Inelastic diffractive event classification in proton-proton collision with the CMS experiment Zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften bei der KIT-Fakultät Physik des Karlsruher Instituts für Technologie (KIT) genehmigte DISSERTATION von Melike Akbiyik aus der Türkey, Yüksekova Tag der mündlichen Prüfung: 13.07.2018 Referent: Prof. Dr. Thomas Müller Korreferent: Prof. Dr. Dr. h.c. Johannes Blümer Betreuer: Dr. Ralf Ulrich Abstract This thesis is devoted to diffraction physics at the CERN LHC. The experimental part of the work studiesp rapidity gap production in the run 2 data samples recorded by the CMS experiment at s = 13 TeV. A multivariate analysis method based on boosted deci- sion trees is developed to determine the cross-section of individual single diffractive (SD) and double diffractive (DD) processes. Diffractive interactions are characterized by ra- pidity gaps, which are regions in the detector devoid of particles and can be measured with a combination of central tracking and very forward calorimeters such as CASTOR. In hadronic collisions, most of the primary energy flows in the forward direction, which is why the forward region is in particular extremely important to characterize events. An event classifier is used to distinguish which process belongs to a specific inelastic event. This event classification is done with the charged particle tracks reconstructed in the sili- con pixel detector in combination with all calorimeter towers of CMS. The efficiency and purity are optimized. The classification uses global observations made by CMS in the huge acceptance region −6:6 < η < 5:2, and down as close as possible to the noise levels. Such a comprehensive multivariate study has never been attempted before. The advantage is an optimal use of CMS for diffractive event classification and thus, a high precision of the measurement. One result of this thesis is an indication of a very slow rise of the single diffractive cross section at the highest energies. Some event generators are challenged by these data. It could be that this is first signs of protons becoming fully opaque in a central collision. Zusammenfassung Ziel der Arbeit ist es diffraktive Physik am CERN LHC zu untersuchen. Der experi- mentelle Teil der Arbeit befasst sich mit der Untersuchung derp Erzeugung von Rapid- itätslücken in Run 2 Daten, die mit dem CMS Experiment bei s = 13 TeV aufgezeichnet wurden. Eine multivariate Analyse Methode, basierend auf "boosted decision trees" wurde entwickelt, um die Wirkungsquerschnitte von einzel- (SD) und doppel- (DD) diffraktiven Ereignissen zu bestimmen. Diffraktive Ereignisse sind durch Rapiditätslücken gekennze- ichnet, welches Bereiche im Detektor ohne Teilchenerzeugung sind. Solche Lücken können durch eine Kombination von zentralen Spurdetektoren und Vorwärtskalorimetern wie CAS- TOR nachgewiesen werden. In hadronischen Kollisionen wird ein Großteil der erzeugten Energie in die Vorwärtsrichtung gelenkt. Dies ist der Grund dafür, dass Vorwärtsdetek- toren besonders wichtig sind in der globalen Charakterisierung von Ereignissen. Eine Klassifizierung von Ereignissen wurde entwickelt um inelastische Kollisionen entsprechend einzuordnen. Die Klassifizierung basiert auf Messdaten des zentralen Silizium Spurde- tektors, sowie aller Kalorimeter Zellen des CMS Experimentes. Die Charakterisierung verwendet globale Beobachtungen des CMS Experimentes im riesigen Akzeptanzbereich von −6:6 < η < 5:2 und so nahe wie möglich am Rauschlevel. Ein derart umfassender Einsatz multivariater Analysetechniken wurde in CMS bisher noch nicht verfolgt. Die Ar- beit verspricht sich davon einen optimalen Einsatz von CMS Daten für die Klassifizierung von diffraktiven Ereignissen sowie eine hohe Präzision der erzielten Messungen. Ein Ergeb- nis der Arbeit ist ein Anzeichen für einen nur langsamen Anstieg des einzel-diffraktiven Querschnittes hin zu sehr hohen Energien. Einige Ereignisgeneratoren haben mit dieser Schlussfolgerung Schwierigkeiten. Es könnte sein, dass diese Daten ein erster Hinweis da- rauf sind, dass zentrale Proton-Proton Kollisionen vollkommen opak werden. Contents 1 Introduction1 2 Theoretical background5 2.1 Cosmic rays and extensive air showers....................5 2.1.1 Cosmic rays...............................5 2.1.2 Physics of extensive air showers....................6 2.2 The Standard model of particle physics.................... 10 2.3 The kinematics of the colliding particles................... 14 2.3.1 Cross sections.............................. 17 2.3.2 Rapidity and rapidity gaps....................... 26 2.3.3 Modeling of diffraction in MC generators............... 29 2.3.4 Underlying event............................ 31 3 Experimental methods 33 3.1 The large hadron collider............................ 33 3.2 The CMS experiment.............................. 35 3.2.1 The central instrumentation of the CMS detector.......... 36 3.2.2 Forward instrumentation of the CMS detector............ 40 3.2.3 Trigger system............................. 41 3.2.4 Simulations............................... 42 3.3 Multivariate event classification........................ 43 3.3.1 Training phase and application phase................. 44 3.3.2 Boosted decision trees......................... 48 3.4 Event reconstruction and software....................... 52 3.4.1 Introducing software.......................... 52 3.4.2 Track and vertex reconstruction.................... 52 3.4.3 ECAL and HCAL towers energy reconstruction........... 54 3.4.4 CASTOR tower reconstruction.................... 54 4 Performance improvements studies 55 4.1 The luminosity determination in ultra-peripheral collisions in proton-lead collisions..................................... 55 4.2 Proton-lead collisions.............................. 56 4.2.1 Starlight event generation....................... 56 4.2.2 The kinematical description of the dimuon process.......... 58 iii 4.2.3 Exclusive collection........................... 60 4.2.4 Correction for acceptance and efficiency................ 62 4.2.5 Luminosity cross-check......................... 65 4.3 Inter-calibration of CASTOR calorimeter................... 67 4.3.1 CASTOR calorimeter.......................... 67 4.3.2 Inter-calibration............................. 68 4.3.3 Channel by channel inter-calibration................. 71 4.3.4 Absolute energy scale.......................... 79 5 Data analysis 83 5.1 LHC run 2.................................... 85 5.2 Event selection................................. 89 5.3 Rapidity gap definition............................. 93 5.4 The kinematics of the diffractive process................... 97 5.5 Diffractive processes identification....................... 100 5.6 Multivariate analysis.............................. 105 5.7 Classifier response................................ 114 5.8 Integrated cross section DD and SD...................... 118 5.9 The Influence of the variable selection on the training............ 129 5.10 Extrapolation of the visible SD and DD cross sections............ 143 5.11 Systematic uncertainties............................ 144 5.12 Results...................................... 151 6 Summary 153 7 Bibliography 157 8 Acknowledgements 163 1 Introduction Diffractive processes form a significant contribution to the total inelastic cross section and therefore it is very important to understand the mechanism behind them. The aim of this thesis is to develop a selection for single diffractive (SD) and double diffractive (DD) events, and, in addition to this, study some of the properties of these events. Inclusive diffractive interactions cannot be calculated within perturbative quantum chromodynamics (pQCD) [1–4], and traditionally have been described by models based on Regge theory [5]. It is very difficult to understand theoretically [6]. However, very important because of the large cross section and contribution e.g. to the underlying event and pileup in particular also at high luminosity LHC [7]. Furthermore, at ultra-high energies in cosmic ray air showers, diffraction is a major effect with important consequences for the interpretation of cosmic ray data [8]. In this thesis I present the firstp observation of soft diffractive dissoci- ation in proton-proton collisions of LHC Run 2 at s = 13 TeV using a novel multivariate event classification. It is significantly important to choose a well-motivated parameter set since the performance of the classification method depends on the used variables. I use tracks, energies, and towers from forward hadronic and CASTOR [9] calorimeter as input variables. Several input sets of the classification variables are compared to see the perfor- mance using the different simulations such as EPOS [10] and Pythia 8 [11]. Diffraction is classified from the two final state hadronic systems separated by the largest rapidity gap in the event. Runs are selected by requiring that the relevant components of the CMS [12]detector were fully functional, in particular, the CASTOR forward calorimeter, HF, and pixel tracker. The application of multivariate classification on global event prop- erties using all components of the CMS experiment is by itself a big challenge. All aspects of the CMS experiment contribute and must be studied. The thesis provides a basic theo- retical introduction needed for the work in Chapter 2. A brief overview of the extensive air showers, cosmic rays, the Standard Model and Quantum Chromodynamics [13] is given. In the chapter 3, we will take a brief look at experimental methods that were used for this thesis.