W + Jets Analysis

Download the tar.gz file that contains the relevant files, and unpack it somewhere. It automatically creates the analysis_SPRACE directory.

Create an EDAnalyzer called VnjetAnalyzer (the name doesn't really matter, it's just to keep source file compatibility). From the $CMSSW_BASE dir (the CMSSW_1_6_0 dir in this case), issue the following commands:

$ mkdir VnjetAnalyzer
$ cd VnjetAnalyzer
$ mkedanlzr VnjetAnalyzer

This creates the following directory hierarchy:

Copy the EDAnalyzer (in our case, the VnjetAnalyzer.cc source file) from analysis_SPRACE to the src directory - overwriting the one you just created. Study the structure of the file, and how it uses the Analysis Tools. For more information, check the CMS Workbook in Analysis Tools. Also, copy the BuildFile to the (deeper) VnjetAnalyzer directory.

Issue the scramv1 b -r command to compile the EDAnalyzer - it should compile fine. If it does not, you probably misplaced the BuildFile. Correct the mistake and issue the command again. The -r option tells the scramv1 b command to reread all files - if the =BuildFile=s have not changed, it is usually safe to skip that option.

Copy the Configuration Fragments w0.cff, w1.cff, ..., to the $CMSSW_BASE/share directory. These Configuration Fragments contain the location of the sources (ROOT files) for our analysis. The sources contain w + jets Events. The number following the letter w is the number of hard partons that were simulated at matrix-element level. After that, the final state is subject to parton showering and hadronization. These files are NOT used at the moment, but they may be used in the future if the samples are stored in the SPRACE cluster at some point in the future.

Copy the Configuration Files w0.cfg, w1.cfg to the VnjetAnalyzer directory. These Configuration Files accomplish the following tasks: scan through each Event in the source, select all partons, create parton-level jets ("clusters") with a given cone radius (default is 0.5), cut on the pt of the jets (default is 20 GeV), and produce a series of histograms. Let's focus our attention in two particular histograms: "totalJets", with the number of jets per Event, and "pt_tJets" with the cumulative pt distribution of jets for all Events. Each .cfg is linked to one of the .cff, and produces a ROOT file named wnj_ic_pt20_R05.root, with n the same as in the .cfg and the .cff.

Copy this ROOT macro: group.C to the main directory. What this macro does is to group all the ROOT files produced by the .cfg files in a single ROOT file. It also sums the histograms for number of jets per Event and cumulative pt distribution in both exclusive and inclusive ways. For instance, in the "nexclusive" histogram which is created, each bin corresponds to the number of Events that have EXACTLY that number of jets. (There is also a luminosity scale factor involved, so that for a given .cfg file the "totalJets" histogram corresponds to 100/pb). The "ninclusive" histogram works in a similar fashion, except that each bin corresponds to the number of Events that have AT LEAST that number of jets.

Run all the Configuration Files: cmsRun w0.cfg, ... This will produce the wnj_ic_pt20_R05.root files.

Run the ROOT macro: root -l -q group.C. This will produce the "allw_ic_pt20_R05.root" file.

Plot the resulting file with ROOT: root -l allw_ic_pt20_R05.root, and select the "nexclusive" histogram. Plot it in log scale to see that the log of number of events with n jets is a linear function of n. That is because the cross-section for n jets scales as a^n, where a is the strong coupling constant, ("alpha_strong"). If one now fits a linear function to that histogram, the slope should be ln a. It works similarly for the "ninclusive" histogram.

-- ThiagoTomei - 16 Nov 2007