Asymmetries in mixed beauty decays

JACCO ANDREAS DE VRIES

Copyright c• 2017 Jacco de Vries, all rights reserved.Asymmetries in mixed beauty decays“Asymmetrieen in gemengde schoonheidsvervallen”Thesis, Vrije Universiteit, Amsterdam

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This work is part of the research programme of the Foundation for Fundamental Researchon Matter (FOM), which is part of the Netherlands Organisation for Scientific Research(NWO). The work is carried out at the National Institute for Subatomic Physics (Nikhef)in Amsterdam, The Netherlands.

VRIJE UNIVERSITEIT

Asymmetries in mixed beauty decays

ACADEMISCH PROEFSCHRIFT

ter verkrijging van de graad Doctor aande Vrije Universiteit Amsterdam,op gezag van de rector magnificus

prof.dr. V. Subramaniam,in het openbaar te verdedigen

ten overstaan van de promotiecommissievan de Faculteit der Betawetenschappen

op woensdag 14 maart 2018 om 13.45 uurin de aula van de universiteit,

De Boelelaan 1105

door

Jacco Andreas de Vries

geboren te Groningen

promotor: prof.dr. M.H.M. Merk

copromotor: dr. J.A.N. van Tilburg

Voor Karin

Contents

1 Introduction 11.1 The Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1.1 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.1.2 The quark sector . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.2 CP violation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2.1 Sakharov conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2.2 Direct CP violation . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.2.3 Neutral-meson mixing . . . . . . . . . . . . . . . . . . . . . . . . . 151.2.4 Time evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.2.5 CP violation in interference . . . . . . . . . . . . . . . . . . . . . . 201.2.6 Standard Model expectations and beyond . . . . . . . . . . . . . . 201.2.7 History of CP violation in mixing . . . . . . . . . . . . . . . . . . . 221.2.8 Measuring ad

sl and as

sl . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2 The LHCb experiment 312.1 The LHC and conditions at point 8 . . . . . . . . . . . . . . . . . . . . . . 332.2 Tracking of charged particles . . . . . . . . . . . . . . . . . . . . . . . . . . 35

2.2.1 Magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.2.2 Vertex Locator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.2.3 Tracker Turicensis . . . . . . . . . . . . . . . . . . . . . . . . . . . 382.2.4 Inner Tracker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.2.5 Outer Tracker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.2.6 Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.2.7 Track reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2.3 Particle identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432.3.1 Ring-imaging Cherenkov detectors . . . . . . . . . . . . . . . . . . . 442.3.2 Calorimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.3.3 Muon stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452.3.4 Particle ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.4 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.5 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Contents

3 Detection asymmetries 513.1 The asymmetry of µ+fi≠ pairs . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.1.1 The J/Â tag-and-probe method . . . . . . . . . . . . . . . . . . . . 543.1.2 The Dú partial-and-full method . . . . . . . . . . . . . . . . . . . . 583.1.3 Comparing the methods . . . . . . . . . . . . . . . . . . . . . . . . 61

3.2 The asymmetry of K+fi≠ pairs . . . . . . . . . . . . . . . . . . . . . . . . 623.2.1 The asymmetry of K+K≠ pairs . . . . . . . . . . . . . . . . . . . . 66

3.3 Hadron identification asymmetry . . . . . . . . . . . . . . . . . . . . . . . 663.4 Trigger and muon PID asymmetry . . . . . . . . . . . . . . . . . . . . . . 70

3.4.1 Asymmetry of two trigger lines . . . . . . . . . . . . . . . . . . . . 73

4 CP violation in mixing of B0 mesons 774.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.2 Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.2.1 Removal of identifiable backgrounds . . . . . . . . . . . . . . . . . . 794.2.2 Mass fit and signal yields . . . . . . . . . . . . . . . . . . . . . . . . 82

4.3 Simulation and background studies . . . . . . . . . . . . . . . . . . . . . . 834.3.1 Signal simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.3.2 Contributions from peaking backgrounds . . . . . . . . . . . . . . . 844.3.3 Determination of the B+ fraction . . . . . . . . . . . . . . . . . . . 85

4.4 Decay-time model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 864.4.1 Determination of the k-factors . . . . . . . . . . . . . . . . . . . . . 874.4.2 Decay-time resolution . . . . . . . . . . . . . . . . . . . . . . . . . . 884.4.3 Signal model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.4.4 Simulation studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 924.4.5 Background models . . . . . . . . . . . . . . . . . . . . . . . . . . . 934.4.6 Full model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

4.5 Detection asymmetries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 964.5.1 The µ+fi≠ asymmetry . . . . . . . . . . . . . . . . . . . . . . . . . 974.5.2 The K+fi≠ asymmetry . . . . . . . . . . . . . . . . . . . . . . . . . 974.5.3 The muon trigger and PID asymmetry . . . . . . . . . . . . . . . . 994.5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

4.6 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014.7 Systematic uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

4.7.1 B+ background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1034.7.2 Other backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044.7.3 Uncertainties on the k-factor . . . . . . . . . . . . . . . . . . . . . . 1054.7.4 Other fit-related systematic uncertainties . . . . . . . . . . . . . . . 1054.7.5 Further consistency checks . . . . . . . . . . . . . . . . . . . . . . . 108

Contents

5 CP violation in mixing of B0

s mesons 1115.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.2 Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

5.2.1 Removal of identifiable backgrounds . . . . . . . . . . . . . . . . . . 1135.2.2 Peaking backgrounds . . . . . . . . . . . . . . . . . . . . . . . . . . 1175.2.3 Fit strategy and measured asymmetry . . . . . . . . . . . . . . . . 118

5.3 Simulation and peaking backgrounds . . . . . . . . . . . . . . . . . . . . . 1215.3.1 Signal simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215.3.2 Contributions from peaking backgrounds . . . . . . . . . . . . . . . 1215.3.3 Asymmetries in the background modes . . . . . . . . . . . . . . . . 1225.3.4 E�ect on as

sl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1235.4 Detection asymmetries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

5.4.1 The µ+fi≠ asymmetry . . . . . . . . . . . . . . . . . . . . . . . . . 1245.4.2 The K+K≠ asymmetry . . . . . . . . . . . . . . . . . . . . . . . . . 1245.4.3 PID asymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1265.4.4 The trigger and muon PID asymmetry . . . . . . . . . . . . . . . . 1285.4.5 Total detection asymmetry . . . . . . . . . . . . . . . . . . . . . . . 129

5.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

6 Discussion 1336.1 The picture after LHCb run 1 . . . . . . . . . . . . . . . . . . . . . . . . . 1336.2 Thoughts about the dimuon anomaly . . . . . . . . . . . . . . . . . . . . . 1346.3 Future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

6.3.1 Additional charm decay channels . . . . . . . . . . . . . . . . . . . 1366.3.2 Additional beauty decay channels . . . . . . . . . . . . . . . . . . . 1376.3.3 Reducing errors due to calibration samples . . . . . . . . . . . . . . 1376.3.4 Reducing errors due to backgrounds . . . . . . . . . . . . . . . . . . 1386.3.5 Beyond run 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

6.4 Closing remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

A Time integration of the assl

analysis 143B Additional material in the ad

slanalysis 145

C Additional material in the assl

analysis 153D Using hadronic decays 155

References 157

Summary 167Samenvatting 169Acknowledgements 173

Contents