Rex

Rex offers a fast emulation of the LHCb detector simulation. From truth level kinematics to reconstructed quantities and all commonly used reconstruction quality variables.

Table of Contents

• About
• Installation
• Usage

About

A full description of the tool can be found in this 📖 arXiv preprint.

📖 pypi | 📖 Project Docs | 💬 Mattermost Chat | 📬 Developer Email

Installation

On lxplus

Set some environment variables

export REX_DIR=/afs/cern.ch/work/X/XXXXXXX/Rex_install
export REX_WEIGHT_DIR=$REX_DIR  # note without this variable set weights default to ~/.cache/lhcb_rex/
export RAPIDSIM_ROOT=$REX_DIR/RapidSim

Make package install directory

mkdir $REX_DIR

Install RapidSim

source /cvmfs/sft.cern.ch/lcg/views/setupViews.sh LCG_105 x86_64-el9-gcc11-opt
cd $REX_DIR && git clone git@github.com:gcowan/RapidSim.git && cd RapidSim && mkdir build && cd build && cmake .. && make -j4 && cd $REX_DIR

for full use of the package we also need an EVTGEN install evtgen.hepforge.

export EVTGEN_ROOT=....

Create new localised python environment and install lhcb_rex

export UV_CACHE_DIR=$REX_DIR/.uv-cache # overwrite UV cache directory, default is ~/.cache/uv but quota is small here
uv venv --python 3.9
source $REX_DIR/.venv/bin/activate
uv pip install lhcb_rex

remove any reference to /cvmfs/ from PYTHONPATH (had problemes with cross-contamination)

export PYTHONPATH=$(echo "$PYTHONPATH" | tr ':' '\n' | grep -v "/cvmfs/sft.cern.ch" | paste -sd ':' -)

Check install has succeeded

python -c "import lhcb_rex"

or

python -c 'import lhcb_rex; lhcb_rex.run(events=1000,decay="Bs0 -> mu+ mu-",naming_scheme="MOTHER -> DAUGHTER1 DAUGHTER2",decay_models="PHSP -> NA NA",workingDir="./Bs_mumu",)'

the above took ~10 mins on lxplus, woefully slow. It took about 50 seconds (including all initialisation time) on cpu at my university cluster, 20 seconds on GPU node (mostly configuration time).

Create ~/rex_setup.sh to configure environment next time, something like this

export REX_DIR=/afs/cern.ch/work/X/XXXXXXX/Rex_install
export REX_WEIGHT_DIR=$REX_DIR
export RAPIDSIM_ROOT=~/RapidSim
source /cvmfs/sft.cern.ch/lcg/views/setupViews.sh LCG_105 x86_64-el9-gcc11-opt
export PYTHONPATH=$(echo "$PYTHONPATH" | tr ':' '\n' | grep -v "/cvmfs/sft.cern.ch" | paste -sd ':' -)
source $REX_DIR/.venv/bin/activate

then on login

source ~/rex_setup.sh

Note, if you want progress bars printed whilst RapidSim runs you need to install this patch

cd $RAPIDSIM_ROOT
git apply <(curl -L https://raw.githubusercontent.com/alexmarshallbristol/Rex-image/refs/heads/master/print_n.patch)
cd build
make

Usage

Examples below

import lhcb_rex

lhcb_rex.run(
    events=1000,
    decay="Bs0 -> mu+ mu-",
    naming_scheme="MOTHER -> DAUGHTER1 DAUGHTER2",
    decay_models="PHSP -> NA NA",
    workingDir="./Bs_mumu",
)

Installation for development

git clone
uv sync
uv run pip install -e .

📬 Have questions? Reach out at alex.marshall@bristol.ac.uk

Notes for me

/dice/users/am13743/fast_vertex_quality/graveyard/ /dice/users/am13743/fast_vertex_quality/checkpoints/

Relevant tags/checkpoints:

• 21_03_2025 - Plotting for paper draft

rm -rf dist/
uv build
uv publish
uv run pip install --no-deps --upgrade --force-reinstall -e . # to update the version number as in pyproject.toml