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[[Image: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 '''Wu experiment''' was a [[particle 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
 |url=http://iktp.tu-dresden.de/uploads/media/Experimental_test_of_parity_in_beta_decay_-_Wu.pdf
 |journal=[[Physical Review]]
 |volume=105 |issue=4 |pages=1413–1415
 |bibcode=1957PhRv..105.1413W
 |doi=10.1103/PhysRev.105.1413
}}</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 interaction|electromagnetic]] and [[strong interaction|strong]] [[fundamental interaction|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.  This result was not expected by the physics community, which had previously regarded parity as a [[conserved quantity]].  [[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. 

== History ==
{{multiple image
| direction = vertical 

| image1    = Parity clocks - P-conservation.svg<!-- file name only; no "File:" or "Image:" prefix -->
| width1    = 300

| image2    = Parity clocks - P-violation.svg<!-- file name only; no "File:" or "Image:" prefix -->
| width2    = 300

| 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= |issue= |pages=375–381
 |bibcode=
 |doi=
}}
:Reproduced in {{cite book
 |last=<!-- --> |first=<!-- -->
 |chapter=<!-- -->
 |editor1-last=Wightman |editor1-first=A. S.
 |year=1993
 |title=The Collected Works of Eugene Paul Wigner
 |volume=Vol. A |pages=84
 |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 you built a mirrored version of it).

This principle was widely accepted by physicists, and ''P''-conservation was experimentally verified in the [[electromagnetic interaction|electromagnetic]] and [[strong interaction|strong]] [[fundamental interaction|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]].<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
|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
 |url=http://iktp.tu-dresden.de/uploads/media/Parity_conservation_in_weak_interactions_-_Lee_Yang.pdf
 |journal=[[Physical Review]]
 |volume=104 |issue=1 |pages=254–258
 |bibcode=1956PhRv..104..254L
 |doi=10.1103/PhysRev.104.254
}}</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 thereafter contacted [[Henry Boorse]] and [[Mark W. Zemansky]], who had extensive experience in [[low-temperature physics]]. 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 December 1956 in the NBS' low-temperature laboratories.<ref name=Hudson/>

Lee and Yang, who prompted the Wu experiment, were awarded the Nobel prize in physics in 1957, shortly after the experiment was performed.

== The experiment ==
[[File:Wu experiment.jpg|thumb|275px|Principle of Wu experiment 1956 to detect parity violation in nuclear beta decay]]
The experiment itself monitored the decay of [[cobalt-60]] atoms, cooled to near [[absolute zero]] and aligned in a uniform magnetic field.<ref name=Hudson/> Cobalt-60 (<sup>60</sup>Co) 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>). This changes the cobalt-60 nucleus into a nickel-60 nucleus. 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 nuclei is an [[Electromagnetism|electromagnetic]] (EM) process. This is important because EM was known to respect P-conservation. Hence, the distribution of the emitted gamma rays acted as a [[Scientific control|control]] for the polarization of the emitted electrons ''via'' the weak interaction, as well as an indicator of the uniformity of the cobalt-60 atoms. Wu's experiment compared the distribution of gamma and electron emissions with the nuclear spins in opposite orientations. If the electrons were always found to be emitted in the same direction and in the same proportion as the gamma rays, ''P''-conservation would be true. If there were a bias in the direction of decays, that is, if the distribution of electrons did not follow the distribution of the gamma rays, then ''P''-violation would be established.

===Materials and methods===
[[File:Wu-Experiment (English).png|thumb|275px|Schematic illustration 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 (compared with electrons) very small magnetic moments of the nuclei, high 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 lowered 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===
In the experiment carried out by Wu, the gamma ray polarization was approximately 60%.<ref name="Wu1957"/> That is, approximately 60% of the gamma rays were emitted in one direction, whereas 40% were emitted in the other. If ''P''-conservation were true in beta decay, electrons would have that same 60%/40% breakdown. However, not only did Wu observe that there was a mismatch between the directions in which the electrons were emitted relative to the gamma rays, but that most of the electrons favored a very specific direction of decay, opposite to that of the nuclear spin.<ref name="Wu1957"/> It was later established that ''P''-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"
 |url=http://link.springer.com/content/pdf/10.1007%2Fs10773-006-9168-2.pdf
 |journal=[[International Journal of Theoretical Physics]]
 |volume=45 |issue=11 |pages=1993–2050
 |bibcode=2006IJTP...45.1993Z
 |doi=10.1007/s10773-006-9168-2
}}</ref>

The results greatly surprised the physics community.<ref name=Hudson/> 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
}}</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
}}</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 ''P''-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==
The parity violation observed in this experiment resulted from the [[W and Z bosons|W and Z]] [[gauge boson]]s of the [[weak interaction]] only interacting with [[Chirality (physics)|left-handed]] matter particles and right-handed antimatter particles. The combined parity and spin operations generate the [[Helicity (particle physics)|helicity]] of the particle.{{citation needed|date=November 2013}}

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]] and would interact only gravitationally, possibly forming a portion of the [[dark matter]] in the universe.{{citation needed|date=November 2013}}

Although ''P''-violation had been shown, it was believed that the [[operator (mathematics)|operator]] ''CP'', the combination of spatial reflection (''P'') and [[charge conjugation]] (''C''), formed an unbroken [[CP-symmetry]] (''CP''-conservation). ''CP''-conservation would imply a perfect symmetry between matter and antimatter, and had been shown to hold for all [[lepton]] interactions in the [[Standard Model]].{{citation needed|date=November 2013}} However, [[CP-violation|''CP''-violation]] is now known to occur in [[strong interaction|strongly interacting]] particles and has been observed extensively in [[Kaon#CP violation|kaon]] and [[B–Bbar oscillation|B meson]] decays.{{citation needed|date=November 2013}} In order to restore an unbroken symmetry, [[time-reversal symmetry]] (''T'') must be invoked to create a combined ''CPT''-symmetry, which has, thus far, been found to be [[CPT theorem|conserved in all known interactions]].<ref name=Potting2013>
{{cite journal
 |last1=Potting |first1=R.
 |year=2013
 |title=Lorentz and CPT violation
 |journal=[[Journal of Physics: Conference Series]]
 |volume=447 |issue=1 |pages=012009
 |bibcode=2013JPhCS.447a2009P
 |doi=10.1088/1742-6596/447/1/012009
}}</ref>

== See also ==
* [[Beta decay]]
* [[Neutrino]]
* [[Fermi's interaction]]
* [[Weak interaction]]
* [[Electroweak interaction]]

== References ==
{{reflist}}

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