Neutrinos and their Role in Astrophysics
George Mason University
About the Lecture
A long-standing problem in astrophysics has been the apparent disagreement between a model of energy production in the sun and experimental data on the flux and energy spectrum of neutrinos at the earth. In the last several years, the results from three neutrino experiments have, together, provided strong evidence of a resolution of the problem. The key to the solution has been the determination that neutrinos are not massless, as previously thought. The disagreement is resolved through the phenomenon of quantum-mechanical oscillations of neutrino type. This presentation will describe the basic features of the particles and processes involved in the solution of the problem.
About the Speaker
Robert Ellsworth is a Professor in the Department of Physics and Astronomy at George Mason University. He received his PhD in 1966 from the University of Rochester. His research interests are in experimental particle and cosmic-ray physics. Since the early 1990's he has been a collaborator on the SuperKamiokande neutrino experiment.
The 2160th meeting of the Philosophical Society of Washington was called to order Friday March 21st at 8:15 PM in the Powell Auditorium of the Cosmos Club. President Haapala was in the chair. The president introduced the speaker for the evening, Robert Ellsworth, a professor of Physics and Astronomy at George Mason University. The speaker received a Ph.D from the University of Rochester in 1966. He conducts research in cosmic-ray and particle physics and has been a collaborator on the SuperKamiokande neutrino experiment.
Mr. Ellsworth began with a description of neutrinos and their properties. In the sequence of cosmology neutrinos provided the basic building blocks for the atomic particles such as protons and neutrons we were once taught were indivisible elements of matter. Neutrinos are similar to the more familiar electron, however they do not carry electric charge. Because of their small size and lack of charge they are not affected by the electromagnetic forces and are thus able to pass through large extents of matter without being affected by it. Most of the neutrinos generated by the Sun, which reach the Earth, pass right through and out the other side. The Solar Neutrino Problem is essentially that the calculation of the physics of solar energetics produces many more neutrinos that have been detected. Initially the neutrino was considered to have no mass although it now seems that the neutrino is not massless. A particle that interacts with other matter so infrequently is extremely difficult to study. This talk concentrated on two detectors in Japan and one in Sudbury, Ontario, specifically designed to detect neutrinos from the atmosphere , the sun, and other astrophysical sources.
The fundamental particles are known as quarks and leptons. A proton is composed of three quarks with separate “flavors”. Neutrinos are a form of lepton. Each of the three flavors of neutrinos is associated with one of the charged particles. The electron neutrino is associated with the electron, the muon neutrino associated with the muon and the tau neutrino associated with the tau particle.
The neutrino was postulated to exist by Wolfgang Pauli in 1930 to explain the lack of conservation of energy and momentum in radioactive beta decays. In 1934 Enrico Fermi included Pauli's hypothetical particle in Fermi's comprehensive theory of radioactive decay. It was Fermi who coined the term neutrino (Italian for little neutral one), Fred Reines received a Nobel Prize in 1995 for his discovery, along with Clyde Cowan, of a particle fitting the expected properties of the neutrino. Additional experiments have shown that the total number of leptons in a nuclear reaction stays constant and the total number of leptons with a given flavor also stays constant.
Neutrinos from fusion reactions in the Sun were first detected in a large tank of liquid carbon tetrachloride, deep in a mine in South Dakota. Collisions of the neutrino with the chlorine molecule produced argon atoms, which were measured.
Muons are produced by cosmic ray disintegration. Muons are capable of penetrating to great depths in the earth The structure of protons and neutrons are studied using high energy beams of neutrinos generated in linear accelerators. New questions arose concerning neutrinos.
Are the laws of conservation absolute?
Do neutrinos have mass?
Are the neutrino and the anti-neutrino distinct?
Could symmetry violations for leptons explain the matter – anti-matter asymmetry in the Universe?
More efficient detectors needed to be developed. This led to the SNO project (Sudbury, CN Neutrino Observatory) using deuterium (heavy water) as a detector, and the Kamiokande and SuperKaminokande detector in Japanese mines which use (light) water as a detector. The different detectors detect neutrinos in different energy ranges. Other experiments have used gallium the target material. The SuperKamiokande detector, which was finished in 1996, has much larger volume and better energy and angular resolution. Many independent experiments report not finding enough neutrinos to match the calculated expectations.
Several explanations are possible for the solar neutrino deficit:
The standard solar model is wrong.
The experiments are wrong.
Neutrinos are changing flavor between the time of creation in the Sun and reaching the Earth.
The SNO experiment has measured (a) the flux for all flavors and also (b) the flux of electron neutrinos plus 1/6 the flux of muon neutrinos. The Kamland experiment measured the anti-neutrinos produced by a near by reactor. The SuperKamiokande experiment measured flux (b).
The results of the three experiments are consistent with flavor oscillations produced by a single set of mass differences.
It now appears that an explanation of the Solar neutrino problem involves the fundamental properties of the neutrinos First, neutrinos are not massless as previously thought. The non-zero masses permit oscillations of neutrino flavors. The flavor of a neutrino appears not fixed but a given neutrino may spend some portion of its time as an electron, muon or tau neutrino. (One experimental result indicates there may be a 4th neutrino flavor: a sterile neutrino, which does not react with matter at all. But this result is controversial.)
The speaker concluded that neutrino oscillations exist and that the solar model works.
The Society thanked the speaker who then took questions from the floor:
Q- How many events per day are detected in Super-K?
A- 20 events per day.
Q- It takes neutrinos 8 minutes to get from the Sun. Does this have an effect?
A- Not directly. But there is a Day / Night effect. Neutrinos can come through the earth or down from the top. This difference is seen in SuperK and a small effect seen by SNO.
Q- If objects oscillate in periods of days would we see it go from Schrodinger's cat and back?
A- In that analogy, Schrodinger's cat would oscillate between a live cat and a dead cat.
Q- How do you distinguish between neutrinos from the Sun and from other sources?
A- We use the angle of the event with respect to the Sun. We know where we are at that time of day.
Q- Why do you have to postulate the 4th neutrino type if you have already established oscillation?
A- Another experiment shows that the only way to accommodate the results is to have another type of neutrino.
Q- How many flavors are assigned to spin?
A- Spin is angular momentum. Neutrinos can have a spin of ½. You are free to define the reference point for up or down. Flavor is a property independent of spin.
Q- Are the difference in mass between neutrinos because neutrinos change mass or because of two neutrinos with different masses interacting?
A- That question is quite deep. An electron neutrino, for example, does not have perfectly defined mass. It is a superposition of at least two states with well-defined masses. You can measure one or measure the other but in an individual experiment you do not measure the average.
The President thanked the speaker, presented him with one year of complementary membership in the society and made the requisite parking and beverage control announcements. President Haapala then adjourned the meeting at 9:46 to the social hour.
Temperature: 12.8° C
Weather: Rainy and cool