Editing Penguin diagram

{{short description|Class of Feynman diagrams}}
[[Image:Penguin diagram.JPG|thumb|Example of a penguin diagram superimposed on an image of a [[Gentoo penguin]].]] 
In [[quantum field theory]], '''penguin diagrams''' are a class of [[Feynman diagrams]] which are important for understanding [[CP violation|CP violating]] processes in the [[standard model of particle physics|standard model]]. They refer to [[one-loop Feynman diagram|one-loop process]]es in which a [[quark]] temporarily changes [[Quark flavor|flavor]] (via a W or Z loop), and the flavor-changed quark engages in some tree interaction, typically a [[Strong interaction|strong one]]. For the interactions where some quark flavors (e.g., very heavy ones) have much higher interaction amplitudes than others, such as CP-violating or [[Higgs mechanism|Higgs interaction]]s, these penguin processes may have amplitudes comparable to or even greater than those of the direct [[tree process]]es. A similar diagram can be drawn for [[lepton]]ic decays.<ref>{{cite web |author=Flip Tanedo |title=Dissecting the Penguin |date=2012-03-19 |url=http://www.quantumdiaries.org/2012/03/19/dissecting-the-penguin/ |website=Quantum Diaries |access-date=2015-04-30 |archive-url=https://web.archive.org/web/20151125034902/http://www.quantumdiaries.org/2012/03/19/dissecting-the-penguin/ |archive-date=2015-11-25 |url-status=live }}</ref>

They were first isolated and studied by [[Mikhail Shifman]], [[Arkady Vainshtein]], and Valentin Zakharov.<ref>{{cite journal |last1=Vainshtein |first1=A. I. |last2=Zakharov |first2=V. I. |last3=Shifman |first3=M. A. |title=A possible mechanism for the ΔT = 1/2 rule in nonleptonic decays of strange particles |journal=[[Pisma Zh. Eksp. Teor. Fiz.]] |volume=22 |page=123 |year=1975 |bibcode=1975ZhPmR..22..123V }}
<br/>Translated in {{cite journal |last1=Vainshtein |first1=A. I. |last2=Zakharov |first2=V. I. |last3=Shifman |first3=M. A. |title=A possible mechanism for the ΔT = 1/2 rule in nonleptonic decays of strange particles |journal=[[JETP Letters]] |volume=22 |page=55 |year=1975 |bibcode=1975JETPL..22...55V }}</ref><ref>{{cite journal |journal=[[Nuclear Physics B]] |volume=120 |issue=2 |page=316 |year=1977 |bibcode=1977NuPhB.120..316S |title=Asymptotic freedom, light quarks and the origin of the ΔT = 1/2 rule in the non-leptonic decays of strange particles |last1=Shifman |first1=M. A. |last2=Vainshtein |first2=A. I. |last3=Zakharov |first3=V. I. |doi=10.1016/0550-3213(77)90046-3 }}</ref>

The processes which they describe were first directly observed in 1991 and 1994 by the [[CLEO (particle detector)|CLEO]] collaboration.{{citation needed|date=May 2021}}

== Origin of the name ==
[[John Ellis (physicist, born 1946)|John Ellis]] was the first to refer to a certain class of Feynman diagrams as "penguin diagrams" in a 1977 paper on b-quarks.<ref>{{Cite journal|url=https://doi.org/10.1016/0550-3213(77)90374-1|doi = 10.1016/0550-3213(77)90374-1|title = The phenomenology of the next left-handed quarks|year = 1977|last1 = Ellis|first1 = J.|last2 = Gaillard|first2 = M.K.|last3 = Nanopoulos|first3 = D.V.|last4 = Rudaz|first4 = S.|journal = Nuclear Physics B|volume = 131|issue = 2–3|pages = 285–307|bibcode = 1977NuPhB.131..285E}}</ref> The name came about in part due to their shape, and in part due to a legendary bar-room bet with [[Melissa Franklin]]. According to John Ellis:<ref name="ellis">{{cite arXiv |eprint=hep-ph/9510397 |author1=Mikhail Shifman |title=ITEP Lectures in Particle Physics |year=1995}}</ref>

{{blockquote|
"[[Mary K. Gaillard|Mary K. [Gaillard]]], [[Dimitri Nanopoulos|Dimitri [Nanopoulos]]] and I first got interested in what are now called penguin diagrams while we were studying [[CP violation]] in the [[Standard Model]] in 1976... The penguin name came in 1977, as follows.

In the spring of 1977, [[Mike Chanowitz]], Mary K and I wrote a paper on [[grand unification theory|GUTs]] predicting the [[bottom quark|b quark]] mass before it was found. When it was found a few weeks later, Mary K, Dimitri, [[Serge Rudaz]] and I immediately started working on its phenomenology. That summer, there was a student at [[CERN]], [[Melissa Franklin]] who is now an experimentalist at Harvard. One evening, she, I, and Serge went to a pub, and she and I started a game of darts. We made a bet that if I lost I had to put the word [[penguin]] into my next paper. She actually left the darts game before the end, and was replaced by Serge, who beat me. Nevertheless, I felt obligated to carry out the conditions of the bet.

For some time, it was not clear to me how to get the word into this b quark paper that we were writing at the time. Then, one evening, after working at CERN, I stopped on my way back to my apartment to visit some friends living in [[Meyrin]] where I smoked some illegal substance. Later, when I got back to my apartment and continued working on our paper, I had a sudden flash that the famous diagrams look like penguins. So we put the name into our paper, and the rest, as they say, is history."<ref name="ellis"/>
}}

==See also==
*[[John Ellis (physicist, born 1946)|John Ellis]]

==References==
{{reflist|1}}

{{DEFAULTSORT:Penguin Diagram}}
[[Category:Electroweak theory]]
[[Category:Diagrams]]