First observation of the semileptonic decay B ̄→ D_1hn+_1tns K ̄l ̄_̄n63l with the BABAR detector [Elektronische Ressource] / vorgelegt von Heiko Jasper

Sujets

• Physik

Informations

• Publié par: technischen_universitat_dortmund
• Publié le: 01 janvier 2009
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

First observation of the
semileptonic decay
+B ! D K ‘ with the‘s
BABAR detector
D I S S E R T A T I O N
zur Erlangung des akademischen Grades
Doctor rerum naturalium
(Dr. rer. nat.)
vorgelegt von
Heiko Jasper
geboren am 06.02.1980 in Dresden
Lehrstuhl fur Experimentelle Physik 5
Fakultat Physik
der Technischen Universitat Dortmund
20091. Gutachter : Prof. Dr. Bernhard Spaan
2. Gutachter : Dr. Jochen Dingfelder
Datum des Einreichens der Arbeit : 14.08.2009Abstract
+This thesis reports on the search for the semileptonic decay B ! D K ‘ including‘s
nal states with electrons and muons. The analysis uses a data sample of about 377 million
+BB pairs, recorded with the BABAR detector at the PEP-II asymmetric energy e e collider
at the SLAC National Laboratory in the years 1999-2006. The D meson is reconstructeds
( ){+ 0exclusively in three channels: D ! , with the decaying into K K , D ! K K withs s
( ) ( ){ {0 0the K being reconstructed in the charged K nal state and D ! K K, where only thes S
( ){ 0 +K decay into is reconstructed. Several Multi Layer Perceptron neural networks areS
used for the event selection. The missing mass with respect to the nominal B meson mass is
used for the extraction of the signal yield. An extended simultaneous maximum likelihood
t of the D signal-regions and sidebands of the three D reconstruction channels leads tos s
a nal result with a signi cance larger than 5 , thus claiming the rst observation of the
+signal decay. The branching ratios for the decay B ! D K ‘ are calculated to be‘s
+1:30+ 4BR(B ! D K e ) = (5:81 (stat:) 0:54(syst:) 0:49(BR(D ))) 10 ;e ss 1:30
+ +1:72 4BR(B ! D K ) = (6:68 (stat:) 0:69(syst:) 0:56(BR(D ))) 10 ; ss 1:69
+ +1:26 4BR(B ! D K ‘ ) = (6:13 0:51(BR(D ))) 10 ;‘ ss 1:24
where the errors are statistical, systematic and due to the limited knowledge of the branching
ratios of the D and its daughters, respectively. The rst error of last branching ratios
represents the total statistical and systematic error of the combined measurement.Contents
Contents i
Pictures iv
Tables viii
1 Introduction 1
2 Theoretical Background 3
2.1 The Standard Model of Particle Physics . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Electroweak Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 CKM-Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1.3 Strong Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 B meson physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 Semileptonic B decays . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2 The decay B! D K‘ . . . . . . . . . . . . . . . . . . . . . . . . . . 11s ‘
3 The BABAR experiment 15
+3.1 PEP-II asymmetric energy e e collider . . . . . . . . . . . . . . . . . . . . . 16
3.2 The BABAR detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.1 Silicon Vertex Tracker (SVT) . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.2 Drift Chamber (DCH) . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.3 Cherenkov Detector (DIRC) . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.4 Electromagnetic Calorimeter (EMC) . . . . . . . . . . . . . . . . . . . 21
3.2.5 Instrumented Flux Return (IFR) . . . . . . . . . . . . . . . . . . . . . 22
3.3 Track reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.4 Monte Carlo simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.5 Particle identi cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4 Data sample 29
4.1 Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.2 Monte Carlo samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.2.1 Generic BB Monte Carlo . . . . . . . . . . . . . . . . . . . . . . . . . 30
4.2.2 continuum Monte Carlo . . . . . . . . . . . . . . . . . . . . . 30
4.2.3 Signal Monte Carlo simulation . . . . . . . . . . . . . . . . . . . . . . 30
iCONTENTS
5 Reconstruction and event selection 33
5.1 Optimization of the selection . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2 of composite particles . . . . . . . . . . . . . . . . . . . . . . 34
5.3 Separation of continuum events . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.4 of cascade leptons and fake leptons . . . . . . . . . . . . . . . . . 39
5.5 Combinatorial background suppression . . . . . . . . . . . . . . . . . . . . . . 40
05.5.1 Selection of D ! K K decays . . . . . . . . . . . . . . . . . . . . . . 40s S
05.5.2 of D ! and D ! K K decays . . . . . . . . . . . . . 42s s
5.5.3 Reconstruction of the B meson . . . . . . . . . . . . . . . . . . . . . . 49
5.5.4 True-D background rejection . . . . . . . . . . . . . . . . . . . . . . . 52s
5.5.5 Multiple candidates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6 Data - Monte Carlo comparison of cut variables 59
6.1 Event shape Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
06.2 D ! K K reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62s S
6.3 D ! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65s
06.4 D ! K K . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68s
6.5 B candidate reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
6.6 Suppression of Upper vertex decays . . . . . . . . . . . . . . . . . . . . . . . . 73
6.7 Lepton momentum distributions . . . . . . . . . . . . . . . . . . . . . . . . . 74
7 Analysis method and t 75
7.1 Neutrino reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
7.2 Fit technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.2.1 Development of the simultaneous t . . . . . . . . . . . . . . . . . . . 79
7.2.2 Fit validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
7.2.3 Signi cance expectation from Toy Monte Carlo results . . . . . . . . . 97
7.3 Fitting the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8 Systematic uncertainties 103
8.1ties due to the t method . . . . . . . . . . . . . . . . . 103
8.1.1 Parametrization of the True-D fraction . . . . . . . . . . . . . . . . . 103s
8.1.2 Fit error of the m(D ) ts . . . . . . . . . . . . . . . . . . . . . . . . . 104s
8.1.3 Width of the signal gaussian . . . . . . . . . . . . . . . . . . . . . . . 104
8.1.4 Constraints on the single channels signal yields . . . . . . . . . . . . . 105
8.1.5 Bias correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
8.1.6 Total systematic uncertainties arising from the tting method . . . . . 106
8.2 Systematic uncertainties due to MC simulation and reconstruction . . . . . . 106
8.2.1 Signal Monte Carlo model . . . . . . . . . . . . . . . . . . . . . . . . . 106
8.2.2 Monte Carlo statistics . . . . . . . . . . . . . . . . . . . . . . . 108
8.2.3 Particle identi cation . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
8.2.4 Track and Photon reconstruction . . . . . . . . . . . . . . . . . . . . . 110
08.2.5 K corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110S
8.2.6 Radiative corrections - PHOTOS . . . . . . . . . . . . . . . . . . . . . 111
8.2.7 B counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
8.2.8 D branching ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111s
8.3 Total systematic uncertainties relative to e ciencies . . . . . . . . . . . . . . 112
iiCONTENTS
9 Results 113
9.1 Total signi cance of the result . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
9.2 Calculation of the branching ratio . . . . . . . . . . . . . . . . . . . . . . . . 114
10 Conclusions 117
Appendix 119
A 119
A.1 Study of D contributions to the signal . . . . . . . . . . . . . . . . . . . . . 119s
A.2 Fit of sideband subtracted data . . . . . . . . . . . . . . . . . . . . . . . . . . 121
A.3 Toy MC validation plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
References ix
Acknowledgments xv
iiiList of Figures
2.1 Summary of the Standard Model of Particle Physics. . . . . . . . . . . . . . . 4
2.2 The CKM unitary triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
=0 ()0=+2.3 Feynman diagram of B ! D ‘ . . . . . . . . . . . . . . . . . . . . . 9‘
+2.4 Spectator of the B ! D K ‘ decay. . . . . . . . . . . . . . 11‘s
02.5 Feynman diagram for B oscillation. . . . . . . . . . . . . . . . . . . . . . . . 12
3.1 PEP-II area at SLAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 Integrated luminosity of the BABAR experiment. . . . . . . . . . . . . . . . . 17
3.3 Overview of the BABAR detector. . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.4 Schematic view of the Silicon Vertex Tracker (SVT). . . . . . . . . . . . . . . 18
3.5 Sc view of the drift chamber (DCH). . . . . . . . . . . . . . . . . . . . 19
3.6 Schematic view of the Cherenkov detector (DIRC). . . . . . . . . . . . . . . . 20
3.7 Layout and schematic view of the crystals of the electromagnetic calorimeter
(EMC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.8 Schematic view of the Instrumented Flux Return. . . . .