“We’re looking for a massive photon,” explains MIT physics professor Richard Milner. That may seem like a contradiction in terms: Photons, or particles of light, are known to be massless. If it does exist, that would represent a major discovery, Milner says. “It’s totally beyond anything we understand about the physical world. A massive photon would be totally different” from anything allowed by the Standard Model, the bedrock of modern particle physics. "It’s a tiny effect,” Milner adds, but “it can have enormous consequences for our theories and our understanding. It would be absolutely groundbreaking in physics.”
However, an exotic particle that resembles a photon, but with mass, has been proposed by some theorists to explain dark matter — whose nature is unknown but whose existence can be inferred from the gravitational attraction it exerts on ordinary matter, such as in the way galaxies rotate and clump together.
Now, an experiment known as DarkLight, developed by MIT physics professpr Peter Fisher and Milner in collaboration with researchers at the Jefferson National Accelerator Laboratory in Virginia and others, will look for a massive photon with a specific energy postulated in one particular theory about dark matter, Milner says. If the planned experiment detects the A’ particle, says Roy Holt, a distinguished fellow in the physics division at Argonne National Laboratory says, “it would signal that dark matter could actually be studied in a laboratory setting.”
To prove the existence of the theorized particle, dubbed A’ (“A prime”), the new experiment will use a particle accelerator at the Jefferson Lab that has been tuned to produce a very narrow beam of electrons with a megawatt of power. That’s a lot of power, Milner says: “You could not put any material in that path,” he says, without having it obliterated by the beam. For comparison, he explains that a hot oven represents a kilowatt of power. “This is a thousand times that,” he says, concentrated into mere millionths of a meter.
The new paper confirms that the new facility’s beam meets the characteristics needed to definitively detect the hypothetical particle — or rather, to detect the two particles that it decays into, in precise proportions that would reveal its existence. Doing so, however, will require up to two years of further preparations and testing of the equipment, followed by another two years to collect data on millions of electron collisions in the search for a tiny statistical anomaly.
While DarkLight’s main purpose is to search for the A’ particle, it also happens to be well suited to addressing other major puzzles in physics, Milner says. It can probe the nature of a reaction, inside stars, in which carbon and helium fuse to form oxygen — a process that accounts for all of the oxygen that now exists in the universe.
“This is the stuff we’re all made of,” Milner says, and the rate of this reaction determines how much oxygen exists. While that reaction rate is very hard to measure, Milner says, the DarkLight experiment could illuminate the process in a novel way: “The idea is to do the inverse.” Instead of fusing atoms to form oxygen, the experiment would direct the powerful beam at an oxygen target, causing it to split into carbon and helium. That, Milner says, would provide an indirect way of determining the stellar production rate.
In 2012, Simona Vegetti, a physics fellow at MIT, discovered an entire galaxy made of dark matter just outside the Milky Way. The dark galaxy may host a luminous galaxy made invisible by the dark matter. “The thing people like about dark matter is that it’s been able to explain so many observations,” Vegetti said.
Because dark matter reflects no light, the galaxy is elusive. Vegetti worked with an international team of scientists including three from the U.S. and two from the Netherlands. Using the Keck Telescope in Hawaii, they detected the galaxy by studying ripples in the patterns of light rays from the Milky Way, a method known as gravitational lensing.
“It’s a dark matter-dominated object,” Vegetti said, “So there might be stars but very little.”
There are thought to be more than 10,000 satellite galaxies attached to our Milky Way galaxy, but only 30 of them are visible, she said. The image at the top of the page shows the Sagittarius Dwarf Galaxy, named for the constellation in which it is seen from the earth, in the process of colliding and merging with our own Milky Way.
“The question becomes are these satellites missing because they don’t exist or because they are purely dark? And that’s one question we’re trying to answer,” she said.