Tuesday, October 29, 2013

Is invisible dark matter detectable?

October 29, 2013
A prototype device designed by the MIT team to produce a very narrow, high-powered beam of electrons for an experiment called DarkLight (credit: MIT)
Scientists at MIT and elsewhere have developed a tool that could test to see if dark matter is detectable.
That will be a challenge: dark matter, believed by physicists to outweigh all the normal matter in the universe by more than five to one, is by definition invisible.
However, the MIT researchers have come up with a workaround, described in a paper in the journal Physical Review Letters co-authored by MIT physics professors Richard Milner and Peter Fisher and 19 other researchers.
“We’re looking for a massive photon,” Milner explains. That may seem like a contradiction in terms: Photons, or particles of light, are known to be massless.
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 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 it does exist, that would represent a major discovery, Milner says. “It’s totally beyond anything we understand about the physical world,” he says. “A massive photon would be totally different” from anything allowed by the Standard Model, the bedrock of modern particle physics, he says.
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.
“It’s a tiny effect,” Milner says, but “it can have enormous consequences for our theories and our understanding. It would be absolutely groundbreaking in physics.”
Probing other physics puzzles
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.
Roy Holt, a distinguished fellow in the physics division at Argonne National Laboratory in Illinois, says this work is “a novel and significant technical development that not only opens a new window to search for a new [particle], but also for new studies in nuclear physics.” If the planned experiment detects the A’ particle, he says, “it would signal that dark matter could actually be studied in a laboratory setting.”
The work, which also included researchers from the Jefferson National Accelerator Facility and Hampton University in Virginia, Arizona State University, and MIT’s Laboratory for Nuclear Science, was supported by the U.S. Department of Energy.

Abstract of Physical Review Letters paper
High-power, relativistic electron beams from energy-recovering linacs have great potential to realize new experimental paradigms for pioneering innovation in fundamental and applied research. A major design consideration for this new generation of experimental capabilities is the understanding of the halo associated with these bright, intense beams. In this Letter, we report on measurements performed using the 100 MeV, 430 kW cw electron beam from the energy-recovering linac at the Jefferson Laboratory’s Free Electron Laser facility as it traversed a set of small apertures in a 127 mm long aluminum block. Thermal measurements of the block together with neutron measurements near the beam-target interaction point yielded a consistent understanding of the beam losses. These were determined to be 3 ppm through a 2 mm diameter aperture and were maintained during a 7 h continuous run.

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