# Event selection cuts and cross section relation

I was working on experimental HEP project where I am bit confused. I generated 1000 MC events with say "x" cross section and without selection cuts. Now suppose I put some selection cuts while writing the C++ code for the data analysis of these events. Because of this, the number of events now drops down, say "y" (<1000) which is logical but while doing any computation should I write the cross section now as (y.x)/1000 ?

• Do you want to say "cross section, given cuts x" , or "over all probability"? That changes the answer. – Floris Aug 10 '17 at 13:38
• @Floris I meant that say 10pb is the cross section of 1000 events without selection cuts. Now, I write a C++ code where I put a selection cut which reduces the events to 900. So now, should I also normalize the cross section to 900*10/1000 = 9pb and use this value for further analysis instead of 10pb? – kg__ Aug 10 '17 at 13:41
• If you are looking for the probability of an event that matches your cut (e.g. "scatters by at least 10°"), then yes - you have just calculated that that probability is (obviously) smaller than the probability of "any" scatter, including the ones you don't care about – Floris Aug 10 '17 at 14:16
• @Floris Should I equate probability with cross section and conclude that my new cross section after selection cut is 9pb? – kg__ Aug 10 '17 at 14:19 