Introduction

This resource contains the full statistical model from the search for charged, long-lived supersymmetric particles in final states with one or more disappearing tracks CMS-SUS-21-006. The instructions below include a few basic examples on how to extract the significance and signal strength measurements, for more details please consult the Combine documentation.

Software instructions

General installation instructions for Combine can be found in the Combine documentation.

A container image is provided to ensure reproducible results. The results in this README are obtained using v9.2.1:

docker run --name combine -it gitlab-registry.cern.ch/cms-cloud/combine-standalone:v9.2.1

A slim version of the container image is also available at gitlab-registry.cern.ch/cms-cloud/combine-standalone:v9.2.1-slim. Versions of packages in the slim container image do not match exactly with the ones in the default container, so small differences in the output of commands with respect to the ones shown below are to be expected.

You can copy files (such as the datacards and other inputs for combine) using docker cp as documented here.

For the commands below, you may require running ulimit -s unlimited; ulimit -u unlimited to avoid memory issues.

Datacards

This resource contains the datacard input/datacard.txt for all models and parameter points. In the datacard all signal rates are set to 1., and are evaluated for different mass points with --setParameters mChine=180,dm=0.193 --freezeParameters mChine,dm as shown in the example below. Cross section values for different values of model parameters are store in RooSplineND objects in input/signal_xs_and_unc/model_<model_name>.root files.

In addition, the background-only datacard is provided as input/bkg-only-datacard/datacard.txt. In this datacard, all signal yields are set to 0, and only uncertainties modifying background processes are present.

Limits

The limits for this analysis are calculated using the AsymptoticLimits method. This analysis contains several signal models T5btbtLL, T6btLL, T6tbLL, PureHiggsino, PureWino with multiple parameters points listed in the model_info.json file. For example to extract the limits using PureHiggsino signal model for chargino-LSP mass difference (dm) of 0.193 GeV and the chargino mass(mChine) of 180 GeV the following commands have to be used:

export model=PureHiggsino
combine -M AsymptoticLimits input/datacard.txt  --setParameters mChine=180,dm=0.193 --freezeParameters mChine,dm

Note that it is important to set the model environment variable export model=PureHiggsino. In the datacards, in order to improve numerical stability during limit calculation, signal yields were scaled with a so-called SHRINKFACTOR which is listed the model_info.json file for each parameter point. This factor needs to be taken into account in order to obtain the correct limits on cross sections, as:

xseclimit = refXsec*limit*shrinkfactor

where refXsec is cross section of the signal model and limit is any of the observed or expected limits output by combine. Note that this scaling procedure is optional, and does not need to be used in studies with new signals.