1. Simulating the Signal Events
*********************************
1.1. Event Generation
=======================

In order to run the simulation we first of all need to generate events. This can be done on the fly with the simulation macro, the
only thing we need is a decay file for !EvtGen, which defines our decay.

In our example the decay file is given by **pp_jpsi2pi_jpsi_mumu.dec** which lists as::

  noPhotos

  Decay pbarpSystem
    1.0 J/psi pi+ pi-   PHSP; 
  Enddecay

  Decay J/psi
    1.0 mu+ mu-      VLL;
  Enddecay

  End

(NB: Since we will run the simulation at the psi(2S) energy, the analysis later will consistently use *psi(2S)* as name for the reconstructed pbar-p system - don't be confused about that.) The very first command ``noPhotos`` suppresses initial and final state radiation handled by the generator for simplicity of the analysis. 

As you can see it basically defines, that our start resonance ``pbarpSystem`` decays to ``J/psi pi+ pi-``, and ``J/psi`` to ``mu+ mu-``. The names of the resonances and particles in the decay file have to match those in the file **$VMCWORKDIR/pgenerators/EvtGen/evt.pdl**. Make sure to define the decay chain up to final state particles (e+, mu+, pi+, K+ and p+ and their charge conjugates). For any non-final-state-particle, by default all possible decays listed in **$VMCWORKDIR/pgenerators/EvtGen/DECAY.DEC** are taken into account.

When you e.g. don't define the J/psi decay in the upper example, the J/psi would, according to **DECAY.DEC**, decay into::

  Decay J/psi
  0.0593    e+   e-                PHOTOS  VLL;
  0.0588    mu+  mu-               PHOTOS  VLL;
  0.0042    rho0 pi0               VVS_PWAVE 1.0 0.0 0.0 0.0 0.0 0.0;
  0.0043    rho+ pi-               VVS_PWAVE 1.0 0.0 0.0 0.0 0.0 0.0;
  0.0043    rho- pi+               VVS_PWAVE 1.0 0.0 0.0 0.0 0.0 0.0;
  0.0036    a_20  rho0             PHSP;
  0.0036    a_2+  rho-             PHSP;
  0.0036    a_2-  rho+             PHSP;
  ...

More details concerning EvtGen have been discussed in earlier tutorials and can be found under the link EvtGen.

1.2. Simulation, Digitization, Reconstruction and Particle Identification
===========================================================================

To run the simulation for e.g. 100 events, you simply execute two macros from the directory  **tutorials/rho**  sequentially::

  > root -l -b -q 'tut_sim.C(100)'

To change the decay file (here **pp_Jpsi2pi_Jpsi_mumu.dec**), you need to give the alternative file name as a parameter for **tut_sim.C**, like

::

  > root -l -b -q 'tut_sim.C(100, "signal", "my_decay.dec")'

The second parameters "signal" is an arbitrary prefix for the output files. Be sure to put single quotes around the macro name when using ROOT in batch mode (**root -b**), otherwise the shell command is spoiled by the brackets. To run the digitization, reconstruction and PID
reconstruction you call the macro::

  > root -l -b -q tut_aod.C

without parameters for the default output name, or correspondingly for the use of another prefix::

  > root -l -b -q 'tut_aod.C(100, "signal")'

There is also shortcut by simply executing the shell script::

  > ./tut_runall.sh 100

which contains both commands from above.

After you ran these macros you should end up with a couple of new files in your directory, including 

  * **signal_sim.root** - simulation stage of analysis
  * **signal_pid.root** - the reconstructed PndPidCandidates and PID information. *This file contains the AOD and McTruth for the analysis!*
  * **signal_par.root** - geometry parameters, magnetic fields, etc


1.3. Running Fast Simulation
==============================

A simplified simulation (e.g. for testing purpose or rougher studies) can be done by using Fast Simulations. There, the initial particles from the generator(s) are either removed based on the detector acceptance or their properties are smeared according to statistical characteristics of fully simulated events. This runs at least 2-3 orders of magnitudes faster, since no particle transport with e.g. Geant or digitization/reconstruction is needed. To run it simply type

::

  > root -b -q 'tut_fastsim.C(1000)'

The output file **signal_fast.root** is created, which can be analysed afterwards with the macro **tut_ana_fast.C** instead of **tut_ana.C**. The major difference in the simulation output are the names of the PID arrays (see sec. 2.2.3), so be aware that you have to use
``PidChargedProbability`` in all places, where a PID algorithm name is required.