Simulation of a four-top-quark process
As the first step in our study we want to investigate a simple description of a collision event with four top quarks in the final state, allowing to probe the existance of heavy top-philic resonances.
In the hands-on project, you can find the prepared dataset at
The following paragraphs are not necessary for the hands-on project. The dataset is provided already, so there is no need for you to produce it.
However, if you are curious how it was obtained, the following section provides some background information about how collision events are simulated for physics processes at particle colliders.
Hard scattering process
To this end, we rely on the description of such a generic top-philic resonance which is provided in the paper "Probing TeV scale Top-Philic Resonances with Boosted Top-Tagging at the High Luminosity LHC" (arXiv:1604.07421v2). The authors of that publication propose an extension of the Standard Model which extends the repertoire for building Feynman graphs by an additional ttZ' vertex. The ttZ' vertex couples the top quark to the heavy resonance, the Z' boson, thereby allowing for its production and decay to top quarks.
In theory, we could now take the Lagrangian of the model, derive the Feynman rules, draw all possible Feynman graphs leading to a four-top-quark final state (at leading order in the strong coupling constant), take out pen and paper and start computing the differential cross-section for certain observables, e.g the transverse momentum pt.
In practice, there are tools designed by theoretical physicists which do exactly this for us. We just have to specify the initial state and the final state for a given model. The tools then take the Feynman rules for the model, determine all possible Feynman graphs for the final state, and numerically compute the cross-section.
Until this point, the physics process is described from first principles: starting from a Lagrangian density, the collision event is described using the parton model. This part of the event is called the hard scattering process because of the momentum scale at which the event occurs, mostly involving large momentum transfers. However, the hard scattering process is only half of the story happening for collisions at the LHC.
Parton shower
The four top quarks in the final state further decay into b-quarks and W bosons. The latter also decay into either leptons or quarks. From this point, the event description consists of a few high-momentum individual objects: four top quarks. In the simple formulation of the research problem, the event description ends with the four top quark final state "at parton level".
Of course, the story is not over yet. As the top quarks carry colour charge, they participate in the strong interaction and can radiate off gluons which in turn can create new quark antiquark pairs. This process leads to the formation of a shower of particles, referred to as parton shower. Because of a property of QCD called colour confinement, there are no free particles carrying colour charge. Therefore, the quarks in the final state which carry bare colour charges undergo process called hadronisation. In the end, there are a large numbers of mesons which appear in collimated sprays of particles, so called jets. The description of the parton shower and the hadronisation is not calculated from first principles but instead from empirical parametrisations using different tools.
The description of the collision event in the advanced statement of the research problem includes the parton shower and the jets composed from final state particles, therefore we refer to this description of the event based on the top quark decay products as "at particle level".
In conclusion, the picture above illustrates the different aspects of a proton collision at the LHC. Two incoming protons (shown here with three lines, representing the valence quarks) are shown in light green in the horizontal center of the illustration. They radiatie off gluons which take part in the hard interaction, shown in red. Then the parton shower (dark red) results in the creation of several particles with relatively low momenta which in turn undergo hadronisation (bright green).
Getting your hands dirty
In case you want to simulate the four top process in the simple problem statement by yourself, have a look here:
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