According to [this answer][1], the reason why $SU(2)_L$ weak theory does not have a theta vacuum is because any theta term can be reabsorbed with a phase redefinition of the left-handed quark field. 

However, I heard another equivalent argument saying that there is a $B+L$ anomaly in the Standard Model which uniquely selects one of the many topologically distinct vacua as the one true vacuum. An instanton of $SU(2)$ lets one initial state tunnel into another in a different $B+L$ sector and, therefore, does not represent a vacuum to vacuum process.

I am trying to get the overall picture and understand how these two arguments are related. Could someone elucidate on that and explain what the $B+L$ anomaly is and how it selects a unique vacuum and how that is equivalent to a phase redefinition?

 


  [1]: https://physics.stackexchange.com/a/27463/76347