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$\Sigma^0$ Analysis

Description

The following code is used to study properties of the $\Sigma^0$ hyperon (u,d,s quarks, I = 1) via its electromagnetic decay into a $\Lambda$ hyperon (same quark content, but I = 0):

$$\Sigma^0 \rightarrow \Lambda \gamma$$

The $\Lambda$ hyperon is then reconstructed via its most common (BR $\approx$ 64%) decay into a proton and a charged pion, using information on energy losses of the decay products in the Time Projection Chamber (TPC):

$$\Lambda \rightarrow p \pi^-$$

The photon is registered via the photon conversion method (PCM): a photon has some probability to create an electron-positron pair while traversing the material of the Inner Tracking System (ITS). This pair is then detected.

THe analysis procedure is coded using C++ by creating a structure Sigma0Reconstruction that has six main parts:

  • definition of configurables and filters;

  • function init that books histograms;

  • function processCollisions, where general information on collisions is obtained;

  • function processGeneratedCollisions, where information on particles generated in 4$\pi$ geometry is obtained;

  • function processConversionPhotons, where one reconstructs photons using the PCM;

  • function processLambdas, where the $\Lambda$ hyperons are reconstructed.

Repository Content

This repository contains several files:

  • This README file;

  • Sigma_0_Analysis.cxx, containing the analysis procedure's description (namely, the structure's methods);

  • config.json, containing the configuration settings of the analysis;

  • a bash script run.sh, used to initialize the analysis.

The full code lisence can be found in Sigma_0_Analysis.cxx.

Prerequisites

In order to use the following code one must first install O2 and O2Physics via AliBuild. After installing O2Physics it can be verified by typing

alienv enter VO_ALICE@O2Physics::latest

If no errors appear, the installation has been successful.

Usage

Tip

In order to avoid the necessity to add a cmake file to the O2Physics repository, one can simply place the analysis code into some unused .cxx file already present there, e.g. replace the contents of ~/O2Physics/Tutorials/PWGMM/myExampleTask.cxx with the new code. The build can then be completed withoud modifying the cmake files. ROOT, JSON, and bash files do not require any extra actions either way

Code Modification

Any modification of the code requires a rebuild of the existing installation of O2Physics. To do that, move to the directory where O2Physics is installed and run the following command:

aliBuild build O2Physics --debug

This will (the debug option allows to get a full backtrace, shall an error occur). If the build is successful, one can run the analysis.

Running Code Locally

Move to the folder where your code is located. Running locally requires having an input file of type AO2D.root. Enter the O2Physics shell by typing

alienv enter VO_ALICE@O2Physics::latest

Now run the bash script triggering the analysis (this may require changing the ownership of the file via chmod 777):

./run.sh

This will initialize the analysis by invoking relevant tables and using the settings from the configuration file.

After the execution is over, the folder will contain a AnalysisResults.root file containing all output histograms and tables.

Further Reading

Please refer to the O2 Analysis Framework Documentation, for detailed analysis and troubleshooting instructions.