The neutrino, poster child for science

In a comment under The Neutron Dance, I glossed on the mysterious particle called the neutrino. This particle has such importance in physics and cosmology that I decided it needed its own post. Besides, the neutrino can serve as a poster child for how science works.

Before 1930, the neutrino did not exist, as least as far as science was concerned. Wolfgang Pauli proposed its existence as a hypothesis, to explain the curious behavior of nuclei undergoing beta decay. In this type of radioactive decay, an unstable nucleus spits out a beta particle (also known as an electron) when a neutron decays into a proton and an electron. According to very well established laws of energy and momentum conservation, when the beta shoots out, the nucleus should recoil in the opposite direction.

Yet experiments failed to show any such recoil. It would be like firing a loaded pistol, but feeling no kickback. Pauli proposed that the neutron was also spitting out an as yet unseen particle that was essentially stealing the missing momentum and energy, eliminating the requirement that the nucleus recoil. Pauli’s hypothetical particle was believed to have zero mass, zero electrical charge and a speed equal to that of a photon of light. In 1934, Italian Enrico Fermi named the Pauli particle the “neutrino,” for little neutral one.

[Backgrounder: In classical (pre-1900) physics, matter particles have momentum = mass x velocity and kinetic energy = 1/2 x mass x velocity-squared. Max Planck in 1901 demonstrated that atoms have energies that are whole-number multiples of a minimum value, a constant x the atom’s frequency of vibration. Einstein in 1905 showed that photons of light, which must be massless, have energy = Planck’s constant x their frequency. Arthur Compton subsequently demonstrated that photons also have momentum = Planck’s constant / their wavelength. Compton used this concept to analyze collisions between X-ray photons and electrons as if they were billiard ball collisions. So, saying a massless particle can have both momentum and energy is not so strange.]

Physics identifies four fundamental forces, or interactions, gravitation, electromagnetism, the weak nuclear force and the strong nuclear force. Neutrinos only “feel” gravity and the weak nuclear force, so they are like ghosts, sailing through matter as if it were not there. (Matter, appearances notwithstanding, is mostly empty space. Neutrinos basically have to hit an atomic nucleus dead-on for the weak force to have any effect on them.) This slippery behavior meant experimental confirmation of the neutrino’s existence had to wait until 1956.

The detection then, as now, depended on that occasional interaction between a neutrino and an atomic nucleus. When a beta decay neutrino collides with a proton, the products are a neutron and a positron, the antimatter version of an electron. The positron inevitably collides with an electron, and the two annihilate each other, producing a gamma ray photon, which like a flash of light can be detected with the appropriate equipment. Given a large enough quantity of neutrinos, say 1012 or more per second per cm2, statistically one should expect a few neutrino collisions per day. The predicted value and the experimental value were within 5% of each other, which by experimental standards is a clincher.

So, the neutrino entered the canon of particle physics. Now physicists had to refine the properties of the neutrino, in particular the value of its mass.

The canon of physics right now is the Standard Model of Particles and Interactions. The model identifies two main classes of particles, bosons and fermions, named after Indian physicist S.N. Bose and Fermi. Bosons carry the four forces, and include the photon. Fermions are matter particles, and are further subdivided into two subclasses, six quarks and six leptons and their antimatter counterparts.

Quarks do not exist independently, but are bound together by the strong force to make up heavier particles, including protons and neutrons. Leptons, which include electrons, are fundamental particles and exist on their own. Leptons are immune to the strong force.
The Standard Model identifies three “flavors” of neutrino, associated with the three other leptons that are involved in their production, the electron of beta decay, the muon and the tau. For most of the 20th century, the Standard Model presumed members of the neutrino family had no mass and a speed equal to light.

Experimental evidence since 1956, however, indicates that in fact the neutrinos do have a tiny, but detectable mass. The most recent evidence was just announced last week. A non-zero mass means their speed must be slower than light, as explained by the theory of relativity.
Science progresses in just this fashion. Hypotheses are proposed, and then verified or not. If they are verified, then the results must fit into a broader scientific understanding, a theory or a model. If the neutrinos have mass, which it appears they do, the Standard Model has to be amended to account for the experimental evidence.

Neutrino mass will also have a ripple effect across other scientific models, including those of stellar fusion, stellar death and the Big Bang.

The sun and all other stars produce “solar neutrinos,” as part of the fusion process that turns hydrogen into helium. Neutrino mass will help astronomers understand better how stellar fusion operates, and how long fusion processes can last.

When stars die in supernovae, they release a prodigious number of neutrinos. The supernova of 1987 sent such a flood of neutrinos toward the earth. Neutrino detectors around the world saw spikes in their neutrino counts as SN1987A brightened, then dimmed. Neutrino mass will help astronomers refine their understanding of the nuclear processes that precede, accompany and follow a supernova.

There are neutrinos left over from the Big Bang still zinging through space. One of the mysteries of cosmology is the amount of mass in the universe. Sufficient mass could lead eventually to a collapse, a Big Crunch. Much of the universe’s mass is unobservable. Neutrino mass can now account for some of that “missing” mass, and provide further clues to the mechanism of the Big Bang.
So, the neutrino has evolved from “non-existence,” to a hypothesis, to an observable fact, to a cornerstone of modern physics and astronomy. Not bad for a little neutral one.

Enrico Fermi: And the Revolutions of Modern Physics (Oxford Portraits in Science) Enrico Fermi: And the Revolutions of Modern Physics

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