Wu experiment

The experiment established that conservation of parity was violated (P-violation) by the weak interaction, providing a way to operationally define left and right without reference to the human body.

This result was not expected by the physics community, which had previously regarded parity as a symmetry applying to all forces of nature.

Tsung-Dao Lee and Chen-Ning Yang, the theoretical physicists who originated the idea of parity nonconservation and proposed the experiment, received the 1957 Nobel Prize in Physics for this result.

In 1927, Eugene Wigner formalized the principle of the conservation of parity (P-conservation),[3] the idea that the current world and one built like its mirror image would behave in the same way, with the only difference that left and right would be reversed (for example, a clock which spins clockwise would spin counterclockwise if a mirrored version of it were built).

[4] Theoretical physicists Tsung-Dao Lee and Chen-Ning Yang did a literature review on the question of parity conservation in all fundamental interactions.

[5] Shortly after, they approached Chien-Shiung Wu, who was an expert on beta decay spectroscopy, with various ideas for experiments.

Wu realized the potential for a breakthrough experiment and began work in earnest at the end of May 1956, cancelling a planned trip to Geneva and the Far East with her husband, wanting to beat the rest of the physics community to the punch.

Most physicists, such as close friend Wolfgang Pauli, thought it was impossible and even expressed skepticism regarding the Yang-Lee proposal.

At the behest of Boorse and Zemansky, Wu contacted Ernest Ambler, of the National Bureau of Standards, who arranged for the experiment to be carried out in 1956 at the NBS' low-temperature laboratories.

[4] After several months of work overcoming technical difficulties, Wu's team observed an asymmetry indicating parity violation in December 1956.

[1] The aim of Wu's experiment was to determine if this was the case for the weak interaction by looking at whether the decay products of cobalt-60 were being emitted preferentially in one direction or not.

Hence the overall nuclear equation of the reaction is: Gamma rays are photons, and their release from the nickel-60 nucleus is an electromagnetic (EM) process.

The experiment then essentially counted the rate of emission for gamma rays and electrons in two distinct directions and compared their values.

If the counting rates for the electrons did not differ significantly from those of the gamma rays, then there would have been evidence to suggest that parity was indeed conserved by the weak interaction.

If, however, the counting rates were significantly different, then there would be strong evidence that the weak interaction does indeed violate parity conservation.

Radioactive cobalt was deposited as a thin surface layer on a crystal of cerium-magnesium nitrate, a paramagnetic salt with a highly anisotropic Landé g-factor.

The salt was magnetized along the axis of high g-factor, and the temperature was decreased to 1.2 K by pumping the helium to low pressure.

Gamma ray polarization was continuously monitored over the next quarter-hour as the crystal warmed up and anisotropy was lost.

Wolfgang Pauli upon being informed by Georges M. Temmer, who also worked at the NBS, that parity conservation could no longer be assumed to be true in all cases, exclaimed "That's total nonsense!"

Temmer assured him that the experiment's result confirmed this was the case, to which Pauli curtly replied "Then it must be repeated!

"[4] By the end of 1957, further research confirmed the original results of Wu's group, and P-violation was firmly established.

The Wu experiment has finally solved the Ozma problem which is to give an unambiguous definition of left and right scientifically.

Thus, the concentration of beta rays in the negative-z direction indicated a preference for left-handed quarks and electrons.

These neutrinos would not couple with the weak Lagrangian and would interact only gravitationally, possibly forming a portion of the dark matter in the universe.

[26] The AAUW called it the “solution to the number-one riddle of atomic and nuclear physics.”[27] Beyond showing the distinct characteristic of weak interaction from the other three conventional forces of interaction, this eventually led to general CP violation, the violation of the charge conjugation parity symmetry.

[29] This is since the lack of symmetry gave the possibility of matter-antimatter imbalance which would allow matter to exist today through the Big Bang.

[31] To further quote the impact it had, Nobel laureate Abdus Salam quipped, If any classical writer had ever considered giants (cyclops) with only the left eye.

[One] would confess that one-eyed giants have been described and [would have] supplied me with a full list of them; but they always sport their solitary eye in the middle of the forehead.