# Feynman diagram of secondary Sigma plus decay

1. Oct 5, 2011

### stmb

Can someone describe the Feynman diagram for the Sigma plus decay to a positive pion and a neutron? I'm fine with the primary decay to the neutral pion and a proton. There appears to be insufficient down quarks created.

2. Oct 6, 2011

### Hepth

uus > ud~ u d d

Well one d d~ set can be made with a photon/gluon, while the s-> d can be a loop-level FCNC.

OR

uus : s---u
uus : u---d
via a W exchange, and the d dbar pair is made with a photon/gluon emission.

3. Oct 6, 2011

Staff Emeritus
Look at the BF's - they are equal for p pi0 and n pi+, so we're not talking about FCNC loops.

You have uus --> uud + ddbar (p pi0)
and uus --> udd + udbar (n pi+)

This is just a rearrangement, which explains their equality.

4. Oct 6, 2011

### stmb

Thanks guys. The BF's are actually not 50/50 but 52/48. Why should they be slightly different? Does the fact that alpha for p pi0 (36 deg) and for n pi+ (167 deg) relevant for this? Appreciate your comments.

5. Oct 6, 2011