Z boson
The exchange particles responsible for the weak interaction are the three carrier particles the charged W+, W- and the neutral Z. Z bosons are produced in different ways during proton-proton collisions in the LHC. The following picture gallery introduces them. Feynman diagrams are used for a better illustration. Learn about the Feynman diagrams here.

Decay of Z bosons
The Z boson is heavy (91,2 GeV/c2, about 100 times heavier than the proton) and decays immediately after its creation. It can decay in many different ways, and although we will only concentrate on two of these possibilities later, we will still tell you about them all here.

The most important is to remember that since Z is neutral the sum of the charges of its decay products have to be 0. This is because in nature, charge is conserved. This gives us the following decay possibilities:

  1. In 10% of the Z-decays, charged lepton-antilepton pairs are produced. The three possible charged lepton pair types are electron-positron, muon-antimuon, and tau-antitau pairs. Each pair is equally probable.
  2. The Z boson decays in 20% of the cases into a neutral (electric charge = 0) lepton-antilepton pair, namely neutrino-antineutrino pair. Our detector is not capable of detecting neutrinos since they almost don't interact with anything. The neutrinos are therefore invisible to us and the only way we can “see” them is when we measure that there is some energy or transverse momentum missing after the collision (since we know that both transverse momentum and energy should be conserved in the collision).
  3. In 70% of its decays, a quark-antiquark pair is produced. These appear as particle showers called “jets“ in the detector.
Out of all these possible decay-modes, we will only study decays of Z bosons into electron-positron or muon-antimuon. These decays are illustrated by the following Feynman diagrams:



Events with an electron pair or a muon pair as illustrated by the two diagrams above are our signal events in our data samples. They are an unambiguous indication that a Z boson 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 Z bosons can be created but for example a W boson or a top quark as well. These particles also decay immediately after their production, since they are also very heavy. A way we can distinguish between a Z signal event and for instance a W event is to study the ways the other particle can decay. The simple way to do this is do look at the Feynman diagrams:

  • So you can see: A W boson will typically decay to only one charged lepton, not two as for the Z boson. And in addition, there is a neutrino. As we learned above, a neutrino will only be noticed due to missing energy in our energy balance.
  • Looking at the top-antitop case, the picture is even more complicated. We can see in total two charged leptons, but also neutrinos and quarks. So even though we also here will have two charged leptons as in the Z boson decay, we in addition will see jets (from the quarks) and missing energy (from the neutrinos), and can therefore distinguish this collision type from the Z boson type.


It's time to practice identifying events with HYPATIA!