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Nutzung Der Gefundenen Werte Far F(1) | | Und P a Bestimmt Worden Reconstruction of B D*°e ve Decays and Determination of \Vcb\ DISSERTATION zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr.rer.nat.) vorgelegt der Fakultat Mathematik und Naturwissenschaften der Technischen Universitat Dresden von Diplom-Physiker Jens Schubert geboren am 11.03.1977 in Pirna 1. Gutachter : Prof. Dr. Klaus R. Schubert 2. Gutachter : Prof. Dr. Michael Kobel 3. Gutachter : Dr. Jochen C. Dingfelder Eingereicht am: 01.12.2006 Abstract In this analysis the decay B- ^ D* 0e-Ve is measured. The underlying data sample consists of about 226 million BB-pairs accumulated on the Y(4S) resonance by the BABAR detector at the asymmetric e+e- collider PEP-II. The reconstruction of the decay uses the channels D*° ^ D0n0, D° ^ K-n+ and n0 ^ 77. The neutrino is not reconstructed. Since the rest frame of the B meson is unknown, the boost w of the D*° meson in the B meson rest frame is estimated by w. The W spectrum of the data is described in terms of the partial decay width dr/dw given by theory and the detector simulation translating each spectrum dr/dw into an expectation of the measured w spectrum. dr/dw depends on a form factor F(w) parameterizing the strong interaction in the decay process. To find the best descriptive dr/dw a fit to the data determines the following two parameters of dr/dw: (i) F(1)|VCb|, the product between F at zero D* 0-recoil and the CKM matrix element |Vcb|; (ii) p A , a parameter of the form factor F(w). The former parameter scales the height of dr/dw and pA1 varies the shape of it. The determined values of F(1)|Vcb|, p A1 and B (B- ^ D* 0e-Ve) are F(1)|VCb| = (35.8 ± 0.5 ± 1.5) x 10-3, pA1 = (1.08 ± 0.05 ± 0.09) and B (B- ^ D* 0e-Ve) = (5.60 ± 0.08 ± 0.42)%, where the uncertainties are statistical and systematic, respectively. The value of B (B- ^ D*°e -Ve) has been determined by an integration of dr/dw over the allowed w range using the fitted values of F(1)|VCb| and pA1. Kurzfassung In dieser Analyse wird der Zerfall B- ^ D* 0e-Ve gemessen. Der zugrundeliegende Datensatz umfasst ca. 226 Millionen BB-Paare, die im Prozess Y(4S) ^ BB am asymmetrischen e+e-- Speicherring PEP-II erzeugt und vom BABAR-Detektor gemessen wurden. Der analysierte Zerfall wird in den Kanalen D* 0 ^ D0n0, D0 ^ K-n+ und n0 ^ 77 rekonstruiert. Das Neutrino wird nicht rekonstruiert. Da das B-Ruhesystem unbekannt ist, muss der Boost w vom D* 0-Meson im B-Ruhesystem durch w abgeschatzt werden. Das w-Spektrum der Daten wird beschrieben durch die aus der Theorie bekannte partielle Zerfallsbreite dr/dw und durch die Detektorsimulation, die jedes dr/dw in eine Erwartung far die gemessenen w-Spektren abersetzen kann. dr/dw hangt von einem Formfaktor F(w) ab, welcher die starke Wechselwirkung im Zerfallsprozess parametrisiert. Um das am besten beschreibende dr/dw zu finden, bestimmt eine Funktionsanpassung an die Daten folgende zwei Parameter von dr/dw: (i) F(1)| |, das Produkt zwischen F bei minimalem D*°-Rackstofi und dem CKM-Matrixelement |Vcb|; (ii) p A , ein Parameter des Formfaktors F(w). Der erstgenannte Parameter skaliert die Hohe von dr/dw und p A variiert die Form. Far F(1)|Vcb|, pA und B (B- ^ D* 0e-Ve) wurden die Werte F(1)|VCb| = (35.8 ± 0.5 ± 1.5) x 10-3, pA1 = (1.08 ± 0.05 ± 0.09) und B (B- ^ D* 0e-Ve) = (5.60 ± 0.08 ± 0.42)% bestimmt, wobei die Unsicherheiten statistisch und systematisch sind. Der Wert far B (B- ^ D* 0e-Ve) ist aus der Integration von dr/dw aber den erlaubten w-Bereich unter Be- nutzung der gefundenen Werte far F(1) | | und p A bestimmt worden. Contents 1 Outline 1 2 Theory 2 2.1 The Standard Model of Particle Physics ................................................................................. 2 2.1.1 The Electroweak Interaction and the Higgs Mechanism ......................................... 3 2.1.2 The CKM Matrix ............................................................................................................. 6 2.1.3 The Strong Interaction ...................................................................................................... 9 2.2 The Decay B- ^ D* 0e-Ve ......................................................................................................... 9 2.2.1 Why Semileptonic Decays ............................................................................................... 10 2.2.2 Heavy Quark Symmetry and Isgur-Wise Function ................................................ 12 2.2.3 The Partial Decay Width of B- ^ D* 0e-Ve............................................................. 14 2.2.4 |VCb | Determination ............................................................................................................. 19 3 The BABAR Experiment 23 3.1 B Meson production at the e+e- Storage Ring PEP-II ...................................................... 23 3.2 The BABAR Detector ....................................................................................................................... 24 3.2.1 The Silicon Vertex Tracker ............................................................................................ 27 3.2.2 The Drift Chamber............................................................................................................. 28 3.2.3 The Detector of Internally Reflected Cherenkov Light ......................................... 28 3.2.4 The Electromagnetic Calorimeter .................................................................................. 31 3.2.5 The Superconducting Solenoid and the Instrumented Flux Return..................... 32 4 Candidate Selection 34 4.1 The Data Sample .............................................................................................................................. 35 4.1.1 Data Events.......................................................................................................................... 36 4.1.2 Monte Carlo Events......................................................................................................... 36 4.2 The Reconstruction of Charged Particles.................................................................................. 38 4.2.1 The Reconstruction of a Track..................................................................................... 38 4.2.2 The Electron Identification ............................................................................................ 39 4.2.3 The Kaon Identification ................................................................................................... 40 4.3 The Reconstruction of Neutral n Mesons ................................................................................. 43 4.4 Efficiency Differences between Data and MC ....................................................................... 47 4.4.1 The Tracking Efficiency ................................................................................................... 49 4.4.2 The Particle Identification Efficiency for Charged Particles ...................................49 4.4.3 The Neutral Pion Reconstruction Efficiency ............................................................. 50 4.5 The Selection Criteria.................................................................................................................... 54 i CONTENTS 4.5.1 The Event Selection ......................................................................................................... 55 4.5.2 The Candidate Selection ................................................................................................... 56 4.5.3 Multiple Candidates per Event ..................................................................................... 60 4.6 The Definition of cos 0BY and w................................................................................................... 61 5 Fit Method 68 5.1 Introduction ........................................................................................................................................ 68 5.2 The Fit Function .............................................................................................................................. 69 5.2.1 General Ansatz in the Am cos d BY Plane ................................................................. 70 5.2.2 Extraction of Am PDFs and cos 0BY PDFs for the BMC and for the RMC . 74 5.2.3 Corrections to the Expectation on Data Relatively to the RMC .......................... 79 5.2.4 Description of the Expectation Function on Data for one w Bin............................. 81 5.2.5 The Final Fit Function ................................................................................................... 83 5.3 The Reweighting Technique ......................................................................................................... 84 5.3.1 The Reweighting Technique for B- ^ D* 0e-Ve...................................................... 84 5.3.2 The Reweighting Technique for B ^ D0ev ............................................................. 87 5.4 The Fit to the Data ....................................................................................................................... 90 6 Fit Result 91 6.1 Monte Carlo Validation ................................................................................................................ 91 6.2 The Fit Result ................................................................................................................................. 94 6.2.1 Central Values and Statistical Uncertainties............................................................. 94 6.2.2
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