Editing Weak interaction (section)

== Violation of symmetry ==
[[File:Right left helicity.svg|thumb|right|280px|[[Chirality (physics)|Left- and right-handed particles]]: {{mvar|p}} is the particle's momentum and {{mvar|S}} is its [[Spin (physics)|spin]]. Note the lack of reflective symmetry between the states.]]
The [[physical law|laws of nature]] were long thought to remain the same under mirror [[reflection (physics)|reflection]]. The results of an experiment viewed via a mirror were expected to be identical to the results of a separately constructed, mirror-reflected copy of the experimental apparatus watched through the mirror. This so-called law of [[parity (physics)|parity]] [[Conservation law (physics)|conservation]] was known to be respected by classical [[gravitation]], [[electromagnetism]] and the [[strong interaction]]; it was assumed to be a universal law.<ref>
{{cite book
 |first=Charles W. |last=Carey
 |year=2006
 |chapter=Lee, Tsung-Dao
 |title=American scientists
 |page=225
 |publisher=Facts on File Inc.
 |isbn=9781438108070
 |chapter-url=https://books.google.com/books?id=00r9waSNv1cC&pg=PA225
 |via=Google Books
}}
</ref> However, in the mid-1950s [[Chen-Ning Yang]] and [[Tsung-Dao Lee]] suggested that the weak interaction might violate this law. [[Chien Shiung Wu]] and collaborators in 1957 discovered that the weak interaction violates parity, earning Yang and Lee the [[Nobel Prize in Physics#1950s|1957 Nobel Prize in Physics]].<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=1957
 |website=NobelPrize.org
 |publisher=Nobel Media
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1957/
 |access-date=26 February 2011
}}
</ref>

Although the weak interaction was once described by [[Fermi's theory]], the discovery of parity violation and [[renormalization]] theory suggested that a new approach was needed. In 1957, [[Robert Marshak]] and [[George Sudarshan]] and, somewhat later, [[Richard Feynman]] and [[Murray Gell-Mann]] proposed a '''V&nbsp;−&nbsp;A''' ([[vector (geometry)|vector]] minus [[axial vector]] or left-handed) [[Lagrangian (field theory)|Lagrangian]] for weak interactions. In this theory, the weak interaction acts only on left-handed particles (and right-handed antiparticles). Since the mirror reflection of a left-handed particle is right-handed, this explains the maximal violation of parity. The ''V&nbsp;−&nbsp;A'' theory was developed before the discovery of the Z&nbsp;boson, so it did not include the right-handed fields that enter in the neutral current interaction.

However, this theory allowed a compound symmetry '''[[CP violation|CP]]''' to be conserved. '''CP''' combines parity '''P''' (switching left to right) with charge conjugation '''C''' (switching particles with antiparticles). Physicists were again surprised when in 1964, [[James Cronin]] and [[Val Fitch]] provided clear evidence in [[kaon]] decays that ''CP'' symmetry could be broken too, winning them the 1980 [[Nobel Prize in Physics]].<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=1980
 |website=NobelPrize.org
 |publisher=Nobel Media
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/1980/
 |access-date=26 February 2011
}}
</ref> In 1973, [[Makoto Kobayashi (physicist)|Makoto Kobayashi]] and [[Toshihide Maskawa]] showed that ''CP'' violation in the weak interaction required more than two generations of particles,<ref name="KM">
{{cite journal
 |first1=M. |last1=Kobayashi
 |first2=T. |last2=Maskawa
 |year=1973
 |title=CP-Violation in the Renormalizable Theory of Weak Interaction
 |journal=[[Progress of Theoretical Physics]]
 |volume=49  |issue=2  |pages=652–657
 |doi=10.1143/PTP.49.652  |doi-access=free
 |bibcode = 1973PThPh..49..652K  |hdl=2433/66179
 |url=http://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/66179/1/49_652.pdf
}}
</ref> effectively predicting the existence of a then unknown third generation. This discovery earned them half of the 2008 Nobel Prize in Physics.<ref>
{{cite web
 |title=The Nobel Prize in Physics
 |year=2008
 |website=NobelPrize.org
 |publisher=Nobel Media
 |url=http://nobelprize.org/nobel_prizes/physics/laureates/2008/
 |access-date=17 March 2011
}}
</ref>

Unlike parity violation, ''CP''&nbsp;violation occurs only in rare circumstances. Despite its limited occurrence under present conditions, it is widely believed to be the reason that there is much more matter than [[antimatter]] in the universe, and thus forms one of [[Andrei Sakharov]]'s three conditions for [[baryogenesis]].<ref>
{{cite book
 |author-last=Langacker |author-first=Paul 
 |orig-year=1989
 |chapter=CP violation and cosmology
 |editor-last=Jarlskog  |editor-first=Cecilia 
 |year=2001
 |title=CP Violation
 |page=552
 |location=London, [[River Edge]]
 |publisher=World Scientific Publishing Co.
 |isbn=9789971505615
 |url=https://books.google.com/books?id=U5TC5DSWOmIC
 |via=Google Books
}}
</ref>