Challenges of measuring the mass of neutrino

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Neutrinos are elementary particles. They travel with a velocity close to that of light. They are similar to electrons but do not have any charge. As neutrinos are electrically neutral, they are not affected by the electromagnetic forces and are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos are affected only by a “weak” sub-atomic force of much shorter range than electromagnetism. Neutrinos are created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms.

Neutrinos are said to have very, very small but non zero mass. So far trying to measure the precise mass of neutrino has been a very challenging task. This is due to the fact that neutrino oscillations are sensitive only to the difference in the squares of the masses. Two experiments involving beta-decay have given concrete evidence of the fact that the neutrino mass is less than 2.2 eV.

Mark Raizen at the University of Texas at Austin and Joshua Klein at the University of Pennsylvania have proposed a new way of calculating the mass of neutrino by making extremely precise measurements of the motions of the various particles involved in the beta decay of tritium.

According to the proposal of Raizen and Klein first a gas of tritium atoms is to be cooled to near absolute zero (to within a few millionths of a degree of absolute zero). This is to reduce the motion of the atoms of the tritium gas as mush as possible. Using this system that is cooled to near absolute temperature they have proposed two ways by which the neutrino mass can be measured.

 The first of these methods involves studying the “boundstate” beta decay of tritium. In this process, the emitted electron becomes bound into the daughter helium atom rather than escaping. This makes the decay a straightforward two-body process, in which the energy of the emitted neutrino is equal to the energy difference between the initial tritium atom and the daughter helium atom. By measuring the time it takes for the daughter helium atom to arrive at a detector placed some known distance from the tritium source one can measure the velocity of the nuclear recoil. Using the velocity of the nuclear recoil the neutrino mass can be calculated. One must keep in mind that this so called “boundstate” beta decay of tritium has so far not yet been observed.

The second method involves studying the three-body beta decay of tritium. Basically this method involves measuring the momenta of the daughter helium ion and the emitted electron using various detectors. The neutrino mass is then calculated directly from these quantities.

Using computer simulation they have realized that of the two methods proposed it is the second method that has the potential of calculating the mass of the neutrino more accurately.

So far these interesting ideas proposed by Raizen and Klein are still theoretical. Much work needs to be done to over come engineering challenges, such as careful control of systematics, to put these ideas into experimental reality.

In Germany the Karlsruhe Tritium Neutrino Experiment called KATRIN experiment is being designed to measure the mass of the electron neutrino directly with a sensitivity of 0.2 eV. It is a next generation tritium beta-decay experiment scaling up the size and precision of previous experiments by an order of magnitude as well as the intensity of the tritium beta source. This experiment is to begin in 2012.

KATRIN is a joint effort of several European and U.S. institutions. Currently there are about 100 scientists, engineers, technicians and students involved, including most of the groups that have worked on tritium beta-decay experiments in recent years. KATRIN is being built at Forschungszentrum Karlsruhe in Germany where much of the required technical infrastructure is already available, especially for the tritium source.

February 9, 2009