HEP’s non-accelerator research program is at the forefront of the Cosmic Frontier, with an emphasis on the dark matter and dark energy, which together comprise about 95% of the universe (normal matter the other 5%) and high-energy cosmic rays. Recommendations from recent advisory panels, including the Particle Astrophysics Science Assessment Group (PASAG), a HEPAP subpanel which reported in 2009, and the NRC’s Decadal Survey of Astronomy & Astrophysics (Astro2010), which reported in 2010, are being used to guide the program in the cosmic frontier. Its portfolio of experiments—some operating, others under construction, and still others in the early planning stages—give the breadth of this unique program. Neutrino properties are studied at the Intensity Frontier using neutrinos from nuclear reactors and underground search for neutrinoless double beta decay. A partial list of these activities follows:
There are several ongoing non-accelerator physics experiments that search for dark matter, investigate dark energy, and measure the highest-energy particles that come from the cosmos. A few representative experiments are
- Baryonic Oscillation Spectroscopic Survey (BOSS)—an above-ground survey that will map out the mass distribution of the universe with unprecedented precision. Changes in this distribution with cosmic time give information on the growth of the universe, which in turn provides a measurement of dark energy properties. Jointly funded by the Arthur P. Sloan Foundation and by DOE, NSF, and NASA.
- Cryogenic Dark Matter Search-II—located deep underground in the Soudan Mine in Minnesota, this experiment looks for dark matter candidates known as Weakly Interacting Massive Particles (or WIMPs), This experiment detects WIMPs by measuring the tiny amount of ionization and phonon energy imparted to a Ge nucleus by its collision with a WIMP. Jointly supported by NSF and DOE.
- Fermi Gamma ray Space Telescope--a space-based observatory used to perform gamma-ray astronomy observations. Its main instrument is the Large Area Telescope (LAT), which will be used to perform an all-sky survey to study such phenomena as active galactic nuclei, pulsars, other high-energy sources and dark matter. A joint NASA and DOE/HEP supported project.
- Pierre Auger cosmic-ray Observatory--located in Argentina, measures the properties of the highest-energy cosmic rays produced in the Universe.
Experiments under construction or being commissioned include
• Dark Energy Survey (DES)—an above-ground experiment that will map out the large-scale structure of the universe in order to untangle how dark energy and gravity have co-shaped the universe.
• Daya Bay—currently under construction in China, commercial power reactors along with detectors located at various distances with respect to the reactor cores will be used to measure the survival probability of electron antineutrinos produced in the reactors, which will provide crucial information on the physics of neutrino oscillations This work is also supported by the Chinese Academy of Sciences.
• The Large Underground Xenon (LUX) dark matter experiment --will use scintillation and ionization signatures in a liquid xenon detector medium to search for WIMP (dark matter) interactions.
• The Enriched Xenon Observatory (EXO) --will search for neutrinoless double beta decays of 136Xe in 200 kg of isotopically enriched liquid xenon. Detection of such decays could provide a measure of the neutrino mass and would identify the neutrino as its own antiparticle.
Some possible future experiments:
• The Large Synoptic Survey Telescope (LSST, to be located in Chile, will investigate dark energy via weak lensing and Type Ia supernovae measurements. Its data will also be used for a wide variety of astronomical measurements including solar system, optical transients, and galactic structure research. DOE, partnering with NSF Astronomy Division, has made significant R&D contributions to this observatory.
• Wide Field Infrared Survey Telescope (WFIRST)— science goals are the investigation of dark energy (via weak lensing, baryon acoustic oscillations, and Type Ia SNe) and the search for Earth-sized planets using the technique of planetary microlensing. DOE has made major contributions in partnership with NASA on the design of a future space-based dark energy mission.
• Dark matter experiments—the next generation (“G2”) dark matter experiments will either detect dark matter particles or will lower the dark matter-nucleon cross section limits by an order of magnitude from current limits.
Relationship to Other Programs:
The non-accelerator research program plays a complementary role to that of HEP’s accelerator-based research and theory programs. As noted in Discovering the Quantum Universe (3.0MB), “it took results from astrophysical and cosmological observations to reveal that most of the universe is made of dark matter and dark energy. . . . Results from accelerator experiments must agree with astrophysical observations and results from underground. Discovering the quantum universe requires combining the most powerful and insightful observations in each of these different scientific approaches.” Non-accelerator physics research also benefits from the HEP Advanced Technology R&D subprogram through its advanced detector research, for example the development of noble gas detectors, cryogenic detectors, and large area photomultipliers. Research and project activities supported by HEP’s non-accelerator program include partnerships with other entities outside DOE as well, particularly NASA and NSF, as well as private institutions that support astronomy and astrophysics research, and several foreign agencies.
The HEP non-accelerator based research program has major scientific challenges in each of its major thrusts.
• Dark Energy—understanding why the universe’s expansion rate continues to accelerate is one of the fundamental challenges facing physics and cosmology today. Either three quarters of the energy density of the Universe is of a completely unknown form – dubbed dark energy – or General Relativity breaks down on cosmological scales and must be replaced with a new theory of gravity. Either way, there are profound implications for fundamental physics.
• Dark Matter--The direct detection and understanding of dark matter remains one of the most important scientific priorities of particle physics. The evidence for dark matter is clear, but so far it has been inferred only through its gravitational influence and its origin and nature are unknown. The existence of dark matter implies new particles beyond the Standard Model. Two leading candidates for dark matter are axions and weakly interacting massive particles (WIMPs).
• High energy cosmic particles--The major scientific thrust of this area is to understand the sources, acceleration mechanisms and propagation of the cosmic rays at extremely high energies, up to 108 TeV. By studying the high-energy particles produced by these cosmic accelerators, we are exploring the physics of extreme conditions in the universe.
• Neutrino oscillations—Neutrino oscillations may contain the clue to the baryon asymmetry of the Universe, which may have occurred via “leptogenesis” due to CP violation in the neutrino sector. High fluxes of antineutrinos from commercial nuclear power plants are being used to measure a critical mixing parameter, which in concert with neutrino data from accelerator experiments may provide the first direct evidence for CP-violation in the neutrino sector.
Research into both dark matter and dark energy, including collaborative efforts with other organizations outside DOE will remain the non-accelerator physics subprogram’s highest priorities. Efforts to investigate the nature of neutrinos via neutrino oscillation and double beta decay experiments, and to study the behavior and sources of the cosmos’ highest energy particles, will continue to receive strong support in this program.