Comment by ars

This is significant because binding 2 heavy quarks together is very hard to do because they decay so fast.

All the particles made from just 3 light quarks have been found. And I think all of the ones made with 1 heavy quark and 2 light ones have also been found.

This is only the 2nd particle made from 2 heavy quarks that has ever been found.

It's not a coincidence that both of those were made with charm quarks, because charm quarks are the lightest of the heavy ones.

None made of 3 heavy quarks has been found (yet).

Also: A particle made with a top quark is not possible - it decays too quickly.