# Particle Physics: Why are Mesons a type of Gauge Boson?

1. Sep 24, 2011

### MarcAlexander

If a Meson is a Hadron that contains a Quark and an Anti-Quark which are both Fermions then why are Mesons a type of Gauge Boson?

2. Sep 24, 2011

### Staff: Mentor

Which mesons are you thinking of? I don't know of any that are also gauge bosons.

In the Standard Model, the gauge bosons are the photon, Z0, W+, W- and the gluons. None of these are mesons.

3. Sep 24, 2011

### MarcAlexander

According to Wikipedia, Mesons are Bosonic. Maybe that just means they have properties like Bosons.

4. Sep 24, 2011

You are mixing bosons and gauge bosons.
Boson is a general name for particles with integral spin (0, 1, 2). They can be elementary or composite particles. Mesons are composite particles with spin 1 so they are bosons. Gauge boson is a boson mediating interaction in a gauge theory. I'm not sure if they can be composite, but the known gauge bosons are all elementary. Photon, gluon, Z, W+, W-.

EDIT: The spin of a composite particle is "sum" of the spins of constituent particles. Though this is not ordinary sum, with mesons it's pretty simple. Each quark has spin 1/2. The spins can be either parallel (you add spins) or anti-parallel (you subtract spins). Thus a two quark particle can have spin either 0 or 1.

Last edited: Sep 24, 2011
5. Sep 24, 2011

### MarcAlexander

Thank you. I now have another query: I thought Gauge Boson and Higgs Boson(theoretical) were types of Boson, if so then how can a Meson be a Boson and the Bosons on the Standard Model(Photon, Gluon, Z, W+, W-) be Bosons??

I'm just confused at what the difference is in the terms themselves.

6. Sep 24, 2011

### Staff: Mentor

Boson and meson are parts of different types of classification.

One classification is bosons (spin 0, 1, 2, etc.) versus fermions (spin 1/2, 3/2, etc.).

Another classification is leptons (fundamental pointlike particles: electron, mu, tau, neutrino) versus hadrons (combinations of quarks) versus gauge bosons. Hadrons are further sub-classified into mesons (quark plus antiquark) and baryons (three quarks or three antiquarks).

All leptons are fermions. All gauge bosons are bosons. Mesons are all bosons because they're two-quark combinations, quarks have spin 1/2, and two spin 1/2 particles can combine to give only total spin 0 or 1. Baryons are all fermions because they're three-quark combinations and three spin 1/2 particles can give only total spin 1/2, 3/2, etc.

7. Sep 24, 2011

Particles are divided into two large groups:
Bosons - spin 0, 1, 2, 3, ...
Fermions - spin 1/2, 3/2, 5/2, 7/2, ...
So it's just a question of spin. Photon has spin 1 - it's a boson, electron has spin 1/2 - it's a fermion, Higgs has spin 0 - it's a boson, proton has spin 1/2 - it's a fermion, Pi meson has spin 0 - it's a boson, etc.
Gauge bosons are special family of bosons (a subset of all bosons).

8. Sep 24, 2011

### MarcAlexander

What exactly is spin, besides a property of a particle? As in what affect does it have on the particle?

9. Sep 27, 2011

### kurros

It is not easy to explain what spin is, but the most important thing it affects is whether or not a particle obeys the Pauli exclusion principle. Spin half particles (fermions) cannot doubly occupy a quantum state, so you can't put two electrons in the same atomic orbital and you can't put your hand through walls etc. Spin one particles you can do such things with no problem, in analogous situations. In superconductors for instance the electrons pair up into composite particles with spin 1, allowing them all to occupy the same ground state.

On a more mundane note spin is a type of angular momentum, as the classical-sounding name suggests. If you have charge it gives you a magnetic moment. It affects a lot of things.