
 Timestamp:

Mar 8, 2019, 12:04:37 PM (7 months ago)
 Author:

mosel
 Comment:


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v16

v17


38  38  ''The weights can also be negative''. This happens, e.g., in the case of pion production on nucleons. In this case the cross section is determined by the square of a coherent sum of resonance and background amplitudes and as such is positive. In the code the resonance contribution is separated out as the square of the resonance amplitude and as such is positive as well. The remainder, i.e. the sum of the square of the background amplitude and the interference term of resonance and background amplitudes, can be negative, however. This latter contribution is just the event type labeled 32 and 33 in the code that describes the 1pi bg plus interference. 
39  39  
40   == How to compute cross sections from the perturbative weights == 
 40  == How to compute cross sections from the perturbative weights for neutrinoinduced reactions == 
41  41  The output file `FinalEvents.dat` contains all the events generated. Per event all the fourmomenta of final state particles are listed together with the incoming neutrino energy, the 'perWeight' and various other useful properties (see documentation for `FinalEvents.dat`). In each event there is one nucleon with perWeight=0 which represents the hit nucleon; for 2p2h processes the second initial nucleon is not written out. 
 42  
 43  The final state nucleons may have masses which are spread out around the physical mass in a very narrow distribution. There are two reasons for that: 1. nucleons may still be inside the potential well and thus have lower masses. These nucleons can be eliminated from the final events file by imposing a condition that they are outside the nuclear potential (the spatial coordinates of all particles are also given in the `FinalEvents.dat` file). 2. For numericalpractical reasons the nucleons are given a BreitWigner mass distribution with a width of typically 1 MeV around the physical mass when calculating the QE cross section. 
42  44  
43  45  As an example we consider here the calculation of the CC inclusive differential cross section dsigma/dE_mu for a neutrinoinduced reaction on a nucleus; E_mu is the energy of the outgoing muon. In `FinalEvents.dat` the lines with the particle number 902 contain all the muon kinematics as well as the perweight. In order to produce a spectrum one first has to bin the muon energies into energy bins. This binning process must preserve the connection between energy and perweight. Then all the perweights in a given energy bin are summed and divided by the bin width to obtain the differential cross section. If the GiBUU run used  for better statistics  a number of runs >1 at the same energies, then this number of runs has to be divided out to obtain the final differential cross section. 
