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Possible Fifth Force Would
Make Direct Detection of Dark Matter Unlikely |
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Possible Fifth
Force Would Make Direct Detection of Dark Matter Unlikely
By Lisa Zyga

In this image of the Bullet Cluster, the blue area shows what is
thought to be dark matter. Physicists are investigating connections
between a possible fifth force and the direct detection of dark
matter. Image source: NASA / CXC / CIA / STSci / Magellan / Univ. of
Ariz. / ESO.
(PhysOrg.com) -- No one knows exactly what a
fifth force might be, but studies have shown that, if a long-range
fifth force does exist, it could have surprising effects on the
universes structure formation. A fifth force could reduce
discrepancies between theory and observation in several areas of
cosmology.
Now, as new research has shown, a fifth
force could
also be connected to dark
matter. In a paper published in Physical
Review Letters, physicists Jo Bovy and Glennys Farrar
were surprised to discover that a fifth force in the dark sector
could place constraints on dark matter that essentially exclude its direct
detection through
spin-independent interactions. Conversely, if future experiments do
detect a spin-independent interaction of dark matter, then any fifth
force in the dark sector must be so weak as to be astrophysically
irrelevant.
Our study shows that we can strongly constrain some properties of
dark matter, i.e., the combination of its interaction with the
visible sector and the strength of a long-range fifth force between dark
matter particles, through experiments with ordinary
matter, Bovy, a Ph.D. student at New York University,
told PhysOrg.com.
As for which scenario appears to be more likely - a fifth force
excluding direct detection of dark matter, or direct detection of
dark matter excluding a relevant fifth force - Bovy and Farrar said
that its impossible to say in advance. Both would be very
interesting both theoretically as well as observationally, Bovy
said.
Previous research has suggested the possibility that a new
long-range, attractive fifth force might exist, which arises in
several extensions of the standard model. Although most dark matter
models predict that the force between dark matter particles is a
short-range force, other models such as supersymmetry and
string theory allow for the existence of a very light boson which
could carry a long-range force in the dark sector.
In the current study, the theorized fifth force - a
nongravitational, long-range attractive force - would couple
directly to dark matter but not to ordinary visible matter. However,
if dark matter particles interacted nongravitationally with ordinary
matter, quantum corrections would make the fifth force emerge in the
visible sector, as well. This quantum correction occurs when two
virtual dark matter particles are coupled to both the fifth force
scalar and to two quarks.
Our study reveals a hitherto unrecognized connection between dark
matter interactions with ordinary matter, and dark matter
self-interactions, said Farrar, a physics professor and Director of
the Center for Cosmology and
Particle Physics at New York University.
As Bovy and Farrar explain in their study, in order to be
astrophysically relevant (i.e., on the order of the gravitational
force), the fifth force carrier must have a vacuum expectation value
on the order of the Planck mass. In turn, this requires that dark
matter be very heavy in most scenarios. In direct detection
experiments, scientists look for dark matter by observing ordinary
matter particles recoiling due to being scattered by dark matter
particles. As the mass of dark matter particles increases, the
bounds on the coupling of dark matter and quarks become more
stringent, making the direct detection of dark matter more
difficult.
Besides its implications on the direct detection of dark matter, a
fifth force has also been hypothesized to impact large-scale
structure formation. If a fifth force was attractive and had a very
long range, it would effectively increase the strength of
gravitational interaction, and thus accelerate structure formation.
As previous studies have shown, such a force could reduce
discrepancies between observations and predictions in several areas,
such as by increasing the number of galaxy clusters and
superclusters and reducing voids, which would agree better with
observations.
As the scientists explain, the impacts of a possible fifth force do
not demand its existence, but they do motivate researchers to
explore the implications of a fifth force.
Jo Bovy and I are looking at what the analogous constraints are,
between possible dark matter annihilation signals (searched for by
Fermi-GLAST and other satellite experiments like PAMELA and ATIC)
and a fifth force, Farrar said. Also, in collaboration with
others, I'm studying the effects of a long-range dark matter force
on cosmology and attempting to improve constraints on it from the
Bullet cluster.
Bovy added that hes also investigating the implications of
short-range attractive forces between dark matter particles, which
may enable the detection of dark matter annihilation from satellite
galaxies of the Milky Way.
More information: Bovy,
Jo and Farrar, Glennys R. Connection between a Possible Fifth Force
and the Direct Detection of Dark Matter. Physical
Review Letters 102,
101301 (2009).
Source: http://www.physorg.com/news157292373.html
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