In this tutorial, we use the H>WW>lnln analysis as an example to illustrate how to build a physics analysis based on both cutncount and shape method and calculate the upper limits, significance. For simplification, we use ideal MC sample without reconstruction and include only one background (continuum WW) in the search.
Disclaimer: Numbers in this tutorial are motivated by the HWW analysis, but do not have onetoone correspondance to the HWW results. Please do not be alarmed that your result is not the same as in the official note or AN.
note: as for the rest of CMSSW software, this requires scientific linux 5, so you should log into lxplus5.cern.ch and not lxplus.cern.ch
setenv SCRAM_ARCH slc5_amd64_gcc472 # use export SCRAM_ARCH=slc5_amd64_gcc472 for bash cmsrel CMSSW_6_1_1 cd CMSSW_6_1_1/src cmsenv addpkg HiggsAnalysis/CombinedLimit V030112 scramv1 b
After this, you have an excutable called "combine" that can be used to do tons of statistical calculations. Use combine help
to see the ultimate list of options.
In a cut and count analysis, we need to know what is the number of signal and background events expected, the number of events observed in data and associated systematic uncertainties. Each source of the systematic uncertainty is labelled as a nusiance. It is important to notice that even the MC statistic error for a given background is assumed as a nusiance.
Datacard is the input to run the statistical tools in the Higgs analysis in CMS. This can be adapted for other search program as well. There are two different softwares in CMS that does this computation, one is in LandS and other is in HiggsLimit/combination. The common data card for a cutbased analysis is given below.
Here is an example of the card called hww_20fb_cut.txt. In this example we consider 20% systematic uncertainty for the signal and 10% for the background.
imax 1 number of channels jmax * number of background kmax * number of nuisance parameters Observation 505 bin 1 1 process HWW qqWW process 0 1 rate 90.000 430.000 uncert_HWW lnN 1.200 1.000 uncert_qqWW lnN 1.000 1.100
combineCards.py of0j=hww_20fb_cut.txt > hww_20fb_cut_comb.txt
Note that if you have other channel (such as hww_20fb_1j_cut.txt), this can also combine them together by append other channels such as
combineCards.py of0j=hww_20fb_cut.txt of1j=hww_20fb_1j_cut.txt > hww_20fb_cut_comb.txt
combine d hww_20fb_cut_comb.txt M Asymptotic Output:  Asymptotic  Observed Limit: r < 1.8221 Expected 2.5%: r < 0.6019 Expected 16.0%: r < 0.8016 Expected 50.0%: r < 1.1289 Expected 84.0%: r < 1.6194 Expected 97.5%: r < 2.2782
This indicates that the 95% upperlimit observed is 1.8, while the median expected is 1.1 with 1sigma band [0.8, 1.6] and 2sigma band [0.6, 2.2]. This result tells us that we observed an excess in data at the level of 12 sigma.
combine d hww_20fb_cut_comb.txt M ProfileLikelihood v 1 significance expectSignal=1 t 1 m 125 n Expected Output:  Profile Likelihood  Significance: 1.80964 (pvalue = 0.035176)
combine d hww_20fb_cut_comb.txt M ProfileLikelihood v 1 significance expectSignal=1 t 1 m 125 Output:  Profile Likelihood  Significance: 1.52713 (pvalue = 0.063364)
A maximum likelihood scan is performed to get the +/ 1 sigma error.
combine d hww_20fb_cut_comb.txt M MaxLikelihoodFit Output:  MaxLikelihoodFit  Best fit r: 0.832828 0.535222/+0.559076 (68% CL) nll S+B > 6.19048 nll B > 5.02441

