Anchored MC

Note that anchoring MCs are currently not supported on Mac!

Prerequisites

Please consider installing/loading one of the following alidist packages

• O2sim,
• O2PDPSuite.

These are meta packages which pull everything that is needed.

The steering script for anchored simulations is called anchoredMC.sh in O2DPG. This script respects various environment variables which are also explained in the test scripts (see links below). Among those, a very useful and powerful one is ALIEN_JDL_ANCHOR_SIM_OPTIONS which can be used to add configurations to the creation of the simulation workflow (for a very concrete example see below).

NOTE that the anchorMC.sh is not meant to be changed to introduce different configurations or features. If there is something needed that cannot be achieved with what is explained here, please contact the developers and explain your request.

Run an anchored simulation

In "anchored" MC simulations, conditions are set to match those during a real data taking run at a given time such as LHC filling scheme, included ALICE detectors, interaction rate etc. Anchored MC productions are crucial for physics analyses to have realistic simulated samples.

An anchored simulation is bound to a run which is identified by a run number. A run in turn spans from the start-of-run (SOR) to the end-of-run (EOR). This is sketched in the figure.

One anchored simulation run corresponds to one specific CYCLE of one SPLITID and contains a given number of timeframes as indicated by the big blue box on the bottom right. A full RUN is covered when all CYCLES have been produced for all SPLITIDS.

There are two examples for anchored simulations:

• PbPb,
• pp.

Not done in the scripts above, but a possible way to utilise the environment variable ALIEN_JDL_ANCHOR_SIM_OPTIONS could be to run an external generator that is configured via an ini file. In addition to the other environment variables, one would achieve this with

export ALIEN_JDL_ANCHOR_SIM_OPTIONS="-gen external -ini <path/to/config.ini>"

Pre- and post-processing steps, special configurations

To add any functionality, required pre- or post-processing steps to your simulation, please do not try to add those by editing core O2DPG scripts or files. There are two specific places where one can add additional functionality or configurations. Inp articular when preparing production requests and running tests, the official productions cannot run with custom O2DPG scripts.

1. The anchorMC.sh steering script should be called inside another shell script where, before or after running anchorMC.sh, additional pre- or postprocessing steps can be done.
   • For instance it might be necessary to copy certain files before(after) a simulation from(to) another place.
2. Add specific arguments to be forwarded to the simulation workflow creation via the environment variable ALIEN_JDL_ANCHOR_SIM_OPTIONS (see also examples in the sections below).

Run Pythia with a different collision system

If you need to run Pythia8 with a different collision system than the one during data taking, you can make use of the above method as well. As a first step, you need to provide your custom cfg file for Pythia8. For examples, please have a look here. Then, configure it via

export ALIEN_JDL_ANCHOR_SIM_OPTIONS="-gen pythia8 -confKey GeneratorPythia8.config=<pth/to/pythia_config.cfg>"

Such a configuration can also be build by using the mkpy8cfg.py. For instance, you could run that first to produce your configuration and pass it on. Here is an example

${O2DPG_ROOT}/MC/config/common/pythia8/utils/mkpy8cfg.py --output pythia8.cfg --seed 5 --idA 2212 --idB 2212 --eCM 5020 --process jets

# required ALIEN_JDL_* variables
export ALIEN_JDL_ANCHOR_SIM_OPTIONS="-gen pythia8 -confKey GeneratorPythia8.config=$(pwd)/pythia8.cfg"

# the following are example values
export NTIMEFRAMES=2
export NSIGEVENTS=50
export SPLITID=100
export PRODSPLIT=153
export CYCLE=0

# on the GRID, this is set, for our use case, we can mimic any job ID
export ALIEN_PROC_ID=2963436952

# run the central anchor steering script; this includes
# * derive timestamp
# * derive interaction rate
# * extract and prepare configurations (which detectors are contained in the run etc.)
# * run the simulation (and QC)
${O2DPG_ROOT}/MC/run/ANCHOR/anchorMC.sh

In addition to the collision system, one can also influence other parameters, like in this case the centre-of-mass energy.

Environment variables

The anchorMC.sh script is sensitive to the following environment variables. Please set/export them, especially in case they are required.

Behind the scenes

The procedure steered behind the scenes is quite involved. The following figure shall provide some overview.