The Wu experiment was a particle and nuclear physics experiment conducted in 1956 by the Chinese-American physicist Chien-Shiung Wu in collaboration with the Low Temperature Group of the US National Bureau of Standards. The experiment's purpose was to establish whether conservation of parity, which was previously established in the electromagnetic and strong interactions, also applied to weak interactions. If parity conservation were universal, particle decays governed by the weak interaction would behave similarly to particle decays involving the other interactions. A parity transformation negates the coordinates in a theory, creating a mirror image. If the predictions of the theory are not altered, it is said to "conserve parity". Objects spinning clockwise appear to spin counterclockwise in their mirror image. The Wu experiment created an atomic system with spin, then compared weak-interaction particle decay for spins clockwise and counterclockwise.
The experiment established that conservation of parity was violated by the weak interaction, thus providing a way to operationally define left and right. This result was not expected by the physics community, which had previously regarded parity as a symmetry that applied 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. While not awarded the Nobel Prize, Chien-Shiung Wu's role in the discovery was mentioned in the Nobel Prize acceptance speech of Yang and Lee, but she was not honored until 1978, when she was awarded the first Wolf Prize.
History
The modern concept of parity was developed in 1927 when Eugene Wigner formalized the principle of the conservation of parity (P-conservation). The motivation for this theoretical principle was empirical observations by Otto Laporte for iron and Henry Norris Russell for titanium. They noted that certain transitions between atomic energy levels are not observed in experimental spectra, a concept now called a selection rule. Wigner showed that the atoms have two kinds of levels. Those of positive parity have the same functional form if the spatial coordinates are reversed (x → −x, y → −y, and z → −z). Those of negative parity change the sign of this form under this reversal of coordinates. Transitions between levels of the same parity are not observed because any emitted photon will have negative parity. The products of the parities before and after the transition have to be equal to "conserve" parity.
This principle was widely accepted by physicists because it simply reflected the intuitive idea that our world and its mirror image would behave in the same way, with the only difference being that left and right would be reversed (for example, a clock that spins clockwise would spin counterclockwise if a mirrored version of it were built). P-conservation was experimentally verified in the electromagnetic and strong interactions. However, during the mid-1950s, two different decays of seemingly identical kaon particles were observed. The "τ" kaon decayed into three pions, but the "θ" kaon decayed into two pions. The pion was known to have odd parity (–1): unless the relative motion of the products was unusual, decay into three pions predicted odd "τ" kaon parity [(–1)3 = –1], but decay into two pions predicted even "θ" kaon parity [(–1)2 = 1].
As more data was accumulated on the properties of the decay products, it became clear that every property of these particles was identical except for their parity. It seemed possible that these were the same particle except for this violation of parity. This was known as the τ–θ puzzle.
Theoretical physicists Tsung-Dao Lee and Chen-Ning Yang did a literature review on the question of parity conservation in all fundamental interactions. They concluded that in the case of the weak interaction, experimental data neither confirmed nor refuted P-conservation. This information was missing for two reasons: first, the weak interaction is revealed in beta decay and the whole process must be studied for parity conservation and second, the beta decay involves a neutrino whose mass cannot be measured directly, preventing some approaches to parity checks. In the summer of 1956 Lee and Yang approached Chien-Shiung Wu, who was an expert on beta decay spectroscopy, with various ideas for experiments. They settled on the idea of testing the directional properties of beta decay in cobalt-60. Wu understood 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.
The key problems that needed to be solved for a successful experiment was creating and maintaining extreme cryogenic temperature while measuring beta decay from nuclei with spin. Wu had to contact Henry Boorse and Mark W. Zemansky, who had extensive experience in low-temperature physics, to perform her experiment. Boorse and Zemansky suggested that Wu contacted Ernest Ambler, of the National Bureau of Standards. Ambler arranged for the experiment to be carried out in 1956 at the NBS low-temperature laboratories. After several months of work to overcome technical difficulties, in December 1956 Wu's team observed an asymmetry that indicated parity violation.
Lee and Yang, who prompted the Wu experiment, were awarded the Nobel Prize in Physics in 1957, shortly after the experiment was performed. Wu's role in the discovery was mentioned in the prize acceptance speech. Many were outraged that she had been overlooked for the prize, from her close friend Wolfgang Pauli, to Lee and Yang, with 1988 Nobel Laureate Jack Steinberger labeling it as the biggest mistake in the Nobel committee's history. Wu did not publicly discuss her feelings about the prize, but in a letter she wrote to Steinberger, she said, "Although I did not do research just for the prize, it still hurts me a lot that my work was overlooked for certain reasons." She was not honored until 1978, when she was awarded the inaugural Wolf Prize.
Theory
The aim of Wu's experiment was to determine whether parity is conserved for the weak interaction that governs beta decay. Wigner's mathematical concept of parity conservation, the equivalence of an object or process when the coordinate system is inverted, is equivalent to a mirror reflection followed by a 180° rotation in the plane of the mirror. Cobalt-60 nuclei have spin so the concept of the experiment can be imagined with a spinning ball. A ball spinning in a right-hand rule direction would have a mirror image with a left-hand spin, but this mirror image ball would look identical to an upside down ball spinning according to the right hand rule: after the 180° rotation there would be no physical differences. However, if the spinning ball had a mark on top or bottom, the reflection would no longer look the same. If the emitted electron from decay products of cobalt-60 were being emitted preferentially either along or against the cobalt spin axis direction, this would create a difference analogous to the mark. A difference in emission probability would signify a violation of parity symmetry. As stated by Wu et al.:
If an asymmetry in the distribution between θ and 180° − θ (where θ is the angle between the orientation of the parent nuclei and the momentum of the electrons) is observed, it provides unequivocal proof that parity is not conserved in beta decay.
The idea can also be described using the properties of vectors. The cobalt-60 nucleus has spin, and spin angular momentum does not change direction under parity. A vector with this property is called an axial vector. Conversely, the direction in which the decay products are emitted is changed under parity and their linear momentum vector is called polar vector. In other words, if in one view the cobalt-60 nuclear spin and the decay product emissions were both in roughly the same direction, then in a mirror image world, they would be in roughly opposite directions, because the emission direction would have been flipped, but the spin direction would not.
Conceptually the experiment is "relatively simple": orient 60Co by means of a magnetic field and measure the amount of beta emission directed along or against the direction of the magnetic field. Among the numerous challenges were maintaining and monitoring the orientation, avoiding systematic bias in the orientation, and detecting the electrons emitted in each direction. Measures to address these challenges are often difficult to apply simultaneously in practice. For example, maintaining the nuclear spin orientation requires a temperature below that achievable by liquid helium evaporation, which makes simultaneous detection in opposite directions difficult, and use of the magnetocaloric effect for cooling requires removal of a magnetic field, while alignment of the nuclei requires application of a magnetic field.
Experiment
The experiment monitored the decay of cobalt-60 (60Co) atoms that were aligned by a uniform magnetic field (the polarizing field) and cooled to near absolute zero so that thermal motions did not randomize the alignment. Cobalt-60 is an unstable isotope of cobalt that decays by beta decay to an excited state of the isotope nickel-60 (60Ni). During this decay, one of the neutrons in the cobalt-60 nucleus decays to a proton by emitting an electron (e−) and an electron antineutrino (νe). The resulting excited nickel nucleus promptly decays to its ground state by emitting two gamma rays (γ) in quick succession. Hence the overall nuclear equation of the reaction is:
Using a magnetic field to orient the cobalt-60 nuclei in one direction and using a reversed field to orient the nuclei in the opposite direction places the detectors in opposite hemispheres with respect to the nuclear spin. If the pattern of electron emission (beta rays) differs in these two conditions, parity is not conserved.
The gamma rays also play a crucial role in the experiment. Gamma rays are photons, and their release from the nickel-60 nucleus is an electromagnetic process. This is significant because electromagnetism was known to respect parity conservation, and therefore the gamma-ray emission pattern should be independent of changes in parity. The gamma rays are emitted in a distribution peaked around the two directions of the cobalt-60 nuclear spin axis: the degree to which the gamma rays were not distributed perfectly equally in all directions (the "anisotropy" of their distribution) can be used to determine how well the cobalt-60 nuclear spins had been aligned. Spin alignment is necessary to observe the anisotropy in the electron emission. If the cobalt-60 nuclei were randomly oriented, the experiment would detect equal numbers of electrons emitted in every direction.
The experiment then counted the rate of emission of electrons along the magnetic field, and of gamma rays along the magnetic field and perpendicularly to it. These rates were then compared with the polarizing field oriented in the opposite direction. If the electron counting rates differed significantly for the two field orientations, there would be strong evidence that the weak interaction does violate parity conservation.
In addition, the experiment was monitored over time as the apparatus slowly warmed up. The gamma ray anisotropy, measured in the direction of the magnetic field (polar direction) and perpendicular to this (equatorial direction) slowly vanished, indicating loss of spin polarization with increasing temperature. The observed asymmetry of the electron counts in the two directions of the polarizing field should track the gamma ray anisotropy results.
Materials and methods
The experimental challenge in this experiment was to obtain a high directional orientation or polarization of the 60Co nuclei.
For normal materials in spin state I the fraction of polarization is proportional to the magnet moment of the nuclei, , the applied field, H and inversely proportional to temperature:
Due to the very small magnetic moments of the nuclei, a magnetic field of about 1 T (10000 G) is required at extremely low temperatures (around 0.01 K), a combination that is very difficult to achieve.
In a paramagnetic element the unpaired 3d or 4f electrons create a strong magnetic field, about 0.1 T (1000 G), at the nucleus.
In that case, the fractional polarization becomes proportional to the fractional polarization of the electrons but is otherwise independent of the applied field.
Thus for nuclei with magnetic moments in a paramagnetic material, aligning the electrons with a strong magnetic field has the side effect of aligning the nuclei. The concept can be paired with adiabatic demagnetization, in which a paramagnetic salt is placed in an external magnetic field and heat caused by the alignment of the electrons is extracted by pumping the liquid helium to low pressure, giving a temperature 1.2 K. When the external field is removed, randomization of the electron spins cools the salt. A temperature of 0.01 K can be achieved by this process. Thus internal paramagnetism is manipulated to both cool the sample and provide the field to align the nuclei. This concept became known as the Rose–Gorter method.