Scientists from KIPAC and the SLAC theory department have demonstrated that astrophysical observations from the Fermi Gamma-ray space telescope can probe the validity of a class of famous particle physics theories known as supersymmetry.
Example of SUSY models allowed given current Fermi-LAT observations, in the plane of lightest supersymmetric particle mass (X-axis) and Fermi-LAT observed upper limit (Y-axis). Every point is a SUSY model with different parameters.
Over the last 80 years, astrophysical evidence has accumulated suggesting that most of the matter - and nearly 25 percent of the total energy - in the Universe is in the form of dark matter. Dark matter has thus far evaded all attempts at electromagnetic detection and has become one of the most compelling arguments for particle physics beyond the Standard Model. Some of the most popular theories predict that dark matter is made up of a new neutrally-charged particle with about the mass of a gold atom (~100 GeV). When two such particles come close to each other, there is the potential for them to annihilate and produce gamma rays. Thus, regions of high dark matter density could potentially shine as gamma-ray sources.
The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, for which KIPAC is the primary science institute, is one of the most sensitive instruments for detecting a gamma-ray signal from dark matter. Some of the cleanest and most promising dark matter targets for LAT observations are the dwarf spheroidal galaxies that orbit our own Milky Way. These small galaxies are not expected to produce gamma-rays through astrophysical processes, but host large concentrations of dark matter.
Since the LAT scans the entire sky, it collects data from the locations of all known Milky Way dwarf satellite galaxies every 3 hours. Since the same type of dark matter is assumed to exist in each of these galaxies, these independent observations can be combined to increase the LAT's sensitivity to a dark matter signal. These observations allow the LAT collaboration to set some of the most stringent constraints on dark matter. For some dark matter models with particle masses less than 30 GeV, these limits rule out the cosmologically plausible thermal relic cross section for annihilation (the cross section expected for a simple dark matter particle evolving with the expansion of the Universe).
Now, building on Fermi's success in constraining dark matter models, LAT Collaboration members at KIPAC - including graduate student Alex Drlica-Wagner, postdoctoral researcher Simona Murgia, and professor Elliott Bloom - have teamed up with scientists Randel Cotta, Thomas Rizzo, and JoAnne Hewett of the SLAC theory group to investigate how these new LAT limits can shed light on supersymmetry. Supersymmetry ("SUSY" in particle physics parlance) is a theoretical framework originally proposed to solve the so-called gauge hierarchy problem in particle physics and was soon found to naturally produce new particles that could constitute the dark matter. Unfortunately, the full parameter space of supersymmetry has >100 free parameters, making it difficult to form predictions. However, the team at the SLAC theory department has found a way to narrow down this space to only 19 free parameters by identifying and eliminating parameters that are free in principle, but not relevant in practice for this or collider studies. This problem becomes even more tractable when constraints from accelerators and direct detection experiments are imposed, leading to a picture of a valid supersymmetric candidate.
Together, the team of KIPAC astrophysicists and high-energy theorists show that the LAT constraints on dark matter annihilation in dwarf galaxies provide a unique new way to examine SUSY. Annihilation into gamma rays can probe some supersymmetric models which are inaccessible to direct detection experiments or even large accelerators like the LHC. That Fermi-LAT observations can be used to constrain SUSY models shows again the intimate and nuanced connections between astrophysics and particle physics.
This work is described in a paper submitted to the Journal of Cosmology and Astroparticle Physics and available from astro-ph at arXiv:1111.2604.
Tidbit author: Alex Drlica-Wagner and Jack Singal