If neutrinos have mass, one of the biggest mysteries of cosmology may be solved
several experiments are being conducted by researchers to measure the mass of neutrinos -- one of the most weakly interacting particles. Mass is a quantity which a body contains. These particles do not possess any charge and interact through their weak nuclear force. A force that acts between the nucleons is called nuclear force. If the neutrino's mass is discovered then it will lead the way to a new theory of elementary particles and forces. Neutrino was first proposed by a scientist in 1930 to explain some puzzles of radioactive decays. Later in 1956, the existence of the particle was finally proved ( Science , Vol 276, No 5320).
One possible method to find the mass of neutrinos is to detect oscillations between three different kinds of neutrinos, one associated with the electron, the muon and the tau particle. A particle can oscillate only if it possess some mass. Thus, by ascertaining oscillation properties of neutrinos, researchers will be able to detect the mass of the particle.
Los Alamos in usa , Super Kamiokande in Japan and the Soudan-2 experiment in Minnesota, are some of the most important oscillation experiments being conducted to measure the mass of neutrinos. Last year, the Los Alamos group had reported evidence of oscillations in the particle. However, the claim was disputed by other researchers. No unanimous decision on the aspect has come out so far. It is said that the findings of Los Alamos and Super Kamiokande Soudan-2 may lead to the evidence of mass in neutrinos.
Scientists are also evaluating a rare process that occurs in certain radioactive materials known as 'neutrinoless double beta decay'. In ordinary beta decay, a neutron in radioactive nucleus decays into a proton giving out an electron (beta particle) and an antineutrino. In a double beta decay, two neutrons decay into two electrons and antineutrinos. But in neutrinoless double beta decay, antineutrinos are absorbed by protons in the nucleus and only the electrons come out.
By studying the electron energy, one can estimate the mass of neutrinos. These experiments have an edge over oscillation experiments because oscillation experiments may help to find out the difference in masses of the three-neutrino types, whereas the double beta decay experiments may help measure the mass of neutrinos.
The Max Planck Institute for Nuclear Physics in Heidelberg, Germany and the Russian Science Centre Kurchatov Institute in Moscow, are conducting an experiment using a large quantity of radioactive substances called 'Germanium-76' that is monitored with sensitive detectors. It has reported an upper limit on the electron neutrino to be 0.48 electron volts. An electron volt is a measure of energy and hence its mass. The mass of electron, the next lightest particle, is about 511 kilo electron volts. The group is trying to improve the apparatus so as to achieve a sensitivity level of 0.1 eV.
Researchers at the Osaka University's Research Centre for Nuclear Physics in Japan, are expected to conduct an experiment using a different isotope, called Calcium-48 to detect the elusive neutrinoless double beta decay. They are confident of achieving a sensitivity of 0.6 eV.
Europe and Russia are jointly setting up a project called nemo, which is expected to begin in 1998. Scientists will use a new detector alongwith a radioactive isotope, Molybdenum-100, in the Frejus Underground Laboratory in the Alps along the French-Italian border. The group is planning to reach a sensitivity level of 0.1 eV.
These experiments may soon help ascertain the mass of neutrinos, which will have a significant impact on the understanding of micro and macro physics. In the field of cosmology, a non zero neutrino mass will have immense bearing. If neutrinos have mass, no matter how small, then a significant part of the matter of the Universe will be in the form of neutrinos. This will lead scientists to rethink the missing mass problem in cosmology that has plagued many theories in physics.
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