This crate implements the cell resampling algorithm for the elimination of negative weights in Monte Carlo collider event samples. The algorithm is described in
Unbiased Elimination of Negative Weights in Monte Carlo Samples J. Andersen, A. Maier arXiv:2109.07851
If Rust and Cargo are installed on your system, run
cargo install cres
Precompiled executables are available on hepforge.
The basic usage is
cres -a JETALGO -R JETR --jetpt JETPT -o OUT.HEPMC2 IN.HEPMC2
This takes a file IN.HEPMC2 in hepmc2 format with mixed-weight
events and produces a file OUT.HEPMC2 where all event weights are
positive. The input file can be compressed with bzip2, gzip, zstd, or
lz4.
We recommend to set the jet algorithm JETALGO, jet radius
JETR, and minimum jet transverse momentum JETPT to the same values
that were used to generate the input events. The supported jet
algorithms are anti-kt, kt, and Cambridge-Aachen.
cres assumes that the total cross section is given by the sum of
all event weights. If this is not the case one should pass a
normalisation factor via the --weight-norm option.
To see a full list of options with short descriptions run
cres --help
The most important options are
--max-cell-size can be used to limit the size of the generated
cells. This ensures that weights are only transferred between events
which are sufficiently similar. The downside is that not all
negative event weights will be removed. If you use this option, we
recommend values that are not too much smaller than the median
radius that cres shows during a standard run.
--ptweight specifies how much transverse momenta affect distances
between particles with momenta p and q according to the formula
d(p, q) = \sqrt{ ptweight^2 (p\perp - q\perp)^2 + \sum (pi - qi)^2 }
--strategy sets the order in which cell seeds are selected. The
chosen strategy can affect generation times and cell sizes
significantly. It is not clear which strategy is best in general.
With --minweight events are also unweighted in addition to the
resampling. Events with weight w < minweight are discarded with
probability 1-|w|/minweight and reweighted to sign(w) * minweight
otherwise. Finally, all event weights are rescaled to exactly
preserve the original sum of weights. The seed for unweighting can
be chosen with the --seed option.