I understand that jets come from the process of a proton-proton collision and the QCD confinement which create more quark-anti quark pair that emit this jets of particles. These jets of particles what are they exactly? Do the jets scatter by the electromagnetic detector and hadronic detector so that is why they are mistaken as leptons? Any help I am fairly new to these concepts.

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1 Answer 1


The quarks and antiquarks rearrange themselves into colour-neutral hadrons. These are mostly pions (3 types) with some kaons (4 types) and occasionally heavier particles.

The EM calorimeter contains high Z material which causes electrons and photons to shower and give a big signal. Pions, kaons etc don't interact so much and give a smaller signal. But these are random processes and the distinction is not always clear so sometimes a pion/kaon jet can happen to give a big signal and fool you into thinking this is an electron. This happens especially if the jet happens to contain lots of pizeros, which decay to photons.

What's left of the jet then passes into the hadron calorimeter where there is lots of material with high nuclear cross section and the pions, kaons etc interact and shower and are absorbed. The electrons have already gone, so the only particles which don't get stopped by the HCAL are muons. Hence anything that makes it out of the back of the calorimeter into the chambers behind must be a muon. But again, this isn't absolutely clear cut as the processes are random. If the particles in a jet happen to interact late in the HCAL, some of the shower may emerge from the back and fool you into thinking this is a muon. This happens especially if one of the kaons (or pions, but more likely kaons) decays in flight to a muon.

  • $\begingroup$ What are Z material and pizeros? $\endgroup$ Jun 8, 2021 at 16:52
  • $\begingroup$ Also, so basically due more to the limits of the measuring devices the hadrons from the jets are mistaken as leptons ? Or sometimes these hadron jets provide a strong signal shower in one of the lepton layers of measurement $\endgroup$ Jun 8, 2021 at 16:54
  • $\begingroup$ High-Z here means high atomic number. That means lots of atomic electrons, lots of charge, and high probability that a high-energy electron will interact. (This is loose language: don't push it too hard.). A pizero is the neutral pion (as opposed to the pi+ and pi-). It decays very rapidly to two photons. Thus a hadronic jet has an electromagnetic component. $\endgroup$ Jun 9, 2021 at 18:35
  • $\begingroup$ Yes. The behaviour of the hadron jet in the detector, the pattern it leaves, is to some extent random. Sometimes (hopefully not often) the random fluctuations make a pattern that looks like an electron, sometimes (hopefully not often) like a muon. Exactly how often depends on the design of the detector and the analysis software. $\endgroup$ Jun 9, 2021 at 18:38

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