Astronomy and Particle Physics Theory Meet Again in Dark Matter Lines

May 5, 2015

Gamma-ray observations of the Universe by the Fermi Gamma-ray Space Telescope have enabled another astrophysical constraint on the properties of particle dark matter.

The Fermi Gamma-ray Space Telescope continues to bridge astronomy and particle physics.

Many observations in astrophysics make it clear that the matter content of the Universe is dominated by dark matter, which doesn't glow but does affect the matter that does by its gravity.  It can then be tempting to view uncovering the nature of the dark matter itself as a particle physics problem, and indeed results from the CERN collider are eagerly anticipated in this regard.  However, astrophysics has much more to say about dark matter, including properties from direct detection techniques such as the CDMS experiment, and from microwave and gamma-ray observations of dense places such as the Galactic center and the centers of satellite galaxies, where the results of dark matter collisions and annihilations may be seen or limited.
 
Another promising avenue for astrophysical dark matter detection is gamma-ray 'lines' that would result from dark matter particle decays or annihilations.  All known astrophysical processes produce gamma rays across a range of energies, however dark matter interactions could in theory produce gamma rays preferentially at one or more specific energies, resulting in a 'line' in a plot of the intensity versus energy of gamma rays.
 
Fermi's Large Area Telescope (LAT) detects gamma rays in the energy range 10 MeV to >300 GeV.  According to particle physics theories, this energy range is promising for the detection of photons from the annihilation or decay of dark matter candidates known as 'WIMPs'.  KIPAC professor Elliott Bloom and graduate student Yvonne Edmonds have searched Fermi's data for excesses gamma-rays with energies from WIMP annihilation or decay directly into a photon and a second particle, such as a neutrino, a Z boson, another photon, or the Higgs particle.  In the absence of a detection of such lines, they have obtained flux limits and the derived annihilation cross-section and lifetime limits that importantly constrain the properties of WIMPs.  The limits achieved by Bloom and Edmonds in fact challenge recent dark matter models that have been invented to explain results in astrophysics such as on the ratio of positrons to electrons in cosmic rays.
 
The determination of WIMP properties in an astrophysical search, such as Bloom and Edmonds' gamma-ray line search with the Fermi-LAT, is complementary to information gained from accelerator searches.  If the LHC experiments provide estimates of the WIMP mass and couplings, it will significantly narrow the mass and cross section search regions for astrophysical experiments.  Furthermore, if the WIMP mass is previously known and a gamma-ray line is detected or constrained from WIMP annihilation or decay into a photon and an undiscovered particle (e.g. Higgs particle), one can make a novel measurement of or constraint on the mass of the new particle.  Additionally, collider experiments are well-suited to determine several, but not all WIMP properties.  For example if a WIMP is long-lived, it is not possible to measure its lifetime in an accelerator, but this can be constrained with astrophysical limits.  The limits on gamma-ray lines from Fermi-LAT observations are another example of a crucial input to particle physics theory from astrophysics.
 
This work is based in part on a paper submitted for publication to Physical Review D.
 
Science Contact:
Elliott Bloom
KIPAC
elliott@slac.stanford.edu
 
Tidbit Authors: Elliott Bloom and Jack Singal