# Flavor conservation in EM interactions

1. Mar 29, 2009

### sizzleiah

Hi Friends,

I am an undergrad currently taking my first course in particle physics.

In thinking about various possible reactions, I have run across what appears to be a rather basic question: Why are reactions of the form $$e^- + e^+ \rightarrow \gamma \rightarrow q + q^'$$ where q and q' are different quarks or antiquarks, prohibited (i.e. flavor changing EM interactions)? Of course, such reactions will be followed by fragmentation.

Assuming q and q' are chosen so that charge is conserved, it seems that the answer must either come from kinematics or must be empirical. The former seems as though it might be possible to overcome in some special cases (e.g. choose the velocities of q and q' so that momentum is conserved, and this may serendipitously lead to energy conservation).

Presumably there is something trivial I am overlooking; be gentle....

2. Mar 29, 2009

### humanino

Hi sizzleiah, and welcome to PF,

there is no better answer IMHO than what you just said : it is empirical. In the standard model however, the photon is diagonal in the flavor basis, it does not allow transitions between different generations.

Note that energy-momentum conservation does not allow you to forbid this process, provided there is enough energy the masses could be different.

3. Mar 29, 2009

### sizzleiah

Thanks very much for your response/welcome, humanino. Ok, so it comes, theoretically, from the fact that the photon is diagonal in the flavor basis; this makes sense!

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