I Why Are There No Down/Anti-Down Mesons?

[edguy99]

In list of Mesons like this you see charm/anti-charm, strange/anti-strange and bottom/anti-bottom. You don't see up/anti-up or down/anti-down. Instead you see up/anti-down or (up/anti-up and down/anti-down) in supposition.

Why no down/anti-down meson?

 

[Orodruin]

Ever heard of neutral pions? They are a linear combination of $u\bar u$ and $d\bar d$. Then there are the eta mesons, which are linear combinations also involving $s\bar s$.

 

[edguy99]

Ever heard of neutral pions? They are a linear combination of $u\bar u$ and $d\bar d$. Then there are the eta mesons, which are linear combinations also involving $s\bar s$.

Yes, I mentioned in my original post: (up/anti-up and down/anti-down) in supposition.

The question remains about this particular meson: Why no down/anti-down meson?

 

[dukwon]

Because the up and down quarks have such similar masses, which are very small in comparison to the mass of mesons. You can think of it like the strong interaction can replace a pair of $u\overline{u}$ valence quarks with a $d\overline{d}$ pair without violating any conservation laws.

 

[edguy99]

Because the up and down quarks have such similar masses.

Makes sense. Does that mean
1. we just can't tell the up/anti-up and down/anti-down particles apart, or
2. the up/anti-up and down/anti-down particles do not exist by themselves and there exists only a single particle that is a linear combination (or supposition) of the two?

 

[dukwon]

The second one. The light unflavoured neutral hadrons are superpositions of $u\overline{u}$, $d\overline{d}$ and sometimes $s\overline{s}$. Hadrons are dynamic interacting bound systems. If no conservation law forbids the changing of the valence quark flavours then it will happen.

 

[mfb]

Another way to count particles: We have $\pi$, $\eta$ and $\eta'$ and three light quarks - fits. The particles are not pure "one quark and its antiquark" states, and the extremely similar up and down masses lead to a single light meson (with negligible strange/antistrange content) and two that are a bit heavier (with significant strange/antistrange content).