With the identification of the different decays of the Z0 at LEP you are well prepared to find the Z0 in collisions of other particles. For example in energetic proton-proton collisions at LHC, many more particles are produced than at LEP, so that the analysis of the events is considerably more difficult. In the colliding protons at the LHC with the interaction of a quark and a gluon a Z 0 can be emitted. At the same time many other particle jets are emerged from the rest of the quarks. In the following exercise, you should try to find some Z0 bosons among all the particles in the complex events.. To make it easier, let us just look for decays of a Z0 to an electron-positron or muon-antimuon pair. Electrons and muons can be identified via the same characteristic patterns as in the section Identification of particle-antiparticle pairs at LEP.

In the following exercise, you should try to find some Z⁰ bosons among all the particles in the complex events.. To make it easier, let us just look for decays of a Z⁰ to an electron-positron or muon-antimuon pair. Electrons and muons can be identified via the same characteristic patterns as in the section Identification of particle-antiparticle pairs at LEP.

The detected pair should have opposite charge. Clicking on the single tracks in Pick mode you get all necessary information about their charge and energy in the Track Momenta Window .

Example of a Z⁰→μ⁺μ^- decay
(Click to enlarge)

Example of a Z⁰→e⁺e^- decay
(Click to enlarge)

Another complication at LHC is the existence of so-called background processes, many of which occur much more frequently than the processes for which we are looking. For events with an electron-positron or muon-antimuon pair, the most prominent backgrounds are pairs of top-antitop quarks. They can decay according to t+t→ μ⁺+ν+ μ^-+ν+… or t+t→ e⁺+ν+ e^-+ν+…, where … stands for further particles in quark jets. In order to suppress this background, one exploits characteristic differences to events with Z⁰ bosons

• In contrast to events with electron-positron or muon-antimuon pairs from Z⁰ decays, the background from t+t always contains two undetectable neutrinos that take away considerable energy from the collision. Therefore all events with a ETMiss larger than 10 GeV should be considered as background.
• Your identified pair of particles must have a total energy corresponding to the sum of the unknown kinetic energy of the Z⁰ boson plus its rest energy of 91.2 GeV, stemming from the Z⁰ mass. Therefore you should require that the sum of the energies of the pair in the Track Momenta Window exceeds 91.2 GeV. Events which fail this requirement are again considered as background.

Example of a tt decay
(Click to enlarge)

You can now start the program Hypatia.

Please classify the events in 3 categories as

• Z⁰ decays to either electron-positron or
• muon-antimuon, or
• as background from top-antitop pairs,

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