# What's the meaning of "non-prompt" particle?

I am currently studying methods for fake-rate determination, and as a source of the so-called reducible background "non-prompt" leptons coming from decays of heavy-flavor mesons & mis-reconstructed jets are mentioned.

What are "non-prompt" leptons?

• Doing a quick Google, Jon Butterworth says "There are several different ways photons can be produced in a collision at the LHC. One of the most common is in the decay of neutral pions, which are hadrons created copiously when protons are smashed up. By "prompt" though, we means photons which are produced promptly in the collision, before the quarks and gluons have had time to form hadrons, and well before those hadrons decay." I assume the definition of a prompt lepton is similar. Dec 12 '17 at 14:34
• Mentioned where? Dec 12 '17 at 14:51
• indico.cern.ch/event/508711/contributions/1188807/attachments/… Dec 12 '17 at 15:14
• the link @M.A.B is to a mohamed ali talk Dec 12 '17 at 15:41
• The correct source for knzhou's comment is: theguardian.com/science/life-and-physics/2013/nov/07/… Nov 11 '18 at 14:46

As mentioned by knzhou, a prompt lepton is a lepton originating from the main collision taking place in the event, as a direct product of the particular decay you're after. An analysis looking for a particular final state containing leptons is really looking for prompt leptons.
Non-prompt leptons come later: either through the decay of the hadronized quarks (so-called jets), or a as a "mis-ID". The former basically means that e.g. a $B$-hadron coming from a $b$-quark might decay, before or within the detector, into one or more leptons. These will leave tracks and hits in the relevant parts of the detector as would a prompt lepton, but correlating those hits with nearby jets or $b$-tagged activity could be a way of removing those unwanted non-prompt leptons.