Proposed Particle Help Explains Odd Galactic Photons

In 2002, a satellite called INTEGRAL was launched by the European Space Agency with an instrument on board to detect and measure gamma rays from space. Four years later, it yielded some intriguing data: An unusually high number of gamma-ray photons from the galactic center carried the same energy—a particular, significant energy—and nobody could figure out why.

The energy is 511 keV (kilo electron volts), and it happens to be the exact rest-mass energy of the positron, the electron anti-particle, which emits a 511 keV photon when it annihilates with an electron. The gamma rays, then, are a telltale sign of positrons in the galactic center. Where those positrons come from, however, is not so clear.

At one point, scientists thought the positrons might originate as electron-positron pairs often referred to as positronium, an "element" that exists for a very short time before the electron and positron annihilate. For positronium to be the source of the gamma rays, there would have to be lots of it, but the usual positron sources, such as black holes and supernovae, don't emit enough of them.

"Theorists have thought that the 511 keV line might be from dark matter annihilation located at the center of our galaxy, but all the well known standard dark-matter candidates, such as the lightest super-particle, could not fit the data," said astrophysicist Seong Chan Park to PhysOrg.com.

Park and colleagues Ji-Haeng Huh, Jihn E. Kim, and Jong-Chul Park are researchers at Seoul National University in Korea, and the group recently proposed an explanation that involves a new particle.

The group suggests that the positrons come from a "millicharged fermion," a very lightweight particle with a tiny electric charge. The suggestion isn't groundbreaking, as many physicists think that there may be some new physics—a new particle, specifically—underlying the gamma-ray production.

"Most preferred interpretations of the 511 keV gamma rays involve the introduction of new particles, and a millicharged fermion has been proposed before, as far back as 20 years ago, as a dark-matter candidate." said Park. "But in this work we have handled the urgent problem of interpreting the 511 keV anomaly using millicharge fermions, within certain physical constraints."

The proposed particle can produce positrons, which would emit 511 keV gamma-ray photons by decaying into positrons and electrons, which would annihilate and emit the photons. Alternatively, it could annihilate with its own antiparticle to yield an electron and positron that would then annihilate.

The millicharged fermion, if it exists, would have escaped detection at collider experiments because its electric charge is so minute, say Park and his colleagues. They also suggest the particle as a dark-matter candidate, possibly making up 10 percent of the dark matter in the universe.

"In the beginning, we worried if the idea of 'millicharged' dark matter might be inconsistent with any existing experimental data," says Park. "As it turns out, that it is not the case and there is room to fit all of them, which is interesting."

This research is described in the June 3, 2008, online edition of Physical Review D.

Citation: Phys. Rev. D 77, 123503 (2008)

Copyright 2008 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Related stories:

Integral discovers the galaxy’s antimatter cloud is lopsided
The shape of the mysterious cloud of antimatter in the central regions of the Milky Way has been revealed by ESA’s orbiting gamma-ray observatory Integral. The unexpectedly lopsided shape is a new clue to the origin of the antimatter.
Scientists Determine the Nature of Black Hole Jets
NASA and Italian scientists using Swift have for the first time determined what the particle jets streaming from black holes are made of.
Scientists take the sharpest image ever made with light
(PhysOrg.com) -- A team of scientists from the Technische Universität Dresden (Germany) and the ESRF in Grenoble (France) has produced the image of an object at the highest resolution ever achieved with X-ray light. A 100-nanometre gold particle fixed on a substrate was reconstructed with 5 nanometre resolution. Contrary to other techniques, X-ray imaging works also in real-life environments like chemical processing or in the presence of high magnetic fields. The team reports its findings in the newest issue of Phys. Rev. Lett. dated 5 September 2008 (published online 29 August 2008).
T-REX is monster light source with multiple applications
When it comes to laser-based light sources, there are few brighter than T-REX, an LLNL project developed jointly by the NIF & Photon Science Principle Directorate and the Physical Sciences Directorate.
Scientists Nudge Closer to the Edge of a Black Hole
NASA scientists and their international partners using the new Japanese Suzaku satellite have collected a startling new set of black hole observations, revealing details of twisted space and warped time never before seen with such precision.
Where are the supermassive black holes hiding?
European and American scientists, on a quest to find super-massive black holes hiding in nearby galaxies, have found surprisingly few. Either the black holes are better hidden than scientists realised or they are lurking only in the more distant universe.
Sandia work shows live cells influence growth of nanostructures
Far above the heads of Earthlings, arrays of single-cell creatures are circling Earth in nanostructures. The sample devices are riding on the International Space Station (courtesy of Sandia National Laboratories and the University of New Mexico, NASA and US Air Force) to test whether nanostructures whose formations were directed by yeast and other single cells can create more secure homes for their occupants — even in the vacuum and radiation of outer space — than those created by more standard chemical procedures.
New record set for smallest X-ray nano-spot
An award-winning device developed at the U.S. Department of Energy's Argonne National Laboratory has set a world's record for tiny spot size with a hard X-ray beam.

News discussion:

Physics news