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Questions and answers

User Support Questions&Answers sessions can be held on Tuesdays at 9:30 am. in 40-RA-10. The next session will be held when need arises. You can use this page to submit your questions: add your question here using the following format (you can edit this page by clicking on the Edit button top right) and send me a short mail with your subject just to notify. The answers will be given in the User Support session and they will appear in this page. If you are not at CERN, and cannot attend the sessions, you can still add your questions here (indicate that you will not attend the session). Please check the existing documentation, especially the CMSWorkbook before adding your question.

Some questions (and answers) concerning common problems have been included here from the Hypernews forums.


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Problems accessing more than one file

hn-cms-swDevelopment, Christian Veelken, Wed, 13 Sep 2006

I have two Monte Carlo files that were produced independently and each contain 500 events. I have written a class derived from EDAnalyzer to make plots of the file content. When I run over one of the two files at a time, everything is fine, no problem. But when I run over both MC files in the same cmsRun job, cmsRun crashes when processing the first event in the second file.


My guess is the program crashes in your EDAnalyzer. Are you by chance filling your own ROOT file from within the EDAnalyzer? If so, then other have found that when the PoolSource goes from one input ROOT file to another one their code crashes. What was determined is unless you've explicitly openned your own ROOT file and tied your histogram, ntuples, etc. to your file, then ROOT will tie those things to the first 'input' file and when that input file goes away the pointers to your histograms, ntuples, etc. then point to object that no longer exist (since ROOT deleted them when the file closed). So it look like you need to modify the ROOT calls you are making in your EDAnalyzer. (Chris Jones)

If Chris' diagnosis is correct (very likely), you can have a look at RecoEcal/EgammaClusterProducers/src/ where I had a similar problem and is now solved by correctly binding the histos and trees to my root file w/o interfering with PoolSource. (Shahram Rahatlou)

Where can I find the data?

How do I know what data is available (real data from MTCC or simulation data) and how can I access it?


For the MTCC data (cosmic muons at CMS) see the instructions in the IGUANA web page, and in particular the data file location.

There are no large centrally produced simulated data sets, yet. For each release of the CMSSW packages, a set of of small event samples are produced which are mainly used to validate the release. Find the instructions how to access this data in the release validation samples page.

How to save cross section in the root file?

Sami Lehti, 21 Jul 2006

In an analysis containing several channels (signal+backgr), it is very helpful if some normalization constants, like cross section and preselection efficiency, can be stored in the root file. How is this done in practise (a working example would be nice)? How is the cross section read from the root file? Can the stored cross section be changed by hand afterwards? For example if i have 1M events in root file(s), and i get a new cross section from theorists, do i have to rerun the 1M events, or can i change the cross section value in the root file(s)? If yes, how?


1) How to add cross-section?

The mechanism to add a cross-section to the run or job (i.e. one number per run) is not yet ready, it is planned but not yet there (it is foreseen by the end of August, at the earliest)

Meanwhile, you can try the following solution which writes a number to each event.

2) How to change an entry in a root file?

-- KatiLassilaPerini - 13 Jul 2006

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Topic revision: r14 - 2006-09-15 - KatiLassilaPerini

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