Are bottomonium and charmonium the only quarkonium systems?

Question

I had a question:

Are bottomonium and charmonium the only quarkonium systems?

I was searching the internet and found that these two systems have been experimetally observed but the toponium has not been observed, as it decays quickly. Wiki says that these are the only two quarkonia, while this paper suggests that more quarkonium systems have been discovered, but does not name any of those other quarkonia. However, in both the above pages, as well as other sites, there is mention of excited states of the charmonium and bottomonium mesons; I don't think that these are what they call as 'separate particles'. So, which one is correct?

EDIT: The part I am talking about is the last line of this image-

Answer 1

I will try to address all the aspects of your question.

1. Quarkonium is by definition a $q\bar{q}$ bound state, with a quark and its anti-quark. Thus particles such as $B_c$ which is a $\bar{b}c$ bound state, i.e. just another $B$ meson, does not qualify. This is just a matter of classification, nothing fundamental.

2. You remark that "the toponium has not been observed, as it decays quickly". A more accurate statement would be that the toponium will never be observed because it does not have time to form. Basically the top quark, and in that it is unique among its peers, does never experience any hadronisation because this takes place on a time scale far too long compared to the life time of the top [*].

3. We might wonder why the higher orbitals of the Hydrogen atom are not considered as different particles whereas they are considered so for bound states of $c\bar{c}$ or $b\bar{b}$. The reason is conventional: in particle physics, if the mass of X is different from the mass of Y, we call X and Y different particles. There is some quantitative logic at work here though: for the Hydrogen atom, the separation between the energy levels is of the order of the eV whereas for $c\bar{c}$ or $b\bar{b}$ bound states, it is of the order of 100 MeV at least. So from the point of view of a particle physicist, naming the 1s and 2s states of the Hydrogen atom as different particles would be meaningless.

4. But new quarkonia are not only just higher orbitals of the same $c\bar{c}$ or $b\bar{b}$ bound state. There are also the so-called hybrids, a recent example of which being Y(4260) which was discovered circa 2005. The best model fitting all its decays is that it is a bound state $c\bar{c} + \text{gluon}$. The Particle Data Review list Y(4260) in its section Heavy Quarkonium Spectroscopy, so it definitively belong in this answer!

[*] As a side note, this has an immense benefit: perturbative calculations are particularly effective for processes involving the top quark as this is all there is. I am only talking about the production and decay mechanism of course, as the decay product of the top quark may include lighter quarks which will hadronise.