In wikipedia page about hadrons the following image appears:

enter image description here

I can understand why the intersection between hadrons and fermions are baryons, as a way to say a baryon is a kind of hadron composed of several quark fermions.

However, what is the meaning of the intersection between hadron and bosons labeled in the picture as mesons? If I understand correctly, a meson consists of one quark and one antiquark, nothing related to any boson.


Since a meson is composed of two spin 1/2 particles, its total spin must be an integer, which makes it a boson.


(Anti)quarks are spin 1/2, so two of them is either spin-0 or spin 1: bosons.

But it's more than that. The pions are (one of) the force carrying boson in quantum hadrodynamics, which is an effective field theory of nuclear interactions. Moreover, reactions like:

$$ p \rightarrow n + \pi^+ $$

look a lot like:

$$ \nu_e \rightarrow e^- + W^+ $$

in terms of coupling isospin and weak-isospin.

There is also something called "Vector Meson Dominance" used to describe higher energy photons' interaction with hadronic matter. Basically, the photon can be dressed as vector (spin-1) meson: $\rho$, $\omega$, and $\phi$. At the quark level, this is:

$$ \gamma \rightarrow q\bar q $$

while it the effective field theory level, the vector mesons are mixed into the photon state.

So they are very much bosons.


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