Some Musings About Nomeclature, Mesons And Forces

Almost all of the ordinary matter in the universe (as opposed to "dark matter") is made up of protons and neutrons assembled together into atomic nuclei of atoms with one or more protons and sometimes with sometimes also with some neutrons each.

The atom is completed with one electron per proton in orbit around the nucleus (strictly speaking, "orbit" is a bit misleading as a classical approximation of the actual quantum physics behavior of electrons associated with an atomic nucleus, but it is good enough for the purposes of this post).  If the correspondence between protons and electrons in an "atom" is other than one to one, it is conventional to use the term "ion" rather than "atom" to describe it.

Protons and neutrons are composite particles made up of three quarks each, which are bound together by gluons which are emitted and absorbed by the quarks in the proton or neutron respectively (the generic name that encompasses both protons and neutrons is a "nucleon" often abbreviated "N").

It turns out that protons and neutrons are actually just the two most common examples of a larger class of composite particles made up of three quarks bound together by gluons which are call "baryons".

There is a still more general class of composite particles made up of quarks (not necessarily three) that are bound together by gluons which are called "hadrons" (there is also a hypothetical class of composite particles made up solely of gluons bound together in the absence of quarks often called "glueballs", but I'm not sure if they count as hadrons or not).

Hadrons made up of two quarks (or of blended combinations of two quark pairs) are called "mesons", a term that was coined in the 1930s when the need for a force carrying particle to bind protons in atomic nuclei was hypothesized many years before the first meson was actually observed.

Mesons made up of two different kinds of quarks are usually named based upon the heaviest quark in the mesons.  When that quark is a bottom quark (formerly also known as a beauty quark), the meson is, logically, known as a B meson.  But, most of the lighter mesons were discovered and named before the quark theory of the Standard Model was worked out and the quarks were assigned names.  So, their names are quite arbitrary.

If the heaviest quark in a meson is a charm quark it is usually known as a D meson (but prior to 1986, a meson with a charm quark and a strange quark was known as an F meson). It is also curious that the physics community managed to abolish the historical irregular name for the D_s meson, but not many other of the historical irregular names of other hadrons.

If the heaviest quark in a meson is a strange quark, it is usually known as a "kaon" abbreviated "K".

If we were renaming mesons today, knowing what we do about the Standard Model, they would probably have been called S mesons, C mesons, and B mesons, respectively. But, historical accident and the immense amount of education needed to do the physics that makes knowing these names relevant has allowed the irregular historical monikers to survive.

Different naming conventions apply to "quarkonia", in which a meson and antimeson of the same flavor are both present.

I'd welcome comments from anyone who can explain how it is that Kaons, D mesons, F mesons and any of the other irregular hardon names were assigned (baryon naming, for what it is worth, has fewer irregular hadron names, perhaps because more of them were discovered after the quark model was in place).

As in other areas of language, irregular names for hadrons seem to have persisted more strongly for the most common mesons, than for the rare ones.

We still have arbitrary meson names not precisely tied to quark content for scalar and axial vector mesons, whose quark content is not well understood.

There are several kinds of mesons that contain only up and down quarks (or are at least dominantly comprised of up and down quarks).  The lightest are the pi mesons, also known as pions.  Another kind of meson containing only (or at least dominantly) up and down quarks is the rho meson.

Pions and rhos bring us from the land of history and language in physics to the land of QCD (the physics of the strong force that binds quarks with gluons called "quantum chromodynamics").

It turns out that the force hypothesized back in the 1930s that binds protons and neutrons into atomic nuclei called the "nuclear strong force" is not a fundamental force of nature.  Instead, it is basically a second order effect of the fundamental strong force which is mediated by gluons to hold hadrons together "leaking" out of hadrons to bind them to other nearby hadrons.

The nuclear strong force is mediated not by gluons, which are fundamental in the Standard Model, but by mesons, which are composite particles, although, like gluons, mesons are bosons (a class of particles that has one kind of quantum mechanical behavior) rather than fermions (a class of particles including all baryons and quarks and leptons such as electrons that has another kind of quantum mechanical behavior).

What caught my eye as I was looking into the history of the odd nomenclature of mesons was that while the nuclear strong force is mediated primarily (virtual) pions, it is also mediated in part by other kinds of mesons, especially (virtual) rhos.

This made me think about different kinds of mediator particles for different forces. Electromagnetism is mediated by photons, which are mostly identical, but can differ from each other in frequency, polariziation or helicity.  The strong force is mediated by gluons, which are also mostly identical to each other, but can come in eight different combinations of color charges.

The weak force, in contrast, is mediated by both W bosons (which come in W+ and W- varieties that are antiparticles of each other), and Z bosons which differ in mass and charge from W bosons.  In this respect, the weak force is a bit like the nuclear strong force, which has more than one kind of mediating boson, although the non-fundamental and emergent nuclear strong force, in principle at least within the Standard Model, can have more kinds of potential mediator mesons than the weak force had mediator weak force bosons.

It is interesting to consider how the Standard Model might be subtly modified to reflect the existence of additional weak force bosons that appear much less frequently the W and Z bosons in much the way that rhos and other mesons mediate the nuclear strong force much less frequently than pions do.

There are hypothetical W' and Z' particles which have been searched for experimentally (and thus far, not discovered).  But, it isn't entirely clear that those models are sufficient to account for the kinds of properties we would predict if the collection of particles that mediate the weak force are analogous to the array of mesons that can mediate the nuclear strong force.

Also, the analogy of the nuclear strong force to the weak force suggests that the W and Z bosons, unlike photons and gluons, might be composite particles, rather than being fundamental. (Electroweak unification involves a concept of blending of more fundamental particles to create the W, the Z and the photon, as opposed to a true composite concept.)  This seems like an interesting line of conjecture to extend to see how far one could take it.

And, of course, we've omitted discussion of the Higgs boson, which shows every indication of being basically a part of the electroweak unification scheme that is not closely related to QCD at all, and gravity, which just plain doesn't play well with the Standard Model, but mostly manages to stay out of the way in circumstances where gravity and the Standard Model might clash with each other.