Pions and Quarks: The Mystery of the Neutral Pion

[QueenFisher]

why is it that the pion with no charge (the one with the 0 in the top corner) actually exists? cos if it's made of an up anti-up or a down anti-down quark, shouldn't they annihilate each other?

 

[Norman]

In a certain sense they do. This is not a stable particle, it has a very short lifetime, about 10^-16 s. It decays 99% of the time to two photons.
Cheers,
Ryan

 

[QueenFisher]

but how can it actually exist at all for any length of time, no matter how small?

 

[Norman]

Well this annihilation process cannot happen instantaneously- it takes some time. And 10^-16 secs. is a VERY short amount of time. Have a look at "An introduction to elementary particles" by griffiths for a good chapter on bound states.

Cheers,
Ryan

 

[samalkhaiat]

but how can it actually exist at all for any length of time, no matter how small?

The neutral pion is guilty of living secret,hidden colourful life (bound state of the colour force between quarks). So, it dies by electrocution(decays through electromagnetic force)

cheers

sam

 

[Rade]

Queenfisher: You may want to read this elementary presentation on pion exchange in "Physics Education" (2002):
http://teachers.web.cern.ch/teachers/archiv/HST2002/feynman/Pion exchange.pdf
The neutral pion is very interesting, being the only pion that shows a superposition of two states (up+anti up quark) and (down+anti down quark). The positive pion is (up+anti down quark), the negative pion (down+anti-up quark). Perhaps the superposition property of the neutral pion helps explain why it is less stable (lifetime ~ 10 -16) than either the positive or negative pion (~ 10 -8) ? And, as to lifetime, 10 -16 is fast, but not all that fast, since J/Psi meson has lifetime of ~ 10 -20, Rho+ meson ~ 10 -23.

 

[arivero]

Well this annihilation process cannot happen instantaneously- it takes some time. And 10^-16 secs. is a VERY short amount of time. Have a look at "An introduction to elementary particles" by griffiths for a good chapter on bound states.

Cheers,
Ryan

Actually the speed of the disintegration process, too fast, puzzled the physicists for a long time. It is dubbed the "Pi0 Anomaly".

 

[humanino]

why is it that the pion with no charge (the one with the 0 in the top corner) actually exists? cos if it's made of an up anti-up or a down anti-down quark, shouldn't they annihilate each other?

The unbearable lightness of the pion
There are deeper reasons for this particle to exist, and those reasons are not fully understood as of today. As arivero wrote, the "pi0 anomaly" or "chiral anomaly" enters the game here. We talk about "anomaly" whenever a classical symmetry is broken at the quantum level.
The mere existence of the pions is linked to the breakdown of a very fundamental symmetry : the "chiral" symmetry, mapping left-movers to right-movers (think of circularly polarized light for instance : it can be left or right. In everyday life, the corkscrew turns right when you screw it, and not left.)
Anyway, there is a theorem (Goldstone) which states that when symmetries are broken, some massless particles come to life ! The very light pions are instances of such particles. The kaons and the eta are also appearing in the process. There are eight Goldstone pseudoparticles total appearing in the breakdown of chiral symmetry.
I stated that the reasons for spontaneous symmetry breaking are not fully understood : you must realize that we can make many very efficient computations using the fact that chiral symmetry is broken. Besides, we can demonstrate that this breaking _must_ occur with several different methodes. Following one of them in details, it is very clear that one must actually choose between chiral symmetry, and fermion number conservation : that is either you have fermions poping out of the vacuum and/or disapearing without notice, or you give up on left-movers to right-movers symmetry. If you were a theorist, which would you choose ? However, it is also well-known that chiral symmetry breaking is linked to confinement. Many models are either based on chiral symmetry breaking or some sort of mechanism for confinement, but very few models are based on both, at least with a non-trivial scenario for confinement. It would be very desirable to gain insight into the non-perturbative structure of the vacuum of QCD by clarifying the situation. For instance clarifying the issue of large Nc limit vs chiral limit : it does matter in which order they are performed (they do not commute). Another very "hot" issue is Gribov scenario for confinement, sort of a revival of "bootstrap" schemes.
For chiral symmetry and the vacuum, see for instance
http://en.wikipedia.org/wiki/QCD_vacuum
or
Introduction to Chiral Symmetry by Volker Koch

 

[QueenFisher]

thanks for all the replies, but being only 17 and not particularly good at physics, i don't understand very much of them i think i'll get the book though.