When dark matter is destroyed, it leaves behind a burst of
exotic particles, according to theory. Now scientists have found a possible
signature of these remains. The discovery could help prove the existence of
dark matter and reveal what it's made of.
No one knows what dark matter is, but scientists think it
exists because there is not enough gravity from visible matter to explain how
galaxies rotate.
An Italian satellite called PAMELA
(Payload for Antimatter Matter Exploration and Light nuclei Astrophysics), launched in
2006 to measure radiation in space, found an overabundance of particles
called positrons, which are the antimatter counterpart to electrons (matter
and antimatter annihilate each other).
This positron signature could have a variety of causes, but
a prime candidate is dark matter, the
intangible stuff thought to make up about 98 percent of all matter in the
universe. When two dark
matter particles collide they can sometimes destroy each other and release
a burst of energy that includes positrons.
"PAMELA found a number of positrons much higher than
expected," the mission's principal investigator Piergiorgio Picozza told SPACE.com.
"Many think this could be a signal from dark matter, because for positrons
this behavior fits very well with many theories of dark matter."
The finding, detailed in tomorrow's issue of the journal Nature,
is not a total surprise, but it could be a huge splash, if confirmed.
"This kind of signal for dark matter has been predicted
as a possible leading signature for over two decades, and [the PAMELA
scientists] are seeing just the kind of things one might expect," said
University of Michigan astrophysicist Gordon Kane, who was not involved in the
research. "There's a very good chance that this is the most important
discovery in basic physics for decades."
Positrons are often created when cosmic rays interact with
atoms in the gas and dust between stars. But this source cannot produce enough
positrons to account for PAMELA's findings. Another possibility is that the
positrons PAMELA found were produced by dense spinning stars called pulsars. To
distinguish between this option and dark matter, more data will be necessary,
either from PAMELA or from the Fermi Gamma-ray Space Telescope, launched last
year.
"We hope to have detected dark matter, but now we need
other verification coming from other experiments," Picozza said.
Even so, some scientists are excited to have come so close
to possibly discovering the presence of dark matter, which has eluded researchers
since it was first conceived in the 1930s.
Kane emphasized that the results, though still not certain,
could be significant not just as proof that dark matter exists, but also for
clues about what makes up this mysterious substance, which cannot
be directly seen and is only detected via its gravitational tug on other
things.
Kane's personal bet for the particle behind dark matter in
these findings is called a wino (pronounced WEE-no) — a specific type of
neutralino, which is a theorized category of particles that could exist as
"supersymmetric partners" for all the Standard Model particles such
as electrons, quarks, etc. The wino is the supersymmetric partner of a particle
called the W boson.
"It does particularly well at producing positrons in
the annihilation, and the positrons have energies that are about right for
these results," Kane said in a phone interview.
If dark matter is made up of neutralinos, then dark matter
particles would be their own antimatter particles, because the anti-neutralino
is simply a neutralino. Thus, when two dark matter particles collide, they can
self-destruct like any other interaction of matter and anti-matter.
Luckily, this does not happen very often. Dark matter
particles are thought to be extremely tiny, and the chances of them hitting
each other perfectly square on, and under the right conditions for destruction,
are very low. This fact allows dark matter to clump together throughout the
universe, scaffolding up galaxies and clusters, without destroying itself every
time two dark matter particles come near each other.
Even though annihilations are rare, the positrons they
produce could survive for up to a few million years, so they can stick around
long enough for detectors like PAMELA to find them.
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