Neutrinos are the lightest and most mysterious of the basic particles that make up the universe as described by particle physicist's Standard Model. They are very small particles, with a mass so tiny that there is still a debate as to whether they even have mass. Neutrinos are very hard to detect, because they are electrically neutral and can only interact via the weak force; they are famously able to traverse the distance from the sun to earth if it were filled with lead without an interaction. There are three flavors of neutrinos: electron neutrinos, tau neutrinos, and muon neutrinos, corresponding to the other members of the lepton family.
For particle physicists, neutrinos are very unique particles. For example, the observation of neutrino oscillations — which imply neutrinos have mass — was the first evidence for physics beyond the Standard Model of particle physics.
For astronomers, neutrinos can open up a new window to the universe. Currently, the only neutrinos that have been detected outside our solar system came from Supernova 1987A, and their detection showed that most of the energy from the supernova is carried away by neutrinos. At high energies, neutrinos are the only particles which are neither absorbed by background radiation nor deflected by intergalactic magnetic fields.
The Greisen-Zatsepin-Kuzmin (GZK) Effect provides a source of high energy neutrinos from the interactions between cosmic rays and photons in the Cosmic Microwave Background. This effect predicts a limit on how high the energy of cosmic rays from very distant sources can be. Theoretically, cosmic rays from distant sources with energies higher than the GZK limit will not be detected on Earth.
Cherenkov Radiation is electromagnetic radiation produced by a charged particle travelling through a medium with a velocity greater than the speed of light in that medium. It is much like a "shock wave" of light, akin to a sonic shock wave created by a jet flying faster than the speed of sound.
A particle shower is the result of the interaction of a high-energy particle or photon with a medium. When the high-energy particle enters this medium, pair production and bremsstrahlung processes convert this particle into an electrically neutral collection of positrons, electrons and photons.
The Askaryan Effect is observed when a particle shower occurs in a dense, radio transparent medium, such as ice, sand or salt. In this case, the positrons, electrons and photons generated by pair production and bremsstrahlung are able to interact with the atomic electrons in the medium. The positrons annihilate with the electrons of the medium, while other electrons of the medium are upscattered to join the shower. This creates a negative charge asymmetry which may be considered to be a charge moving faster than the speed of light in the medium which will emit Cherenkov radiation. The emissions will be coherent for wavelengths on the order of the size of the shower, and thus in the radio frequencies.
ANITA is equipped with 32 quad-ridged horn antennas, designed to detect the radio pulses generated in interactions of high-energy neutrinos with the Antarctic ice. Ice is extremely radio-transparent, so use of the Antarctic continent gives a possible volume of order 106 km in which to detect these interactions.Antarctica also has very little radio noise, unlike densely populated regions of the world.
When a high-energy neutrino interacts within the ice, the resulting cascade emits coherent radio Cherenkov radiation, due to the Askaryan Effect. The ANITA antennas and RF system are designed to see these radio pulses as the balloon floats 30 km above Antarctica in the circumpolar winds.
ANITA-LITE was the ANITA prototype balloon that flew from McMurdo in Antarctica, piggybacked on another experiment, TIGER in 2003. This prototype used only two antennas, and it did not detect any Askaryan signals nor any radio background events that would impact the detection of a signal from a neutrino interaction. ANITA-Lite currently sets the best limits on the high energy neutrino flux.