The Antarctic Impulsive Transient Antenna (ANITA) experiment has been designed to study ultra-high-energy (UHE) cosmic

neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...

s by detecting the radio pulses emitted by their interactions with the

Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...

ice sheet. This is to be accomplished using an array of radio antennas suspended from a

helium balloon A gas balloon is a balloon that rises and floats in the air because it is filled with a gas lighter than air (such as helium or hydrogen). When not in flight, it is tethered to prevent it from flying away and is sealed at the bottom to prevent t ...

flying at a height of about 37,000 meters. ANITA-4 prior to launch

The neutrinos, with energies on the order of 10¹⁸ eV, produce radio pulses in the ice because of the

Askaryan effect The Askaryan radiation also known as Askaryan effect is the phenomenon whereby a particle traveling faster than the phase velocity of light in a dense dielectric (such as salt, ice or the lunar regolith) produces a shower of secondary charged par ...

. It is thought that these high-energy cosmic neutrinos result from interaction of ultra-high-energy (10²⁰ eV)

cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

s with the photons of the

cosmic microwave background radiation In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all space ...

. It is thus hoped that the ANITA experiment can help to explain the origin of these cosmic rays.

Experimental time frame

ANITA-I launched from

McMurdo Vice-Admiral Archibald McMurdo (24 September 1812 – 11 December 1875) was a Scottish naval officer and polar explorer after whom Antarctica's McMurdo Sound, McMurdo Station, McMurdo Ice Shelf, McMurdo Dry Valleys and McMurdo–South Pole ...

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

in the summer of 2006–07. The array should travel around the continent with the circumpolar winds for approximately a month before being recovered by the CSBF. Each successive mission (if funded) would be at two-year intervals. ANITA-II, a modified instrument with 40 antennas, launched from McMurdo Station in the summer of 2008–2009. ANITA-III, expected to improve sensitivity by a factor of 5–10, launched in December 2014. ANITA-IV launched in December 2016, with a lighter overall build, tunable notch filters and an improved trigger system.

Funding

ANITA is a collaboration of multiple universities, led by UH Manoa and funded through grants by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

and the U.S. Department of Energy.

Results

ANITA flew four times between 2006 and 2016 and set the most competitive limits on the ultrahigh-energy diffuse

flux above several tens of

exa A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decadic. Each prefix has a unique symbol that is prepended to any unit symbol. The pre ...

electronvolt In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defi ...

(EeV). In addition to its constraints on the diffuse neutrino flux, each ANITA flight has observed dozens of ultrahigh-energy cosmic rays via the geomagnetic radio emission from cosmic-ray-induced extensive air showers which ANITA typically observes in ''reflection off'' the surface of the ice. ANITA-I and ANITA-III also each detected anomalous radio signatures that were observationally consistent with upcoming extensive air showers emerging from the surface. Upcoming extensive air showers are predicted to be created by the decay of upcoming tau leptons generated via incident tau neutrinos during their propagation through the Earth. However, the angles at which these events were observed are in tension with Standard Model neutrino properties as the Earth should strongly attenuate the neutrino flux at these steep emergence angles. A follow-up study by the

IceCube The IceCube Neutrino Observatory (or simply IceCube) is a neutrino observatory constructed at the Amundsen–Scott South Pole Station in Antarctica. The project is a recognized CERN experiment (RE10). Its thousands of sensors are located under ...

experiment, which searches for neutrinos with significantly less energy than ANITA, could not detect any significant source of neutrinos from the location of these events. As of 2016, these events remain unexplained. The fourth flight of ANITA, ANITA-IV, also detected four events that were observationally consistent with upcoming tau-induced extensive air showers. Unlike the events from ANITA-I and ANITA-III that were observed at steep angles below the horizon, the ANITA-IV events were observed very close to the horizon where tau-induced events are most likely to occur.

Collaborators

The current ANITA collaboration team includes members from the

University of Hawaii at Manoa A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States, t ...

;

University of California, Los Angeles The University of California, Los Angeles (UCLA) is a public land-grant research university in Los Angeles, California. UCLA's academic roots were established in 1881 as a teachers college then known as the southern branch of the California St ...

;

Ohio State University The Ohio State University, commonly called Ohio State or OSU, is a public land-grant research university in Columbus, Ohio. A member of the University System of Ohio, it has been ranked by major institutional rankings among the best publ ...

;

The University of Delaware The University of Delaware (colloquially UD or Delaware) is a public land-grant research university located in Newark, Delaware. UD is the largest university in Delaware. It offers three associate's programs, 148 bachelor's programs, 121 ma ...

;

The University of Kansas The University of Kansas (KU) is a public research university with its main campus in Lawrence, Kansas, United States, and several satellite campuses, research and educational centers, medical centers, and classes across the state of Kansas. Tw ...

;

Washington University Washington University in St. Louis (WashU or WUSTL) is a private research university with its main campus in St. Louis County, and Clayton, Missouri. Founded in 1853, the university is named after George Washington. Washington University is r ...

; the NASA

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...

;

University College London , mottoeng = Let all come who by merit deserve the most reward , established = , type = Public research university , endowment = £143 million (2020) , budget = ...

;

University of Chicago The University of Chicago (UChicago, Chicago, U of C, or UChi) is a private research university in Chicago, Illinois. Its main campus is located in Chicago's Hyde Park neighborhood. The University of Chicago is consistently ranked among the b ...

;

National Taiwan University National Taiwan University (NTU; ) is a public research university in Taipei, Taiwan. The university was founded in 1928 during Japanese rule as the seventh of the Imperial Universities. It was named Taihoku Imperial University and served d ...

; and the California Polytechnic State University.

References

External links