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Neutrinos are the least understood and the most elusive of the known fundamental particles in the Standard Model of particle physics, though they are the second most abundant in the universe. On average there are about 3 million neutrinos in a cubic meter, but they are difficult to detect because they interact with matter so rarely. Neutrinos are produced in cosmic ray interactions in the atmosphere and in particle accelerators on Earth, but the only cosmic neutrinos that been detected have originated either from our Sun or the exploding star Supernova 1987a.


Current research experiments in neutrino physics are investigating: neutrino oscillations, a phenomenon where neutrinos change from one type to another as they propagate through space, revealing that neutrinos do have mass; the nature of the neutrino, that is, whether the neutrino is its own antiparticle; searches for cosmic neutrinos which can probe deeper in the universe than other types of cosmic messengers.

Neutrino Astronomy

Since neutrino interactions are so scarce, neutrino telescopes need detection volumes of a cubic kilometer or more to have any chance of observing cosmic neutrinos. Antarctic ice is an attractive medium for neutrino telescopes due its large volume and purity, as is sea water. The Icecube experiment at the South Pole and the ANTARES in the Mediterranean sea both seek neutrinos from black holes, supernova remnants, or Gamma Ray Bursts, the most energetic processes in the universe.

ANITA is an Antarctic balloon experiment that will launch its second flight in December 2008, viewing the ice sheet from 37 km altitude and viewing 1.5 million cubic kilometers of ice at once. ANITA is searching for a radio signature from neutrinos associated with the highest energy particles ever observed above 10ERROR 18 ERROR eV. Neutrinos at these energies may reveal the sources of the highest energy cosmic rays, and their interactions occur at higher energies than probed at the LHC.

While ANITA may be the first to detect ultra-high energy neutrinos, several proposed experiments aim to measure a sample of cosmic neutrinos that is large enough to extract precise particle physics and astrophysics information from their properties. IceRay is a proposed array of antennas that would surround IceCube at the South Pole. SalSA is a proposed antenna array that would be deployed in one of the many salt formations that occurs in many places around the world.

Acoustic experiments such as ACORNE are searching for an audible signal that neutrinos would emit as they impact the earth. ACORNE has an array of sensors deployed on the sea floor off the coast of Scotland.

LOFAR is an international collaboration deploying a array of 25,000 antennas spanning many European countries with unprecedented sensitivity for astronomical observations at radio frequencies below 250 MHz. LOFAR will also make world-class measurements in atmospheric science, planetary science and particle astrophysics. LOFAR can be used to search for neutrinos interacting in the surface of the moon, and can measure the radio signal emitted by cosmic ray interactions in the atmosphere.


Neutrino Oscillations

As early as the late 1950s, physicists suggested that neutrinos could have mass, and, if they do, would be able to transform from one flavour to another, such as from a muon neutrino to a tau neutrino. The MINOS experiment is an international collaboration of 200 physicists that set out to answer two major questions: What is the maximum fraction of a neutrino beam that can change from one flavor to another? For example, how much of the beam will transform from muon neutrinos to tau neutrinos? How great or small is the oscillation length? The oscillation length is the distance a beam of neutrinos, traveling at near the speed of light, would take to transform from one neutrino flavor to another and back again. In order to measure these parameters, a neutrino beam is made from a proton beam at Fermilab near Chicago in the US and the neutrinos are measured with a near detector at Fermilab and compared with a far measurement in Soudan, Minnesota 735 km away.

Neutrinoless Double Beta Decay

All the fermions in the Standard Model are Dirac particles by nature, where the antiparticle is the charge conjugate of the particle and has equal but opposite quantum numbers. Neutrinos, however, have an extra possibility due to their neutral charge. It is not forbidden for neutrinos to have a Majorana mass, which would imply that the neutrino and antineutrino are the same particle. This possibility seems to be favoured by the theoretical community, indeed it is a requirement of many Grand Unified Theories. The Majorana nature of the neutrino leads to the non-conservation of the full lepton number and can address many fundamental questions. In particular it can explain a tiny assymetry between matter and antimatter shortly after the Big Bang, which was necessary to make the world surrounding us evolve the way it did.

Experiments which can answer both questions are searching for neutrinoless double beta decay — 0νββ. In fact, 0νββ is the only practical way to understand whether the neutrino is a Majorana or a Dirac particle. NEMO 3 is one such experiment, now installed at the Fréjus Underground Laboratory (LSM) in Modane (Savoie, France). It is a very low radioactive background detector. Its performances enable the observation of DBD processes with lifetime at the sensitivity of 1024-25 years. Scientists are analysing data from the NEMO 3 detector, searching for very rare neutrinoless double beta decay events. The SuperNEMO next generation experiment is currently in R&D phase.


Some Possible Origins Projects

  • Studying whether the highest energy cosmic messengers originate from sources that follow the large scale structure of the universe or whether they are uniform on the sky, and thus whether they originate from astrophysical sources or new fundamental particles
  • Searching for neutrinos interacting in the moon and cosmic ray particles interacting in the atmosphere while making new measurements of properties of the ionosphere with LOFAR
  • Searching for a possible time-dependent variation of a fundamental constant of nature called the Fermi constant by measuring the rate of double-beta decay in rocks of varying age
  • Developing a radio experiment to search for ultra-high energy cosmic neutrinos with future moon missions such as MoonNEXT, MoonLITE and the International Lunar Network and characterising the electromagnetic properties of the moon surface and the lunar radio environment
  • Designing and developing a mega-scale water Cherenkov detector for observing astrophysical sources, measure fundamental neutrino properties and investigate Grand Unification

Recent papers of interest

Title: Constraints on cosmic neutrino fluxes from the ANITA experiment
Authors: ANITA Collaboration (S. Barwick et al.)

Title: First results of the search of neutrinoless double double beta decay with the NEMO 3 detector
Authors: NEMO Collaboration (R. Arnold et al.)

Title: A Study of Muon Neutrino Disappearance Using the Fermilab Main Injector Neutrino Beam
Authors: MINOS Collaboration (P. Adamson et al.)

Title: Forecasting neutrino masses from combining KATRIN and the CMB: Frequentist and Bayesian analyses
Authors: Ole Host, Ofer Lahav, Filipe B. Abdalla, Klaus Eitel

Title: Neutrino Mass, Dark Energy, and the Linear Growth Factor
Authors: Angeliki Kiakotou (UCL), Oystein Elgaroy (Oslo Uni.), Ofer Lahav (UCL)

This page last modified 17 February, 2009 by Jakub Bochinski