Using Combine at Purdue AF¶

Combine is a RooStats / RooFit based software framework widely used in LHC experiments for statistical analysis of experimental data.

Combine can be used either in a CMSSW environment or in standalone mode in Pixi or Conda environments. At Purdue AF, we recommend using standalone mode, because some CMSSW releases are not compatible with the operating system, and loading other operating systems — while possible via Apptainer — can cause unexpected issues.

The fastest way to try Combine is the global Pixi environment, which includes the cms-combine package (its version is listed in the environment's pixi.toml):

$ pixi shell --manifest-path /work/pixi/global/pixi.toml
$ combine --help

Combine in Pixi environments¶

Install from conda-forge (recommended)¶

You can easily install Combine from the conda-forge distribution into your own Pixi environment by adding the cms-combine package to your Pixi project. Run the following command in the project directory (which contains the pixi.toml file):

pixi add cms-combine

OR add the package explicitly to the [dependencies] section of the pixi.toml file, optionally pinning a version:

[dependencies]
cms-combine = "*"

Install from source¶

You can also install Combine from source by adding a "task" (custom command) to your Pixi project. This can be useful if you want to test features in an unreleased version of Combine.

All you need to do is copy the following code block into the [tasks] section of the pixi.toml file:

[tasks]
install_combine = """
sh -c '
set -e

# Delete existing Combine installation, if present
rm -rf HiggsAnalysis/CombinedLimit

# Clone latest Combine version from GitHub
git clone https://github.com/cms-analysis/HiggsAnalysis-CombinedLimit.git HiggsAnalysis/CombinedLimit
cd HiggsAnalysis/CombinedLimit

# Get the Python major.minor version from the active pixi env
PY_VER=$(python - << \"PY\"
import sys
print(f\"{sys.version_info.major}.{sys.version_info.minor}\")
PY
)

# Install Combine into the environment
cmake -S . -B build \
-DCMAKE_INSTALL_PREFIX=\"$CONDA_PREFIX\" \
-DCMAKE_INSTALL_PYTHONDIR=\"lib/python${PY_VER}/site-packages\" \
-DUSE_VDT=OFF
cmake --build build -j\"$(nproc --ignore=2)\"
cmake --install build

cd -
'
"""

Then, once your environment is built, you can install Combine by running:

pixi run install_combine

Combine in custom Conda environments¶

If you want to use Combine in your own Conda environment, you can similarly install it from conda-forge by adding the cms-combine package to the environment.yaml file.

To install Combine from source into a Conda environment, follow the instructions below:

git clone https://github.com/cms-analysis/HiggsAnalysis-CombinedLimit.git HiggsAnalysis/CombinedLimit
cd HiggsAnalysis/CombinedLimit

# configure conda-forge as the preferred channel
conda config --set channel_priority strict
conda config --add channels conda-forge

# activate the environment
conda activate <your-conda-environment>

# install Combine's dependencies:
conda install root=6.34 gsl boost-cpp vdt eigen tbb cmake ninja
# NOTE: if your environment is managed using an environment.yaml file,
# add these dependencies to it and rebuild the environment, instead of installing them here.

# configure and build with CMake
cmake -S . -B build -DCMAKE_INSTALL_PREFIX=$CONDA_PREFIX -DCMAKE_INSTALL_PYTHONDIR=lib/python3.12/site-packages -DUSE_VDT=OFF
cmake --build build -j$(nproc --ignore=2)
cmake --install build

After installation, to use Combine in a new Terminal you only need to activate the environment.