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Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy.

Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. Angelika Fertig University of Cambridge Supervisor: Klaus Mönig DESY Zeuthen, 09.09.2010. Outline. Introduction Data simulation and reconstruction Results Conclusions and outlook.

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Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy.

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  1. Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. Angelika Fertig University of Cambridge Supervisor: Klaus Mönig DESY Zeuthen, 09.09.2010

  2. Outline Introduction Data simulation and reconstruction Results Conclusions and outlook

  3. Introduction Jet = collimated bunch of hadrons. Most commonly: 2-jet events. Elementary two-parton interactions: Multijets (2 or more jet events), via higher order QCD processes, e.g. final state radiation of gluons. Due to colour confinement, quark-antiquark pairs are created out of the vacuum => Complicated events. The data was simulated with Pythia 6, a Monte Carlo event generator and a jet algorithm was used to identify jets. Graphics by Torbjörn Sjöstrand

  4. Data simulation and reconstruction Jet selection criteria During the analysis cuts are applied that remove jets with low energy. Three different types: 1. Leading jet: pT,min = 60 GeV; all other jets: pT,min = 30 GeV. 2. Cut at HT/5 (HT = sum of the pT of all jets), and either pT,min = a) 1 GeV or b) 30 GeV for all jets. 3. pT,min = 24 GeV and 40% of the pT of the leading jet, for all jets. Graphic by Torbjörn Sjöstrand

  5. Fraction of cross sections for inclusive 2-jet events gg, qg and qq scattering most dominant, fraction changes with energy. Only virtual antiquarks => small σ for qqbar interactions. qg sc. gg sc. qq sc. gg -> qqbar qqbar -> qqbar qqbar -> gg

  6. Fraction of cross sections for inclusive 3-jet events Only one cut shown, others similar. σ of inclusive 3-jets similar to σ of inclusive 2-jets, except for qq and gg behaviour. qg sc. gg sc. qq sc. gg -> qqbar qqbar -> qqbar qqbar -> gg

  7. Ratio of cross sections for inclusive 3-jet and inclusive 2-jet events = 3 = 4/3

  8. Conclusions and outlook Inclusive multijet cross sections and their ratios for different processes and cuts have been studied. The different elementary processes vary in cross section. The ratios of the cross sections of inclusive 3-jet and 2-jet events vary due to the amount of colour charges the partons carry (gluons have twice the colour charge of quarks). The simulated MC events agree with real data taken from ATLAS. In the future, these simulations can help to eventually calculate the running of the strong coupling constant, αs.

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