physics
posted by Phyllis .
Compute the average number of interactions per bunch crossing for a luminosity L=2.5E31 cms1,if the total cross section is sigma =20mb and bunch crossing occur every T=4.What is the probability of having zero interactions in a bunch?

What is a bunch crossing? Bunch of what?
What are mb units? Millbarnes?
cm/s is not a unit of luminosity
What is T? What are its units?
this is jibberish 
Trying to sort out the units.
20 mb = 20 millibarns (mbarn)?
the luminosity is presumably cm2 s^1
because by definition
event rate = luminosity * cross section
Not sure of units of T ns?
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