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## Main Question or Discussion Point

The pseudoscalar mesons have [itex]J^P = 0^-[/itex]

They form a nonet: for S = ±1, I (isospin) = 1/2 and so there are two particles for each value of strangeness. This account for 4 particles: the ground-state Kaons.

For S=0, I can be 0 or 1. I=1 gives a triplet: [itex]\pi^\pm \mbox{ and } \pi^0[/itex].

For S=0 and I = 0 however there are two particles: the [itex]\eta \mbox{ and the } \eta^\prime[/itex]

As far as I can see the eta and the eta prime have exactly the same characteristics. My question is: why is there an eta prime? All the other mesons seem justified by the quark model. Why are there two [itex]I^CJ^P = 0^+0^-[/itex] particles and what distinguishes them?

They form a nonet: for S = ±1, I (isospin) = 1/2 and so there are two particles for each value of strangeness. This account for 4 particles: the ground-state Kaons.

For S=0, I can be 0 or 1. I=1 gives a triplet: [itex]\pi^\pm \mbox{ and } \pi^0[/itex].

For S=0 and I = 0 however there are two particles: the [itex]\eta \mbox{ and the } \eta^\prime[/itex]

As far as I can see the eta and the eta prime have exactly the same characteristics. My question is: why is there an eta prime? All the other mesons seem justified by the quark model. Why are there two [itex]I^CJ^P = 0^+0^-[/itex] particles and what distinguishes them?