# What exactly is background in terms of data analysis in particle physics? [closed]

I often come across the term "background" in particle physics. Could someone explain what it exactly means?

## closed as too broad by ZeroTheHero, AccidentalFourierTransform, Jon Custer, Yashas, user259412Apr 26 '17 at 0:03

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• Context of how it's used would help – scrappedcola Apr 24 '17 at 16:43
• I am looking at a process where a higgs boson decays into two photons . To analyse the decay five monte carlo background samples were used for the higgs production (gluon gluon Fusion, vector boson fusion, etc.) Now, what exactly is "backround"? – Mark Apr 24 '17 at 16:47
• Background is the events in your selected data sample which aren't your signal. Often they can be just random combinations of final-state particles (combinatorial background) or misidentified processes. – dukwon Apr 24 '17 at 17:17
• And how can someone "identify" the background? – Mark Apr 24 '17 at 17:18
• There isn't really one generic answer for that. Unless you expect zero background or zero signal, you never know on an event-by-event basis whether a particular event is signal or background. We only really care about distributions of events. In $H→\gamma\gamma$, the background is just modelled as a smooth polynomial in $m(\gamma\gamma)$ and the Higgs as a Gaussian or similar peaking shape. – dukwon Apr 24 '17 at 17:23

## 1 Answer

Any general definition is bound to be abstract. I usually go with something like

Everything in a subset of your data where you have (or expect) a signal of some physics that isn't that signal is a background.

What that might consist of depends on the particular case. It is also worth noting that many data sets are subject to multiple analyses, and data that is signal in one may be background in another and vice versa.

Nor is this definition in any way special to particle physics, it's nearly ubiquitous in experimental work.

• I'd rephrase "a signal of some physics that isn't that signal". Perhaps the first "signal" could be "sign". – dukwon Apr 24 '17 at 19:43