October 29, 2013
Experiment Seeks To Shed Light On Elusive Dark Matter Particle
As we peer into the darkness of space we find stars, planets, nebulae, and gas clouds. These objects are all made of normal matter – protons, neutrons, electrons, and other elementary particles. But it is the matter that we don’t see that has confounded astronomers for decades.
An estimated 80 percent of the stuff that makes up the Universe does not interact electromagnetically. This aptly named dark matter is consequently difficult to detect. Nonetheless, scientists are convinced of its existence because of the effect that it has on everything from galaxy rotation to gravitational lensing. The challenge now is to simply figure out what it is.
So far, experiments have failed to identify the culprit, leaving several competing theories. The most often discussed possibility is the WIMP – Weakly Interacting Massive Particle – that would have characteristics similar to a very heavy neutrino. But another theoretical solution gaining steam is the A’ – pronounced A Prime – particle that is akin to a massive photon.
That sounds like a non sequitur, since photons, by definition, have no mass. “It’s totally beyond anything we understand about the physical world,” says MIT physicist Richard Milner, one of the theorists that have proposed an experiment to potentially detect the A Prime particle. “A massive photon would be totally different.”
To investigate its existence, a team of scientists has proposed an experiment called DarkLight that would involve modifying a particle accelerator at the Jefferson Laboratory to produce a very narrow beam of electrons with a megawatt of power. At this specific threshold the beam would cause the A Prime particle to decay into a particle pair with specific properties that can be detected at the facility.
However, the number of events would be very small, therefore requiring millions upon millions of interactions to be compiled in order to detect the statistical anomaly that would signify a definitive detection. “It’s a tiny effect,” says Milner, but “it can have enormous consequences for our theories and our understanding. It would be absolutely groundbreaking in physics.”