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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 CandJetMatcher (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 CandJetMatcher
$ cd CandJetMatcher
$ mkedanlzr CandJetMatcher

This creates the following directory hierarchy: (put a picture here). From now on, we will always work with the (deeper) CandJetMatcher directory as our main directory, except when told otherwise.

  • Copy the EDAnalyzer from analysis_SPRACE to the src directory - overwriting the one you just created.

  • Issue the scramv1 b command to compile the EDAnalyzer - it should compile fine.

  • 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, but NOT to hadronization - therefore, the Events have partons as "final-state particles".

  • Copy the Configuration Files w0.cfg, w1.cfg to the main 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 2 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: agrupar_pt20_R05.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 an EXCLUSIVE way. 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).

  • Run all the Configuration Files: cmsRun w0.cfg , ... . This step can be quite time-consuming, because most of the source files reside either at CERN and/or FNAL, and they must be locally cached before analysis can begin. This will produce the wnj_ic_pt20_R05.root files.

  • Run the ROOT macro: root -l -q agrupar_pt20_R05.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.

ThiagoTomei - 09 Oct 2007
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Topic revision: r3 - 2007-10-09 - ThiagoTomei

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