View of universe from NASA’s Fermi Gamma-ray Space Telescope. Brown physicists studied seven dwarf galaxies, circled in white. Observations indicate the dwarf galaxies are full of dark matter because their stars’ motion cannot be fully explained by their mass alone, making them ideal places to search for dark matter annihilation signals. Image: Koushiappas and Geringer-Sameth/Brown University |
If dark matter exists in the universe, scientists now have set the
strongest limit to date on its mass.
In a paper to be published in Physical
Review Letters, Brown
University assistant
professor Savvas Koushiappas and graduate student Alex Geringer-Sameth report
that dark matter must have a mass greater than 40 GeV in dark-matter collisions
involving heavy quarks. Using publicly available data collected from an
instrument on NASA’s Fermi Gamma-ray Space Telescope and a novel statistical
approach, the Brown pair constrained the mass of dark matter particles by
calculating the rate at which the particles are thought to cancel each other
out in galaxies that orbit the Milky Way galaxy.
“What we find is if a particle’s mass is less than 40 GeV, then it cannot
be the dark matter particle,” Koushiappas says.
The observational measurements are important because they cast doubt on
recent results from dark matter collaborations that have reported detecting the
elusive particle in underground experiments. Those collaborations—DAMA/LIBRA,
CoGeNT, and CRESST—say they found dark matter with masses ranging from 7 to 12
GeV, less than the limit determined by the Brown physicists.
“If for the sake of argument a dark matter particle’s mass is less than 40
GeV, it means the amount of dark matter in the universe today would be so much
that the universe would not be expanding at the accelerated rate we observe,”
Koushiappas says, referring to the 2011 Nobel prize in physics that was awarded
for the discovery that the expansion of the universe is accelerating.
The Fermi-LAT Collaboration, an international scientific collaboration,
arrived at similar results, using a different methodology. The Brown and
Fermi-LAT collaboration papers will be published in Physical Review Letters.
Physicists believe everything that can be seen—planets, stars, galaxies,
and all else—makes up only 4% of the universe. Observations indicate that dark
matter accounts for about 23% of the universe, while the remaining part is made
up of dark energy, the force believed to cause the universe’s accelerated
expansion. The problem is dark matter and dark energy do not emit
electromagnetic radiation like stars and planets; they can be “seen” only
through their gravitational effects. Its shadowy profile and its heavy mass are
the main reasons why dark matter is suspected to be a weakly interacting
massive particle (WIMP), which makes it very difficult to study.
What physicists do know is that when a WIMP and its anti-particle collide
in a process known as annihilation, the debris spewed forth is comprised of
heavy quarks and leptons. Physicists also know that when a quark and its
anti-quark sibling annihilate, they produce a jet of particles that includes
photons, or light.
Koushiappas and Geringer-Sameth in essence reversed the annihilation chain
reaction. They set their sights on seven dwarf galaxies which observations show
are full of dark matter because their stars’ motion cannot be fully explained
by their mass alone. These dwarf galaxies also are largely bereft of hydrogen
gas and other common matter, meaning they offer a blank canvas to better
observe dark matter and its effects. “There’s a high signal-to-noise ratio.
They’re clean systems,” Koushiappas says.
The pair analyzed gamma ray data collected over the last three years by the
Fermi telescope to measure the number of photons in the dwarf galaxies. From
the number of photons, the Brown researchers were able to determine the rate of
quark production, which, in turn, allowed them to establish constraints on the
mass of dark matter particles and the rate at which they annihilate.
“This is the first time that we can exclude generic WIMP particles that
could account for the abundance of dark matter in the universe,” Koushiappas
says.
Geringer-Sameth developed the
statistical framework to analyze the data and then applied it to observations
of the dwarf galaxies. “This is a very exciting time in the dark matter search,
because many experimental tools are finally catching up to long-standing
theories about what dark matter actually is,” says Geringer-Sameth, from
Croton-on-Hudson, N.Y. “We are starting to really put these theories to the
test.”