Some useful links and slides for better understanding bTag and the methods used to perform it:
Cern Twiki:
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Principal algorithms used: https://twiki.cern.ch/twiki/bin/view/CMSPublic/WorkBookBTagging
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More detailed information, such as how to modify and rerun the b tagging algorithms: https://twiki.cern.ch/twiki/bin/view/CMSPublic/SWGuideBTagging
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b Tag & Vertexing Physics Object Group (require a CERN account): https://twiki.cern.ch/twiki/bin/viewauth/CMS/BtagPOG
Slides: