[+]
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.
DarkLight
“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.
(¯`*• Global Source and/or more resources at http://goo.gl/zvSV7 │ www.Future-Observatory.blogspot.com and on LinkeIn Group's "Becoming Aware of the Futures" at http://goo.gl/8qKBbK │ @SciCzar │ Point of Contact: www.linkedin.com/in/AndresAgostini
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