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I am looking at some papers in particle physics and came across 2 terms called QCD Background and ABCD method (control region and signal region). When I searched them on Google, could not get much information about them. Could someone either of those terms in a simple way for a non-physicist?

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In simple terms QCD as a "background" usually refers to LHC research where hadronic jets create a lot of particles that clutter up the results you're trying to see. I think it has become a slang term and the use is discouraged.

ABCD method is a tool used to separate the particles of interest (signal) from the "other stuff" (background) made by the jets. It is a set of boundaries that relies on the fact that you have two independent distributions to distinguish between signal and background. See section 5.3 here http://dare.uva.nl/document/221955

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First of all thanks for your reply. So, can I say that everything other than what I am looking for in the final state is a background? –  Damian Dulkawie Apr 10 at 3:24
    
The term "background" is not discouraged, at least not that I've ever heard. Its use is ubiquitous in the particle physics community. –  David Z Apr 10 at 3:30
    
@DavidZ I meant the mix of terms "QCD Background" where QCD is a theory and background don't really go together. I think the CMS doesn't like it used in publications. –  user6972 Apr 10 at 3:48
    
@DamianDulkawie Yes, "background is a blanket term that covers the non-signal". We used to say "this year's signal next year's background" :). 30 years ago QCD was the sought for signal and the parton model provided the background. –  anna v Apr 10 at 4:23
    
@user6972 well, that may be something CMS does, but in particle physics in general, "QCD backgrounds" is still a reasonably common phrase. –  David Z Apr 10 at 5:44
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