Measurement of the photon polarization using B0 s ! ϕγ at LHCb Mostafa Hoballah To cite this version: Mostafa Hoballah. Measurement of the photon polarization using B0 s ! ϕγ at LHCb. Other [cond- mat.other]. Université Blaise Pascal - Clermont-Ferrand II, 2015. English. NNT : 2015CLF22555. tel-01176223 HAL Id: tel-01176223 https://tel.archives-ouvertes.fr/tel-01176223 Submitted on 15 Jul 2015 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Numéro d’Ordre : DU 2555 PCCF T 1502 EDSF : 817 UNIVERSITÉ BLAISE PASCAL DE CLERMONT-FERRAND (U.F.R. Sciences et Technologies) ÉCOLE DOCTORALE DES SCIENCES FONDAMENTALES THÈSE présentée pour obtenir le grade de DOCTEUR D’UNIVERSITÉ Spécialité : PHYSIQUE des PARTICULES par Mostafa HOBALLAH 0 Measurement of the photon polarization using Bs → φγ at LHCb. Thèse soutenue publiquement le 03 Mars 2015, devant la commission d’examen: M. A. FALVARD Président du jury M. A. A. GALLAS TORREIRA Rapporteur Mme. E. TOURNFIER Rapporteur M. K. TRABELSI Examinateur M. O. DESCHAMPS Directeur de thèse M. P. PERRET Directeur de thèse i Résumé 0 → Cette thèse est dédiée à l’étude des désintégrations Bs φγ au LHCb afin de mesurer la polarisation du photon . Au niveau des quarks, ces désintégrations procèdent via une transition pingouin b → sγ et sont sensibles aux eventuelles contributions virtuelles de Nouvelle Physique. La mesure de la polarisation du photon permet de tester la structure V − A du couplage du Modèle Standard dans les processus des diagrammes de boucles de pingouin. Cette mesure peut être réalisée en étudiant le taux de dés- intégration dépendant du temps des mésons B. Une analyse délicate a été faite pour comprendre la distribution du temps propre et l’acceptance de sélection qui affecte cette distribution. Afin de contrôler l’acceptance de temps propre, des méthodes basées sur les données ont été développées. Plusieurs stratégies utilisées dans la mesure de la po- larisation des photons sont introduites et des résultats préliminaires sont présentés. De plus, une étude de certains effets systématiques est discutée. Dans le cadre de l’étude des désintégrations radiatives, une nouvelle procedure d’identification de photons a été développée et nous avons fourni un outil pour calibrer la performance de la variable de séparation photon/pion neutre sur la simulation. Ces outils sont d’intérêt général pour la collaboration LHCb et sont largement utilisés. Mots clés: LHCb detector - Heavy Flavor Physics - Radiative Decays - 0 Effective Field Theories - Bs → φγ - Photon Polarization - Proper Time - Photon Identification - Photon/π0 separation. iii v Abstract 0 → This thesis is dedicated to the study of the photon polarization in Bs φγ decays at LHCb. At the quark level, such decays proceed via a b → sγ penguin transition and are sensitive to possible virtual contributions from New Physics. The measurement of the photon polarization stands also as a test of the V − A structure of the Standard Model coupling in the processes mediated by loop penguin diagrams. The measurement of the photon polarization can be done through a study of the time-dependent decay rate of the B meson. A delicate treatment has been done to understand the proper time distribution and the selection acceptance affecting it. To control the proper time accep- tance, data driven control methods have been developed. Several possible strategies to measure the photon polarization are introduced and preliminary blinded results are pre- sented. A study of some of the systematic effects is discussed. In the context of studying radiative decays, the author has developed a new photon identification procedure and has provided a tool to calibrate the performance of the photon/neutral pion separation variable on simulation. Those tools are of general interest for the LHCb collaboration and are widely used. Keywords: LHCb detector - Heavy Flavor Physics - Radiative Decays - Effective Field 0 Theories - Bs → φγ - Photon Polarization - Proper Time - Photon Identifi- cation - Photon/π0 separation. vii Remerciements M y acknowledgements go to everyone who has contributed in some way to the development of this work. I begin by thanking the director of the LPC, Alain Falvard, for granting me the chance to prepare my thesis in good conditions. I want to thank the région d’Auvergne for having financially supported this work during three years and also for having granted me extra financial support to participate to con- ferences and present my work. I am so happy to have been part of the LHCb team in the LPC. I would like to thank all the members of the team especially Olivier Deschamps and Régis Lefevre who have worked hard with me so as to elaborate this work and make it in a good condition. In the past three years I have experienced the most difficult but also the most beautiful moments, memories that will stay engraved in my mind till the day I die. I would like also to thank all the members of the radiative analysis working group in the LHCb collaboration your help is very well appreciated. My fellow students and friends in the LPC, Ibrahim, Marouen, Mohamad, Jan.. I would like to thank you for all your help and all your time that you have spent with me. Marouen, I will never forget all the times that we spent on the balcony smoking -me being as a passive smoker- those moments were the best moments of a working day. Also, I am very grateful to Alain Falvard, Karim Trabelsi, Edwige Tournefier and Abra- ham Gallas for having accepted to be a part of my thesis jury. ix x I am to mention my daughter Ilyana since her presence has made me realize more and more the importance of my parents in my life. Since the day I knew she was coming I started to live a new kind of emotions, emotions that I knew my father has felt when he knew that I was coming, I can not just pass by such situation without realizing how much my parents cared about me and how much they sacrificed. Thanking them would not express the enormous amount of gratitude that I have towards them. I can now feel how much they are happy to see that their son has achieved something in this life. On several unthought occasions your shadow would come to my mind flashing back a series of beautiful memories that I have lived by your side. Your smile has given me a lot of energy when I was feeling stressed. I would like to thank you for having knew you... Mariam. 1 V(RR1K4/)VR.RITA (04%8 V3S&1/))@4V V1SE4)(0&4 -T))R]+(T T3V+(TTR *4S3V+(S VRNS>)( `àffl ”m`affl f´a‹m˚i˜l¨l´e, `àffl M`a˚r˚i`a‹mffl, `àffl I˜l›y´a‹n`affl... 1. «Comme si la vie n’a jamais existé, et que la vie de l’au-delà est toujours là» , Hussain ibn Ali Ibn Abi Taleb. Quand Hussain ibn Ali Ibn Abi Taleb s’est rapproché de sa mort, il s’apercevait plus que jamais que le temps qu’il a passé en vie n’était qu’un voyage vers un autre monde plus joli, et surtout, un monde éternel. Contents Introduction 1 1 The Electroweak interaction 3 1.1TheStandardModel............................. 4 1.2 Radiative B hadrondecays.......................... 14 2TheLHCb 25 2.1TheLHC................................... 26 2.2Datatakingperiodsandoperatingconditions............... 28 2.3TheLHCbdetector.............................. 29 2.4TheLHCbtriggersystem.......................... 48 2.5TheLHCbluminositymeasurements.................... 52 2.6TheLHCbsoftware.............................. 53 2.7DataflowinLHCb.............................. 54 3 Photon reconstruction and identification 55 3.1Photonreconstruction............................ 56 3.2 π0 reconstruction............................... 59 3.3Multivariateapproaches........................... 60 3.4Photonidentification............................. 63 3.5 γ/π0 separation................................ 76 0 → 4 The measurement of the photon polarization in Bs φγ 87 4.1Introduction.................................. 88 xiii xiv CONTENTS 4.2Signalandcontrolchannelsselection.................... 91 4.3Thepropertime................................102 4.4 Proper time acceptance study on MC ....................113 5 Preliminary results 131 5.1Fitstrategies................................. 132 0 ∗ 0 5.2 Extracting τB0 with B → K (892) γ events................135 5.3 Sensitivity on AΔ ...............................141 5.4Systematicuncertaintiesstudy........................153 5.5Conclusions..................................161 Conclusion 163 Appendices 169 A B → Vγ and B → VJ/ψ time acceptance alignment 169 A.1Kinematicalbasisoftheprocedure.....................169 A.2 Alignment of the proper time acceptance ..................177 B Photon calibration 179 C Definition of the different variables used for selections 181 Introduction T he Standard Model (SM) of particle physics provides a successful interpretation of most of the observed phenomena. There are several theoretical or experimental open questions as the origin of the mass hierarchy, the leptons family number, the electric charge quantification, the existence of dark matter and the asymmetries in the universe. These open questions may suggest that more fundamental symmetries are at play and justify the search for new phenomena beyond the standard model. This New Physics search can be done either by searching for new heavy particles or with precision valida- tion of the SM predictions. The validity of the SM can be checked in many ways one of which is the study of b hadron decays. One of the main experiments that study b flavor is the LHCb at CERN. The b quark transitions proceeding via Flavor-Changing Neutral Current (FCNC) loop processes provide an efficient indirect probe for the search of new particles possibly prop- agating inside the virtual loops.