Neutrinos and the Quest for Grand Unity in Physics
Rabindra Nath Mohapatra
Department of Physics, University of Maryland
The President Mr. Ohlmacher called the 2042nd meeting to order at 8:25 p.m. on March 17, 1995. The Recording Secretary read the minutes of the 2041st meeting and they were approved. The President then read a portion of the minutes of the 434th meeting March 16, 1895.
The President introduced Mr. Rabindra N. Mohapatra of the Department of Physics, University of Maryland, to discuss “Neutrinos and the Quest for Grand Unity in Physics”
Mr. Mohapatra introduced himself by explaining there are two kinds of particle physicists, the kind who do experiments and his kind, the theorists. Recent findings concerning neutrinos have caused some excitement. He wanted to discuss these findings and how they related to an ultimate objective of explaining everything from microphysics to macrophysics in one law.
Early philosophers, from Anaximander who said there were four elements to Anaximanes who said there was one, believed that all the variety of substance could be explained as arising from something ultimately more simple. Galileo was among the first to advance the idea that there were universal physical laws. Newton using Galileo's observations and arguments formulated and unified the laws of motion and gravity. Later efforts by physicists to find universal unified laws include Maxwell's unified theory of electricity and magnetism in the last century and Einstein's unified theory of space and time in this century.
Physicists currently recognize three “kinds of things” in the universe, particles, forces and space-time. The smallest chemically characterizable parts of matter are molecules, which are composed of atoms and more than 100 different kinds of elemental atoms have been discovered. This variety is explained by atoms that are composed of essentially three types of particles, electrons, protons and neutrons. It has been convincingly demonstrated that electrons do not have parts. Protons and neutrons, on the other hand, do seem to have constituent parts and theory now proposes that they consist of three each of two types of particles called quarks, specifically u or up quarks and d or down quarks. In order to preserve the conservation laws of energy and momentum Pauli in 1930 hypothesized the existence of particles called neutrinos that had no charge and no, or almost no, mass. They were experimentally demonstrated by Reines and Cowan in 1956. Current theories suggest there may be as many as 100 million neutrinos for every atom in the universe. They almost never interact with the other forms of matter; the mean free path of neutrinos in water is 1018 meters.
Now it is thought that there are three different kinds of neutrinos. The up and down quarks, the electron and the neutrino form a family of particles, and there are three such families. The three neutrinos are the electron-type, the muon-type and the tauon-type. The twelve different kinds of particles all have a quantum mechanical behavior characterized as half-integer spin and are called fermions. This spin quantum number property can be visualized as the particle with its spin axis aligned either parallel or antiparallel to an experimental direction like its direction of motion. These particles interact through four forces, nuclear (or “color"), electromagnetic, “weak”, and gravitational. Yukawa and Feynman showed that the forces could be explained as the exchange of particles called bosons with the quantum property of integer spins.
The principles of symmetry were first discussed by the Pythagoreans, and it is now recognized as a key to unification. Neutrinos are produced by the weak force in a process that violates the left/right symmetry of the universe, or parity. Lee and Yang showed that in the beta decay of radioactive nuclei anti- neutrinos are produced with their spins aligned in only one direction. If neutrinos have mass then beta decay must be symmetric, but the difference in production rate for the opposite spin symmetry must be so large that it can only be observed at very high energy.
If neutrinos have mass then there may be a process whereby the neutrinos of the three different families oscillate or interconvert. This may explain the apparent shortage in neutrinos produced by the sun. While a photon produced by nuclear fusion in the core of the sun takes about 1 million years to reach the surface, the neutrino produced in the same reaction takes less than 1 second to emerge. We think we understand the fusion process in the sun very well and the flux of neutrinos from the sun should be 6.1x1010 per cm2 sec. Using various detectors consisting of very large masses of 37Cl, 71Ga, 98Mo or 2H, only between one-third and one-half of the expected number of electron anti-neutrinos are seen. There are four possible reasons for this shortage: (1) something is wrong with the sun (extremely unlikely), (2) the neutrinos oscillate between electron, muon and tauon types (the detectors are only for the electron-type), (3) neutrinos decay, or (4) neutrinos have magnetic moment. This most likely explanation no appears to be that the neutrinos have mass, and oscillate or interconvert. The difference in their masses appears to be less than about 3 eV which means that the grand unification of forces must take place at energies greater than 1010 GeV which is well beyond the range of present technologies.
The speaker kindly answered questions from the audience. The President thanked the speaker on behalf of the Society. The President then announced the speaker for the next meeting, restated the parking policy, and adjourned the 2042nd meeting at 10:00 p.m.
John S. Garavelli