UNIVERSITÀ DEGLI STUDI DI MILANO - BICOCCA DIPARTIMENTO DI FISICA G. OCCHIALINI CORSO DI DOTTORATO IN FISICA E ASTRONOMIA CICLO XXVII

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1 UNIVERSITÀ DEGLI STUDI DI MILANO - BICOCCA DIPARTIMENTO DI FISICA G. OCCHIALINI CORSO DI DOTTORATO IN FISICA E ASTRONOMIA CICLO XXVII Measurement of the branching fraction of B c + J/ψπ + π π + relative to B c + J/ψπ + and of the production cross-section of J/ψπ + relative to B + J/ψK + with the CMS experiment B + c Settore scientifico disciplinare FIS/01 Tesi di Dottorato di: SARA FIORENDI Tutore: Prof. L.Moroni Anno Accademico

2 Abstract The pseudoscalar B c + (Bc ) meson, the ground state of the bc (b c) system, is the lightest particle containing two heavy quarks of different flavors, and thus represents a unique laboratory to study heavy-quark dynamics. The investigation of B c + meson properties (charge conjugation is implied throughout this thesis) is of special interest compared to the flavor symmetric heavy quarkonium ( bb, cc) states, and provides a new testing ground for predictions in the context of effective models inspired by quantum chromodynamics. The decay processes of the B c + meson can be generically divided into three classes: those involving the decay of the b quark, the decay of the c quark, and the annihilation of the b and c quarks. The b c transition, accounting for about 20% of the decay rate, offers an easily accessible experimental signature, having a high probability to produce a J/ψ meson. The first B c + experimental observations were indeed made by the CDF collaboration in the semileptonic channel B c + J/ψl + ν (l = e, µ). The physics of the B c + meson has entered a new era with the advent of the CERN Large Hadron Collider. Thanks to the unprecedented energy and instantaneous luminosity provided by the LHC, the available B c + statistics is much higher than in previous experiments. A rich program of measurements involving new decay modes is being carried out by the LHCb collaboration, and the ATLAS experiment has recently observed a new state whose mass is consistent with the predicted mass for the second S-wave state of the B c + meson. The CMS experiment, due to the excellent muon identification system and tracking detectors, is particularly suited to the study of final states containing J/ψ mesons, where J/ψ µ + µ. In this thesis the analyses of the multi-body B c + J/ψπ + π π + final state and the two-body B c + J/ψπ + channel are presented, along with the ratio of their branching fractions R Bc B(B c + J/ψπ + π π + )/B(B c + J/ψπ + ). The measurement of the relative branching fractions of the two hadronic decays enables an investigation of the B c + properties independent of the production cross section. The B c + J/ψπ + mode is also compared to the B + J/ψK + decay, which has a similar vertex topology; the ratio of their production cross section times branching fraction R c/u σ(b c + ) B(B c + J/ψπ + )/σ(b + ) B(B + J/ψK + ) is measured in the central rapidity region, y < 1.6, complementary to that accessible in the LHCb experiment, and for B c + transverse momentum greater than 15 GeV. This measurement can give guidance to improve theoretical calculations still affected by large uncertainties, and constrain the B c + production models The analysis was performed on a 5.1 fb 1 sample of data collected by the CMS experiment at s = 7 TeV, using a trigger based on the selection of two muons coming from a J/ψ decay. The reduction of the copious combinatorial background is achieved by exploiting both kinematic and topological selections on the B c + candidate. The reconstruction efficiency has been accurately parametrized and measured on dedicated MonteCarlo samples, which well

3 3 simulate the data-taking conditions. Various sources of systematic uncertainties have been evaluated and are described in details in the text. This thesis is organized as follows. A short review of the Standard Model is discussed in Chapter 1. Motivations for B c + meson studies are described in Chapter 2, where B c + properties are examined, along with the status of theoretical predictions and the current experimental scenario. Chapter 3 is devoted to the illustration of the experimental apparatus used to perform the measurement. The LHC accelerator and the CMS detector are described, focusing the attention on the CMS inner tracker and muon stations, which are used to reconstruct the final state particles of the B c + decays. The detailed description of the measurements is outlined in Chapter 4. The first part is dedicated to an overview of the procedure used to extract the B c + J/ψπ + π π +, B c + J/ψπ + and B + J/ψK + signals. The measurements of R c/u and R Bc are then described in the second part of the Chapter, along with the efficiency parametrization and the evaluation of systematic uncertainties. The results are compared with the available theoretical predictions and with measurements from other experiments at the end of the Chapter.

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5 Contents 1 Review of Standard Model concepts The SM Lagrangian The EW sector The QCD sector The B + c meson B + c production B + c decays and lifetime Semileptonic and leptonic modes Non-leptonic modes B c J/ψ + nπ decays Experimental scenario The Large Hadron Collider and the CMS experiment The Large Hadron Collider The LHC layout The LHC luminosity LHC operations The CMS experiment The superconducting magnet The tracking system The electromagnetic calorimeter The hadronic calorimeter The muon system The CMS Trigger Track and vertex reconstruction Muon reconstruction Measurement of R c/u and R Bc Overview Dataset and triggers Montecarlo samples Online selection Offline event Selection

6 6 CONTENTS The B c + J/ψπ + π π + signal The B c + J/ψπ + and B + J/ψK + signals Data and MC comparison Signal stability studies The R c/u ratio Systematic uncertainty evaluation on R c/u Result The R Bc ratio Systematic uncertainty evaluation on R Bc Result Conclusions 79