I have a few questions: Can a u quark turn into a d quark (heavier) via the weak interaction? If so, how is the mass/energy shortfall made up? How can the (supermassive) carrier bosons (W, Z) be called into being? Where does the energy come from? Or is the energy bill unpaid because they are virtual particles, on the verge of existence? Is it akin to a tunneling effect? Thank you.