View source for Wu experiment

{{short description|Nuclear physics experiment}}
{{good article}}
[[File:Chien-shiung Wu (1912-1997) C.jpg|thumb|[[Chien-Shiung Wu]], after whom the Wu experiment is named, designed the experiment and led the team that carried out the test of the conservation of parity in 1956.]]
The '''Wu experiment''' was a [[particle physics|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]].<ref name="Wu1957" >
{{cite journal
 |last1=Wu |first1=C. S.
 |last2=Ambler |first2=E.
 |last3=Hayward |first3=R. W.
 |last4=Hoppes |first4=D. D.
 |last5=Hudson |first5=R. P.
 |year=1957
 |title=Experimental Test of Parity Conservation in Beta Decay
 |journal=[[Physical Review]]
 |volume=105 |issue=4 |pages=1413–1415
 |bibcode=1957PhRv..105.1413W
 |doi=10.1103/PhysRev.105.1413
 |doi-access=free
}}</ref> The experiment's purpose was to establish whether or not conservation of [[parity (physics)|parity]] (''P''-conservation), which was previously established in the [[electromagnetic]] and [[strong interaction|strong]] interactions, also applied to [[weak interaction]]s. If ''P''-conservation were true, a mirrored version of the world (where left is right and right is left) would behave as the mirror image of the current world. If ''P''-conservation were violated, then it would be possible to distinguish between a mirrored version of the world and the mirror image of the current world.

The experiment established that conservation of parity was violated (''P''-violation) by the weak interaction, providing a way to [[operational definition|operationally define]] [[Body relative direction|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. 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,<ref name="nobelPrize1957">
{{cite web
 |last=Klein |first=O. B.
 |year=1957
 |title=The Nobel Prize in physics in 1957: Award ceremony speech
 |url=https://www.nobelprize.org/prizes/physics/1957/ceremony-speech/
 |publisher=The Nobel Foundation
 |access-date=2 October 2018
}}</ref> but she was not honored until 1978, when she was awarded the first [[Wolf Prize]].

==History==
{{multiple image
|align     = left
|direction = vertical
|image1    = Parity clocks - P-conservation.svg<!-- file name only; no "File:" or "Image:" prefix -->
|width1    = 150
|image2    = Parity clocks - P-violation.svg<!-- file name only; no "File:" or "Image:" prefix -->
|width2    = 150
|footer    = '''Top:''' ''P''-symmetry: A clock built like its mirrored image will behave like the mirrored image of the original clock.<br/>'''Bottom:''' ''P''-asymmetry: A clock built like its mirrored image will ''not'' behave like the mirrored image of the original clock.
}}

In 1927, [[Eugene Wigner]] formalized the principle of the conservation of [[parity (physics)|parity]] (''P''-conservation),<ref>
{{cite journal
 |last=Wigner |first=E.P.
 |year=1927
 |title=Über die Erhaltungssätze in der Quantenmechanik
 |url=http://www.digizeitschriften.de/dms/img/?PPN=PPN252457811_1927&DMDID=dmdlog30
 |journal=Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch Physikalische Klasse
 |volume=1927|pages=375–381
 |postscript=;
}}
<br/>{{*}}Reprinted in {{cite book
 |editor1-last=Wightman |editor1-first=A.S.
 |year=1993
 |title=The Collected Works of Eugene Paul Wigner
 |volume=A |pages=84–90
 |publisher=[[Springer (publisher)|Springer]]
 |doi=10.1007/978-3-662-02781-3_7
 |isbn=978-3-642-08154-5
}}</ref> 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).

This principle was widely accepted by physicists, and ''P''-conservation was experimentally verified in the [[electromagnetic]] and [[strong interaction|strong]] interactions. However, during the mid-1950s, certain decays involving [[kaon]]s could not be explained by existing theories in which ''P''-conservation was assumed to be true. There seemed to be two types of kaons, one which decayed into two [[pion]]s, and the other which decayed into three pions. This was known as the [[τ–θ puzzle|{{math|τ–θ}} puzzle]].<ref name=Hudson>
{{cite book
 |last=Hudson |first=R.P.
 |year=2001
 |chapter=Reversal of the Parity Conservation Law in Nuclear Physics
 |editor1-last=Lide |editor1-first=D. R.
 |title=A Century of Excellence in Measurements, Standards, and Technology
 |chapter-url=http://nvlpubs.nist.gov/nistpubs/sp958-lide/111-115.pdf
 |series=NIST Special Publication 958
 |publisher=[[National Institute of Standards and Technology]]
 |isbn=978-0849312472
}}
</ref>

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.<ref name="LeeYang1956">
{{cite journal
 |last1=Lee  |first1=T.D.
 |last2=Yang |first2=C.N.
 |year=1956
 |title=Question of Parity Conservation in Weak Interactions
 |journal=[[Physical Review]]
 |volume=104 |issue=1 |pages=254–258
 |bibcode=1956PhRv..104..254L
 |doi=10.1103/PhysRev.104.254 |doi-access=free
}}
</ref> Shortly after, they 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 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.<ref>
{{cite book
 |last = Chiang |first = Tsai-Chien
 |year = 2014
 |title = Madame Chien-Shiung Wu: The First Lady of Physics Research
 |publisher = World Scientific
 |pages = 136–137
 |isbn = 978-981-4374-84-2
}}
</ref><ref>
{{cite book
 |last=Wu |first=C. S.
 |year=1973
 |editor-last=Maglich |editor-first=B.
 |title=Adventures in Experimental Physics: Gamma Volume
 |location=Princeton
 |publisher=[[World Science Communications]]
 |pages=101–123
 |asin=B000ITLM9Q
}}
</ref><ref>
{{Cite arXiv
 |last1=Lee|first1=T. D.
 |year=2006
 |title=New Insights to Old Problems
 |eprint=hep-ph/0605017
}}
</ref>

Wu had to contact Henry Boorse and [[Mark W. Zemansky]], who had extensive experience in [[low-temperature physics]], to perform her experiment. 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 [[National Institute of Standards and Technology|NBS]]' low-temperature laboratories.<ref name=Hudson/> After several months of work overcoming technical difficulties, Wu's team observed an asymmetry indicating parity violation in December&nbsp;1956.<ref>
{{cite book
 |last=Wu  |first=C.S.
 |year=2008
 |title=Nishina Memorial Lectures
 |chapter=The discovery of the parity violation in weak interactions and its recent developments
 |series=[[Lecture Notes in Physics]]
 |volume=746  |pages=43–70
 |publisher=[[Springer Science+Business Media]]
 |doi=10.1007/978-4-431-77056-5_4
 |isbn=978-4-431-77055-8
 |chapter-url=https://inspirehep.net/record/790221/files/978-4-431-77056-5_4.pdf
}}
</ref>

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,<ref name=nobelPrize1957/> but was not honored until 1978, when she was awarded the inaugural [[Wolf Prize]].<ref name="wolfprize">
{{cite press release
 |title=Chien-Shiung Wu winner of Wolf Prize in Physics - 1978 
 |publisher=[[Wolf Foundation]]
 |url=http://www.wolffund.org.il/index.php?dir=site&page=winners&cs=381
 |access-date=9 December 2019
}}
</ref> Many were outraged, 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.{{sfn|Chiang|2014|p=146}} 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."{{sfn|Chiang|2014|pp=147–149}}

==Theory==
If a particular interaction respects parity symmetry, it means that if left and right were interchanged, the interaction would behave exactly as it did before the interchange. Another way this is expressed is to imagine that two worlds are constructed that differ only by parity—the "real" world and the "mirror" world, where left and right are swapped. If an interaction is parity symmetric, it produces the same outcomes in both "worlds".<ref name="Wu1957"/>

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. This would signify the violation of parity symmetry because if the weak interaction were parity conserving, the decay emissions should be emitted with equal probability in all directions. As stated by Wu et al.:<ref name="Wu1957"/>
{{Blockquote|If an asymmetry in the distribution between ''θ'' and 180°&nbsp;−&nbsp;''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 reason for this is that the cobalt-60 nucleus carries [[Spin (physics)|spin]], and spin does not change direction under parity (because angular momentum is an [[axial vector]]). Conversely, the direction in which the decay products are emitted ''is'' changed under parity because momentum is a [[polar vector]]. In other words, in the "real" world, if the cobalt-60 nuclear spin and the decay product emissions were both in roughly the same direction, then in the "mirror" world, they would be in roughly opposite directions, because the emission direction would have been flipped, but the spin direction would not.<ref>
{{cite web 
 |last=Boyd |first=S.
 |date=20 April 2016
 |title=The Weak Interaction
 |url=https://warwick.ac.uk/fac/sci/physics/staff/academic/boyd/warwick_week/neutrino_physics/weak.pdf
 |publisher=[[Warwick University]]
 |access-date=8 December 2019
 }}</ref>

This would be a clear difference in the behaviour of the weak interaction between both "worlds", and hence the weak interaction could not be said to be parity symmetric. The only way that the weak interaction could be parity symmetric is if there were no preference in the direction of emission, because then a flip in the direction of emissions in the "mirror" world would look no different from the "real" world because there were equal numbers of emissions in both directions anyway.

==Experiment==
[[File:Wu experiment at Bureau of Standards.jpg|thumb|The Wu experiment performed at the Bureau of Standards low temperature laboratory, Washington DC, in 1956. The vertical vacuum chamber, containing the cobalt-60, detectors, and field coil, is being placed into a Dewar before being inserted into the large electromagnet in the background, which will cool the radioisotope near absolute zero by [[adiabatic demagnetization]].]]The experiment monitored the decay of [[cobalt-60]] (<sup>60</sup>Co) atoms that were aligned by a uniform magnetic field (the polarizing field) and cooled to near [[absolute zero]] so that thermal motions did not ruin the alignment.<ref name="Wroblewski">
{{cite journal
 |last=Wroblewski |first=A. K.
 |year=2008
 |title=The downfall of parity: The revolution that happened fifty years ago
 |journal=[[Acta Physica Polonica B]]
 |volume=39 |issue=2 |pages=251–264
 |bibcode=2008AcPPB..39..251W
 |url=https://inspirehep.net/record/807786/files/v39p0251.pdf
}}</ref> Cobalt-60 is an [[unstable isotope|unstable]] [[isotopes of cobalt|isotope of cobalt]] that decays by [[beta decay]] to the stable isotope [[nickel-60]] (<sup>60</sup>Ni). During this decay, one of the [[neutron]]s in the cobalt-60 nucleus decays to a [[proton]] by emitting an [[electron]] (e<sup>&minus;</sup>) and an [[electron antineutrino]] ({{overline|&nu;}}<sub>e</sub>). The resulting nickel nucleus, however, is in an [[excited state]] and promptly decays to its ground state by emitting two [[gamma ray]]s (&gamma;). Hence the overall nuclear equation of the reaction is:

:<math>{}^{60}_{27}\text{Co} \rightarrow {}^{60}_{28}\text{Ni} + e^- + \bar{\nu}_e + 2{\gamma}</math>

Gamma rays are photons, and their release from the nickel-60 nucleus is an [[Electromagnetism|electromagnetic]] (EM) process. This is important because EM was known to respect parity conservation, and therefore they would be emitted roughly equally in all directions (they would be distributed roughly "isotropically"). Hence, the distribution of the emitted electrons could be compared to the distribution of the emitted gamma rays in order to compare whether they too were being emitted isotropically. In other words, the distribution of the gamma rays acted as a [[Scientific control|control]] for the distribution of the emitted electrons. Another benefit of the emitted gamma rays was that it was known that the degree to which they were ''not'' distributed perfectly equally in all directions (the "anisotropy" of their distribution) could be used to determine how well the cobalt-60 nuclei had been aligned (how well their spins were aligned).<ref name=Ambler>{{cite journal
 |last1=Ambler |first1=E.
 |last2=Grace |first2=M. A.
 |last3=Halban |first3=H.
 |last4=Kurti |first4=N.
 |last5=Durand |first5=H.
 |last6=Johnson |first6=C. E.
 |last7=Lemmer |first7=H. R.
 |year=1953
 |title=Nuclear polarization of cobalt 60
 |journal= The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science
 |volume=44 |issue=349 |pages=216–218
 |doi=10.1080/14786440208520296
}}</ref> If the cobalt-60 nuclei were not aligned at all, then no matter how the electron emission was truly distributed, it would not be detected by the experiment. This is because an unaligned sample of nuclei could be expected to be oriented randomly, and thus the electron emissions would be random and the experiment would detect equal numbers of electron emissions in all directions, even if they were being emitted from each individual nucleus in only one direction.

The experiment then essentially counted the rate of emission for gamma rays and electrons in two distinct directions and compared their values. This rate was measured over time and with the polarizing field oriented in opposite directions. 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.

===Materials and methods===
[[File:Wu-Experiment (English).png|thumb|left|Schematic of the Wu experiment.]]
The experimental challenge in this experiment was to obtain the highest possible polarization of the <sup>60</sup>Co nuclei. Due to the very small magnetic moments of the nuclei as compared to electrons, strong magnetic fields were required at extremely low temperatures, far lower than could be achieved by liquid helium cooling alone. The low temperatures were achieved using the method of [[adiabatic demagnetization]]. 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&nbsp;K by pumping the helium to low pressure. Shutting off the horizontal magnetic field resulted in the temperature decreasing to about 0.003&nbsp;K. The horizontal magnet was opened up, allowing room for a vertical solenoid to be introduced and switched on to align the cobalt nuclei either upwards or downwards. Only a negligible increase in temperature was caused by the solenoid magnetic field, since the magnetic field orientation of the solenoid was in the direction of low g-factor. This method of achieving high polarization of <sup>60</sup>Co nuclei had been originated by Gorter<ref name=Gorter1948>
{{cite journal
 |last=Gorter|first=C. J.
 |year=1948
 |title=A New Suggestion for Aligning Certain Atomic Nuclei
 |journal=[[Physica (journal)|Physica]]
 |volume=14 |issue=8 |page=504
 |bibcode=1948Phy....14..504G
 |doi=10.1016/0031-8914(48)90004-4
}}</ref> and Rose.<ref name=Rose1949>
{{cite journal
 |last=Rose |first=M. E.
 |title=On the Production of Nuclear Polarization
 |journal=[[Physical Review]]
 |year=1949
 |volume=75 |issue=1 |page=213
 |bibcode=1949PhRv...75Q.213R
 |doi=10.1103/PhysRev.75.213
}}</ref>

The production of gamma rays was monitored using equatorial and polar counters as a measure of the polarization. Gamma ray polarization was continuously monitored over the next quarter-hour as the crystal warmed up and anisotropy was lost. Likewise, beta-ray emissions were continuously monitored during this warming period.<ref name="Wu1957"/>

===Results===
[[File:Parity transformation.png|thumb|Result of the Wu experiment, wherein a cobalt atom with spin vector ''j'' emits an electron ''e''.]]
In the experiment carried out by Wu, the gamma ray anisotropy was approximately 0.6. That is, approximately 60% of the electrons were emitted in one direction, where as 40% were emitted in the other. If parity were conserved in beta decay, the emitted electrons would have had no preferred direction of decay relative to the nuclear spin, and the asymmetry in emission direction would have been close to the value for the gamma rays. However, Wu observed that the electrons were emitted in a direction preferentially opposite to that of the gamma rays with an asymmetry significantly greater than the gamma ray anisotropy value. That is, most of the electrons favored a very specific direction of decay, specifically opposite to that of the nuclear spin.<ref name="Wu1957"/> The observed electron asymmetry also did not change sign when the polarizing field was reversed, meaning that the asymmetry was not being caused by [[remanent magnetization]] in the samples. It was later established that parity violation was in fact maximal.<ref name=Hudson/><ref>
{{cite journal
 |last1=Ziino |first1=G.
 |year=2006
 |title=New Electroweak Formulation Fundamentally Accounting for the Effect Known as "Maximal Parity-Violation"
 |journal=[[International Journal of Theoretical Physics]]
 |volume=45 |issue=11 |pages=1993–2050
 |bibcode=2006IJTP...45.1993Z
 |doi=10.1007/s10773-006-9168-2
|s2cid=121004619
 }}</ref>

The results greatly surprised the physics community. Several researchers then scrambled to reproduce the results of Wu's group,<ref>
{{cite journal
 |last1=Garwin |first1=R. L.
 |last2=Lederman |first2=L. M.
 |last3=Weinrich |first3=M.
 |year=1957
 |title=Observations of the failure of conservation of parity and charge conjugation in meson decays: the magnetic moment of the free muon
 |url=http://www.fas.org/rlg/021557%20Garwin-Lederman-Weinrich.pdf
 |journal=[[Physical Review]]
 |volume=105 |issue=4 |pages=1415–1417
 |bibcode=1957PhRv..105.1415G
 |doi=10.1103/PhysRev.105.1415
 |doi-access=free
}}</ref><ref>{{cite journal
 |last1=Ambler
 |first1=E.
 |last2=Hayward
 |first2=R. W.
 |last3=Hoppes
 |first3=D. D.
 |last4=Hudson
 |first4=R. P.
 |last5=Wu
 |first5=C. S.
 |year=1957
 |title=Further Experiments on Decay of Polarized Nuclei
 |url=http://puhep1.princeton.edu/~kirkmcd/examples/EP/ambler_pr_106_1361_57.pdf
 |journal=[[Physical Review]]
 |volume=106
 |issue=6
 |pages=1361–1363
 |bibcode=1957PhRv..106.1361A
 |doi=10.1103/PhysRev.106.1361
 |access-date=2013-11-24
 |archive-date=2013-12-03
 |archive-url=https://web.archive.org/web/20131203040203/http://puhep1.princeton.edu/~kirkmcd/examples/EP/ambler_pr_106_1361_57.pdf
 |url-status=dead
 }}</ref> while others reacted with disbelief at the results. [[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!"<ref name=Hudson/> By the end of 1957, further research confirmed the original results of Wu's group, and ''P''-violation was firmly established.<ref name=Hudson/>

==Mechanism and consequences==
[[Image:Beta Negative Decay.svg|thumb|right|The [[Feynman diagram]] for {{SubatomicParticle|Beta-}}&nbsp;decay of a [[neutron]] into a [[proton]], [[electron]], and [[electron antineutrino]] via an intermediate [[W boson|{{SubatomicParticle|W boson-}} boson]].]]
The results of the Wu experiment provide a way to [[operational definition|operationally define]] the notion of left and right. This is inherent in the nature of the weak interaction. Previously, if the scientists on Earth were to communicate with a newly discovered planet's scientist, and they had never met in person, it would not have been possible for each group to determine unambiguously the other group's left and right. With the Wu experiment, it is possible to communicate to the other group what the words left and right mean exactly and unambiguously. The Wu experiment has finally solved the [[Ozma problem]] which is to give an unambiguous definition of left and right scientifically.<ref name="Gardner2005">
{{cite book
 |last=Gardner |first=M.
 |year=2005
 |title=The New Ambidextrous Universe: Symmetry and Asymmetry from Mirror Reflections to Superstrings
 |pages=[https://archive.org/details/newambidextrousu00mart/page/215 215–218]
 |edition=3rd Revised
 |publisher=[[Courier Corporation]]
 |isbn=978-0-486-44244-0
 |url-access=registration
 |url=https://archive.org/details/newambidextrousu00mart/page/215
 }}</ref>
 
At the [[fundamental particle|fundamental]] level (as depicted in the [[Feynman diagram]] on the right), Beta decay is caused by the conversion of the negatively charged ({{nowrap|−{{sfrac|1|3}} [[elementary charge|e]]}}) [[down quark]] to the positively charged ({{nowrap|+{{sfrac|2|3}} e}}) [[up quark]] by emission of a [[W boson|{{SubatomicParticle|W boson-}} boson]]; the {{SubatomicParticle|W boson-}} boson subsequently decays into an electron and an electron antineutrino:
:{{Subatomic particle|down quark}} → {{Subatomic particle|up quark}} + {{SubatomicParticle|Electron}} + {{SubatomicParticle|Electron Antineutrino}}.

The quark has a [[Chirality (physics)|left]] part and a [[Chirality (physics)|right]] part. As it walks across the spacetime, it oscillates back and forth from right part to left part and from left part to right part. From analyzing the Wu experiment's demonstration of parity violation, it can be deduced that only the left part of down quarks decay and the weak interaction involves only the left part of quarks and leptons (or the right part of antiquarks and antileptons). The right part of the particle simply does not feel the weak interaction. If the down quark did not have mass it would not oscillate, and its right part would be quite stable by itself. Yet, because the down quark is massive, it oscillates and decays.<ref name="LedermanHill2013">
{{cite book
 |last1=Lederman |first1=L. M.
 |last2=Hill |first2=C. T.
 |year=2013
 |title=Beyond the God Particle
 |pages=125–126
 |publisher=[[Prometheus Books]]
 |isbn=978-1-61614-802-7
}}</ref>

Overall, as <math>J_\text{Co}=5</math>, the strong magnetic field vertically polarizes the {{Nuclide|Cobalt|60}} nuclei such that <math>J_{z,\text{Co}}=J_\text{Co}=+5</math>. Since <math>J_\text{Ni}=4</math> and the decay conserves [[angular momentum]], <math>J_{z,\text{Co}}=J_{z,\text{Ni}} + J_{z,e^-} + J_{z,\nu}=+5</math> implies that <math>J_{z,e^-}=J_{z,\nu}=+1/2</math>. Thus, the concentration of [[beta ray]]s in the negative-z direction indicated a preference for left-handed quarks and electrons.

From experiments such as the Wu experiment and the [[Goldhaber experiment]], it was determined that massless neutrinos must be left-handed, while massless antineutrinos must be right-handed. Since it is currently known that neutrinos have a small mass, it has been proposed that right-handed neutrinos and left-handed antineutrinos could exist. These neutrinos would not couple with the weak [[Lagrangian (field theory)|Lagrangian]] and would interact only gravitationally, possibly forming a portion of the [[dark matter]] in the universe.<ref name=RHNeutrinosReview>
{{cite journal
 |last1=Drewes |first1=M.
 |year=2013
 |title=The Phenomenology of Right Handed Neutrinos
 |journal=[[International Journal of Modern Physics E]]
 |volume=22 |issue=8 |pages=1330019–593
 |arxiv=1303.6912
 |bibcode=2013IJMPE..2230019D
 |doi=10.1142/S0218301313300191
|s2cid=119161526
 }}</ref>

==Impact and influence==
The discovery set the stage for the development of the [[Standard Model]], as the model relied on the idea of symmetry of particles and forces and how particles can sometimes break that symmetry.<ref>{{Cite journal|last=Cho|first=Adrian|title=Postage stamp to honor female physicist who many say should have won the Nobel Prize|journal=Science|url=https://www.science.org/content/article/postage-stamp-honor-female-physicist-who-many-say-should-have-won-nobel-prize|date=2021-02-05|access-date=2021-02-01|doi=10.1126/science.abg9557|s2cid=234022248}}</ref>{{sfn|Chiang|2014|p=142}} The wide coverage of her discovery prompted the discoverer of fission [[Otto Robert Frisch]] to mention that people at Princeton would often say that her discovery was the most significant since the [[Michelson–Morley experiment]] that inspired Einstein's [[theory of relativity]].{{sfn|Gardner|2005|p=[https://books.google.com/books?id=kdLG_a8cPPUC&pg=PA217 217]}} The [[American Association of University Women|AAUW]] called it the “solution to the number-one riddle of atomic and nuclear physics.”<ref>{{Cite web|title=Chien-Shiung Wu Overlooked for Nobel Prize|url=https://www.aauw.org/resources/faces-of-aauw/chien-shiung-wu-overlooked-for-nobel-prize/}}</ref> 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.<ref>{{Cite web|title=Chien-Shiung Wu, Physicist Who Helped Change The World|url=https://diversity.lbl.gov/2015/05/19/chien-shiung-wu-physicist-who-helped-change-the-world/|date=2015-05-19}}</ref> This violation meant researchers could distinguish matter from antimatter and create a solution that would explain the existence of the universe as one that is filled with matter.<ref>{{Cite web|title=Antimatter|url=https://home.cern/science/physics/antimatter|date=2021-03-01}}</ref> This is since the lack of symmetry gave the possibility of [[Baryon asymmetry|matter-antimatter imbalance]] which would allow matter to exist today through the [[Big Bang]].<ref>{{Cite web|last=Sutton |first=Christine|title=CP violation |date=1998-07-20 |url=https://www.britannica.com/science/CP-violation}}</ref> In recognition of their theoretical work, Lee and Yang were awarded the [[Nobel Prize for Physics]] in 1957.<ref>{{cite web |title=The Nobel Prize in Physics 1957 |publisher=The Nobel Foundation |url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1957/ |access-date=March 24, 2015}}</ref> To further quote the impact it had, Nobel laureate [[Abdus Salam]] quipped, <blockquote>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. In my view what we have found is that space is a weak left-eyed giant.{{sfn|Gardner|2005|p=[https://books.google.com/books?id=kdLG_a8cPPUC&pg=PA218 218]}}</blockquote> Wu's discovery would pave the way for a unified [[electroweak force]] that Salam proved, which is theoretically described to merge with the strong force to create a total new model and a [[Grand Unified Theory]].

==See also==
* ''[[The Ambidextrous Universe]]'' by [[Martin Gardner]]; book containing a lengthy popular discussion of parity and the Wu experiment
* [[Fermi's interaction]]

==References==
{{reflist}}

==Further reading==
*{{cite web
 |last1=Martin |first1=W. C.
 |last2=Coursey |first2=J.
 |last3=Dragoset |first3=R. A.
 |date=July 1997
 |title=The Fall of Parity
 |url=https://www.nist.gov/pml/general/parity/index.cfm
 |publisher=[[National Institute of Standards and Technology|NIST Physical Measurement Laboratory]]
}}

[[Category:Chien-Shiung Wu]]
[[Category:Electroweak theory]]
[[Category:Physics experiments]]
[[Category:1956 in science]]
[[Category:1956 in Washington, D.C.]]
[[Category:Asymmetry]]