ChrisVer said:
This is wrong. A meson with a charm and an antidown quark would be the D^+ charmed meson.
The c,\bar{d} can still go through a s-channel (with Ws) into other particles, and you can have D^+ \rightarrow l^+ \nu_l. They have a small Branching fraction, but they are not "forbidden". The BR of l=e, \tau haven't been observed and pdg gives upper bounds, but l=\mu has been measured.
A better example for proving that wrong are the Kaon decays. eg K^+ \rightarrow \mu^+ \nu_\mu (63.55%) and K+ is a u\bar{s}.
What you say would be true if there was no possible transition between generations (no CKM matrix). It would be true however to say that J/ψ (cc*) or Y (bb*) do have much shorter lifetime. But it's difficult to make use of it, because they have so many different decay modes (either the mesons or the quarkonia).
When I say that a meson is not able to annihilate, I do not mean that it is not able to decay. What I mean instead when I say that I meson is not able to annihilate is that it cannot decay directly to photons, leaving behind no quarks, directly from its current state without intermediate W boson interactions.
For example, in a case like a B
0 meson decay to photons, you have a b quark and an anti-d quark leaving a neutral charge. For this to decay to photons, you must first have the b quark transition by emitting to W
- boson to, for example, a charm quark, and then have the charm quark emit a W
+ boson while it transforms to a d quark, and then have the d quark and the anti-d quark annihilate, and also have the W+ and W- bosons either produce equal and opposite decay products and annihilate, or directly annihilate each other (if the timing of the sequential interaction permitted, which it probably wouldn't given the short mean lifetime of a W boson and the relatively slow speed of weak force interactions).
I would describe that sequence of events a B
0 meson decaying into a D
+ meson and a W
- boson, followed by the decay of the D
+ meson into a pion (or given that a pion isn't really d-antid, a pair of pions) and a W- boson, followed by the annihilation of the pions and the annihilation of the W
+ and W
- boson products with each other.
Collapsing the original meson to the final decay process oversimplifies what is going on which is something much more complex than annihilation.
In contrast, quarkonium can go directly from meson to a pair of photons without decaying into something else as an intermediate stage of the process. which is fairly called annihilation.
This matters because it isn't all that amazing that something can exist even though after being transformed through multiple future steps of non-instantaneous subsequent reactions, the end decay product can be photons. In contrast, it is amazing that a composite object made up of particles that can instantly annihilate into photons in a single step as is the case in quarkonium.
