Difference: E6Generation (6 vs. 5)

Revision 5
10 Jul 2013 - Main.AaronVermeersch
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Pythia8 Generation of Z' Processes

The download and installation of Pythia8 can be found here. If the user does not have any experience with Pythia8 then it is recommended to follow the Pythia8 installation on that page in order to gain general familiarity with the event generator.
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Once the two changes are in place Pythia8 will generate your events with radiation and produce the correct LHE file. This file will contain thousands of particles for each event due to the radiation of photons and their corresponding decays. Since the photons will decay into electron/positron states there may be multiple pairs of final state electrons and the challenge becomes finding the pair that decayed from the intermediary Z and not the photons. This is complicated because of how Pythia8 handles radiation and how it modifies the particle record. In order to radiate photons Pythia8 will create a chain of the same type of particle with small changes in momentum between them (i.e. electron -> electron -> electron and the same can be true for the Z/Z'/positron). The differences in momentum between the electrons in the chain will correspond to the momentum of a photon in your event which was radiated by the electron. These chains of particles will prevent the user from looking at the first mother of the electron as it may return another electron and not the intermediary boson.
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The simplest method of determining the electron pair that had decayed from the intermediary boson is to select the highest pt electron and positron. This will work the majority of the time and is a useful method when looking at particles that have went through a detector simulation.
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The easiest method of determining the electron pair that had decayed from the intermediary boson is to select the highest pt electron and positron. This will work the majority of the time and is a useful method when looking at particles that have went through a detector simulation. However, if you are looking for one hundred percent accuracy in choosing the correct pair then the user will need a more sophisticated method in finding them.
 
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However if the user wants to find only the electrons coming off of the boson then they will need to exploit the mother and child relationshipos. When converting the LHE file to a root n-tuple using ExRootAnalysis the user will be given two additional variables: Mother1 and Mother2. These designate the position in the particle index where the mother is stored and can be used to find the correct final state particles. There is one caveat: the particle index starts at 0 while the index in Mother1 and Mother2 start at 1. In order to determine the mother's ID of the current final state particle the user should write the following into their code:
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When converting the LHE file to a root n-tuple using ExRootAnalysis the user will be given two additional variables: Mother1 and Mother2. These designate the position in the particle index where the mother is stored and can be used to find the correct final state particles. However there is one caveat: the particle index starts at 0 while the index in Mother1 and Mother2 start at 1. In order to determine the mother's ID of the current final state particle the user should write the following into their code:
 

int RealMotherIndex = Particle_Mother1[i] -1;
int MotherID = Particle_PID[RealMotherIndex];
 
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