Thesis Submitted to Mcgill University in Partial Fulfilment of the Requirements for the Degree of Master of Science
Total Page：16
File Type：pdf, Size：1020Kb
A BABAR sensitivity study on the search for the invisible decay of J= in B± ! K∗± J= Racha Cheaib Physics Department Master of Science McGill University,Montreal September 9, 2011 A thesis submitted to McGill University in partial fulfilment of the requirements for the degree of Master of Science. c Racha Cheaib, 2011 Acknowledgements I am grateful to the BABAR collaboration, physicists and engineers, for the quality and precision of the BABAR detector and BABAR dataset. I would like to thank my supervisor, Steven Robertson, for his continuous help through all stages of this analysis. I also want to thank Dana Lindemann, Wissam Moussa, and other colleagues in the physics department for their valuable input. Also, special thanks to the Leptonic Analysis Working Group for their help and contribution. ii Abstract We present a sensitivity study on the search for J= νν in B± K∗± ! ! J= using data from the BABAR experiment at the SLAC National Accelerator Laboratory. The decay is highly suppressed in the Standard Model and thus is a possible window for new physics such as supersymmetry and dark matter. Hadronic tag reconstruction is employed for the analysis, where one B is fully reconstructed using hadronic decay modes. The remaining tracks and clusters are attributed to the signal B on which the B± K∗± J= cut-based signal ! selection is applied. The associated K∗ is allowed to decay via two modes, Mode 1: K∗± K0 π± and Mode 2: K∗± K± π0. The approach is to ! S ! reconstruct a K∗± candidate, the only signature in a signal event, and calculate the recoiling mass. The data is left blinded in the signal region and only a range of the branching fraction limits is calculated to determine the sensitivity. The result for Mode 1 is an upper limit, at the 90% confidence level, on ( J= B νν) of 9.13 10−2 using the Barlow method and 11.10 10−2 using the ! × × Feldmann-Cousins method. The upper limit for Mode 2, also at the 90% CL, is estimated to be 2.49 10−2 and 2.98 10−2 using Barlow and Feldmann- × × Cousins respectively. The branching fractions thus yield a sensitivity of order 10−2. Although the result is not an improvement on the current J= νν ! limits, this method can be extended to other cc quarkonium modes and could further yield a much better result with data from the newly approved SuperB experiment, the extension of BABAR to higher luminosities. iii Abr´eg´e Nous pr´esentons une ´etude de sensibilit´esur la recherche de J= νν ! dans B± J= K∗± en utilisant les donn´eesde l'exp´erimentation BABAR du ! Centre de l'acc´el´erateur lin´eaire de Stanford. La radioactivit´eest hautement supprim´eedans le mod`ele standard et offre donc de nouvelles opportunit´es physiques telles que la supersym´etrie et la mati`ere noire. Une reconstruction hadronique fut employ´eepour l'analyse, o`uun B fut totalement reconstruit en utilisant les modes de transformations hadroniques. Les pistes et regroupe- ments restants furent attribu´esau signal de B sur laquelle une s´election de signal coup´ee`ala base fut appliqu´ee. Le K∗ associ´eest capable de se trans- former en deux modes, Mode 1: K∗± K0 π± et Mode 2: K∗± K± π0. ! S ! L'approche est de reconstruire un candidat K∗, la seule signature d'un signal, et de calculer la masse de recul. La donn´eeest invisible dans la r´egion du signal et seulement un intervalle des embranchements fractionnaires est cal- cul´epour d´eterminer la sensibilit´e. Le r´esultat pour Mode 1 est une limite sup´erieure, avec un intervalle de confiance de 90%, sur (J= νν) de 9.13 B ! 10−2 en utilisant la m´ethode de Barlow et de 11.10 10−2en utilisant celle × × de Feldmann-Cousins. Le r´esultat pour Mode 2, aussi avec un intervalle de confiance de 90%, est calcul´e`a2.49 10−2 et 2.98 10−2 en utilisant Barlow et × × Feldmann-Cousin respectivement. Les fractions d'embranchements ont donc une sensibilit´ede l'ordre de 10−2. Bien que ce n'est pas une am´elioration des limites actuelles de J= νν, cette m´ethode peut ^etre ´etendue `ad'autres ! modes quarkonium cc et pourrait aboutir `ade meilleurs r´esultats avec les don- n´eesde SuperB, une nouvelle exp´erimentation am´elior´eede BABAR offrant un niveau de luminosit´eplus sup´erieur. iv Contents 1 Introduction 1 2 Theory 3 2.1 The Standard Model . .4 2.1.1 Leptons and Quarks . .4 2.1.2 Gauge bosons . .5 2.1.3 The Higgs Boson . .7 2.2 Beyond the Standard Model . .8 2.2.1 Dark Matter . 10 2.3 J= νν: Significance and Motivation . 11 ! 2.3.1 Related Studies . 14 3 SLAC and the BABAR Detector 16 3.1 PEP-II and SLAC linac . 16 3.2 The BABAR Detector . 17 3.2.1 The Silicon Vertex Tracker . 19 3.2.2 The Drift Chamber . 21 3.2.3 Detector of Internally Reflected Cherenkov Light . 22 3.2.4 The Electromagnetic Calorimeter . 24 3.2.5 Instrumented Flux Return . 26 3.2.6 Trigger System . 28 v 4 Analysis Tools 30 4.1 Hadronic Reconstruction Method . 30 4.2 Ntuple Production . 31 4.3 Event Reconstruction . 32 4.4 BABAR Dataset . 34 4.5 Monte Carlo Background . 35 4.6 Signal Monte Carlo . 37 4.7 J= ! `+`− in B± ! K∗± J= Monte Carlo . 37 5 Background Analysis 39 5.1 Background Events . 39 5.2 BackgroundCuts .......................... 41 5.2.1 Btag Cuts.......................... 41 5.2.2 mES Cut .......................... 43 5.2.3 Continuum Likelihood Cut . 44 5.3 mES Sideband Substitution . 48 6 Signal Selection 52 6.1 Particle Identification . 52 ∗± 0 ± 0 + − 6.2 Mode 1: K ! KS π ;KS ! π π ............. 54 6.2.1 Track Multiplicity and Particle ID cut . 55 6.2.2 Eextra and cos θP miss .................... 56 0 6.2.3 KS reconstruction . 58 6.2.4 K∗ reconstruction . 59 6.3 Mode 2: K∗± ! K± π0 ..................... 61 6.3.1 Track Multiplicity, Particle ID and cos ΘP miss Cut . 61 6.3.2 π0 reconstruction . 62 ∗ 6.3.3 Eextra cut and K Reconstruction . 63 vi 6.4 Signal Selection Cross Check . 64 6.4.1 J= ! `+`− ........................ 65 0 ∗ 6.4.2 D ! K l ν` Study ................... 69 7 Efficiencies 72 8 Uncertainties 76 8.1 Btag Yield.............................. 77 8.2 Continuum Likelihood Suppression . 79 8.3 Track Multiplicity Cut . 80 8.4 ParticleID.............................. 81 8.5 cos ΘP miss Cut ........................... 82 8.6 Eextra Cut.............................. 83 8.7 π0 reconstruction and π0 Mass Cut . 86 0 8.8 KS reconstruction and Mass Cut . 88 8.8.1 Summary of Systematic Errors. 89 9 ( J= νν) 91 B ! 9.1 Expected Background Estimate . 92 9.1.1 Combinatorial Background Estimate . 92 9.1.2 Peaking Background Estimate . 94 9.2 Results and Limits . 94 10 Conclusion 99 11 Appendecies v 11.1 Likelihood Selector . .v 11.2 Decision Tree Selector . vi 11.3 Performance of chosen PID selectors . vii vii Chapter 1 Introduction Over the past few decades, the Standard Model has become a widely accepted description of essentially all the particles observed to date and their inter- actions. It has gained popularity in both the theoretical and experimental segments of particle physics, mainly due to its predictive power and precision measurements. However, many particle physicists still believe that this model is not the complete story and a more fundamental theory awaits discovery. The aim of this thesis is to push forward the ongoing hunt for physics beyond the Standard Model. By measuring the branching fraction of J= νν in B± ! ! K∗± J= , the model's known particle interactions are put to test. This is true since the invisible decay of J= has a highly suppressed branching fraction and occurs only via one annihilation channel. Because the Standard Model rate is below our current sensitivity, any signal, if found, is a clear hint of new physics. The BABAR experiment at SLAC National Accelerator Laboratory completed its last run at the Υ (4S) resonance, in 2008, reaching an integrated luminosity of 429 fb−1. More than 470 million B meson pairs were produced allowing for a branching fraction measurement of J= νν. The aim of this analysis is ! to determine the sensitivity of the BABAR dataset to this branching fraction 1 using Monte Carlo simulation. A hadronic tag reconstruction method is em- ployed, where one B is reconstructed exclusively via hadronic modes in the decay Υ (4S) BB. The remaining tracks and clusters are then used to form ! a K∗ candidate and evidence of an invisible decaying J= is sought by com- puting the invariant mass at the system recoiling against the K∗. Before discussing the details of our analysis approach, chapter 2 of this thesis focuses on the Standard Model and its possible extensions. An overview of the BABAR detector and the PEP-II storage ring is provided in chapter 3. Chapter 4 lists the analysis tools used, including data and Monte Carlo samples as well as the hadronic Btag reconstruction method. The background associated with this analysis is discussed in chapter 5. The signal selection is explained in detail in chapter 6, and the resulting efficiencies are listed in chapter 7. The systematic uncertainties associated with the signal selection are discussed in chapter 8, and the branching fraction calculation is outlined in chapter 9. The final results and conclusions are stated in chapter 10.