Detection and reconstruction of short-lived particles produced by neutrino interactions in emulsion

198 Pages·2007·20.85 MB·English

Detection and reconstruction of short-lived
particles produced by neutrino interactions
in emulsion
Proefschrift
ter verkrijging van
de graad van Doctor aan de Universiteit Leiden,
op gezag van de Rector Magnificus Prof. Mr. P.F. van der Heijden,
volgens besluit van het College voor Promoties
te verdedigen op dinsdag 12 juni 2007
klokke 15:00 uur
door
Johannes Wilhelmus Edmond Uiterwijk
geboren te Maastricht
in 1969
promotor: Prof. Dr. M. de Jong
co-promotor: Dr. J. Panman
referent: Prof. Dr. E. Koffeman
promotiecommissie: Prof. Dr. A. Achu´carro
Prof. Dr. P.J. van Baal
Prof. Dr. J.M. van Ruitenbeek
Dr. J.L. Visschers
The work described in this dissertation is part of the research program of ’het Nation-
aal Instituut voor Kernfysica en Hoge-Energie Fysica (NIKHEF)’ in Amsterdam, the
Netherlands. The Author was financially supported by ’de Stichting voor Fundamenteel
Onderzoek der Materie (FOM)’ and the European Center for Nuclear Research (CERN)
in Geneva, Switzerland. FOM is funded by ’de Nederlandse Organisatie voor Fundame-
teel Onderzoek der Materie (NWO)’
ISBN: 978-90-8666-034-6
iii
Contents
Introduction vii
1 Neutrino masses and oscillations 1
1.1 Neutrino history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.1 The electron neutrino . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 The muon neutrino . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.3 The tau neutrino . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 The standard model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 Particles and forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 Parameters and constants . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.3 Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.4 Neutrino–matter interactions and cross-sections . . . . . . . . . . . 7
1.2.5 Beyond the standard model . . . . . . . . . . . . . . . . . . . . . . 10
1.3 Neutrino masses and oscillation . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.1 Direct mass measurements . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.2 Neutrino oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3.3 Oscillation detection methods . . . . . . . . . . . . . . . . . . . . . 14
1.4 Neutrino oscillation hints . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.4.1 Cosmology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.4.2 Solar neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.4.3 Atmospheric neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.4.4 The 1998 Super-Kamiokande result . . . . . . . . . . . . . . . . . . 22
1.5 The CHORUS oscillation search . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.5.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.5.2 Excluded parameter space . . . . . . . . . . . . . . . . . . . . . . . 24
1.6 Current neutrino experiments . . . . . . . . . . . . . . . . . . . . . . . . . 25
1.6.1 Super-Kamiokande . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1.6.2 SNO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
1.6.3 KamLAND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
1.7 Status of oscillation research . . . . . . . . . . . . . . . . . . . . . . . . . . 31
1.7.1 Atmospheric neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . 31
1.7.2 Solar and reactor neutrinos . . . . . . . . . . . . . . . . . . . . . . 35
1.7.3 Accelerator experiments . . . . . . . . . . . . . . . . . . . . . . . . 40
1.7.4 Cosmic neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
1.7.5 Three-flavour oscillation . . . . . . . . . . . . . . . . . . . . . . . . 43
1.8 Discussion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
iv contents
2 The CHORUS experiment 47
2.1 Detection principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.1.1 Tau identification in emulsion . . . . . . . . . . . . . . . . . . . . . 48
2.1.2 Background processes . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.2 Neutrino beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.3 Experimental setup overview . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.4 Emulsion target and electronic tracking detectors . . . . . . . . . . . . . . 55
2.4.1 Emulsion target considerations . . . . . . . . . . . . . . . . . . . . 55
2.4.2 Interfacing emulsion and electronic tracking detectors . . . . . . . 58
2.4.3 Tracking detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
2.4.4 Target region experimental setup . . . . . . . . . . . . . . . . . . . 60
2.5 Trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
2.6 Downstream detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
2.6.1 Hadron spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . 64
2.6.2 Streamer-tubes and honeycomb detector . . . . . . . . . . . . . . . 65
2.6.3 Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.6.4 Muon spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
2.7 Online monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
2.8 Track reconstruction and scanning predictions . . . . . . . . . . . . . . . . 70
2.9 Emulsion and scanning techniques . . . . . . . . . . . . . . . . . . . . . . 71
2.9.1 Hybrid experiments and automatic scanning . . . . . . . . . . . . . 71
2.9.2 Emulsion characteristics . . . . . . . . . . . . . . . . . . . . . . . . 72
2.9.3 Scanning microscopes . . . . . . . . . . . . . . . . . . . . . . . . . 76
2.9.4 Automatic track recognition . . . . . . . . . . . . . . . . . . . . . . 76
2.10 Reconstructing tracks and vertices in emulsion . . . . . . . . . . . . . . . 77
2.10.1 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
2.10.2 Interaction location by scan-back . . . . . . . . . . . . . . . . . . . 79
2.10.3 Vertex reconstruction with net-scan . . . . . . . . . . . . . . . . . 82
3 Honeycomb tracker 85
3.1 Motivation and requirements . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.2 Detection principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.3 Design and mechanical construction . . . . . . . . . . . . . . . . . . . . . 88
3.3.1 Monolayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
3.3.2 Honeycomb module . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3.3.3 Honeycomb tracker . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.3.4 Prototype measurements . . . . . . . . . . . . . . . . . . . . . . . . 93
3.4 Data-acquisition and read-out electronics . . . . . . . . . . . . . . . . . . 95
3.4.1 The bit-stream principle . . . . . . . . . . . . . . . . . . . . . . . . 95
3.4.2 Chambercards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
3.4.3 Clockcard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
3.5 Read-out system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.5.1 Read-out protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
3.5.2 Read-out program . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
3.5.3 Data compression . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
contents v
3.6 Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
3.6.1 Drift-time calibration . . . . . . . . . . . . . . . . . . . . . . . . . 107
3.6.2 Track finding per module . . . . . . . . . . . . . . . . . . . . . . . 108
3.6.3 Reconstructing 3-D tracks . . . . . . . . . . . . . . . . . . . . . . . 109
3.7 Performance and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 111
3.7.1 Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
3.7.2 Read-out electronics . . . . . . . . . . . . . . . . . . . . . . . . . . 111
3.7.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4 Track finding in emulsion 113
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4.1.1 Microscope optics and stages . . . . . . . . . . . . . . . . . . . . . 115
4.1.2 Tracking input characteristics . . . . . . . . . . . . . . . . . . . . . 118
4.1.3 Algorithm restrictions and requirements . . . . . . . . . . . . . . . 118
4.1.4 Toolkit abstraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.2 Multi-dimensional ordering containers . . . . . . . . . . . . . . . . . . . . 119
4.2.1 Find-in-range algorithm . . . . . . . . . . . . . . . . . . . . . . . . 119
4.2.2 Search trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
4.2.3 Hash table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
4.2.4 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
4.2.5 Timing performance . . . . . . . . . . . . . . . . . . . . . . . . . . 127
4.3 Track-finding algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
4.3.1 Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
4.3.2 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
4.3.3 Tracking time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
4.4 Tracking efficiency for simulated data . . . . . . . . . . . . . . . . . . . . 140
4.4.1 Hit generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
4.4.2 Acceptance criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
4.4.3 Efficiency criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
4.4.4 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . 145
4.5 The track trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
4.5.1 Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
4.5.2 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
4.6 Application in CHORUS emulsion scanning . . . . . . . . . . . . . . . . . . 150
4.6.1 Tracking configuration . . . . . . . . . . . . . . . . . . . . . . . . . 151
4.6.2 Prediction matching on the interface sheets . . . . . . . . . . . . . 152
4.6.3 Local-alignment procedure . . . . . . . . . . . . . . . . . . . . . . . 153
4.6.4 Tracking results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
4.7 Conclusion and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
∗+
5 D production 157
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
5.2 Charm-quark production and fragmentation . . . . . . . . . . . . . . . . . 158
5.3 Vertex reconstruction and charm sample selection . . . . . . . . . . . . . . 161
∗+ 0 +
5.4 Event selection of D → D π . . . . . . . . . . . . . . . . . . . . . . . 163
vi contents
0
5.4.1 D secondary vertex selection . . . . . . . . . . . . . . . . . . . . . 163
+
5.4.2 Primary π selection . . . . . . . . . . . . . . . . . . . . . . . . . . 163
5.4.3 Monte-Carlo simulation . . . . . . . . . . . . . . . . . . . . . . . . 164
5.4.4 Signal extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
5.5 Efficiency and background evaluation . . . . . . . . . . . . . . . . . . . . . 167
5.5.1 Background evaluation . . . . . . . . . . . . . . . . . . . . . . . . . 167
5.5.2 Detection efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
5.6 Results and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
Bibliography 171
Summary 184
Samenvatting 186
Acknowledgments 188
Curriculum Vitae 190
vii
Introduction
Several experiments have detected discrepancies in the fluxes of solar and atmospheric
neutrinos. Neutrino oscillations provide a possible explanation for the measured differ-
ences. This dissertation presents one of the first generation, specific, neutrino-oscillation
experiments using a man-made neutrino source, the chorus experiment at the European
laboratory for particle physics (cern)1.
Chorus was designed to look for νµ → ντ oscillation. Such oscillations can only
happen if the neutrino eigenstates of the weak interaction do not coincide with the
mass eigenstates. In the standard model of elementary particles and interactions, the
neutrinos are assumed to be massless. However, there is no fundamental reason why
neutrinos would be the only elementary fermion with no mass.
In 1998, the Super-Kamiokande experiment published clear evidence for neutrino os-
cillation [86]. That result unfortunately implied that chorus would not observe any
oscillation signal. Since then, the emphasis of the chorus data analysis has shifted to
exploiting the unique potential of the large number (about 100,000) recorded νµ interac-
tions in the emulsion to analyse in detail the production and decay of charmed particles.
In Chapter 1, the key aspects of neutrino masses and oscillations are discussed. A
description of the role of the chorus experiment is given, followed by an overview of the
current status of neutrino-oscillation experiments. Chapters 2 and 3 describe the layout
of the chorus experiment and the design of one particular sub-detector, the honeycomb
tracker. The algorithms for reconstructing particle tracks contained in the emulsion
images and their implementation are the subjects of Chapter 4. The last chapter, Chap-
ter 5, reproduces a published charmed-meson production study. An introduction to the
phenomenology of charm-quark production and fragmentation is given. This chapter
highlights the possibilities of emulsion and current-day scanning techniques to recon-
struct neutrino-interaction vertices and subsequent decay of short-lived particles.
As a final note, in this dissertation all particle masses, branching ratios, and other
particle characteristics have been taken from the Particle Data Group review of 2004 [1].
1The abbreviation stands for ‘Conseil Europ´een pour la Recherche Nucl´eaire’, the original committee
that proposed to built the cern laboratory
viii
1
Chapter 1
Neutrino masses and
oscillations
The neutrino was introduced as a hypothetical particle in 1930 by Pauli to solve
the energy crisis in nuclear β-decay. Because of its extremely small cross-section
to interact with matter, experimental neutrino physics only started much later.
The basic interaction was first described by Fermi’s theory and later unified in the
electro-weak theory of the standard model of elementary particles.
When anomalously low fluxes of solar and atmospheric neutrinos were measured,
neutrino oscillations were presented as a possible explanation for the missing neu-
trinos. In the last two decades, the experiments searching for neutrino oscillations
have become more sensitive, culminating in clear evidence for oscillation of atmo-
spheric neutrinos in 1998. This was followed in 2001 to 2003 by confirmation of
the solar-model predicted flux for 8B neutrinos and determination of the oscillation
parameters causing the solar-neutrino disappearance.
This chapter is mostly dedicated to the physics of neutrino oscillations. It will
introduce several aspects of the physics involving neutrinos, namely: the historic
discoveries of the neutrinos; neutrino masses and mixing, including a simplified
theory of oscillation; the parameter space explored by the CHORUS experiment; and
an overview of the current results of oscillation experiments. This chapter concludes
with a brief discussion of the current knowledge about neutrino mixing and an
outlook to the remaining questions to be answered in the future.
2 neutrino masses and oscillations
1.1 Neutrino history
In the beginning of the 20th century, there was a fundamental problem in physics: energy
seemed not to be conserved in certain radioactive decays. The energy of electrons emitted
in β-decays did not have a definite value but instead a continuous spectrum. In 1930,
Wolfgang Pauli, in his famous letter to the ‘radioactives’, postulated that an unknown
neutral particle was present inside the nucleus which escaped unobserved in β-decays,
carrying away the missing energy and momentum [2]. In 1934, Enrico Fermi developed
a comprehensive theory of radioactive decays [3] involving Pauli’s hypothetical particle
which he named in Italian the ‘little neutron’. Henceforth, this has stayed the particle’s
name: the ‘neutrino’. The symbol used for this particle is the greek letter ν.
1.1.1 The electron neutrino
Fermi’s calculations showed that a neutrino interacts only very weakly (or equivalently
very rarely) with matter. At the time, the neutrino–matter interaction cross-section
was considered too small for the neutrino to be detected. It was only 23 years after its
postulation by Pauli, that the first experiment to detect electron neutrinos was set up
by Reines and Cowan [4]. At a nuclear reactor, a prolific source of νe from β-decays of
the neutron-rich fission products, anti-neutrinos were detected via the inverse β-decay
1
process:
+
νe + p → e + n .
The detector consisted of liquid scintillator and dissolved cadmium-chloride. The light
produced by fast electrons in the liquid scintillator was detected by photo-multiplier
tubes. A neutrino interaction was identified by the delayed coincidence of two pulses:
+
the first one originating from the two gammas from the e annihilation; the second,
required to arrive several microseconds later, being the signal from the neutron captured
by a cadmium nucleus. Within a year the first signals of neutrinos were seen [5], followed
by statistically stronger evidence in 1956 [6].
1.1.2 The muon neutrino
The muon, a heavier version of the electron, was first detected with cloud-chambers in
cosmic-ray experiments around 1935 [7–11]. In its decay, µ → e, the emerging electron
shows a continuous energy spectrum with similar polarization as in radioactive β-decay.
Assuming that Fermi’s theory could also describe muon decay, there had to be three
particles in the final state. It was natural to assume that the two undetected particles
were both neutrinos. One of these neutrinos was associated with the electron (now known
as the νe). The absence of electro-magnetic decay of the muon, µ → e+γ via the process
shown in Figure 1.1, was an indication for the existence of a second neutrino species
associated with the muon, nowadays indicated as νµ.
1The symbols ‘p’ and ‘n’ stand for the proton and neutron. The symbols ‘d’ and ‘α’ will be used
for the deuteron and the helium-4 nucleus. Occasionally, when the focus is on isotope composition, the
proton, deuteron and helium-4 nuclei will be given by the corresponding nuclear symbols: 1H, 2H and
4He.