Listening to dark matter

Oct 16, 2008
Listening to dark matter

(PhysOrg.com) -- A team of researchers in Canada have made a bold stride in the struggle to detect dark matter. The PICASSO collaboration has documented the discovery of a significant difference between the acoustic signals induced by neutrons and alpha particles in a detector based on superheated liquids.

Since neutron induced signals are very similar to dark matter induced signals, this new discovery, published today, Thursday, 16 October, in the New Journal of Physics, could lead to improved background suppression in dark matter searches with this type of detector.

So far, alpha particles have been an obstacle to the detection of dark matter's weakly interacting massive particles (WIMPs) in PICASSO. This detector, which is based on the operation principle of the classic bubble chamber, is sensitive to alpha particles over exactly the same temperature and energy range, therefore making it very difficult to discriminate between the two types of particles.

Alpha particles are relatively common on Earth, emitted by radioactive nuclei such as uranium, and thorium, and are therefore also present in traces in the detector material itself. WIMPs are thought to fill the large spaces between galaxies, concentrating around them in gigantic clouds. As the Earth moves together with the sun through the Milky Way's dark matter cloud, researchers hope to detect occasional collisions of a WIMP particle with an atom in their detectors.

Teams of researchers around the globe work deep underground to create the best conditions to isolate WIMPs from their travelling companions, namely neutrons, which are created by cosmic rays. Underground, teams in the US, Canada, England, Italy, Japan, Korea and Russia have long been sparring over the best detection methods for WIMPs.

The Canadian-American-Czech team based at SNOLAB, using their PICASSO detector, experimented with very sensitive Fluorine-based superheated liquids and analysed acoustic signals following phase transitions induced by alpha particles and WIMP like, neutron induced recoil nuclei. To their surprise they found a significant difference in amplitudes of the acoustic signals, which has never been observed before.

As experiment spokesperson Viktor Zacek (Université de Montréal) said, "When we looked at our calibration data taken with neutrons and compared them with our alpha background data we saw a peculiar difference which we attributed first to some detector instabilities or gain drifts in our electronics. However when we checked the data and refined the analysis the discrimination effect became even more pronounced."

Detection of WIMPs is the first challenge in the struggle to understand dark matter. Much of our understanding until now has been hypothetical. There is convincing astronomical evidence to suggest that 23 per cent of the Universe is made up of dark matter – different from the matter with protons, neutrons and electrons that we are accustomed to.

This dark matter is between a hundred to a thousand times heavier than a proton and interacts extremely weakly with itself and 'ordinary' matter. It is believed it was created during the Big bang and that it now surrounds most galaxies, and also our Milky Way in gigantic clouds.

Journal paper: stacks.iop.org/NJP/10/103017

Provided by Institute of Physics

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User comments : 12

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frigatebird
4.3 / 5 (3) Oct 16, 2008
Can anyone explain why WIMP's interact only weakly with eachother, if they have mass?
kniedzius
2.5 / 5 (2) Oct 16, 2008
To frigatebird:

WIMP's only interact through weak and gravitational interactions. Having mass doesn't mean that particles should interact through strong interaction. All in all, strong interaction is for quarks and gluons.
axemaster
3 / 5 (1) Oct 16, 2008
I have a question that has been bugging me somewhat, that is related to this stuff. Totally nonmathematical unfortunately:

Dark matter is invisible, interacting only through gravity, and possibly also through the weak force (though not confirmed). It also is generally believed to have very high mass, explaining why it has not been detected in accelerators yet.

What else shares this property? Black holes. They interact only through gravity (ignoring accretion disk stuff).

Black holes are believed to have blackbody radiation, given that Earth hasn't been swallowed by cosmic-ray induced micro singularities.

Many (if not all) GUT theories involve extra dimensions as a way to simplify interactions, and also in order to dilute gravity by shunting gravitons into other dimensions.

Isn't it possible that there could be some geometry created by a black hole, where below a certain radius of the event horizon (thus constraining the source of BB radiation), all the radiation is emitted into a closed loop of spacetime? The black hole would reach a certain minimum size, after which any remaining particles would be trapped in a little bubble of space. Gravitons would be able to travel from that plane to our 3D space, creating a "shadow" of the singularity in the form of gravity.

All the frozen microsingularities would tend to orbit masscons, producing the observed effect of dark matter, including the gravitational lensing. It would also explain why we haven't detected them yet, because nothing weaker than the LHC can make black holes. The universe could have been populated with huge amounts of singularities during the Big Bang. Most microsingularities created through interactions near our Solar System would carry insane KE, so they wouldn't stay here, instead orbiting the galaxy, as observed...

I dunno, just thinking about all this...
Velanarris
not rated yet Oct 16, 2008
Also remember that gravity is the weakest of the 4 forces when compared to the nuclear forces and EM.

Axemaster,

If they were black holes you'd detect massive amount of EM and ionization in their general vicinity. Even if the geometry stopped IR you'd still have gamma and microwave to think about as well as the obvious gravitational lensing interactions.
axemaster
1 / 5 (1) Oct 16, 2008
I'm talking about microsingularities, many times smaller than a proton. I'm pretty sure that wouldn't cause ionization. Plus they are in space bubbles (or branes if you prefer) possibly tangent to our normal spacetime. String theory and M theory both use this, where gravity extends as a factor of greater than r^2 into hidden dimensions. So EM in general wouldn't be able to escape, as it would follow the closed curvature of the pocket-space.
GrayMouser
4 / 5 (4) Oct 16, 2008
"Since neutron induced signals are very similar to dark matter induced signals"

Doesn't this depend on IF dark matter exists?
Velanarris
not rated yet Oct 16, 2008
I'm talking about microsingularities, many times smaller than a proton. I'm pretty sure that wouldn't cause ionization. Plus they are in space bubbles (or branes if you prefer) possibly tangent to our normal spacetime. String theory and M theory both use this, where gravity extends as a factor of greater than r^2 into hidden dimensions. So EM in general wouldn't be able to escape, as it would follow the closed curvature of the pocket-space.


Yes, but the existance of singularities in this curved multidimensional space wouldn't necessarily bleed through on such a small scale.

That and if you are referring specifically to microsingularities String theory as it stands now with M theory would make those items impossible outside fo our observed 3d space time.
xen_uno
1 / 5 (1) Oct 16, 2008
I'm a bit skeptical on the existence of dark matter myself, as it appears to have been created as a correction factor for gravitational equations that wouldn't jive otherwise.
M_N
1 / 5 (1) Oct 16, 2008
I'm not a physicist, but it seems to me that slightly "tweaking" some of the existing laws is a much more elegant solution to the observed issues (e.g. galaxy rotation) than having to invent "dark matter" and "dark energy".

It seems to me that even if it is real, dark matter is so elusive that if we think we have directly "detected" it will be a while before we have the confidence level / certainty to say for sure. Anyway, interesting times ahead!
ArtyNouveau
not rated yet Oct 17, 2008
What is a "...dark matter induced signal?"
Alizee
Oct 18, 2008
This comment has been removed by a moderator.
Alizee
Oct 18, 2008
This comment has been removed by a moderator.
smiffy
1 / 5 (1) Oct 18, 2008
Alizee: I find it hard to believe in Dark Matter myself, but the article you reference (thanks) seems to provide a fairly strong indication that something like that exists, even if it's interpretation is disputed. Why do you discount such evidence?
jeffsaunders
not rated yet Oct 20, 2008
What I want to know is how come the amount of the dark matter in the Universe keeps changing.

There is convincing astronomical evidence to suggest that 23 per cent of the Universe is made up of dark matter %u2013 different from the matter with protons, neutrons and electrons that we are accustomed to.


I have read 96% of the universe is dark matter in a number of other articles each by someone that is supposedly an expert.

If the amount can vary by so much - what if the is a negative amount of dark matter in the universe? (tongue in cheek.)