How can the Chi_b (3P) particle exist if matter and anti-matter annihilate?

[feyomi]

The recently discovered Chi_b (3P) is made up of a bottom quark and an anti bottom quark.
I thought, however, that should a matter particle meet with its anti counterpart, they would annihilate into pure energy.

Am I missing something?

Thanks.

 

[AdrianTheRock]

The components of a chi_b(3P) are orbiting around each other - the 3P denotes the orbital quantum state.

Yes, particles and their corresponding anti-particles will annihilate one another if they meet 'directly', but it's also possible for them to form into bound orbital states, even though this is usually only transiently. These states are generally termed '-onium' states. For example positronium states involve an electron and positron orbiting one another, charmonium states have c and cbar components, and the chi_b(3P) is itself one of the various bottomonium states. The π⁰ itself is also an onium state, albeit a mixture of u-ubar and d-dbar, and similarly for various other neutral mesons.

Like a Hydrogen atom, each onium group has a set of allowed orbital energy states. To annihilate, the particles must basically fall out of these orbits.

The other caveat about annihilation that doesn't get mentioned much is that, at energies below the electroweak symmetry breaking scale, the particle and antiparticle involved must be either electrically or colour-charged. The products of the annihilation are the corresponding gauge bosons, ie photons and gluons* - annihilation itself is a straightforward interaction between charged particles and gauge bosons. Because the latter are massless, any particle/antiparticle pair with the corresponding charges can transform into them as there will always be a state of two or three real gauge bosons that has lower energy than that of the particle + antiparticle. Neutrinos and antineutrinos, however, can't do this, because they only carry weak isospin charges and so the only gauge bosons they could interact with, and hence annihilate into, are Ws and Zs, which are very heavy. The cosmic neutrino background contains both neutrinos and antinuetrinos (assuming neutrinos are Dirac particles and not Majorana) but these can't annihilate because they don't have the energy to produce real Ws or Zs.

* sufficiently energetic pairs can also annihilate into single virtual gauge bosons that then produce new, lighter particle pairs eg e⁺e^-.

 

[AdrianTheRock]

PS to clarify the last point a bit more, a neutrino-antineutrino pair can produce a virtual Z⁰, but the only thing the latter can then do is produce another neutrino-antineutrino pair, because neutrinos are the lightest particles that interact with Z⁰s.

 

[feyomi]

Cool, thanks Adrian.