather there is a very high peak where all the mass that we ascribe to particle is located and then there is a very low tail which extends for light years and which is responsible for dark matter effects. This tail would not affect physics on our scales, it would only become important on astronomical scales.
@8: Even assuming a perfectly even distribution of dark matter (in the ratio of 5:1 to normal matter), what gravitational effects are you expecting to see within the solar system?
So where are the CoGeNT slides and/or the arXIv link for the paper?
people are wasting the governments m0ney.A.ll the dark matter in our mature galaxy is long gone. Our galaxy is no longer growimg because it has no fermibosonic matter left to eat. Dark matter is the bosonic half of fermibosonic matter. We must have anther big bang to restore the fermi bosonic entity
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