Wu experiment

The Wu Experiment was an experiment conducted in 1956 by the Chinese American physicist Chien-Shiung Wu in collaboration with the Low Temperature Group of the National Bureau of Standards. The experiment's purpose was to check the parity conservation law in weak interactions experimentally. However, the parity violation of weak interaction was discovered instead.

History

In 1927, Eugene Paul Wigner, constructed the parity operation based on the photon interacting with the electron wave function, since an atom has at least two excited states symmetry in the interaction should imply some conserved operation. This generated the "Law of Conservation of Parity" in elementary particle physics for Electromagnetism. However, it was never implied physically, in non-electromagnetic interactions, that Parity should always be conserved. At Columbia University Chinese-born theoretical physicists Tsung-Dao Lee and Chen Ning Yang, grew to question a parity conservation law in all fundamental interactions. Their research into experimental results convinced them that this "Law" was valid for electromagnetic interactions and for the strong nuclear force. However, this "Law" had not been tested for the weak nuclear force. Moreover, Lee & Yang's own theoretical studies on the Fermi Theory of Nuclear decay showed that universal conservation of parity was probably not true.

In 1956 Tsung-Dao Lee and Chen Ning Yang published a paper stating the theory that the weak interaction, in contrast to the strong and electromagnetic interactions, may not conserve parity. in which they had also proposed a number of specific experiments.

Lee and Yang then turned to experimental physicist Chien-Shiung Wu, an experimental expert on beta decay, for her expertise in choosing and then working out the hardware manufacture, set-up, and laboratory procedures for carrying out the best experiment, now known as the Wu experiment.

The experiment

The Wu experiment monitored the decay of aligned cobalt-60 atoms, cooled to near absolute zero and aligned in a uniform magnetic field.

For this low temperature work, Dr. Wu needed the expertise and the facilities of the National Bureau of Standards as well as people at the Low Temperature Group to aid her. The experiment itself was carried out in the NBS headquarters in Maryland which supplied the equipment to carry out the experiment.

An artistic representation of the experiment is here

Cobalt-60 is an unstable isotope of cobalt that decays by beta decay to the stable isotope nickel-60, an electron and an electron antineutrino. The stable nickel nucleus is activated by the reaction and emits two gamma rays with energies of 1.17 and 1.33 MeV, hence the overall nuclear equation of the reaction is

${}_{27}^{60}{\text{Co}}\rightarrow {}_{28}^{60}{\text{Ni}}^{*}+e^{-}+{\bar {\nu }}_{e}+2{\gamma }.$

The gamma rays are emitted electromagnetically, hence they are released quickly from the activated daughter nucleus. They are distributed uniformly in the field and act as an indicator to the uniformity of the cobalt-60 atoms as well as a control to the polarization of the emitted electrons via the weak interaction.

Any large difference between the polarization of the gamma ray spectrum and the electron spectrum would indicate parity violation.

Using liquid helium, cobalt-60 nuclei were lowered to a temperature of about 10 mK to reduce atomic vibrations to effectively zero. An uniform magnetic field across the sample of cobalt-60 was needed to magnetically align the atomic nuclei so that their spin axes point in a preferred direction (namely, parallel to the magnetic field in the z-direction).

The parent nucleus has the z-component of the spin S_z = +5, the (excited) daughter nucleus S_z = +4. The resulting electron and the antineutrino each carry spin S = 1/2. Because of conservation of angular momentum that is both show their spins in the direction of spin of the cobalt nucleus, and are thus antiparallel to the magnetic field. The external magnetic field therefore determines the spin direction of the emitted electrons and neutrinos - but at T>0 only to a degree corresponding to the polarization of the cobalt nuclei.

The polarization of the cobalt nuclei can be determined by the intrinsic anisotropy of the nickel core emitted photons (decay cascade: 4⁺ → 2⁺ → 0⁺ ). In the experiment carried out by Wu, the gamma ray polarization was approximately 60%

It is now the number of in negative z-direction of emitted electrons are measured. One must distinguish here the following two orientations of the nuclei:

• Forward : The nuclear spins are aligned in the positive z-direction (along the magnetic field). When the cobalt-60 nuclei is decayed via beta decay the electrons are emitted in the negative z-direction (opposite to the nuclear spin). Nothing says to electrons that they should be fly off in the negative z-direction. The spin direction of the electron is along the initial nuclear spin, thus the direction of the momentum and the spin of the electron are in opposite so negative helicity. This can be illustrated as follows (here the double arrow indicates a spin-1/2 orientation, the single arrows for direction of motion):

${\begin{array}{ccccccc}\Rightarrow \Rightarrow &&&&\Rightarrow &&\Rightarrow \\{}^{60}{\text{Co}}&\longrightarrow &{}^{60}{\text{Ni}}&+&e^{-}&+&{\bar {\nu }}_{e}\\&&&&\longleftarrow &&\longrightarrow \\\end{array}}$

• Reverse : Since the nuclear spin ( I_z ) is similar to the angular momentum and it points the same direction under mirror symmetry but the momentum "p" or the direction of motion "r" change sign. Under mirror symmetry  :${\vec {p}}\times {\vec {I_{z}}}\rightarrow -{\vec {p}}\times {\vec {I_{z}}}=-{\vec {p}}\times {\vec {I_{z}}}$ the equation of motion changes a sign. Thus the electrons should fly off in the negative z-direction, meaning the same result as above.

If we set up a "mirror" of the experimental set-up by rotating the nuclear spins with the opposite direction of the magnetic field, the nuclear spins now becomes aligned in the negative z-direction (ie. antiparallel to the magnetic field), but this time the electrons are emitted in the positive z-direction again in the opposite direction of the nuclear spin as follows:

${\begin{array}{ccccccc}\Leftarrow \Leftarrow &&&&\Leftarrow &&\Leftarrow \\{}^{60}{\text{Co}}&\longrightarrow &{}^{60}{\text{Ni}}&+&e^{-}&+&{\bar {\nu }}_{e}\\&&&&\longleftarrow &&\longrightarrow \\\end{array}}$

As shown in the diagrams above, in both cases the spin and angular momentum vectors are conserved. If the parity operation was conserved, the electrons coming off from the mirror symmetric system should go in the negative z-direction along the nuclear spin, but in both cases they tend to fly off in the opposite direction of the nuclear spin of cobalt-60. If beta decay have conserved under the mirror symmetry, it would be the same number of electrons emitted in the direction of the nuclear spins and in the opposite direction. Wu experimentally observed, however, that almost all the electrons are emitted against the direction of spin of the nuclei, which corresponds to a maximum parity violation.

Results

The violation of parity was not found as a small correction, but was strongly observed from the experimental set up. The results of the measurements showed a strong asymmetry in the distribution of the beta electrons. Moreover, the counting rate for beta emission reverses upon reversing the magnetic field. This contrasted with the anisotropic distribution of gamma rays, which were the same in the forward and reversed fields. Hence parity violation is inherent in the weak interaction.

Consequences

The reason for parity violation observed in this experiment is due to the W and Z gauge bosons of the weak interaction coupling only to left-handed matter particles and the right-handed antimatter particles. The combined parity and spin operations generate the helicity of the particle, which determines whether the particle spirals either right or left handed.

From experiments such as the Wu experiment and Goldhaber experiment it was determined that if neutrinos are massless, neutrinos must be left-handed, while antineutrinos must be right handed. Since neutrinos have been determined to have a small, albeit negligible, mass it has been proposed that right-handed neutrinos and left-handed antineutrinos could exist. These neutrinos would not couple with the weak Lagrangian and would intereact only gravitationally, possibly forming a portion of the dark matter in the universe.

After the violation of mirror symmetry had been shown, denoted by the parity operator P, it was believed still that the operator CP, the combination of spatial reflection and charge conjugation, was an unbroken symmetry. The CP symmetry implies a perfect symmetry between matter and antimatter and is unbroken for all lepton interactions in the Standard Model.

However, CP symmetry breaking does occur in strongly interacting particles and CP-violation has been observed extensively in kaon-decay. This gives an important hint into the matter-antimatter asymmetry seen in the universe. In order to restore the unbroken symmetry, time reversal symmetry T must be invoked to create the combined symmetry CPT. This symmetry is conserved in all known interactions. This created the concept of the CPT theorem in elementary particle physics, which in the context of quantum field theory can be proved.

The fact that a violation in physics had been identified through some broken symmetry led to further developments of the theory of the weak interaction, together with a large number of follow-up experiments, have led to the unification of the weak and electromagnetic interactions into a unified electroweak interaction. For contributions in describing the physics to the electroweak unification, Abdus Salam, Sheldon Glashow and Steven Weinberg were awarded the Nobel Prize in Physics in 1979.

See also

• Beta decay
• Neutrino
• Fermi's interaction
• Weak interaction
• Electroweak interaction

External links

• Reversal of the Parity Conservation Law in Nuclear Physics