Peter's last post on Maurice Goldhaber stimulates me to write about one of the most brilliant experiment of the twentieth century, one he performed in 1957 with Grodzins and Sunjar, and which has become a cornerstone of the physics of weak interactions and of particle physics in general.
The reader be warned: although I kept this at a level which should be accessible to anybody who knows what conservation of momentum is, explaining why the Goldhaber experiment is so phenomenal requires delving into details and many will get bored before reaching the end...
Anyway here we go. During the fifties, physicists were busy trying to understand weak interactions of elementary particles. At the time, the first accelerators were starting to produce evidence for the existence of many new unexpected particles. One used to smash protons against targets, and study the bodies produced downstream. Many new "resonances" were thus found as peaks in the distribution of the invariant mass of the final state particles: the mass is a well defined attribute of a particle, and it can be reconstructed by measuring energy and momentum of all the bodies created in the particle's disintegration.
Physicists were hoping that by studying the properties of these new found bodies they could understand the details of the weak interaction, the one which governed the decay of the neutron and of many other "strange" particles - strange because they were produced copiously by the strong interaction, but were slow (due to the weakness of the interaction responsible) to disintegrate. Enrico Fermi, with brilliant intuition, had already laid down in the thirties the correct formalism for making theoretical calculations of the decay rate of these particles, but had ignored the fact that the weak interaction violates a symmetry of nature called Parity, a fact postulated by Lee and Yang in 1956 and quickly demonstrated afterwards by Wu and her collaborators at the National Bureau of Standards, where she observed a clear anisotropy in the direction of emission of decay electrons from radioactive Cobalt 60 isotopes, kept at low temperature and with their spins aligned along the direction of a strong magnetic field.
If weak interactions violated the parity symmetry, Fermi's "V" formalism had to be modified. Two options were borne by experimental data: the interaction was either "V-A" or "S-T", these letters identifying the symmetry properties of the interaction potential. If the interaction was V-A, then beta decay (the mechanism through which radioactive atoms decay, by transforming one neutron into a proton, an electron, and a neutrino) had to produce (anti)neutrinos which spinned clockwise; if the interaction was S-T, the antineutrinos would all spin counterclockwise.
Neutrinos are the most elusive particles we know. They are enormously copious in the universe, since the fusion mechanisms in the star cores emit gazillions of them. But they almost never interact with matter when they traverse it: their cross section, or probability of yielding an interaction, is so tiny that most of the time they cross the entire earth unscathed. So if they do not interact, they cannot be seen! We in fact detect particles by observing their interaction with matter. How then, thought Goldhaber and company, can we be so clever as to measure their spin, which is an elusive property to begin with ?
Enter Europium 152, a radioactive isotope with quite peculiar characteristics. Fast to decay, it has zero total angular momentum (J=0: no net spin), and transmutes into excited Samarium by capturing one of its own inner electrons, thereby emitting a neutrino in one direction, and a Samarium nucleus in the other -the two bodies balancing the total impulse, thanks to conservation of momentum.
Excited Samarium has a total spin J=1 (in units of Planck's constant), and is also quite peculiar: it decays into its fundamental state - it de-excitates - by emitting a 960 keV photon so quickly (in 0.07 picoseconds) that its direction of motion has no time to be lost by thermic effects, even in a solid sample of Europium. The photon, a particle with one unity of spin, has the spin either aligned or anti-aligned with its own direction of motion, and, due to conservation of angular momentum, aligned with the original spin of excited Samarium. If the photon has been emitted along the direction of motion of excited Samarium, its spin will be aligned with the original spin of the neutrino emitted in the original decay of Europium, and it will have the same sign! This means that if we measure the photon spin properties, we infer the neutrino's spin properties. Bingo number 1!
But how can we select those photons emitted exactly in the direction of excited Samarium ? Bingo number 2 is to realize that the photons do not possess all the energy of the de-excitation of Samarium: when the photon is emitted, the Samarium atom recoils, keeping to itself some of the energy released in the form of kinetic energy. So these photons, when they encounter another atom of Samarium, will not be able to pump it back to the excited state, UNLESS... Unless the photon was emitted by Samarium traveling in the same direction, thereby being "kicked forwards", doppler-shifted to higher energy. So it will be only those photons that are able to do resonant scattering with a Samarium nucleus. Bingo number 3 is the fact that the momentum acquired by the Samarium nucleus in the decay of Europium is roughly equal to the amount necessary for the photon to gain just the needed kick forward!
To summarize: if you want to measure the neutrino's spinning property (AKA its polarization) you just need to measure the polarization of those photons which do resonant absorption by Samarium atoms (those that were kicked forward).
Ok, but how did they do that ? That is easy! Photons with energy of about a MeV have different interaction properties in a sample of iron which is magnetized parallel or antiparallel to the photons direction. That is due to the electrons in the iron being more likely to absorb photons of a given spin if their spin is antiparallel to it.
The end result ? Take a sample of Europium 152, place it before a block of magnetized iron. Place a sample of Samarium behind it, and an instrument to detect the secondary radiation from de-excitation of the Samarium undergoing resonant scattering (a photomultiplier will do). Count the ticks of the tube, turn the iron by 180 degrees, count again. Easy as pie!!
The picture above shows a sketch of the experimental apparatus. A small source of Europium is placed in the niche at the left, immersed in a block of magnetized iron. a big lead shielding prevents photons from reaching th photomultiplier directly from the Europium source. The rectangles labeled "Sm2O3" identify a ring of Samarium oxide, which is used as the scatterer, where photons can do resonance absorption. The photomultiplier tube is connected to a sodium iodide scintillating crystal, which records the photon signal.
Here is the original paper (click on the gif thumbnails to enlarge):
Through inventiveness and cleverness, Goldhaber and company designed and carried out one of the most extraordinary experiments in modern Physics. Reducing the measurement of one of the most elusive properties of subnuclear matter to a mere count of ticks of a photomultiplier tube is an enormous achievement! The neutrino is spinning counterclockwise, antineutrinos are clockwise. The interaction is V-A: a fundamentum of Particle Physics.