Wu experiment

The Wu experiment was carried out in 1956 by the Chinese-American physicist Chien-Shiung Wu in collaboration with the low-temperature group of the National Bureau of Standards in order to experimentally check the preservation of parity in the weak interaction . It was established as described by Tsung-Dao Lee and Chen Ning Yang was suspected in contrast to the prevailing theory that the parity conservation in the weak interaction is not true ( parity injury ).

prehistory

In 1927, Eugene Paul Wigner introduced the parity quantum number as a symmetry property of the wave functions of the states of the atom . This quantum number is retained for physical processes that would take place in a mirrored form. It was considered certain that there would be no exception at all.

In 1956, Tsung-Dao Lee and Chen Ning Yang published the conjecture that parity is not preserved in the weak interaction in contrast to gravitation , in the strong and electromagnetic interaction . They had also suggested several special experiments.

The experiment

Principle of the proof of parity violation in the Wu experiment

⁶⁰ Co atomic nuclei are magnetically aligned at a temperature of about 10  mK in such a way that their spins point in a preferred direction (namely parallel to the magnetic field, i.e. in the positive z-direction). The cobalt isotope under consideration decays in a beta-minus decay to nickel-60:

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

The mother 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 have a spin S  = 1/2. Because of the conservation of angular momentum, their spins both point in the spin direction of the cobalt nucleus and are thus parallel to the magnetic field.

Schematic representation of the structure of the Wu experiment.

The experimental challenge in this experiment was to achieve the highest possible degree of polarization of the ⁶⁰ Co-nuclei. Due to the very low magnetic moment of the nuclei - compared to electrons - extremely low temperatures and high magnetic fields are necessary, which could not be achieved by cooling with liquid helium and using a coil. But this was achieved with the help of the Gorter-Rose method. For this purpose, ⁶⁰ Co nuclei were stored in a paramagnetic salt (CeMg nitrate), which has a strongly anisotropic g-factor and was kept in a cryostat by liquid helium and pumping off gaseous helium at a temperature of approx. 1.2 Kelvin.

First, the salt was magnetized by a magnetic field along the axis with the larger g-factor and then adiabatically demagnetized , which resulted in a temperature decrease to approx. 0.003 Kelvin. The salt was then magnetized along the low g-factor direction (z-direction), causing only a negligible rise in temperature. Due to the polarization of the electron shell of the cobalt ions and the associated magnetic field, there is a significantly higher magnetic field in the vicinity of the core, so that a degree of polarization of the ⁶⁰ Co nuclei of approx. 60% was achieved. The ⁶⁰ Co polarization degree can be determined from the anisotropy of the photons emitted by the excited daughter nucleus ⁶⁰ Ni (decay cascade: 4 ⁺  → 2 ⁺  → 0 ⁺  ). The Gorter-Rose method had already been successfully demonstrated with ⁶⁰ co-nuclei in 1953 .

The number of electrons emitted is now measured with a detector in the negative z-direction, once with a magnetic field in the + z-direction and once in the opposite direction. For reasons of angular momentum conservation, the spins of the electron and neutrino must point in the direction of the original ⁶⁰ co-spin. The external magnetic field thus 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 now distinguish (the horizontal arrows indicate the orientation to the z-direction):

• Field in + z direction : The nuclear spins are aligned in the positive z direction. The electrons detected in the negative z-direction are thus emitted against the direction of the ⁶⁰ Co spin and thus also against the direction of their spin (that is, with negative helicity ). This can be illustrated as follows (here the double arrow stands for a spin 1/2 component, the single arrows for the direction of movement):

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

• Field in -z direction : Only the nuclear spins are now polarized in opposite directions. So the electrons are detected that were emitted in the direction of the ⁶⁰ Co spin, i.e. with positive helicity:

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

The reversal of the orientation of the nuclear spins to the electron speed corresponds to a mirroring, i.e. the parity operation (see screw movement in the mirror). If parity had been preserved, both scenarios would be equally likely: just as many electrons would be emitted in the direction of the nuclear spin as in the opposite direction. Wu found, however, that significantly more electrons are emitted antiparallel to the spin direction of the nuclei than parallel to it. The difference was the theoretically maximum possible size.

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

The result

The violation of parity is not a small correction, but a maximum in the weak interaction. It is, so to speak, one of their characteristics.

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

After the violation of the space reflection symmetry  P had been shown, it was assumed that the operator  CP , the combination of space reflection and charge exchange , is an unbroken symmetry, until a violation was found here too , the CP violation during kaon decay. The combined symmetry  CPT on the other hand ( T for Time denotes the time reversal ) is preserved in all interactions. This is the statement of the CPT theorem , which can be proven within the framework of quantum field theory .

See also

• Goldhaber experiment

Individual evidence

1. ↑ CS Wu, E. Ambler, RW Hayward, DD Hoppes, RP Hudson: Experimental Test of Parity Conservation in Beta Decay . In: Physical Review . 105, 1957, pp. 1413-1415. doi : 10.1103 / PhysRev.105.1413 .
2. ↑ TD Lee, CN Yang: Question of Parity Conservation in Weak Interactions . In: Physical Review . 104, 1956, pp. 254-258. doi : 10.1103 / PhysRev.104.254 .
3. ^ CJ Gorter: A New Suggestion for Aligning Certain Atomic Nuclei . In: Physica . 14, 1948, p. 504.
4. ^ ME Rose: On the Production of Nuclear Polarization . In: Physical Review . 75, 1949, p. 213.
5. ^ E. Ambler, MA Grace, H. Halban, N. Kurti, H. Durand, CE Johnson: Nuclear Polarization of Cobalt 60 . In: Philosophical Magazine . 44, 1953, p. 215.

Web links

• Reversal of the Parity Conservation Law in Nuclear Physics (English)