At the Cosmic Frontier our understanding of dark energy, dark matter, the accelerating universe, neutrino properties, and a host of other physics topics advances through a strong multi-pronged, experimental and theoretical program by
1. The measurement of the properties and origins of known particles.
These particles include high energy gamma rays, cosmic rays, and neutrinos. As one might expect, from such a host of particles, many questions are being investigated. For example, cosmic rays have been studied for more than 100 years. But the highest energy cosmic rays, those that occur very infrequently, have been a mystery as to their origin and production mechanism. Recent results indicate that they may come from Active Galactic Nuclei, that is, those galaxies with super massive black holes at their centers. High energy gamma rays are also a puzzlement. They are not produced by ordinary stars. That is, there production is not driven by how hot the star shines but by non-thermal production methods. For example, dwarf spheroidal galaxies are known to be rich in dark matter. A search for an enhanced high energy gamma ray signal from these galaxies may reveal a dark matter decay mechanism for their production. Whereas gamma rays and cosmic rays are extraterrestrial in origin, neutrino research under the Cosmic Frontier takes place deep underground. There Xenon-136 detectors look for what physicists call neutrinoless double beta decay. If it’s found, it may have far-reaching consequences on why we live in a world that has any ordinary matter in it at all.
For more information about experiments currently supported by HEP click on the following links:
2. The direct and indirect detection of theoretically postulated particles that may comprise dark matter.
Ordinary matter—everything that makes up the things we are, see and touch—forms only about 4.9% of the matter-energy composition of the Universe. The remainder consists of mysterious substances called dark energy (68.3%) and dark matter (26.8%) (These percentages are the recent Planck results). Candidate particles are WIMPs and axions.
WIMPs--The term WIMP stands for Weakly Interacting Massive Particle, which sums up what we know about them: They are dark because they are a kind of matter that does not emit light and interacts very weakly with ordinary matter, making it difficult to detect with ordinary observation methods. But it is susceptible to gravity, which is how we became aware of its existence in the first place through observations of stellar rotations within galaxies. Generally WIMPs are searched for either by direct detection or indirect detection. The former often involves a cryogenic detector filled with Xenon or Germanium with Silicon. Massive WIMPs are thought to interact with normal matter through nuclear collisions that produce either scintillation light or phonons (call this Signal 1 or S1) in conjunction with ionized electrons (S2). A characteristic S1/S2 ratio is thought to be a signature for a WIMP interaction. Of course background radiation, primarily from neutrons, must be accurately known. Other experimental techniques are also being deployed.
Axions--First postulated to solve a problem with strong CP violation (why is it so small?), axions have been proposed as a dark matter candidate. Unlike WIMPS, axions are believed to be very light in mass. They are searched for through a variety of mechanisms, most notably through their decay into photons in the presence of a strong magnetic field. So far no axions have been detected with certainty although specific mass ranges have now been excluded.
For more information about experiments currently supported by HEP, click on the following links:
3. The use of a wide variety of techniques and detectors located deep underground, on mountain tops, and even on satellites circling the earth.
A number of experimental techniques have been discussed above, such as deep underground cryogenic detectors for dark matter searches; large arrays of detectors located on mountain sides used for cosmic ray experiments; and the gathering of light from distant astronomical objects. In the case of neutrinoless double beta decay, the decay energy released is split between the emitted electrons. If the sum of the electron energies equals the mass difference between the parent (Xenon-136) and daughter (Ba-136) isotopes (the so-called Q-value), then this is the signature for that type of decay. Similarly, physicists who search for axions will know they have found them if they sum the energies of expected axion decay products and find a well defined spectral line for a specific mass. Astrophysicists have developed a number of sophisticated methods for deriving conclusions from their raw data. One of those methods is known as weak gravitational lensing. This technique is used in astronomical dark matter searches. It is well known and experimentally verified that light will bend its path around a massive object such as a planet or some other non-luminous body. The light that went one way around the object will combine with light that went the other way to produce a signal with increased amplitude or brightness—the lensing effect. Unlike strong lensing, the background light from many sources is statistically analyzed for signs of dark matter in the foreground. All-sky surveys are the best ways to quantify the amount of dark matter in the universe via this method.
These are just a few of the methods physicists use to obtain and analyze their data. A look at the above links to many of the Cosmic Frontier experiments will provide more details.