# what is QCD Background?

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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?

This post imported from StackExchange Physics at 2014-04-13 14:34 (UCT), posted by SE-user Damian Dulkawie

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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

This post imported from StackExchange Physics at 2014-04-13 14:34 (UCT), posted by SE-user user6972
answered Apr 10, 2014 by (40 points)
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?

This post imported from StackExchange Physics at 2014-04-13 14:34 (UCT), posted by SE-user Damian Dulkawie
The term "background" is not discouraged, at least not that I've ever heard. Its use is ubiquitous in the particle physics community.

This post imported from StackExchange Physics at 2014-04-13 14:34 (UCT), posted by SE-user David Z
@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.

This post imported from StackExchange Physics at 2014-04-13 14:34 (UCT), posted by SE-user user6972
@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.

This post imported from StackExchange Physics at 2014-04-13 14:34 (UCT), posted by SE-user anna v
@user6972 well, that may be something CMS does, but in particle physics in general, "QCD backgrounds" is still a reasonably common phrase.

This post imported from StackExchange Physics at 2014-04-13 14:34 (UCT), posted by SE-user David Z

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