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Copernicus
2015-Aug-26, 10:04 AM
In string theory are pions represented by 10 dimensions or do they only require 7 dimensions? Has anybody ever studied this? Are different pions different this way as some decay by just the weak force but other decay through multiple forces.

antoniseb
2015-Aug-26, 12:21 PM
In string theory are pions represented by 10 dimensions or do they only require 7 dimensions? Has anybody ever studied this? Are different pions different this way as some decay by just the weak force but other decay through multiple forces.
Given that pions are usually treated as two quarks (e.g. up + anti-down) I'm not sure why you single them out for looking at a number of dimensions. Can you give us more about what you are asking about?

Shaula
2015-Aug-26, 12:27 PM
The only reason some decay via the weak force alone is because some are charged. Photons are not, which closes off the main electromagnetic decay route to the charged pions.

And in string theory particles are modelled as one dimensional objects. These objects are set within a high dimensional spacetime but that spacetime is not different for different particles.
Edit: As Antoniseb says mesons are not fundamental.

Are these questions motivated by pions acting as force carriers for the residual strong force?

Jeff Root
2015-Aug-26, 01:32 PM
The only reason some decay via the weak force alone
is because some are charged. Photons are not, which
closes off the main electromagnetic decay route
to the charged pions.
Could you explain the bolded part in more detail for those
don't get why you brought up photons, which as you say
are uncharged yet are the carriers of the electromagnetic
force. I'm not getting the idea you meant to convey.

-- Jeff, in Minneapolis

Shaula
2015-Aug-26, 02:07 PM
Could you explain the bolded part in more detail for those who don't know much about this? Going beyond that, I don't get why you brought up photons, which as you say are uncharged yet are the carriers of the electromagnetic force. I'm not getting the idea you meant to convey.
-- Jeff, in Minneapolis
The only decay option for an electromagnetic decay in pions is to photons (because of all the quantum numbers the EM force cannot change) - all I was saying is that because there are pions with +1 and -1 charge they cannot decay to photons like the neutral pions can due to charge conservation laws. The charged ones instead have to decay via the weak force.

So the decay modes for the two types of pion can be summarised as:
{\pi}^{0} \to 2\gamma (\approx99\%)
{\pi}^{0} \to 2\gamma \to \gamma + {e}^{+} + {e}^{-} (\approx1\%)
{\pi}^{0} \to 2\gamma \to 2{e}^{+} + 2{e}^{-} (\ll1\%)
and
{\pi}^{+} \to {\mu}^{+} + {v}_{\mu}
{\pi}^{-} \to {\mu}^{-} + {\bar{v}}_{\mu}

Edit:
What is actually happening in the weak interaction (which is the only one that allows flavour changing) is that the two quarks are interacting to form an intermediate boson, which then decays:
u + \bar{d} \to {W}^{+} \to {\mu}^{+} + {v}_{\mu}
\bar{u} + d \to {W}^{-} \to {\mu}^{-} + {\bar{v}}_{\mu}

There are some incredibly rare other decay mode for charged pions, but they are all weak force mediated too for the same reason.

Copernicus
2015-Aug-26, 07:06 PM
Given that pions are usually treated as two quarks (e.g. up + anti-down) I'm not sure why you single them out for looking at a number of dimensions. Can you give us more about what you are asking about?

good point, quarks would be in the same category. I'm just wondering if string theory requires everything to be in 10 dimensions, or is the mass part of the quarks modeled in lesser amounts of dimensions.