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A BDT scores: Signal vs Background

  1. Jul 4, 2017 #1

    ChrisVer

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    I have one question:
    Many times, in order to test whether an outcome should be considered as "Signal" (S) or "Background" (B), we are using BDTs which we have trained on known outcomes...
    I was wondering though, the outcome of being S/B is binary : either it's signal or it's not... the only way I can interpret the BDT variable after that is as "how likely it is to be signal or not", since a particular cut on the BDT corresponds to some particular background rejection and signal efficiency (ROC curves).
    Is my interpretation correct? Would a "signal" object with a BDTscore=0.89 be a less likely signal than one with BDTscore=0.95 ? If not, then is there a way to compare the two events? I.e. the higher I place a cut on BDT the stronger/tighter signal events I'm selecting?
     
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  3. Jul 4, 2017 #2

    mfb

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    It doesn't have to be. You can make multiple categories with increasing S/B ratio (increasing BDT score) and study them separately. LHCb does this for ##B_s \to \mu \mu## measurements, ATLAS and CMS do this for ##H \to \gamma \gamma## measurements for example. It improves your sensitivity compared to a single cut.
     
  4. Jul 4, 2017 #3

    ChrisVer

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    Sorry I didn't mean the ratio between Signal and Background, but being Signal or being Background (shortly I wrote S/B for S or B)
     
  5. Jul 4, 2017 #4

    mfb

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    I know, but what the BDT gives you is a ranking in terms of S/B ratio.
     
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