Re: Pythia and generated particles

From: Mario Kadastik <>
Date: Mon, 14 Feb 2005 23:08:38 +0200

Rene BRUN wrote:

>We already have many examples with TClonesArray in the tutorials.
>See, for instance: tcl.C, Jets.C, pythiaExample.C

well it was quite difficult to find a way to actually address the n-th element of the TClonesArray. I finally just tried ->At(index) and found out that it worked :) I checked into the tutorial examples and it was quite difficult to actually understand that with the At() subroutine you can access the real objects that are in the TClonesArray. I am new to root and may be looking at a different viewpoint, but to me it seems that a lot of important aspects haven't been covered :) For example I haven't found a nice way to do these things:

  1. after writing an array as a branch, to read it back into an array without SIGSEGV
  2. write a simple Float_t variable into a root file

For example I'm currently stuck with the next big thing :) Namely I want to generate a number of events with Pythia, write all the generated particles into root tree for later analysis and in addition I want to save the total process cross section also into that root tree.

What I would have expected is that I can access the cross section with this: pythia->GetXSEC(isub, 3)
as it seems to be missing I wen't for the Pyint5 common block. But for some reason it's not working either:

root [2] TPythia6 *pythia = new TPythia6
root [3] generalProperties(pythia)
root [4] useDoubleHiggs(pythia,200,400,1,8,8)
root [5] pythia->Initialize("cms","p","p",14000)
<skipped the info text>
           I                                      I                 I
           I  ISUB  Subprocess name               I  Maximum value  I
           I                                      I                 I
           I                                      I                 I
           I   96   Semihard QCD 2 -> 2           I    2.0977E+04   I
           I  349   f + fbar -> H_L++ + H_L--     I    1.6608E-09   I
           I                                      I                 I

well for some reason I couldn't access the cross section information while I did find someone else use an earlier version of root similarily:

#if __PYTHIA_VERSION__ >= 6

   Pyint5_t *pyint5=fPythia->GetPyint5();    for(i=0;i<fNUMSUB;i++){




     fNGEN[i]=fPythia->GetNGEN(ns[i], 3);
     fXSEC[i]=fPythia->GetXSEC(ns[i], 3);

(taken from:

I'm unable to understand how should I access the XSEC information from pythia. And also how to store just one float number in a root file for later usage???

Mario Received on Mon Feb 14 2005 - 22:08:04 MET

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