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.