The Physics
Opus in profectus

Quantum Flavordynamics

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reactions of interest

Beta decay, also called beta minus decay

n0 → p+ + e + ν0

Positron emission, also called beta plus decay, since it's a kind of decay

p+ → n0 + e+ + ν0

Electron capture, described as a decay process, but it's really more of a reaction

e + p+ → n0 + ν0

Inverse beta decay — should be called neutrino capture. It's another even that would be better described as a reaction.

ν0 + p+ → n0 + e+

Electron-neutrino scattering (should this be here, or in another subsection?)


È un neutrone? No, è un neutrino.

The name neutrino is a a bit of wordplay attributed to the Italian physicist Edoardo Amaldi (1908–1989). To understand the joke, you need to know a little Italian grammar.

An augmentative is a form of a word that indicates bigness or intensity. In Italian this is done by adding the suffix -one to a word. Some augmentative words in Italian that readers might recognize are…

Replace -one with -ino and you have a diminutive. This makes a word small and cute and innocuous. Some diminutive words in Italian that readers might recognize are…

What makes the word neutrino witty in Italian is that it's grammatically incorrect. The new particle really should have been called a neutronino, since the word neutrone is not an augmentative. When Amaldi used the wrong diminutive for the newer word (neutr-ino) it implied that older word was an augmentative (neutr-one), which it isn't. Thus the joke.

This is the best quote I can find to support this.

The name "neutrino" (a funny and grammatically incorrect contraction of "little neutron" in Italian: neutronino) entered the international terminology through Fermi, who started to use it sometime between the conference in Paris in July 1932 and the Solvay Conference in October 1933 where Pauli used it. The word came out in a humorous conversation at the Istituto di Via Panisperna. Fermi, Amaldi and a few others were present and Fermi was explaining Pauli's hypothesis about his "light neutron". For distinguishing this particle from the Chadwick neutron Amaldi jokingly used this funny name, — says Occhialini, who recalls of having shortly later told around this little story in Cambridge.

Eduardo Amaldi, 1984

disorganized notes

  1. Certain materials had been observed to emit electrons (beta decay). Since both the atom and the nucleus have discrete energy levels, it is hard to see how electrons produced in transition could have a continuous spectrum (see 1930 for an answer.)
  2. Wolfgang Pauli suggests the neutrino to explain the continuous electron spectrum for beta decay.
  3. Enrico Fermi puts forth a theory of beta decay that introduces the weak interaction. This is the first theory to explicitly use neutrinos and particle flavor changes.
  4. Frederick Reines and Clyde Cowan detect antineutrinos
  5. Julian Schwinger, Sidney Bludman, and Sheldon Glashow, in separate papers, suggest that all weak interactions are mediated by charged heavy bosons, later called W+ and W. Actually, it was Yukawa who first discussed boson exchange twenty years earlier, but he proposed the pion as the mediator of the weak force.
  6. Experiments verify that there are two distinct types of neutrinos (electron and muon neutrinos). This was earlier inferred from theoretical considerations.
  7. Leon Lederman shows that the electron neutrino is distinct from the muon neutrino
  8. Steven Weinberg puts forth his electroweak model of leptons
  9. Gerard 't Hooft shows that the Glashow-Salam-Weinberg electroweak model can be renormalized

Quotes from forgotten sources.

Quote that must be paraphrased

from the Nobel website…
The neutrino was postulated in 1930 by Pauli to explain the continuous energy spectrum of electrons emitted in nuclear β decay. Fermi's theory for weak interactions was developed during the 1930's. At the time, neutrino interaction cross-sections were considered too small for neutrino detection. However, the large neutrino fluxes that later became available with nuclear reactors opened the field of neutrino physics. The experimental confirmation of the neutrino came in an experiment in 1955 by Cowan and Reines. Pontecorvo and Alvarez (Pontecorvo 1946, Alvarez 1949) suggested reactor experiments using the reaction ν + 37Cl → 37Ar + e to detect the neutrino. At the time there was no clear distinction between neutrino and antineutrino. The reaction often goes by the name the Davis-Pontecorvo reaction. The threshold energy is 0.81 MeV and the Argon isotope decays via electron capture with a half-life of 35 days.

Gravity was long believed to be the energy source of the sun. By 1920 it was known that the sun was mainly composed of helium and hydrogen and Eddington proposed that nuclear fusion was the energy source. However, it took until 1938 before there was a complete theory of the nuclear reactions within the sun (Bethe and Chritchfield 1938, Bethe 1939). It was a challenge to prove experimentally that nuclear burning was the energy source of the sun.

Raymond Davis Jr did his first reactor-based chlorine neutrino experiment at Brookhaven in the early 1950s. He used a tank filled with 3,900 liters of CCl4 as a target. Helium was bubbled through the tank to remove the few argon atoms. The radioactive argon could then be removed from the helium by passing the gas through a charcoal trap with liquid nitrogen (−196 °C) which adsorbs the argon quantitatively and allow helium to pass.

Neutrinos are very abundant in the Universe. Indeed, the ratio between neutrinos and nucleons (protons or neutrons) in the Universe is about 109. On the Earth, the dominant source of neutrinos is our sun. Every second more than 10 billion (1010) neutrinos pass through every cm2, the majority with low energy (< 0.4 MeV). Only 0.01% of the solar neutrinos have an energy larger than 5 MeV.

A new and 100 times larger experiment was proposed based on Davis' radiochemical method and on Bahcall's calculated rate of 40 ± 20  SNU (1 SNU = 1 Solar Neutrino Unit, 1 capture per second and per 1,036 target atoms) (Davis 1964, Bahcall 1964). Davis experiment was funded and installed in the Homestake Gold Mine, Lead, South Dakota (depth 1,500 m). The tank contained 615 tonnes of C2Cl4, an agent normally used for dry cleaning. It was ready to start data taking in 1967. The extraction of argon by helium was done approximately once every two months to match the half-life of 37Ar.

The first results from Davis came in 1968, based on 150 days of data taking (Davis et al. 1968). An upper limit of the solar neutrino flux of 3 SNU was given, much lower than the then calculated rate of 20 SNU (Davis et al. 1968). In this 1968 paper is discussed different possibilities to improve the sensitivity and in particular how to reduce the background. The experimental challenge, that Davis was so successful in meeting, was to extract an average of only 17 argon atoms among the 2 x 1030 chlorine atoms in the tank every second month.

Davis experiment was running almost continuously from 1970 until 1994. The final results were published in 1998 (fig. 2, Cleveland et al. 1998). During this time it is estimated that a total of 2,200 argon atoms were produced in the tank. Of these 1997 were extracted and 875 counted in the proportional counter. Of the latter, 776 are estimated to be produced by solar neutrinos and 109 by background processes. The production in the tank was 0.48 ± 0.03 (stat.) ± 0.03 (syst.) argon atoms per day, corresponding to 2.56 ± 0.16 (stat.) ± 0.16 (syst.) SNU. Davis was a true pioneer and his successful mastering of the extraction of a few atoms out of 1030 gave birth to a new field of neutrino physics.

Intermediate vector bosons

Source of neutrinos

Necessary extensions to QFD

Neutrinos have no charge, interact infrequently with other particles, and are probably massless. The sun produces huge numbers of neutrinos every second, but detectors on earth haven't seen them in the numbers solar physicists expected. Obviously, we have a problem here.

Is it the sun? That's not even the right question. The sun does what it does no matter what physicists say or don't say. The real question is, are there problems with our current models of the sun? Maybe, but probably not. Models of the sun have otherwise been quite successful. Why should they fail on this one account? Is it the detectors? Probably not. Neutrinos may be governed by the weak force, but this doesn't make them undetectable. When used to measure the neutrino output from other sources they too have been quite successful. If the problem isn't with solar models or neutrino detectors it must lie in the space between the sun and the detectors. Something's happening to the solar neutrinos on their 150 million kilometer journey.

The short answer is it's a flavor thing. When speaking of subatomic particles, it might be correct to say "the electron", but it is not at all correct to say "the neutrino". The word neutrino refers to a small group of three different, but similar particles: the electron neutrino, the muon neutrino, and the tau neutrino.

oscillating neutrinos

Neutrinos are exactly massless in the Standard Model, but recent experimental observations show neutrinos oscillating between different flavors. This implies that neutrinos are very, very light, but not massless. In addition, the mixing ratios hint at the possibility of a fourth neutrino; the sterile neutrino, so called as it does not appear to interact with any other particle. Maybe this really isn't a weak link — more of an unresolved problem.

sterile neutrino

Current experiment: The MINOS (Main Injector Neutrino Oscillation Search) experiment at Fermilab. Neutrinos are created at a known rate at the laboratory in Batavia, Illinois (a western suburb of Chicago) and directed through the Earth to a detector in an abandoned iron mine in Soudan, Minnesota (near the Canadian border). The idea is this: measure the number of neutrinos produced at Fermilab and the number detected at the iron mine in Soudan. The important calculation is the number of neutrinos that were "lost" along the way. Standard Model predictions at the time predict that none should be lost, but earlier experiments hint at the possibility of neutrino oscillations. If the number detected in Minnesota is significantly different from the number generated in Illinois, then the Standard Model as it is currently interpreted is in need of modification.


higgs boson

1964: François Englert and Robert Brout at L'Université Libre de Bruxelles in Belgium; Peter Higgs at the University of Edinburgh in Scotland; and Gerald Guralnik, Carl Hagen, and Tom Kibble at Imperial College, London

According to the Standard Model, the W and Z bosons should be massless, but experimental evidence shows them to be quite heavy. A way around this is to introduce another variety of intermediate vector boson called the Higgs particle, Higgs boson, or (my personal favorite) higgson. When W and Z particles interact with the higgson, they acquire mass.


Four methods for producing a Higgs particle.

Feynman diagrams of mechanisms for producing Higgs bosons