The Wu- experiment was conducted in 1956 by the Chinese-American physicist Chien- Shiung Wu in collaboration with the cryogenic group of the National Bureau of Standards to check the parity conservation experimentally in the weak interaction. It has been found, as had been suspected of Tsung- Dao Lee and Chen Ning Yang in contrast to the prevailing doctrine that in the weak interaction parity conservation does not hold ( parity violation ).


1927, the parity quantum number was introduced as a symmetry property of the wave functions of the states of the atom of Eugene Paul Wigner. In physical processes that would occur in a mirrored form exactly this quantum number is preserved. It was taken for granted that there is no exception to it at all.

1956 published Tsung- Dao Lee and Chen Ning Yang, the assumption that in the weak interaction, in contrast to gravity, the strong and electromagnetic interactions, the parity is not conserved. They had also proposed a number of specific experiments.

The experiment

60Co nuclei are aligned magnetically at a temperature of about 10 milli - Kelvin that their spins pointing in one preferred direction ( ie parallel to the magnetic field, ie in the positive z -direction). The observed cobalt isotope decays into a beta-minus decay to nickel -60:

The parent nucleus, the z- component of the spin Sz = 5, the (excited ) daughter nucleus Sz = 4. The resulting electron and antineutrino each carry spin S = 1/2. Because of conservation of angular momentum thus show their spins both in the direction of spin of the cobalt core and are thus parallel to the magnetic field.

The experimental challenge in this experiment was in achieving the highest possible degree of polarization of the 60Co nuclei. Due to the - compared to electrons - very small magnetic moment of the nuclei extremely low temperatures and high magnetic fields are required, which could not be achieved by cooling with liquid helium and exclusive use of a coil. However, this was achieved with the help of the Gorter - Rose method. To 60Co cores were stored ( CeMg nitrate ) in a paramagnetic salt, which has a highly anisotropic g-factor and kept in a cryostat through liquid helium and pumping of gaseous helium at a temperature of about 1.2 Kelvin.

First, the salt was magnetized by a magnetic field along the axis having the larger g-value, and then adiabatically demagnetized, resulting in a decrease in temperature resulted in about 0,003 Kelvin. Then the salt is taken in the direction of the low- g-factor (z-direction ) has been magnetized so that only a negligible increase in temperature was caused. Due to the polarization of the electron shell of the cobalt ions and the associated magnetic field is present, a significantly higher magnetic field near the core, so that a degree of polarization of 60Co nuclei of about 60% was achieved. The 60Co degree of polarization can about the anisotropy of the radiation emitted from the excited daughter nucleus 60Ni photons: be determined ( decay cascade 4 → 2 → 0 ). The Gorter - Rose method was successfully demonstrated in 1953 with 60Co nuclei.

It is the number of emitted electrons in the negative z - direction is then measured. From angular momentum conservation reasons, the spins of the electron and neutrino must point in the direction of the original 60Co spins. Thus, the external magnetic field also determines the spin direction of the emitted electrons and neutrinos - but only to a certain degree, which corresponds to the degree of polarization of the cobalt nuclei. One must distinguish the following two scenarios here:

  • Scenario: The nuclear spins are oriented in the positive z direction. The detected in the negative z - direction electrons were emitted so opposite to the direction of the 60Co spins and thus their spin ( ie with negative helicity ). This can be illustrated as follows ( here is the double arrow for a Spin-1/2-Anteil, the single arrows for the direction of motion ):
  • Mirrored scenario: Since the nuclear spins are Axialvektoren, they are still facing a mirror in the same direction. Rather than reflect the experimental setup, it is therefore sufficient to rotate the nuclear spins with the magnetic field. It electrons are then detected, which were emitted in the direction of the 60Co spins, ie, with positive helicity:

If the parity condition, both scenarios would be equally likely: It would emit the same number of electrons in the direction of the nuclear spin as in the opposite direction. However, Wu found experimentally that almost all electrons are emitted opposite to the direction of spin of the nuclei, which corresponds to a maximum parity violation.

The reason is that the exchange bosons of the weak interaction couple only to left-handed particles ( and right-handed antiparticles ).

The result

The violation of parity is not a minor correction, but a maximum in the weak interaction. It is as it were one of their license plates.

Later, the Goldhaber experiment showed that there are only left-handed neutrinos and right-handed antineutrinos.

After the violation of space inversion symmetry P had been shown to have assumed that the operator CP, the combination of space reflection and Ladungsvertauschung, an unbroken symmetry is until here also a violation was found, the CP violation in kaon decay. The combined symmetry CPT contrast (T for Time refers to the time reversal) is available in all interactions. Such is the statement of the CPT theorem that can be proved in the framework of quantum field theory.