W Particle
The exchange particles responsible for the weak interaction are the three carrier particles W+, W- and Z0. W particles are produced in different ways during proton-proton collisions in the LHC. The following picture gallery introduces them. Feynman diagrams are used for better illustration. Get to know the diagrams here.
Production of different W particles
  • This Feynman diagram describes the process in which most W+ particles are produced, the quark-gluon interaction. A down quark is also created and causes a jet. Whether the jet is visible or not depends on the direction of flight of the down quark. It is often the case that one cannot seen the jet that resulted from the quark because it is moving close to the beam pipe. In most cases you see either zero or one jet in such an event. Additional gluons may be radiated which then create more jets.
  • This diagram shows the predominant process (quark-gluon interaction) to create W- particles. At least one jet is also created from the up quark.

  • But W+ particles can also be radiated from quarks. This is shown here: an up quark converts into a down quark by radiating a W+ particle.
  • And the same is valid for anti-quarks. Here: an anti-up quark converts into an anti-down quark.

Decay of W particles
The W particle is heavy (80.4 GeV/c2) and decays almost immediately after its production. In two-thirds of its decays, a quark-antiquark pair is produced, which appears as jets in the detector. In one third of the W-decays, a lepton and a neutrino are produced. In these cases, the leptons can be an electron, a muon, or a tau with equal probability. Before the tau can be detected in the detector, it decays as well. In our events, we will only look at decays of W particles into electrons (or positrons) or muons (or anti-muons). Thus we get the following Feynman diagrams:

  • The W+ decays into a positron and an electron neutrino in this case.
  • Here, the W- decays into an electron and an anti-electron neutrino
  • The W+ can also decay into an anti-muon and a muon neutrino.
  • For the W-, there is also the possibility of a decay into a muon and an anti-muon neutrino.

Events with a signature in the detector as illustrated by the last four diagrams will be regarded as signal events in our data samples. Each is an unambiguous indicator of a W particle which has existed for a very short time. All other events have to be categorized as background. Let's have a look at possible background events in this last picture gallery:

Background events
If protons collide not only can W particles be created but, for example, Z0 particles as well. These particles also decay promptly after production:

  • The Z0 particle produced by a proton-proton collision can, inter alia, decay into an electron-positron pair or ...
  • ... into a muon-antimuon pair.

With this information we should be able to identify all W events in MINERVA.