Solar neutrino problem
Encyclopedia
| Solar neutrino problem | |
| Discrepancies in the measurements of actual solar neutrino types and what the Sun's interior models predict. | |
| Former Standard Model | |
| Neutrinos should have been massless according to the then-accepted theory; this means that the type of neutrino would be fixed when it was produced. The Sun should emit only electron neutrinos as they are produced by H–He fusion. | |
| Observation | |
| Only one third to one half of predicted number of electron neutrinos were detected; neutrino oscillation Neutrino oscillation Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates... explains the difference but requires neutrinos to have mass Mass Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:... . |
|
| Resolutions | |
| Neutrinos have mass and so can change type. | |
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s flowing through the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and theoretical models of the solar
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
interior, lasting from the mid-1960s to about 2002. The discrepancy has since been resolved by new understanding of neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
physics, requiring a modification of the Standard Model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of particle physics
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
– specifically, neutrino oscillation
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...
. Essentially, as neutrinos have mass, they can change from the type that had been expected to be produced in the Sun's interior into two types that would not be caught by the detectors in use at the time.
Introduction
The SunSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
is a natural nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactor, powered by a proton–proton chain reaction which converts four hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
(proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s) into helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s, positron
Positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. The positron has an electric charge of +1e, a spin of ½, and has the same mass as an electron...
s and energy. The excess energy is released as gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
s and as kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the particles and as neutrinos — which travel from the Sun's core to Earth without any appreciable absorption by the Sun's outer layers.
As neutrino detectors became sensitive enough to measure the flow of neutrinos from the Sun, it became clear that the number detected was lower than that predicted by models of the solar interior. In various experiments, the number of detected neutrinos was between one third and one half of the predicted number. This came to be known as the solar neutrino problem.
Measurements
In the late 1960s, Ray Davis'sRaymond Davis Jr.
Raymond Davis, Jr. was an American chemist, physicist, and Nobel Prize in Physics laureate.-Early life and education:...
and John N. Bahcall
John N. Bahcall
John Norris Bahcall was an American astrophysicist, best known for his contributions to the solar neutrino problem, the development of the Hubble Space Telescope and for his leadership and development of the Institute for Advanced Study in Princeton.-Early and family life:Bahcall was born in...
's Homestake Experiment
Homestake Experiment
The Homestake experiment was an experiment headed by astrophysicists Raymond Davis, Jr. and John N. Bahcall in the late 1960s. Its purpose was to collect and count neutrinos emitted by nuclear fusion taking place in the Sun. Bahcall did the theoretical calculations and Davis designed the experiment...
was the first to measure the flux of neutrinos from the Sun and detect a deficit. The experiment used a chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
-based detector. Many subsequent radiochemical and water Cerenkov
Cherenkov radiation
Cherenkov radiation is electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium...
detectors confirmed the deficit, including the Sudbury Neutrino Observatory
Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is a neutrino observatory located 6,800 feet underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The detector turned on in May 1999,...
.
The expected number of solar neutrinos had been computed based on the Standard Solar Model
Standard Solar Model
The Standard Solar Model refers to a mathematical treatment of the Sun as a spherical ball of gas...
which Bahcall had helped to establish and which gives a detailed account of the Sun's internal operation.
In 2002 Ray Davis
Raymond Davis Jr.
Raymond Davis, Jr. was an American chemist, physicist, and Nobel Prize in Physics laureate.-Early life and education:...
and Masatoshi Koshiba
Masatoshi Koshiba
is a Japanese physicist. He jointly won the Nobel Prize in Physics in 2002.He graduated from the University of Tokyo in 1951 and received a Ph.D. in physics at the University of Rochester, New York, in 1955...
won part of the Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
for experimental work that found the number of solar neutrinos was around a third of the number predicted by the Standard Solar Model
Standard Solar Model
The Standard Solar Model refers to a mathematical treatment of the Sun as a spherical ball of gas...
.
Changes to the Solar Model
Early attempts to explain the discrepancy proposed that the models of the Sun were wrong, i.e. the temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
and pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
in the interior of the Sun were substantially different from what was believed. For example, since neutrinos measure the amount of current nuclear fusion, it was suggested that the nuclear processes in the core of the Sun might have temporarily shut down. Since it takes thousands of years for heat energy to move from the core to the surface of the Sun, this would not immediately be apparent.
However, these solutions were rendered untenable by advances in both helioseismology
Helioseismology
Helioseismology is the study of the propagation of wave oscillations, particularly acoustic pressure waves, in the Sun. Unlike seismic waves on Earth, solar waves have practically no shear component . Solar pressure waves are believed to be generated by the turbulence in the convection zone near...
, the study of how waves propagate through the Sun, and improved neutrino measurements.
Helioseismology observations made it possible to measure the interior temperatures of the Sun; these agreed with the standard solar models
Standard Solar Model
The Standard Solar Model refers to a mathematical treatment of the Sun as a spherical ball of gas...
. (There are unresolved problems of the structure of what was found with helioseismology. Instead of the old "pot-on-the-stove" model of vertical convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
, horizontal jet streams were found in the top layer of the convective zone. Small ones were found around each pole and larger ones extended to the equator. As might be expected, these had different velocities.)
Detailed observations of the neutrino spectrum from the more advanced neutrino observatories also produced results which no adjustment of the solar model could accommodate. In effect, overall lower neutrino flux (which the Homestake experiment results found) required a reduction in the solar core temperature. However, details in the energy spectrum of the neutrinos required a higher core temperature. This happens because different energy neutrinos are produced by different nuclear reactions, whose rates have different dependence upon the temperature; in order to match parts of the neutrino spectrum a higher temperature is needed. An exhaustive analysis of alternatives found that no combination of adjustments of the solar model was capable of producing the observed neutrino energy spectrum, and all adjustments that could be made to the model worsened some aspect of the discrepancies.
Resolution
Currently, the solar neutrino problem is assumed to have resulted from an inadequate understanding of the properties of neutrinos. According to the Standard ModelStandard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of particle physics, there are three different kinds of neutrinos:
- electron neutrinoElectron neutrinoThe electron neutrino is a subatomic lepton elementary particle which has no net electric charge. Together with the electron it forms the first generation of leptons, hence its name electron neutrino...
s (which are the ones produced in the Sun and the ones detected by the above-mentioned experiments, in particular the chlorine-detector Homestake Mine experiment), - muon neutrinoMuon neutrinoThe muon neutrino is a subatomic lepton elementary particle which has the symbol and no net electric charge. Together with the muon it forms the second generation of leptons, hence its name muon neutrino. It was first hypothesized in the early 1940s by several people, and was discovered in 1962 by...
s, and - tau neutrinos.
In the 1970s, it was widely believed that neutrinos were massless and their types were invariant. However, in 1968 Pontecorvo
Bruno Pontecorvo
Bruno Pontecorvo was an Italian-born nuclear physicist, an early assistant of Enrico Fermi and then the author of numerous studies in high energy physics, especially on neutrinos. According to Oleg Gordievsky and Pavel Sudoplatov , Pontecorvo was also a Soviet agent...
proposed that if neutrinos had mass, then they could change from one type to another.
Thus, the "missing" solar neutrinos could be electron neutrinos which changed into other types along the way to Earth and therefore escaped detection.
The supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
1987A
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...
produced an indication that neutrinos might have mass, because of the difference in time of arrival of the neutrinos detected at Kamiokande and IMB
Irvine-Michigan-Brookhaven (detector)
IMB, the Irvine-Michigan-Brookhaven detector, was a nucleon decay experiment and neutrino observatory located in a Morton Salt company's Fairport mine on the shore of Lake Erie in the United States. It was a joint venture of the University of California, Irvine, the University of Michigan, and...
. However, because very few neutrino events were detected it was difficult to draw any conclusions with certainty. In addition, whether neutrinos have mass or not could have been more definitively established had Kamiokande and IMB both had high precision timers which would have recorded how long it took the neutrino burst to travel through the Earth. If neutrinos were massless, they would travel at the speed of light; if they had mass, they would travel at velocities slightly less than that of light. Because the detectors were not intended for supernova neutrino detection, however, this was not done.
The first strong evidence for neutrino oscillation
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...
came in 1998 from the Super-Kamiokande
Super-Kamiokande
Super-Kamiokande is a neutrino observatory which is under Mount Kamioka near the city of Hida, Gifu Prefecture, Japan...
collaboration in Japan. It produced observations consistent with muon-neutrinos (produced in the upper atmosphere by cosmic rays) changing into tau-neutrinos. What was proved was that fewer neutrinos were detected coming through the Earth than could be detected coming directly above the detector. Not only that, their observations only concerned muon neutrinos coming from the interaction of cosmic rays with the Earth's atmosphere. No tau neutrinos were observed at Super-Kamiokande.
The convincing evidence for solar neutrino oscillation came in 2001 from the Sudbury Neutrino Observatory
Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is a neutrino observatory located 6,800 feet underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The detector turned on in May 1999,...
(SNO) in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. It detected all types of neutrinos coming from the Sun, and was able to distinguish between electron-neutrinos and the other two flavors (but could not distinguish the muon and tau flavours), by uniquely using heavy water
Heavy water
Heavy water is water highly enriched in the hydrogen isotope deuterium; e.g., heavy water used in CANDU reactors is 99.75% enriched by hydrogen atom-fraction...
as the detection medium. After extensive statistical analysis, it was found that about 35% of the arriving solar neutrinos are electron-neutrinos, with the others being muon- or tau-neutrinos. The total number of detected neutrinos agrees quite well with the earlier predictions from nuclear physics, based on the fusion reactions inside the Sun.
Caveats
The crux of the solar neutrino problem, and its resolution, lies in the fact that both the interior of the Sun and the behavior of traveling neutrinos is unknown to begin with. One may assume knowledge of one and determine the other by experiment here on Earth. If one assumes the Standard Solar Model is valid, one can derive the propagation properties of neutrinos, such as neutrino oscillations, given data from solar neutrino experiments. Likewise, if one presumes something about the propagation of solar neutrinos, one may derive some conclusions about the validity of solar models.External links
- Solar neutrino data
- Solving the Mystery of the Missing Neutrinos
- Raymond Davis Jr.'s logbook
- Nova – The Ghost Particle
- The Solar Neutrino Problem by John N. BahcallJohn N. BahcallJohn Norris Bahcall was an American astrophysicist, best known for his contributions to the solar neutrino problem, the development of the Hubble Space Telescope and for his leadership and development of the Institute for Advanced Study in Princeton.-Early and family life:Bahcall was born in...
- The Solar Neutrino Problem, by L. Stockman
- A set of photos of different Neutrino detectors
- John Bahcall's web site