Wednesday, 12 June 2013

50 years of the Cabibbo mechanism

Fifty years ago this week, the journal Physical Review Letters published an article by Nicola Cabibbo entitled "Unitary Symmetry and Leptonic Decays".  It gave a working and quantitative theoretical description of how particles which interact by the strong interaction (one of the fundamental forces of nature) can decay according to the weak interaction (another of them).  It was already known that there were some patterns and rules that seemed to be obeyed, but Cabibbo gave a beautifully simple  explanation that also made successful predictions, and that went on to be extended and incorporated into what is now called The Standard Model of Particle Physics.

50 years ago, the number of observed particles in particle physics experiments was becoming so large that they no longer felt like the basic building blocks of nature.  Just like the complexity of chemistry had been simplified to the building blocks of elements, and the many elements had been explained by the properties of three constituents (protons, neutrons and electrons), so did the increasing number of observed particles suggest that they too must possess an underlying structure.   The patterns and regularity reinforced this idea.

These patterns included the fact the particles, most of which are unstable, seemed to decay in a rather uniform way, with understandable lifetimes.  Only, some of them did not, and these were called strange particles.  However, it appeared that at least the strange particles behaved rather similarly to other strange particles.

A breakthrough came when Murray Gell-Mann and Yuval Ne'eman independently spotted that if one ignored strangeness and electrical charge then the particles could be grouped into sets which more or less looked the same.  For example, a group of so-called mesons, consisting of mostly of things which now bear the name pions and kaons, could be considered as different manifestations of one kind of more fundamental particle, exhibiting different properties only because of their charge and strangeness (whatever that was).  Pretty soon, the quark model was proposed, and all known particles at the time were described in terms of only three underlying quarks, or their mirror-image antiquarks.  These three are known as the up (u), down (d) and strange (s) quarks.  We now say that particles have strangeness because they have a strange quark or antiquark, but the strangeness categorisation existed before the quark hypothesis.

Cabibbo's paper deals with the observed fact that decays in which the weak interaction is in play works in two different ways according to whether on not the strangeness changes during the decay.  In doing this he was able to suppose the very clear and unifying idea that the weak interaction behaves in a very universal way, both for particles that do not interact strongly (leptons, such as electrons and neutrinos) and those that do.  He said that, if there is a universal weak interaction strength G, then

strangeness conserving decays have weak coupling Gcosθc
strangeness changing decays have weak coupling Gsinθc

where θc is another universal parameter called the Cabibbo Angle (don't worry too much about the designation angle.  Partly you can think of it as just the obvious way to parameterise the process by a single number).  Leptons have a weak coupling G with no Cabibbo factor. 

Cabibbo's paper doesn't mention constituent quarks at all, as the paper slightly predates the concept, and today we would rather describe the Cabibbo mechanism via the direct coupling between the weak interaction bosons and the quarks, often drawing Feynman diagrams as I have done to illustrate this post.  We still use the Cabibbo angle, at least as a first approximation.  Cabibbo's key idea was further developed, when more quarks were discovered, into the CKM (Cabibbo-Kobayashi-Maskawa) matrix.  It describes the way the weak interaction interacts with mixed combinations of the strongly-interacting quarks, using generalisations of the Cabibbo angle.

It was not without some controversy when Kobayasi and Maskawa were awarded a share of the Nobel Prize in Physics in 2008, while Cabibbo was not.

As we remember the 50th anniversary of Cabibbo's paper, it is curious to note that Google's PageRank algorithm rates the importance of his paper #1 of all papers published in APS journals - above any Nobel Prize winner.

As a wistful final word, I note that Cabibbo's paper runs to just two pages.  That used to be common in Physical Review Letters.  Now practically every paper I see pushes the 4-page limit.  Or exceeds it.

----- citation appears below ----

Cabibbo, N. (1963). Unitary Symmetry and Leptonic Decays Physical Review Letters, 10 (12), 531-533 DOI: 10.1103/PhysRevLett.10.531