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 violating processes in the standard model. They refer to one-loop processes in which a quark temporarily changes flavor (via a W or Z loop), and the flavor-changed quark engages in some tree interaction, typically a 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 interactions, these penguin processes may have amplitudes comparable to or even greater than those of the direct tree processes. A similar diagram can be drawn for leptonic decays.[1]
The processes which they describe were first directly observed in 1991 and 1994 by the CLEO collaboration.[citation needed]
Origin of the name
John Ellis was the first to refer to a certain class of Feynman diagrams as "penguin diagrams" in a 1977 paper on b-quarks.[4] 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:[5]
In the spring of 1977, Mike Chanowitz, Mary K and I wrote a paper on GUTs predicting the 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."[5]
^Flip Tanedo (2012-03-19). "Dissecting the Penguin". Quantum Diaries. Archived from the original on 2015-11-25. Retrieved 2015-04-30.
^Vainshtein, A. I.; Zakharov, V. I.; Shifman, M. A. (1975). "A possible mechanism for the ΔT = 1/2 rule in nonleptonic decays of strange particles". Pisma Zh. Eksp. Teor. Fiz.22: 123. Bibcode:1975ZhPmR..22..123V.
Translated in Vainshtein, A. I.; Zakharov, V. I.; Shifman, M. A. (1975). "A possible mechanism for the ΔT = 1/2 rule in nonleptonic decays of strange particles". JETP Letters. 22: 55. Bibcode:1975JETPL..22...55V.
