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
Feb 05, 2016
Pythia simulations of multijet events in proton-proton collisions at 7 TeV centre-of-mass energy.
Angelika FertigUniversity of CambridgeSupervisor: Klaus MönigDESY Zeuthen, 09.09.2010
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
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
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: pT,min
= 60 GeV; all other jets: pT,min
= 30 GeV.
> 2. Cut at HT/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. pT,min
= 24 GeV and 40% of the pT of the leading jet, for all jets.
Graphic by Torbjörn Sjöstrand
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
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 -> qqbarqqbar -> gg
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
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.