Etude et impact du bruit de fond corrÃ©lÃ© pour la mesure de l'angle ...

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108 5. Correlated background and a delayed event by neutron capture on Gd. Time and spatial correlations between prompt and delayed events are settle by the neutron thermalization process, therefore they are indistinguishable from those of ¯⌫ e events. The spatial distribution is expected to be rather uniform within the target volume. The proton recoil is expected to show a rather flat spectrum, extending beyond the reactor ¯⌫ e spectrum > 12 MeV. Scintillator quenching and acceptance e↵ects are expected to introduce a slope, either positive or negative depending from which e↵ect dominates. The delayed energy spectrum is due to neutron capture on Gd, therefore it is expected to be indistinguishable from the one produced by ¯⌫ e events. In addition, a muon crossing the detector surrounding generates FN with high multiplicity along its track, then more than one FN could reach the detector and generate a trigger. tel-00821629, version 1 - 11 May 2013 5.0.2 Stopping Muon The SM component arise from muons entering through the detector chimney, stopping in the top of the ID and decaying. The ionization energy deposited by the muon before stopping mimics the prompt event and the decay Michel electron mimics the delayed event. The time correlation between prompt and delayed events is expected to be of the order of the muon live time, ⌧ µ ' 2.2 µs, since the Michel electron deposit quickly its energy in the scintillator. For the same reason, the spatial distance between the two events is expected to be less than few cm and to be dominated by the resolution of the vertex reconstruction algorithm (⇠ 15 cm). In order to escape the ID muon tagging, the muon track length in the chimney is expected to be . 15 cm for a minimum ionising muon, depositing ⇠ 2 MeV/cm. Since the muon is stopping, its ionization energy increase quadratically as the muon velocity decrease and the muon track length is expected to be shorter than a minimum ionising muon. For this reason, prompt and delayed events are then expected to be concentrated in a small region within the detector chimney. Given such acceptance conditions, the amount of SM entering through the chimney and generating correlated background is expected to be small with respect to the total amount of muons interacting in the ID. Due to the complicated acceptance condition, the prompt spectral shape is not known and must be measured experimentally. Michel electrons has awellknow energy spectrum, extending up to ⇠ 60 MeV. The delayed events are due to those Michel electrons depositing energy in [6, 12] MeV, with a di↵erent spectral shape with respect to the Gd capture peak. 5.1 Measuring correlated background Since both FN and SM components shows a prompt energy spectrum extending at high energy, a pure sample of correlated background can be selected
5.1 Measuring correlated background 109 requiring the prompt energy to be above ⇠ 12 MeV, where the ¯⌫ e contamination is negligible, and below 30 MeV, ID muon tagging threshold. The background rate and spectral shape in the ¯⌫ e energy region, [0.7, 12.2] MeV, can be inferred by fitting the high energy sample assuming a flat spectrum and extrapolating the shape to low energy. However, the accuracy of such estimation depends on the validity of the extrapolation of the spectral shape. The selection performed at high energy allow also to study FN/SM separation based on the di↵erent correlation time distribution shown by the two background components, estimating separation e ciencies and purities. Such separation allow to study FN and SM independently. tel-00821629, version 1 - 11 May 2013 The analysis of the high energy tail of the prompt spectrum do not allow to infer directly the shape of the correlated background at low energy. However, the low energy shape is a key ingredient for the rate and shape analysis performed to measure ✓ 13 , thus precise measurements are required. Techniques based on MC simulation of the background and tagging strategy exploiting the capabilities of the DC detector are considered and discussed in the following. 5.1.1 MC technique Dedicated MC simulation, could be performed to obtain the spectral shape at low energy. The total background rate is then obtained by scaling the MC spectrum to the data, by normalise the spectrum to the high energy tail [12, 30] MeV. However, the accuracy of MC simulation are mainly limited by the modeling of scintillator quenching and acceptance e↵ects. The simulation of the FN prompt spectra is, in fact, sensible to the scintillator quenching a↵ecting protons, created via elastic scattering by the neutron during its thermalization. The accurate modeling of the chimney, which is actually a very complicated part of the detector from both geometrical and mechanical point of view, is instead required to accurately simulate acceptance e↵ect for incoming muons generating correlated background. Due to such complications, MC simulations of correlated background have not been considered so far. Accurate MC models are under development within the DC collaboration and will be used in future to crosscheck the current estimations. 5.1.2 Tagging techniques Tagging techniques based on the IV and the OV are exploited to infer spectral shape for FN and SM, using a rather pure sub-sample of correlated background. The tagging e ciencies must be estimated in order to rescale the tagged shapes and obtain the total background rate. Such e ciencies
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UNIVERSITÉ DENANTES FACULTÉ DES S
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tel-00821629, version 1 - 11 May 20
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Abstract tel-00821629, version 1 -
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Acknowledgments Since so many peopl
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Contents 1 Introduction 13 tel-0082
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CONTENTS 11 tel-00821629, version 1
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Chapter 1 Introduction tel-00821629
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15 tel-00821629, version 1 - 11 May
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Chapter 2 Neutrino physics tel-0082
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2.2 Neutrino history in brief 19 ma
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2.3 Neutrino oscillation, the theor
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2.4 Measuring neutrino oscillation
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3-flavour oscillations The dominant
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After the Sun, the other main natur
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2.5 The problem with the neutrino m
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2.6 Summary and open questions 37 s
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In the next future, reactor experim
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Chapter 3 The Double Chooz experime
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3.1 The Double Chooz detector 43 to
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3.1 The Double Chooz detector 45 te
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3.1 The Double Chooz detector 47 it
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3.1 The Double Chooz detector 49 te
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3.2 The detector read-out 51 40 m o
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3.3 The online system 53 IV trigger
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3.3 The online system 55 tions as s
Page 57 and 58: 3.3 The online system 57 tel-008216
Page 59 and 60: 3.4 The calibration system 59 tel-0
Page 61 and 62: loosing their kinetic energy 3 and
Page 63 and 64: 3.5 Reactor neutrino flux simulatio
Page 69 and 70: 3.8 Data reconstruction 69 A dedica
Page 71 and 72: Regarding the pulse information, th
Page 73 and 74: 3.8 Data reconstruction 73 tel-0082
Page 75 and 76: 3.8 Data reconstruction 75 Gain (ch
Page 77 and 78: 3.9 Conclusions 77 tel-00821629, ve
Page 79 and 80: Chapter 4 Measurement of ✓ 13 wit
Page 81 and 82: 4.1 Data sample 81 create a signal
Page 83 and 84: 4.2 ¯⌫ e signal selection 83 Liv
Page 85 and 86: 4.2 ¯⌫ e signal selection 85 Pro
Page 87 and 88: 4.2 ¯⌫ e signal selection 87 Neu
Page 89 and 90: 4.3 Backgrounds 89 energy spectrum
Page 91 and 92: 4.3 Backgrounds 91 tel-00821629, ve
Page 93 and 94: 4.4 Selection e ciencies and system
Page 95 and 96: 4$&14+#':"'$"$468'#21%$;' ! $"$468'
Page 97 and 98: 4.4 Selection ! e ciencies and syst
Page 99 and 100: 4.5 Data analysis summary 99 4.4.3
Page 101 and 102: 4.6 Final fit and measurement of
Page 103 and 104: 4.6 Final fit and measurement of
Page 105 and 106: 4.7 Summary and Conclusions 105 tel
Page 107: Chapter 5 Correlated background tel
Page 111 and 112: 5.2 High energy analysis 111 in a l
Page 113 and 114: 5.2 High energy analysis 113 tel-00
Page 115 and 116: 5.3 OV Tag analysis 115 The t cut v
Page 117 and 118: 5.4 Fast Neutrons analysis 117 bigg
Page 119 and 120: 5.4 Fast Neutrons analysis 119 5.4.
Page 121 and 122: 5.4 Fast Neutrons analysis 121 tel-
Page 123 and 124: 5.4 Fast Neutrons analysis 123 esti
Page 125 and 126: 5.4 Fast Neutrons analysis 125 Corr
Page 129 and 130: 5.4 Fast Neutrons analysis 129 Prom
Page 131 and 132: 5.4 Fast Neutrons analysis 131 lect
Page 133 and 134: 5.4 Fast Neutrons analysis 133 mari
Page 139 and 140: 5.5 Stopping Muon analysis 139 5.5.
Page 141 and 142: 5.5 Stopping Muon analysis 141 tel-
Page 143 and 144: 5.5 Stopping Muon analysis 143 tel-
Page 145 and 146: 5.5 Stopping Muon analysis 145 Sour
Page 147 and 148: 5.5 Stopping Muon analysis 147 belo
Page 149 and 150: 5.5 Stopping Muon analysis 149 The
Page 151 and 152: 5.6 Estimation of correlated backgr
Page 157 and 158: 5.7 Conclusion 157 5.7 Conclusion t
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Chapter 6 Conclusions tel-00821629,
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161 • The SM analysis could be im
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Chapter 7 Contributions to Double C
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165 from this study and from [64] f
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List of Figures tel-00821629, versi
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LIST OF FIGURES 169 tel-00821629, v
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List of Tables 3.1 Mean energy rele
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Bibliography [1] [2] tel-00821629,
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BIBLIOGRAPHY 183 [31] J.K. Ahn et a
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BIBLIOGRAPHY 185 [61] W. Hampel et
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BIBLIOGRAPHY 187 [92] A. Stueken et
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