Measurement of the Mass of the Top Quark in the l+ Jets Channel Using the Matrix Element Method

Transcription

1 Measurement of the Mass of the Top Quark in the l+ Jets Channel Using the Matrix Element Method Carlos Garcia University of Rochester For the DØ Collaboration APS Meeting 2007

2 Outline Introduction Top quark at the Tevatron. The Matrix Element Method Basics of the method used. Top Quark Mass Measurement in the l+jets Channel Results from the 900 pb -1 of data. 2

3 Top Quark At the Tevatron, top quarks are primarily produced in pairs via the strong interaction. Since V tb ~ 1, the top quark almost always decays to Wb (Ws, Wd CKM suppressed) Event topology depends on the W decay mode jet E T mis ν b p b p jet jet jet Experimental signature in the lepton+jets channel: - 1 high p T lepton - 4 jets (2 b-jets) - large E T mis 3

4 Top Quark Identification Background Processes: W+jets ttbar Multi-jet background W+jets production.!! Multi-jet events: leading to fake or mis-characterized lepton and fake missing transverse energy! - Exactly 4 Calorimeter Jets p T > 20 GeV " < Isolated Lepton p T > 20 GeV " e < 1.1, " µ < Missing Transverse Energy E mis T > 20 GeV W+jets ttbar 4

5 The Basics of the Analysis I We calculate a probability per event to be signal or background as a function of the top mass and the Jet Energy Scale (JES). If we had all the parton level information 'y' this probability would be just proportional to the differential cross section. In reality is a bit more complicated: Initial state Differential cross section, based on LO Matrix Element ( qq " tt ) only Transfer Function: probability to measure x, when parton-level y was produced. P tt! ( x;m top,jes) = Normalization: 1 ( ) " m top # dq 1 dq 2 f ( q) f ( q )d"( y;m top )Prob( x, y,jes) Overall JES is a free parameter in the fit, constrained in situ by the mass of the W decaying hadronically Measurements (x): jets and leptons Background probability is conceptually the same but flat in m top and JES. 5

6 The Basics of the Analysis II b-tagging: Weight each jet to parton assignment with b-tagging probabilities. 24 possible weighted assignments between jets and partons Six particle final state P tt N"tag tt ( x;m top,jes) = # W c P tt c=1;24! W c tt = " J =1;4 p J c ( x;m top,jes) If jet J was tagged: If jet J was not tagged:!! Parameterized tagging efficiencies p J = " J ( flavor,#, p T ) p J =1"# J ( flavor,$, p T ) 6

7 The Basics of the Analysis III Probability per event: P N"tag evt ( ) ( x;m top,jes, f ) top = f top P N"tag ( tt x;m top,jes) + ( 1" f top )P bkg x,jes Probability per sample: P N"tag (x;m top,jes) = # P N"tag evt (x;m top,jes) evt f top = f top best (m top,jes) To combine the three b-tagged samples the 2D probabilities are multiplied:! P comb (x;m top,jes) = P 0"Tags (x;m top,jes)p 1"Tag (x;m top,jes)p 2"Tags (x;m top,jes) To use an external constraint in JES we convoluted the original likelihood with the measure in the photon + Jet sample which is represented as a Gaussian likelihood centred at JES = 1 with a width of 3.7% (this width is estimated from ttbar MC sample). P (x;m top,jes) = P comb (x;m top JES)G(JES) 7

8 Calibration Mass Calibration and Pull for Muons D0 Run II Preliminary Mass Calibration and Pull for Electrons D0 Run II Preliminary D0 Run II Preliminary Before applying to data, the method is calibrated for shifts in the mean and uncertainties using ensemble tests on simulated MC events. D0 Run II Preliminary 8

9 Data Result Data Sample Electrons channel (971 pb -1 ): 173 events in 0 Tags 57 events in 1 Tag 19 events in 2 Tags Muons channel (813 pb -1 ): 163 events in 0 Tags 70 events in 1 Tag 22 events in 2 Tags 249 events (28% purity) 255 events (25% purity) 9

10 Data Result Result combining Tags, Calibrated D0 Run II Preliminary m + jets D0 Run II Preliminary e + jets l + jets: 2.4 M top =170.5 ± 2.4 ( stat.+jes)gev 10

11 Data Result Result combining Tags, Calibrated Most Significant Systematic Errors (+) (-) Signal modeling l + jets b fragmentation model b/l ratio (1.5%) JES pt dependence TRF-Tagging MC Signal fraction QCD Contamination GeV l + jets: 2.4 M top =170.5 ± 2.4 ( stat.+jes) 1.2 ± 1.1 ( syst. )GeV 11

12 Top Mass Current Status Impact on Standard Model Higgs boson: M H = "24 GeV; M H <144GeV@95CL! 12

13 CONCLUSIONS D0 Run II Preliminary result for Tags from 900 pb -1 : With prior (3.7%) in JES: 2.4 M top =170.5 ± 2.4 ( stat.+jes) 1.2 ± 1.1 ( syst. )GeV A new improved measurement of the top quark mass allow us to reach the 1% precision (Combined DØ + CDF measurement). The precise measurement of top quark mass helps constrain the mass of the SM Higgs boson, and it is one of the most important measurements at the Tevatron. 13

14 DATA RESULT + PRIOR Combined Tag analysis, Calibrated + Prior m + jets e + jets l + jets: JES =1.032 ± stat. ( ) 14

15 Systematic Errors Error Source up(+) down(-) Physics modeling: Signal modeling b fragmentation model PDF uncertainty Background modeling b/c semileptonic decay Detector Modeling b/l ratio (1.5%) JES pt dependence Trigger b tagging TRF-Tagging MC TRF (signal) TRF (background) Method: Signal fraction QCD contamination MC calibration mass MC calibration JES Total Systematic Errors Total Errors (Stat.+JES+Sys.)

16 DATA 2D likelihood before calibration. Each color represents one sigma. After projection in the two dimensions we apply the calibration curves and adjust the errors using the final pulls. Electrons channel Muons channel 16

17 The Ensembles Example of composition and fluctuations in the ensembles (inputs). Electrons channel composition: 251 events 27.6% purity. Below are the purities for the different Tag samples. UNTAGGED 27.5% 0 TAG 10% 1 TAG 69% 2 TAG 92% 17

18 JES Calibration JES Calibration for Muons JES Calibration for Electrons Tags Tags 18

19 DATA RESULT Combined Tag analysis - Calibrated m + jets e + jets l + jets: JES =1.046 ± stat. ( ) 19

20 DATA RESULT Combined Tag analysis - Calibrated m + jets e + jets l + jets: 2.7 M top =169.2 ± 2.7 stat.+jes ( ) 20

21 DATA RESULT 2.7 M top =169.2 ± 2.7 Errors in Data relative to MC ( stat.+jes)gev! Combined Tag analysis - Calibrated 3.9GeV 3.7GeV 21

22 The DZero Detector and the Tevatron Tracking: Silicon vertex detector (SMT) Central Fiber Tracker (CFT) 2 T Superconducting Solenoid. η < 2.5 Preshowers EM/HAD Calorimeter: Central, η <1.1 Forward, η <4.2 Muon system: 1.8 T iron toroids. η <2.0 3-Level trigger system: Level 1 (hardware): 2 khz Level 2 (hardware): 1 khz Level 3 (software): 50 Hz 22