# The particle/antiparticle pairs in mesons as well as three quarks in baryons?

1. Jun 30, 2012

### WilliamJ

Hi,
Does anyone out there know the rules for mesons when it comes to the particle/antiparticle combinations? Is it simply like an up and anti-up quark that makes up a meson or is it like up and anti-down or down and anti-up? Or can it even be other combinations of the six quarks?
Also I have a question about baryons. I know that these are made up of three quarks, like up up down for a proton and a down down up for a neutron. Well, can it be the combination of any three types of quarks, like up up up or something like up down charm sttrange or some other combination? If someone can answer this and possibly show all of the possible combinations of quarks and anti-quarks in mesons and the possible combinations of quarks when it comes to baryons, I would really like to hear from you so I can be a little more informed than I already am. Another question is is that with protons and neutrons for example, if you have an anti-proton (a P with a bar over the top...called p-bar right? and an anti-neutron, a N with a bar over the top of it, do all of the signs change like the charge of an up quark like +2/3 turning into a -2/3 (which would be like an anti-up quark) or a -1/3 (which would be like an anti-down quark) turning into a +1/3 charge?
I would appreciate it if anyone out there would try and give a stab at the questions I have just presented.
Thank you,

2. Jun 30, 2012

### tom.stoer

A meson can be a combination of x and anti-y, where x and y are arbitary flavors (u,d,s,...). That means that any state like |x, anti-y> is allowed. There can even be linear superpositions of such states. Some simple combinations can be found for the pion and the kaon.

http://en.wikipedia.org/wiki/Pion
http://en.wikipedia.org/wiki/Kaon

Yes, this is possible. A baryon can be a combination of x, y and z, where x, y and z are arbitrary flavors. So any state like |xyz> is allowed.

Some simple combinations can be found for the nucleon and the Delta but there are many more

http://en.wikipedia.org/wiki/Nucleon
http://en.wikipedia.org/wiki/Delta_baryon
http://en.wikipedia.org/wiki/List_of_baryons

Yes, this is what happens. Every quark is replaced by an anti-quarkls with opposite electric charge and opposite color charge (which we never mentioned in this discussion).

3. Jun 30, 2012

### Staff: Mentor

Usually you can form all combinations, and many combinations even exist in multiple different particles (as they can have different quantum numbers). However, there are some exceptions:

- $u \overline{u}$ and $d \overline{d}$ do not exist as individual particles, just the neutral pion as superposition of both exists.
- the formation of mesons and baryons takes some time. While this is extremely small, the decay of the top-quark is even quicker. Therefore, it cannot be part of mesons/baryons in a meaningful way.
- all neutral mesons apart from the pion can oscillate into their own antiparticles. The particle names usually refer to the eigenstates of the weak interaction (the quark content). These are different from the mass eigenstates. It is a bit tricky to define "particle" here.