Description of an Expression

[Mystery111]

I haven't seen this expression before:

 

[math]\frac{-iM\gamma}{\hbar^2k}[/math]

 

Well I can immediately see that [math]\hbar k[/math] is the momentum. What is the gamma here acting coefficient on the imaginary mass?

 

In fact, more generally, what is this expression describing? I am inclined to believe it is simply a momentum, but the gamma is knocking me off: source can be found in the first equation in this link:

 

http://hitoshi.berkeley.edu/221B-S02/sol2.pdf

[Mystery111]

No one? No suggestions?