Separating Signal from Background in D 0 K 0 S + Raymond Tat 1 - PowerPoint PPT Presentation

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Separating Signal from Background in D 0 → K 0 S π + π − Raymond Tat 1 Angelo Di Canto 2 Mark Williams 3 1 University of California, Berkeley 2 CERN 3 University of Manchester August 2018 Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 1

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Table of Contents 1 What is the LHCb experiment? 2 Why D 0 → K 0 S π + π − ? 3 Data Analysis 4 Conclusions Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 2

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Section 1 What is the LHCb experiment? Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 3

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions CP Violation The universe is mostly matter. But why? Standard Model CP violation is not enough to explain the observed asymmetry. So, we search for CP violation beyond what is predicted by the Standard Model. This will be a sensitive probe of new physics. Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 4

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Large Hadron Collider beauty (LHCb) LHCb’s main goal is to investigate CP violation. LHCb’s design is specially suited to study b and c quarks. Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 5

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Section 2 Why D 0 → K 0 S π + π − ? Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 6

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions CP Violation and Mixing with Neutral Charm Mesons No CP violation measured in the charm sector to date. Two phenomena described by four parameters: CP Violation Described by | q / p | and φ Mixing: D 0 → ¯ D 0 Described by x and y D 0 → K 0 S π + π − allows us to measure all 4 parameters. Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 7

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions The Current State of Affairs y (%) Arg(q/p) [deg.] σ !"#$% !"#$% 1 CPV allowed σ 1.2 60 2 &!$'()*+,- &!$'()*+,- σ 3 σ 4 1 σ 5 40 0.8 20 0.6 0.4 0 0.2 −20 0 σ 1 −0.2 −40 σ 2 σ 3 −0.4 σ 4 −60 σ 5 −0.6 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 1.2 0.6 0.8 1 1.2 1.4 1.6 x (%) |q/p| Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 8

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Section 3 Data Analysis Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 9

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions S π + π − Candidates Analysis of D 0 → K 0 S π + π − candidates from LHCb (Run 2, Input: D 0 → K 0 2016-2017) Eventual goal: Perform an analysis of the decay time versus the Dalitz coordinates Goal of this project: Improve signal significance ( ↓ σ stat ) while avoiding large biases or correlations ( ↓ σ syst ) Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 10

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Delta Mass Plot with Backgrounds Candidates 22000 ± B = 3.84 0.29 20000 ± C = 2.64 0.16 ± 18000 cut = -0.7359 0.073 ± mean = 145.4313 0.0041 16000 ± nbkg = 502869 1626 14000 ± nsig = 151485 1518 ± 12000 sigma0 = 0.541 0.014 ± sigma1 = 0.307 0.025 10000 ± sigma2 = 0.979 0.021 8000 ± threshold = 138.962 0.065 ± 6000 var1 = 0.513 0.026 ± var2 = 0.796 0.021 4000 2000 0 data 4 σ 2 (data - model)/ 0 − 2 − 4 140 145 150 155 160 δ 0 ( ) [MeV] m D Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 11

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Multivariate Analysis (MVA) Combines information from many weakly discriminating variables into a more powerful discriminator Test multiple MVA methods using a combination of the libraries TMVA, sklearn, and hep ml. Ex: BDT, neural network, Fischer discriminant analysis Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 12

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions The sPlot technique sPlot is a data-driven method to extract signal and background distributions for variables Fit signal and backgrounds distributions to the delta mass. Also allows us to choose input variables based on signal-background separation. Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 13

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions MVA Performance Evaluation We can assess MVA performance with a Receiver Operating Characteristic (ROC) curve. Area under the ROC curve can be used as a metric for MVA performance Receiver operating characteristic 1.0 0.8 True Positive Rate 0.6 0.4 uGB_bin_tau (area = 0.77) GradBoost (area = 0.78) MLP (area = 0.83) 0.2 AdaBoost (area = 0.78) uGB_bin_dalitz (area = 0.77) Luck 0.0 0.0 0.2 0.4 0.6 0.8 1.0 False Positive Rate Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 14

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Results Candidates Candidates 22000 ± ± B = 3.84 0.29 B = 1.72 0.59 20000 ± ± C = 2.64 0.16 10000 C = 1.93 0.30 ± ± 18000 cut = -0.7359 0.073 cut = -1.230 0.12 ± ± mean = 145.4313 0.0041 mean = 145.4398 0.0042 16000 8000 ± ± nbkg = 502869 1626 nbkg = 35924 378 14000 ± ± nsig = 151485 1518 nsig = 86195 440 ± ± 12000 sigma0 = 0.541 0.014 sigma0 = 0.652 0.072 6000 ± ± sigma1 = 0.307 0.025 sigma1 = 0.417 0.017 10000 ± ± sigma2 = 0.979 0.021 sigma2 = 1.04 0.10 8000 4000 ± ± threshold = 138.962 0.065 threshold = 139.39 0.13 6000 ± ± var1 = 0.513 0.026 var1 = 0.61 0.15 ± ± var2 = 0.796 0.021 var2 = 0.635 0.051 4000 2000 2000 0 0 data data 4 4 σ σ (data - model)/ 2 (data - model)/ 2 0 0 − − 2 2 − − 4 4 140 145 150 155 160 140 145 150 155 160 δ δ 0 0 m ( D ) [MeV] m ( D ) [MeV] Signal efficiency: 57% Background rejection: 93% Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 15

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Boosting to Uniformity For analysis, we want our MVA to have uniform efficiency in decay time and Dalitz coordinates. We can force this to be the case with Uniform Gradient Boosting (uGB) Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 16

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Section 4 Conclusions Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 17

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Summary When a signal peak is clearly visible over a background, sPlot and MVA can help to reduce backgrounds. This analysis is data-driven: no Monte Carlo necessary. Multilayer perception achieves highest performance, but uniform boosting may reduce systematic uncertainty. Eventually, this will help us to perform a decay-time analysis of D 0 → K 0 S π + π − . Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 18

Why D 0 → K 0 S π + π − ? What is the LHCb experiment? Data Analysis Conclusions Image Credits Decision Tree: https://indico.scc.kit.edu/event/48/contributions/ 3410/attachments/1690/2312/BDT_KSETA_Freudenstadt.pdf LHCb image: https://home.cern/about/updates/2013/02/ proton-lead-run-brings-new-physics-reach-lhcb Crab Nebula: https://home.cern/topics/antimatter/ matter-antimatter-asymmetry-problem Tat University of California, Berkeley CERN University of Manchester Separating Signal from Background in D 0 → K 0 S π + π − 19