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Simulations using POWHEG box

POWHEG box is a computer framework for implementation of NLO calculations in Monte Carlo programs according to the POWHEG method. It is coded in FORTRAN and for purpose of this analysis, only Dijet process was utilised. More informations about POWHEG can be obtained here.

Software is ready to compile in $POWHEG/Dijet directory, creating pwhg_main file that will utilise information from powheg.input. This file contains information about simulated collisions, such as are shown in following lines, where number of simulated events colliding particles and beam energies are set up.

  numevts 10000      ! number of events to be generated
  ih1  1               ! hadron 1 (1 for protons, -1 for antiprotons)
  ih2  1               ! hadron 2 (1 for protons, -1 for antiprotons)
  ebeam1 3500d0        ! energy of beam 1
  ebeam2 3500d0        ! energy of beam 2

To use different (external) pdfs, one has to edit Makefile according to instructions before compilation. Then, by editing following lines in powheg.input PDF sets can be set up:

 ! To be set only if using internal (mlm) pdfs
 ! 131 cteq6m
 ! ndns1 131            ! pdf set for hadron 1 (mlm numbering)
 ! ndns2 131            ! pdf set for hadron 2 (mlm numbering)

 ! To be set only if using LHA pdfs
 ! 10550 cteq66-0
 lhans1  10550      ! pdf set for hadron 1 (LHA numbering)
 lhans2  10550      ! pdf set for hadron 2 (LHA numbering)

We are using cteq6.6, which have code 10550-10594 for central dataset + errors. In this analysis, LHAPDF 5.9.8 was used.

Additional parameters of powheg.input file stayed for purpose of this analysis unchanged (except for LHA dataset selection and seeding for parallel simulations). Furthermore, one can set up weighted event generation.

  • bornsuppfact
  • bornktmin

In order to speed up production of jets in wide range of , we use these parameters, setting up bornsuppfact to 70 and bornktmin to 1 GeV. First parameter sets suppression for cross section as a function of kT. Value of suppression factor serves as inverse value of weight of event.

More information can be found here and here.

Running of pwhg_main creates several files from which the important ones are pwgevents.lhe and pwgstat.dat which contain simulated collisions and cross section information respectively. First is Les Houches event file containing information about simulations parameters and events with 5 particles, 2 colliding and 3 created. This file is analysed by Pythia producing final state particles.

Pythia8

For event reconstruction from simulated lhe files from POWHEG Box files, I am using Pythia 8.176 compiled with boost and FastJet libraries. Pythia is set up using following lines:

  • pythia.readString("Beams:frameType = 4");
  • pythia.readString(inputFileName);
No additional setting up is required. Default Tune 4C was used in main analysis. Output particles are analysed using FastJet 2.4.3.

FastJet

FastJet 2.4.3 was used in analysis of final state pythia particles. We used anti-kT jet algorithm with boost invariant pT recombination scheme and best strategy. Area was determined using Voronoi algorithm in order to speed up reconstruction.

Analysis

Particle cuts:
* pT > 150 MeV/c
* | eta | < 0.9

Jet cuts:
* various R : 0.2, 0.3, 0.4, 0.6
* | eta_jet | < 0.9 - R
* pT_jet > 10 GeV/c

Underlying event subtraction

In analysis, we look on jets with & without subtracted underlying event. This is calculated on event by event basis by looking inside cone perpendicular to jet axis in azimuth, while pseudo-rapidity stays constant.

Systematics

In order to evaluate systematic uncertainty on produced NLO predictions, several checks have to be done.

Scale

We have to vary factorisation and renormalisation scales by factors { 0.5, 1.0, 2.0 } each, excluding combinations of highest and lowest values, and case of factor=1.0 in both cases corresponds to default calculations.
These variation yield 15% up and down change to jet energy scale (taking into account only maximal change).

PDF errors

Another systematic chech is usage of error PDFs with respecto central CTEQ6.6m PDF. This error is calculated as:


where f is observable, in our case jet, and f^{0} is jet calculated using central PDF's, f^{+} is jet calculated using error PDF with positive variation of parameter i and similarly f^{-} is jet calculated with error PDF with negative variation of this parameter. CTEQ6.6m contains 1+44 PDF sets, which stand for 1 central set, 22 sets with positive and 22 sets with negative variations.

In our case for every pT bin, was calculated from non-negative diferences between error PDF and central PDF.

Fragmentation variation (variation of PYTHIA tune)

Comparison of NLO with DATA

pp @ 7 TeV

pp @ 2.76 TeV

Interpolation to 5.02 TeV

Citations

* Jet pair production , S. Alioli, K. Hamilton, P. Nason, C. Oleari and E. Re, JHEP 1104 (2011) 081, arXiv:1012.3380.
* T. Sjöstrand, S. Mrenna and P. Skands, JHEP05 (2006) 026, Comput. Phys. Comm. 178 (2008) 852.
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Topic revision: r6 - 2013-10-10 - MichalVajzer
 
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