Dark Matter: Have Its Particles Been Detected?

Dark matter, the invisible, unidentified substance that does not reflect light but makes up about 85-95% of the mass of the universe, is searched for from deep within the heart of underground mountain laboratories. In one such laboratory in Gran Sasso Mountain, scientists believe they have found the signature of dark matter particles.

The story of the recent discoveries begins ten years ago in the same Gran Sasso laboratory of the DAMA project (DAMA stands for DArk MAtter) in Abruzzo, Italy. Dark matter is sought from deep underground because the detectors need to be far from the reach of other particles that could obscure the signature of dark matter. Ten years ago, data collected by DAMA matched the expected signature of one group of particles that are theorized to be the components of dark matter. The news wasn't met with resounding acceptance from the scientific community because no other subterranean particle detectors had found the same result. DAMA reacted by upgrading to DAMA/LIBRA and going for another four years, the results of which produced new data.

Dark matter is in large part, though not solely, collected in invisible halos around galaxies. It is the invisible stuff that, according to Swiss astronomer Fritz Zwicky (1933) who first predicted it, exerts strong influence at galaxies' edges to cause them to twirl as we see them do. This is true for Earth's galaxy, too. Since our solar system is located in an outer position on an arm of the spiral Milky Way Galaxy, which places it essentially within the galactic halo, Earth is in relatively near proximity to the dark matter particles and weakly interacting massive particle (WIMP) "wind" that "blows" through the halo and around our solar system. As The Guardian puts it, DAMA was designed to detect dark matter in space as Earth flies through it.

It is projected by dark matter theory that Earth's switch in direction as it orbits the Sun causes it to alternately travel with the galactic stream of WIMPs (also theorized to comprise dark matter), then against the stream of WMIPs. When Earth is traveling with (in the same direction as) the galactic halo stream of WIMPs in the December cycle, theory predicts that a small number of dark matter particles will collect in the DAMA/LIBRA detector. Conversely, when Earth is traveling against (into) the WIMP stream in the June/July cycle, DAMA/LIBRA will collect a greater number of particles.

The unique signature of these particles was predicted by University of Michigan physicist Katherine Freese, who is an inflation and dark matter theorist. Lead researcher at DAMA, Dr. Rita Bernabei of the University of Rome, acting as spokesperson for DAMA, states that the signatures of the particles that DAMA has collected for seven cycles as the original experiment DAMA and four cycles as the upgraded DAMA/LIBRA are in agreement with the signature Freese predicted for dark matter particles in a galactic halo.

The current most likely suspect for the identity of the signature particle is the theoretical axion, a particle that would be far lighter than a WIMP and that would prevent a detectable "electric dipole moment" (molecules with two points of opposite charge separated by a measurable space). WIMPs, which are also predicted as part of the dark matter composition, are particles that have much larger masses than axions and that interact with baryonic matter (the material world) through gravitation and the weak nuclear force (the force that allows protons and neutrons to decay as in radioactivity).

Underground detectors, of which DAMA is only one of many (e.g., Physics World identifies XENON10 in Italy, ZEPLIN-III in the UK, and CDMS-II and COUPP in the U S), are great, enormous tanks about 1 mile underground that are filled with approximately 100 kg of sodium iodide, which generates pulses of light when its electrons are excited due to being hit by particles. As The Telegraph explains it, the solution is of ultrapure sodium iodide crystals that flash each time a particle collides with it. According to Physics World, DAMA/LIBRA upgraded to 250 kg of sodium iodide. Known particles that can penetrate 1 mile underground leave recognizable signatures of light pulses as they hit the sodium iodide. Unknown particles that leave unrecognizable signatures or theoretically predicted signatures are candidates for being the particle for which the detector was designed to search.

DAMA's data also shows the predicted modulation of particle hits predicted by Freese's dark matter theory, with higher numbers of particle hits in the June/July cycle and fewer in the December cycle. This shows irresistible evidence for the theoretically predicted modulation that is the result of Earth's orbit around the Sun while the Sun and its solar system orbits around the center of the Milky Way, with both all the while moving through what The Telegraph calls "the sea of dark matter that surrounds us."

As is made apparent by Physics World, the rest of the scientific community remains skeptical about the findings, though, saying that proof of modulation is not the same as proof of dark matter particles. As Henrique Araujo of ZEPLIN-III told Physics World: "No one disputes that they observe an annual modulation, but the community needs to be persuaded that dark matter has anything to do with it."

Still, DAMA/LIBRA is claiming greater than 99.9999998% certainty that the data collected points to dark matter particles at the low end of the predicted size range. The first DAMA results were at a 6.3 � confidence level (99.9999998% certainty), while DAMA/LIBRA has an increased 8.2 � confidence level.

The Guardian, however, points out that although the scientific community at large is not convinced the signature is of dark matter, there are dissenters who back Bernabei, such as astrophysicist Frank Halzen of the University of Wisconsin-Madison who told England's New Scientist: "We are pretty sure now that this is not a statistical fluke. We should pay attention to this. We should not just ignore it."

The NASA Hubble Space Telescope relayed images in 2007 that proved definitive for the discussion of dark matter's existence. At that date ,The Telegraph reported that the Hubble images showed a "ghostly ring of dark matter" encircling two ancient galaxies that had at one far distant time experienced a galactic collision and formed a cluster galaxy. The image was so definitive because the galactic collision left little to debate in relation to the presence of an invisible dark ring: both galaxies were surrounded by individual rings of dark matter that exploded and combined during the collision. Has the work of identifying what dark matter is made up of taken a definitive step forward at this year DAMA/LIBRA?

The Guardian:
http://www.guardian.co.uk/science/2008/apr/24/physics.sciencenews

The Telegraph:
http://www.telegraph.co.uk/earth/main.jhtml?view=DETAILS&grid=&xml=/earth/2008/04/23/scidark123.xml
http://www.telegraph.co.uk/earth/main.jhtml?view=DETAILS&grid=&xml=/earth/2008/04/23/scidark123.xml
http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2007/09/13/scidark113.xml

Physics World:
http://physicsworld.com/cws/article/news/33870

Author Phillip Pullman courts dark matter, The Telegraph:
http://www.telegraph.co.uk/connected/main.jhtml?xml=/connected/2005/04/27/ecquest27.xml

Where in the Galaxy are we??
http://www.enchantedlearning.com/subjects/astronomy/solarsystem/where.shtml