Dark Matter WIMP Detection Claimed

Scientists at the University of Rome claim they have discovered evidence for Weakly Interacting Massive Particles (WIMPs). Their paper will be presented on Friday, and of course the verification process will take a while. The claimed particles weigh as much as a nickel atom, and could turn out to be the dark matter that astrophysicists have sought for so many years. All you touch and all you see may be only 20% of the universe. Read the NYTstory (free reg. req.) and then visit the TBTFblog for detailed information.

Comments

Dark Matter?

[kevlar]

The term 'dark matter' is simply matter that we cannot see. When astrophysicists are looking for dark matter, what they're actually trying to do is see gas, dust and dead stars that are not luminous. Dark matter is not a different form of matter, its just matter that has settled into the 3 degrees Kelvin equilibrium of space, and is therefore undetectable, unless heated by an external force.
"Though astronomers have been measuring the gravitational pull of the dark matter since the 1930's, they have never succeeded in detecting it directly."
I assume with this statement they're refering to the velocity vs distance from the center for stars in a galaxy. Its always been known that the fact that the stars in a spiral galaxy rotate with uniform motion, like a disk, simply because of the amount of dust and gas in between the stars.

If you ask me, I call this someones "what if" explaination, and attempted proof, that will quickly be disproved if it does in fact have any scientific basis. Of course, when this happens however, it won't make it to the presses ;)

Re:Dark matter?

[MrGrendel]

The big thing that gives away the amount of dark matter is the gravitational lensing effect near galaxies. If most of the mass in a galaxy could be attributed the the visible stars (including stars that emit only in the non-visible parts of the spectrum) then the lensing effect would be much smaller than what is observed.

The other thing that tipped off the astrophysicists about how much dark matter is out there is the dynamics of spiral galaxies. If normal orbital dynamics were at work, then based on the distributions of stars, the stars in the centers of galaxies should have a much shorter orbital period than the stars on the rim. But what is observed is something much closer to the rotation of a semi-rigid disk -- the stars on the rim don't take much longer to go full circle than the stars very near the center. This suggests that the distribution of mass in spiral galaxies does not correlate with the distribution of stars.

Is it possible that this matter is maybe some form of elementary particle that doesn't give off other particles (ie, the smallest particle which would not give off photons) and that's why we can't see it? Just my own questions on the subject. Wish I knew more about it.

The WIMPs are a type of elementary particle. Unlike protons quarks (making up protons and neutrons) and electrons which respond to gravity, electromagnetism, and nuclear forces (strong and weak), WIMPs only respond to gravity and the weak nuclear force. The absence of electromagnetism is why they don't ever give off any light.

Re:Dark Matter?

[Captn Pepe]

You're slightly missing the point. By observing the gravitational lensing of galaxy clusters, and the rotational rates of stars in galaxies, astronomers notice that they weigh about ten times what they should, based on their luminosity. Observations of our own galaxy, meanwhile, indicate that unless the Milky Way is extremely free of gas and dust relative to all the others, galaxies don't have enough non-luminous ordinary matter (gas and dust) to make up this difference.

The major candidates for this matter thus far have been MACHOS (massive compact halo objects - i.e. brown dwarfs/neutron stars/black holes), neutral gas (neutral hydrogen, in particular, is rather difficult to detect), and WIMPS. In the last few years, more evidence has been accumulating for all three of these classes. Personally, I expect that the missing mass will turn out to be a mixture of gas and WIMPS - if the halo contained enough compact objects to be significant, you'd think we'd see more stars there too.

Headline:

[bkocik]

"Wimps found in physics laboratory"

Sorry, couldn't help myself. I like physics, too. :-)

Regards,

Status of a subject...

[zunger]

Several people seem interested in what dark matter is and whether its existence is a certain thing or a theory. So here's some stuff from the science end --

The matter you can actually see through a telescope is really only luminous matter; things which are directly emitting (a great deal of) light. Namely, stars, quasars, occasionally black holes (which are black but infalling matter creates huge X-ray jets) and things like that. Anything else, by definition, is "dark matter." (So by definition, you and I are made of dark matter - this is not generally that wierd a stuff)

The reason we know dark matter is there in large quantities is by measuring the motion of stars in galaxies and so on. Basically, we understand how gravity works pretty well (at least on astrophysical scales) and so by watching the motions and orbits of luminous objects, we can work backwards and find the distribution of mass in the universe. From this we find that only about 10% of all mass is luminous - the rest is "dark matter."

Now, it turns out we can find out substantially more about dark matter from these gravity measurements. (There are a lot of different kinds of measurements which I won't go into; suffice it to say that they all more or less agree) For one thing, we can tell how it clumps up, and from that deduce some things about its internal structure. For example, dark matter made out of heavy noninteracting particles (say about the mass of an iron nucleus) will move around very differently from dark matter made out of very light fast particles, which will move differently from large lumps of matter each about the size of a star, and so on.

The basic types of dark matter are:

Hot Dark Matter: (HDM) Small light particles moving about at close to the speed of light. Measurements suggest that there isn't much of this around, not enough to make a huge difference. Neutrinos would fall into this category.

Baryonic Cold Dark Matter: (Baryonic CDM) Heavy particles in the form of ordinary nuclei and atoms. Up to and including ourselves. This category also includes "MACHOs" (An acronym whose expansion I can't remember right now), which are essentially star-sized or bigger objects which we can't see. Brown dwarfs, large gas giants, and so on. Large dust clouds also fall into this category.

Non-Baryonic CDM: CDM means that the particles in question are heavy and so move much slower than the speed of light. Non-baryonic means that they're not made up of ordinary nuclei. This category includes what are called WIMPs (Weakly Interacting Massive Particles), which are any sort of big, heavy particle that doesn't interact much with other matter in the universe. (e.g., it can't have an electric charge, since that would make its dynamics very very unlike experimental data)

The reason WIMP searches are so cool is that any particle that turns out to be a WIMP will probably be very interesting in its own right. I can't explain all of the details in something of this length, but there is a symmetry called Supersymmetry (SUSY) which is postulated to exist. There are lots of good theoretical reasons to believe in it (for the technically minded: Grand unification doesn't work entirely right without SUSY, and you can't introduce fermions into string theories without SUSY.) and by now everyone is pretty much expecting to discover it experimentally soon; in fact, a discovery that SUSY doesn't exist would be even more interesting than a discovery that it does.

The reason I bring up this whole dreary story is that SUSY predicts that for every particle of ordinary matter (electrons, protons, photons, etc.) there is another related particle, its superpartner. A direct detection of a superpartner would be both a vivid confirmation of SUSY and incredibly useful experimental data about the structure and nature of the universe. (There are armies of physicists who are ready to strip every imaginable drop of information out of data right now. People have been waiting for this for a while.)

And lo and behold - the superpartner of the photon, called the neutralino, happens to have some properties that would make it a great candidate for a WIMP. It interacts very weakly indeed; for comparison, the Coulomb force between two electrons is proportional to 1/r^2, where r is their separation. The force between two neutralinos would scale something like e^(-r/r0)/r^2, where r0 is a characteristic distance on the order of perhaps 10^-20 meters. They're also stable - due to some conservation laws (analogous to conservation of electric charge, which makes circuits work) they can't decay into anything else, so once they're created, you're pretty much stuck with them drifting through the universe. And they're heavy - experimentally, their mass should be somewhere between 80-a few hundred GeV. (For comparison, a Hydrogen atom has a mass of just over 1GeV)

Now the Rome group is claiming to have detected WIMPs of masses somewhere between 52 and 134 GeV, which are candidates to be neutralinos. This will definitely spark some excitement and a lot of discussion. What happens next is that people are going to be reading this and arguing over every detail of their data analysis and so on, and other people will try to replicate their results. If this is confirmed, it represents a big step in understanding both the large-scale nature of the universe (WIMPs, and the nature and origin of dark matter) and its very small-scale structure. (SUSY, the fundamental interactions of matter)

OTOH, one shouldn't get too excited yet -- this represents an interesting result but it still has to go through a very rigorous checking and repeating process. It has happened (quite a few) times before that interesting signals have been observed which later turn out to be something very ordinary. It'll take some time to tell about this one, but hell - if it works, it's seriously neat.

Yonatan

Re:NYT login

[G27 Radio]

yes, or you may replace the 'www' part of the url with 'partners' to go directly there (as someone pointed out earlier today.) Or click here:

htt p://partners.nytimes.com/library/national/science/ 021900sci-dark-matter.html

No annoying registration...who would've thought it would be that easy?

numb

From the source...

[Captn Pepe]

Here's the abstract, and here's the full preprint paper. It's an interesting, if quite densely technical, read.