
Mirror, Mirror: Discovering Parity Isn't - peter_d_sherman
https://www.aps.org/programs/outreach/history/historicsites/nbs-2011.cfm
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peter_d_sherman
>" _Parity conservation enjoyed exalted status among the most fundamental laws
of physics, including conservation of energy, momentum and electric charge._

It implies that Nature is symmetrical under reflections in a mirror. As a
consequence, two similar radioactive particles spinning in opposite directions
about a vertical axis should emit their decay products with the same intensity
upwards and downwards. Yet although there were many experiments that
established parity conservation in strong interactions, the assumption had
never been experimentally verified for weak interactions like beta decay.
Indeed, when the weak force was first postulated to explain disintegration of
elementary particles, it seemed inconceivable that parity would not hold there
as well.

In the 1950s, high-energy physicists began observing phenomena that could not
be explained by existing theories, most notably the decays of K mesons emitted
in the collision of a high-energy proton with an atomic nucleus. The K meson
appeared in two distinct versions, decaying into either two or three pi
mesons, (which necessarily had opposite parity), although in all other
characteristics they seemed identical. In June of 1956, theoretical physicists
Chen Ning Yang and Tsung Dao Lee submitted a short paper to the Physical
Review raising the question of whether parity is conserved in weak
interactions, and suggesting several experiments to decide the issue.

Lee and Yang's paper did not immediately spark more than passing curiosity
among physicists when it appeared in October 1956. Freeman Dyson later
admitted that while he thought the paper was interesting, "I had not the
imagination to say, 'By golly, if this is true, it opens up a whole new branch
of physics!' And I think other physicists, with very few exceptions, at that
time were as unimaginative as I." Richard Feynman pronounced the notion of
parity violation "unlikely, but possible, and a very exciting possibility,"
but later made a $50 bet with a friend that parity would not be violated.

One of the simplest proposed experiments involved measuring the directional
intensity of beta radiation from cobalt-60 nuclei oriented with a strong
magnetic field so that their spins aligned in the same direction. Parity
conservation demands that the emitted beta rays be equally distributed between
the two poles. If more beta particles emerged from one pole than the other, it
would be possible to distinguish the mirror image nuclei from their
counterparts, which would be tantamount to parity violation.

The NBS team set about performing beta decay experiments. When the results
were in, they arrived at the astonishing conclusion that the emission of beta
particles is greater in the direction opposite to that of the nuclear spin;
parity was clearly demonstrated not to be conserved. Leon Lederman, who at the
time worked with Columbia University's cyclotron, performed an independent
test of parity with that equipment, involving the decay of pi and mu mesons,
and also obtained distinct evidence for parity violation.

Feynman lost his bet.

 _The result shattered a fundamental concept of nuclear physics that had been
universally accepted for 30 years_ ,

thus clearing the way for a reconsideration of physical theories and leading
to new, far-reaching discoveries - most notably a better understanding of the
characteristics of elementary particles, and a more unified theory of the
fundamental forces."

