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. Angelika Fertig | Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. | 09.09.2010 | Page 2 Outline > Introduction > Data simulation and reconstruction > Results > Conclusions and outlook

3. Angelika Fertig | Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. | 09.09.2010 | Page 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. Angelika Fertig | Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. | 09.09.2010 | Page 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: p T,min = 60 GeV; all other jets: p T,min = 30 GeV. > 2. Cut at H T /5 (H T = sum of the p T of all jets), and either p T,min = a) 1 GeV or b) 30 GeV for all jets. > 3. p T,min = 24 GeV and 40% of the p T of the leading jet, for all jets. Graphic by Torbjörn Sjöstrand

5. Angelika Fertig | Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. | 09.09.2010 | Page 5 Fraction of cross sections for inclusive 2-jet events qg sc. qq sc. gg sc. qqbar -> gg qqbar -> qqbar > gg, qg and qq scattering most dominant, fraction changes with energy. > Only virtual antiquarks => small σ for qqbar interactions. gg -> qqbar

6. Angelika Fertig | Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. | 09.09.2010 | Page 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. Angelika Fertig | Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. | 09.09.2010 | Page 7 Ratio of cross sections for inclusive 3-jet and inclusive 2-jet events = 4/3 = 3

8. Angelika Fertig | Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy. | 09.09.2010 | Page 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, α s.