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Analysis of W± Bosons with ALICE: Effect of Alignment on W± Bosons Analysis

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Analysis of W± Bosons with ALICE: Effect of Alignment on W± Bosons Analysis Analysis of W± bosons with ALICE: Effect of alignment on W± bosons analysis by Pieter Johannes Wynand du Toit Department of Physics University of Pretoria and iThemba LABS, Somerset West SA-CERN for the ALICE Collaboration Submitted in partial fulfilment of the requirements for the degree Magister Scientiae in the Department of Physics in the Faculty of Natural and Agricultural Sciences University of Pretoria Pretoria 30 April 2013 © University of Pretoria Analise van W± bosone met ALICE: Effek van oplyning op W± bosone analise deur Pieter Johannes Wynand du Toit Fisika Departement Universiteit van Pretoria en iThemba LABS, Somerset-Wes SA-CERN vir die ALICE Medewerking Voorgelê ter vervulling van ‘n deel van die vereistes vir die graad Magister Scientiae in die Fisika Departement in die Natuur- en Landbouwetenskappe Fakulteit Universiteit van Pretoria Pretoria 30 April 2013 i © University of Pretoria Declaration I, Pieter Johannes Wynand du Toit, declare that the dissertation, which I hereby submit for the degree Magister Scientiae (Physics) at the University of Pretoria, is my own work and has not previously been submitted by me for a degree at this or any other tertiary institution. SIGNATURE: ......................................... DATE: ......................................... SUPERVISOR: Prof C. Theron CO-SUPERVISORS: Dr S. V. Förtsch Dr E. Z. Buthelezi ii © University of Pretoria Verklaring Ek, Pieter Johannes Wynand du Toit, verklaar dat die verhandeling wat ek hiermee vir die graad Magister Scientiae (Fisika) aan die Universiteit van Pretoria indien, my eie werk is en nie voorheen deur my vir 'n graad aan hierdie of 'n ander tersiêre instelling ingedien is nie. HANDTEKENING: ......................................... DATUM: ......................................... PROMOTOR: Prof C. Theron MEDE-PROMOTORS: Dr S. V. Förtsch Dr E. Z. Buthelezi iii © University of Pretoria Abstract Analysis of W± bosons with ALICE: Effect of alignment on W± bosons analysis by Pieter Johannes Wynand du Toit Supervisor: Professor C. Theron Co-supervisors: Doctor S. V. Förtsch and Doctor E. Z. Buthelezi The ALICE (A Large Ion Collider Experiment) detector at the CERN Large Hadron Collider (LHC) is dedicated to studying the deconfined medium called the quark-gluon plasma (QGP), which is formed at extreme energy densities in heavy-ion collisions. ALICE can study hadrons, photons, electrons and muons up to the highest multiplicities expected at the LHC and down to very low transverse momentum ( 30 MeV/c) by employing excellent particle identification (PID) and tracking over a broad momentum range ( 100 MeV/c – 100 GeV/c). It consists of the central barrel which covers mid-rapidity (| | 0.9) and the Muon Spectrometer covering the forward rapidity region ( ). The Muon Spectrometer detects dimuons decaying from heavy quarkonia (e.g. ) which are hard, penetrating probes as well as high- single muons from bosons which are initial-state observables. These probes are essential tools for determining medium induced effects and studying the initial conditions of the interaction. The boson has a high mass of = 80.385 ± 0.015 GeV and is therefore formed in the early stages of the collision. It decays to single muons ( ) which are detected in the high- region (30 – 80 GeV/c). The high centre-of-mass energies (√ ) obtained during proton-proton (pp) and lead-lead (Pb-Pb) collisions at the LHC are sufficient for the formation of the boson. Due to the increase in luminosity for the LHC in 2011 it is now thought possible to perform a data analysis of the boson in ALICE. The results can then be compared to previous performance studies and to results from other LHC experiments (ATLAS, CMS and LHCb). As a first requirement of the analysis, the effect of the alignment of the Muon Spectrometer has to be determined. Misalignment of the Muon Spectrometer could result in a systematic uncertainty in the measurement of the muon track, thereby influencing the efficiency of the detector. By analysing simulations of boson signals generated with PYTHIA in pp collisions at √ = 8 TeV with ideal and residual misalignment configurations of the detector, these alignment effects on the and pseudorapidity ( ) distributions, as well as the ratio (charge asymmetry) were studied using the AliROOT framework. It was found that the misalignment does cause a systematic uncertainty in the distributions and charge asymmetry, especially in the region > 40 GeV/c where the systematic uncertainty grows above 50 %. iv © University of Pretoria Analyses of Pb-Pb collisions conducted in 2011 at √ = 2.76 TeV were then performed on data reconstructed with original alignment information and data refitted with improved alignment information. They were compared to establish the effect of alignment on the single muon distributions. The improved alignment has a limited effect in the high- region and therefore also on possible boson studies. Due to lack of statistics at high- the boson signal and the nuclear modification factor ( ) could not be extracted, but it is foreseen that the extraction will later be possible with the use of 2012 pp and Pb-Pb data. v © University of Pretoria Samevatting Analise van W± bosone met ALICE: Effek van oplyning op W± bosone analise deur Pieter Johannes Wynand du Toit Promotor: Professor C. Theron Mede-promotors: Doktor S. V. Förtsch en Doktor E. Z. Buthelezi Die ALICE (“A Large Ion Collider Experiment”) detektor by CERN se Groot Hadron Versneller (“LHC”) is toegewy tot die bestudering van die onbeperkte medium bekend as die kwark-gluon plasma (KGP), wat by uiterste energie digthede in swaar-ioon botsings gevorm word. ALICE kan hadrone, fotone, elektrone en muone bestudeer tot by die hoogste verwagte meervoudighede by die LHC en tot by baie lae transversale momentum ( 30 MeV/c) deur gebruikmaking van uitstekende deeltjie identifikasie (“PID”) en sporing oor ‘n wye momentum streek ( 100 MeV/c – 100 GeV/c). Dit bestaan uit die sentrale loop wat die middel-rapiditeitsreeks (| | 0.9) dek en die Muon Spektrometer wat die voorwaartse rapiditeitsreeks ( ) dek. Die Muon Spektrometer neem dimuone waar afkomstig van swaar kwarkonia (bv. ) wat hard, indringende waarneembares is, sowel as hoë- enkel muone vanaf bosone wat begintoestand waarneembares is. Hierdie waarneembares is noodsaaklik vir die bepaling van medium geïnduseerde effekte en die bestudering van die aanvanklike voorwaardes van die interaksie. Die boson het 'n hoë massa van = 80.385 ± 0.015 GeV en word daarom in the vroeë stadiums van die botsing gevorm. Dit verval na enkele muone ( ) wat in die hoë- streek (30 - 80 GeV/c) waargeneem word. Die groot massamiddelpunt energieë (√ ) wat behaal word tydens die proton-proton (pp) en lood-lood (Pb-Pb) botsings by die LHC is voldoende vir die vorming van bosone. As gevolg van die toename in luminositeit in 2011 vir die LHC word dit verwag dat dit nou moontlik is om 'n boson data-analise uit te voer in ALICE. Die resultate kan dan vergelyk word met vorige werkverrigtingstudies en met resultate van ander LHC eksperimente (ATLAS, CMS en LHCb). As ‘n eerste vereiste vir die analise moet die effek van die oplyning van die Muon Spektrometer bepaal word. Afwyking van die Muon Spektrometer se oplyning kan moontlik 'n sistematiese fout in die bepaling van die muon se veroorsaak en daardeur die doeltreffendheid van die detektor beïnvloed. Deur simulasies te analiseer van boson seine in pp botsings teen √ = 8 TeV wat met PYTHIA gegenereer is vir ideale en residuele oplyningsfout konfigurasies van die detektor, is hierdie effekte op the en pseudorapiditeit ( ) verspreidings, sowel as die verhouding (lading asimmetrie) bestudeer met die gebruik van die AliROOT raamwerk. Daar is bevind dat die vi © University of Pretoria oplyningsfout wel ‘n sistematiese fout in the bepaling van die verspreidings en lading asimmetrie veroorsaak, veral in die streek > 40 GeV/c waar die sistematiese fout groter as 50% word. Analises van Pb-Pb botsings wat in 2011 uitgevoer is teen √ = 2.76 TeV is daarna voltooi op data gerekonstrueer met oorspronklike oplyning informasie en data aangepas tot verbeterde oplyning informasie. Die twee gevalle is vergelyk om die effek van die oplyning op enkele muon verspreidings te bepaal. Die verbeterde oplyning het 'n beperkte uitwerking in die hoë- streek en dus ook op moontlike boson studies. Weens 'n gebrek aan statistiek by hoë- is die W ± boson sein en die kernwysigingsfaktor ( ) nie onttrek nie, maar daar word voorsien dat die onttrekking later sal moontlik wees met die gebruik van 2012 pp en Pb-Pb data. vii © University of Pretoria Acknowledgements I would like to thank everyone who supported me during my studies. Thanks to Dr Zeblon Vilakazi, Dr Zinhle Buthelezi, Dr Siegfried Förtsch and Dr Deon Steyn at iThemba Laboratory for Accelerator Based Sciences (LABS), Somerset West for all their help and for giving me the opportunity to experience the wonderful field of Particle Physics. Thank you also to Dr Francesco Bossù and my fellow M.Sc. student, Johnson Senosi for all their helpful discussions during the writing of this dissertation. Thanks to everyone from Subatech at Ecole des Mines de Nantes for their help. Especially to Dr Diego Stocco, Dr Philippe Pillot and Dr Gines Martínez-García for their time and for helping me understand the analysis techniques, workings of the Muon Spectrometer and the theoretical background. Also a word of thanks to Dr Nicole Bastid and the rest of the PWG-HF group for all their valuable insights. I would like to thank everyone at ALICE who helped me during my training as shifter at Point 2 and for the wonderful opportunity I received to work on such an intriguing experiment. To Dr Andrew Hamilton from the University of Cape Town who was willing to give me a course on Particle Physics in his spare time, I would like to extend my sincere gratitude. Without the financial help of the National Institute of Theoretical Physics (NITheP) and the National Research Foundation (NRF) I would not have been able to study Particle Physics.
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