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Absolute Luminosity Measurements at Lhcb THESE` Pour obtenir le grade de DOCTEUR DE L’UNIVERSITE´ DE GRENOBLE Specialit´ e´ : Physique Subatomique & Astroparticules Arretˆ e´ ministerial´ : 7 aoutˆ 2006 Present´ ee´ par Plamen Hopchev These` dirigee´ par Bolek Pietrzyk prepar´ ee´ au sein du Laboratoire d’Annecy-le-Vieux de Physique des Particules et de l’Ecole´ Doctorale de Physique, Grenoble Absolute luminosity measurements at LHCb These` soutenue publiquement le 25 Novembre 2011, devant le jury compose´ de : Dr. Lucia Di Ciaccio LAPP (Annecy), President´ Dr. Massimiliano Ferro-Luzzi CERN, Rapporteur Dr. Witold Kozanecki CEA(Saclay), Rapporteur Dr. Jaap Panman CERN, Examinateur Dr. Bolek Pietrzyk LAPP(Annecy), Directeur de these` Abstract Absolute luminosity measurements are of general interest for colliding-beam experi- ments at storage rings. These measurements are necessary to determine the absolute cross-sections of reaction processes and are valuable to quantify the performance of the accelerator. LHCb has applied two methods to determine the absolute scale of its luminosity measurements for proton-proton collisions at the LHC running at a centre- of-mass energy of 7 TeV. In addition to the classic \van der Meer" scan method a novel technique has been developed which makes use of direct imaging of the individ- ual beams using both proton-gas and proton-proton interactions. The beam imaging method is made possible by the high resolution of the LHCb vertex detector and the close proximity of the detector to the beams, and allows beam parameters such as po- sitions, angles and widths to be determined. We describe both methods and compare the two results. In addition, we present the techniques used to transport the absolute luminosity measurement to the full data-taking period. R´esum´e Les mesures de la luminosit´eint´egr´ee pour les exp´eriencesaupr´esde collisionneur ont un int´er^etmajeur. Ces mesures participent `ala d´eterminationdes sections efficaces de production des processus ´etudi´es,elles quantifient ´egalement les performances de l'acc´el´erateuret des exp´eriences.Deux m´ethodes ont ´et´eutilis´eespar l'exp´erienceLHCb pour d´eterminer la mesure de la luminosit´eabsolue enregistr´eedurant la campagne 2010 de prise de donn´eesdes collisions proton-proton `aune ´energiede 7 TeV dans le centre de masse : outre la m´ethode classique applel´ee"van der Meer scan" une nouvelle technique est d´evelopp´eepermettant une d´eterminationdirecte des param`etresde chaque faisceau en localisant les interactions faisceau-faisceau et les interactions faisceau-gaz r´esiduel. Cette m´ethode n'est possible que grace `ala r´esolution du d´etecteurde vertex de LHCb et sa proximit´eavec la zone des faisceaux de protons et les param`etrestels la position, les angles et les largeurs des faisceaux peuvent ^etremesur´es.Les deux methodes sont d´ecriteset leurs r´esultatsdiscut´es.De plus les techniques utilis´eespour ´etendreles mesures de luminosit´eabsolue `al'ensemble de la prise de donn´ees2010 sont d´ecrites. Contents Abstract iii Contentsv Acknowledgements ix Preface xi 1 Luminosity1 1.1 Relevance of absolute luminosity measurements.............2 1.1.1 Hard-scattering QCD formalism..................2 1.1.2 Cross-section measurements....................6 1.1.3 Parton distribution functions...................9 1.2 Methods for absolute luminosity determination.............. 12 1.2.1 Indirect methods.......................... 14 1.2.2 Direct methods........................... 22 2 The Large Hadron Collider 25 2.1 CERN and the Large Hadron Collider.................. 25 2.2 Characteristics and operation....................... 29 2.2.1 Beam dynamics........................... 29 2.2.2 Filling scheme............................ 31 2.2.3 Beam instrumentation....................... 31 2.2.4 Operational aspects......................... 32 3 The LHCb experiment 37 3.1 Overview................................... 38 3.2 Tracking system............................... 42 3.2.1 Vertex Locator........................... 42 3.2.2 Tracker Turicensis, Inner Tracker and Outer Tracker....... 45 3.2.3 Magnet............................... 47 3.2.4 Track reconstruction........................ 48 3.2.5 Primary vertex reconstruction................... 51 3.2.6 Performance............................. 52 3.3 Calorimeters................................. 54 3.4 Trigger system............................... 56 3.4.1 LHCb trigger system........................ 56 3.4.2 Beam gas trigger.......................... 58 3.4.3 Trigger for relative luminosity measurement........... 66 3.4.4 Trigger for van der Meer scans................... 67 4 Measurement of the relative luminosity 69 4.1 Luminosity counters............................ 70 4.2 Methods for the determination of the average number of visible pp in- teractions per crossing........................... 71 4.2.1 Average of the counter distribution................ 71 4.2.2 Fit to the counter distribution................... 71 4.2.3 Zero-count method......................... 73 4.3 Systematic effects.............................. 75 4.4 Data handling................................ 78 5 Beam intensity measurement 79 5.1 Total beam current............................. 79 5.2 Ghost charge measurement......................... 80 5.2.1 Event selection........................... 80 5.2.2 Systematic corrections....................... 85 5.2.3 Ghost charge measurement in LHC fills 1653 and 1658..... 88 5.3 Bunch-by-bunch fractions......................... 90 5.4 Satellite bunches.............................. 91 5.5 Uncertainties................................ 92 6 The van der Meer scan method 93 6.1 Experimental conditions during the van der Meer scans......... 94 6.2 Time stability................................ 96 6.3 Cross-section determination........................ 99 6.4 Length scale calibration.......................... 104 6.5 Systematic errors.............................. 109 6.5.1 Coupling between the x and y coordinates in the LHC beams. 109 6.5.2 Ghost charge and satellite bunches during the VDM scans... 111 6.5.3 Reproducibility of the luminosity at the nominal beam positions 113 6.5.4 Cross-check with the z position of the luminous region..... 114 6.5.5 Summary of the systematic errors................. 114 6.6 Results of the van der Meer scans..................... 114 7 The beam-gas imaging method 117 7.1 Data-taking conditions........................... 119 7.2 Analysis and data selection procedure................... 119 7.3 Vertex resolution.............................. 122 7.3.1 Dependence on the track multiplicity of the vertex........ 124 7.3.2 Dependence on the z position of the vertex............ 125 7.3.3 Resolution unfolding........................ 127 7.4 Measurement of beam profiles and overlap integral............ 128 7.4.1 Measurement of beam profiles................... 128 7.4.2 Measurement of the overlap integral................ 132 7.5 Corrections and systematic errors..................... 134 7.5.1 Resolution.............................. 135 7.5.2 Crossing angle effects........................ 137 7.5.3 Bias due to unequal beam sizes and beam offsets........ 139 7.5.4 Time dependence and stability................... 141 7.5.5 Gas pressure gradient........................ 142 7.6 Results of the beam-gas imaging method................. 144 8 Conclusion 149 Bibliography 153 Thesis summary xv R´esum´ede la th`ese xix Acknowledgements I am greatly indebted to many people for help, both direct and indirect, in writing this PhD thesis. I am grateful to the French Minist`ere de l'Enseignement Sup´erieur et de la Recherche who provided the financial support for the preparation of the thesis, and to Laboratoire d'Annecy-le-Vieux de Physique des Particules (LAPP) who offered me a very friendly and stimulating working environment. I profited a lot, both in personal and scientific aspect, from my interaction with the members of the LAPP{LHCb group. I am truly thankful to Philippe Ghez, Vincent Tisserand and St´ephaneT'Jampens for sharing their expertise and experience with me. Most of all I am grateful to Marie-No¨elleMinard and to my supervisor Bolek Pietrzik for their guidance and vigorous support throughout my thesis. I am obliged to my colleagues from the LHCb luminosity group for numerous fruit- ful discussions and for providing figures and text included in this thesis. I would like to thank Jaap Panman for being a shining example of a researcher and a gentleman, Massimiliano Ferro-Luzzi for his invaluable help during my first steps in LHCb and for providing the opportunity for me to work on the beam-gas imaging method, and Vladislav Balagura for sharing his passion about science and for showing me how to be a better physicist. Being able to work with and learn from them has been an honour. Going back in time I cannot miss to acknowledge my professors and colleagues at the University of Sofia. I am truly indebted and thankful to Leandar Litov, who sparked my interest in particle physics and who supported me even when I decided to leave the CMS group in Sofia. The derni`ere ligne droite may be straight, but may have bumps as well. This is why I owe a sincere thankfulness to Sabine Elles for her limitless support, helpful initia- tives and for all memorable discussions on way to the local restaurant inter entreprises. Finally, I would like to thank my parents Daniela and Hristo, without whose love and support I would not have made it this far. Preface The collision energy and the luminosity are essential characteristics of a particle collider.
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