Difference: E6Generation (r4 vs. r3)

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.

The production of Z decays within the context of the E6 models is a fairly straightforward process since Pythia8 includes the Z inherently within the standalone package. In order to generate the Z decays Pythia8 requires the setting of a number of parameters and hopefully this twiki page should guide the user towards successful generation.

The Pythia8 Online Manual is linked and the Z documentation can be found within the New Gauge Bosons section of the Process Select category.

Parameter Specification

This section will describe the parameters that need to be set in order for Pythia8 to do its job. The parameters included within this section must be modified before event generation and must be included in the generation code through the command:


where value is the parameter to be modified. In order to start the generation of Z decays the user must include:

NewGaugeBoson::ffbar2gmZZprime = on

which will set the process for the generation. Next the user should specify the final state particles that they would want in their event. If the user wants only a single type of particle, for example electron/positron final states, two commands must be in place:

32:onMode = off
32:onIfAny = 11

where the first command switched off all decay modes for the Z, which has an ID of 32, and the second command then switched on the decay mode for only electron/positron final states. This twiki will assume electron/positron final states but the same could be done for muons where the user should input the muon ID into the string.

The mass of the Z that Pythia8 uses is set at 500 GeV by default. The way to modify the mass of the Z is with:

32:m0 = 1500

which will produce a 1500 GeV Z and can obviously be changed to any mass value.

Next up is to select the mode that the user wants to run the generation in. This will specify which of the particles: photon/Z/Z contribute to the event and will impact various distributions. For example, if Z/Z is being used then in the invariant mass of the final state particles there will be two peaks: one for the Z and one for the Z.

Regardless of what is specified here the ID of the intermediary particle will always be assigned a value of 32 and the possible decay channels are specified by the decay channels that are allowed for the Z.

The various modes are:

0: photon/Z/Z with interference
1: pure photon
2: pure Z
3: pure Z
4: photon/Z with interference
5: photon/Z with interference
6: Z/Z with interference

And the argument that needs to be included is:

Zprime:gmZmode = 0

Forward-Backward Asymmetry Note: If you are planning on looking at AFB then it is going to be necessary to run with additional interference from the photon. If the user ran with strictly Z then the AFB will be approximately zero. When I ran my generation with photon/Z/Z' interference the correct AFB plot would eventually be produced.

Finally we get to the couplings for the Z which will define which of the E6 models the user is generating. Pythia8 allows for the specification of the axial and vector couplings to all the quarks and leptons in the Standard Model but the more general setting has the user specifying the couplings for the first generation and then assuming they are the same for the others. In order to accomplish this the user should add to their code:

Zprime:universality = on

So that now the only couplings that need to be specified are the ones for the first generation and Pythia will use these values for the other generations. It should be noted that Pythia8 uses its own convention for the Z couplings. Thus the user will not be able to take the couplings from a theory paper and directly place them into their code. If the user wishes to see the Pythia8 convention and how it compares with other conventions the relevant paper can be found here which will contain the expressions for each of the couplings as a function of the mixing angle on page 6.

And to set the couplings for the various particles the arguments are:

Zprime:vd = 0
Zprime:ad = 0.5055
Zprime:vu = 0
Zprime:au = 0.5055
Zprime:ve = 0
Zprime:ae = 0.5055
Zprime:vnue = 0.2527
Zprime:anue = 0.2527

where the values presented are for the E6 Psi model. Additionally the Z couplings for various E6 models within the Pythia convention already in decimal form can be found here: LINK.

An example of a Pythia8 .cc file that generates Z decays is located here . This file will generate 10,000 events for Z -> e- e+ with photon/Z/Z interference within the E6 Psi model and saves it to a LHE file. The LHE file can then be converted to a root n-tuple through using the use of ExRootAnalysis.

Radiation and the Event Record

The next modification that the user may wish to do is to allow leptons the ability to radiate photons. By default Pythia8 has this option turned off and the LHEF initialization should be:

LHAupFromPYTHIA8 myLHA(&pythia.process, &pythia.info);

If the user wanted to change the default so that leptons could radiate photons the string that must be included is:

TimeShower:QEDshowerByL = on

and the LHEF initialization must also be changed to:

LHAupFromPYTHIA8 myLHA(&pythia.event, &pythia.info);

so that the event record will contain the radidated radiated photons and their corresponding decays. If the LHEF initialization is not modified then the event record may be incomplete and the kinematics might not reflect the changes caused by the radiation.

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 next challenge becomes finding the pair that decayed from the intermediary Z and not the photons. This is complciated by the way complicated because of how that 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 slight 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 a another different electron having undergone radiation and not the intermediary boson.

-- AaronVermeersch - 10 Jul 2013

E6Parameters.shshE6Parameters.shmanage 0.8K 09 Jul 2013 - 16:30AaronVermeersch Calculated Z' Couplings for Various Models
E6_Zprime_Generation.ccccE6_Zprime_Generation.ccmanage 2.5K 09 Jul 2013 - 16:27AaronVermeersch Example of a Script to Generate Z' Processes in E6 Psi Models

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