Feasibility study of performing high precision gamma spectroscopy of _L63 [Lambda] _L63 [Lambda] hypernuclei in the PANDA experiment [Elektronische Ressource] / von Alicia Sanchez-Lorente

Sujets

• Physik

Informations

• Publié par: johannes_gutenberg-universitat_mainz
• Publié le: 01 janvier 2010
• Nombre de lectures: 5
• Langue: English
• Poids de l'ouvrage: 12 Mo

Extrait

Feasibility study of performing
high precision gamma spectroscopy
of ΛΛ hypernuclei
in thePANDA experiment
Doktorarbeit
von
Alicia Sanchez-Lorente
geboren in Murcia (Spanien)
Institut fur¨ Kernphysik
Johannes Gutenberg-Universit¨at Mainz
August 20102
Erster Berichterstatter:
Zweiter Berichterstatter:
Dekan des Fachbereichs Physik:
Datum der mundlic¨ hen Prufung:¨ 30. September 2010Contents
Introduction 7
1 Topics on hypernuclear physics 13
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.2 General Aspects on Λ–Hypernuclei . . . . . . . . . . . . . . . . 14
1.2.1 The Spin–dependent ΛN interaction . . . . . . . . . . . 15
1.2.2 Specific production reactions . . . . . . . . . . . . . . . . 16
1.2.3 Decay modes of Λ–Hypernuclei . . . . . . . . . . . . . . 21
1.3 Spectroscopy of Λ Hypernuclei . . . . . . . . . . . . . . . . . . . 27
1.3.1 Historical background . . . . . . . . . . . . . . . . . . . 28
2 S = -2 System and Double Λ hypernuclei 35
2.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.2 Principles of Double–Λ–Hypernuclei Formation. . . . . . . . . . 36
2.2.1 Decay of double Λ Hypernuclei . . . . . . . . . . . . . . 39
2.2.2 Spectroscopy of double Lambda Hypernuclei . . . . . . . 39
2.2.3 Status of identified ΛΛ–hypernuclei . . . . . . . . . . . . 40
3 The PANDA experiment at FAIR 49
3.1 Physics program of PANDA . . . . . . . . . . . . . . . . . . . . 49
3.2 Detector Overview . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.2.1 Target Spectrometer . . . . . . . . . . . . . . . . . . . . 51
3.2.2 Forward Spectrometer . . . . . . . . . . . . . . . . . . . 63
3.2.3 Data adquisition . . . . . . . . . . . . . . . . . . . . . . 64
4 The Hypernuclear Detector Setup atPANDA 67
4.1 The PANDA Simulation Framework . . . . . . . . . . . . . . . . 67
4.2 Hypernuclear Detector Setup Requirements. . . . . . . . . . . . 71
4.2.1 Choice of the primary target . . . . . . . . . . . . . . . . 71
4.2.2 Active Secondary Target . . . . . . . . . . . . . . . . . . 78
4.2.3 Germanium Array . . . . . . . . . . . . . . . . . . . . . 81
4.2.4 Time of Flight System . . . . . . . . . . . . . . . . . . . 83
34 CONTENTS
5 Performance of the Hypernuclei production mechanism 89
5.1 Double hypernuclei via a statistical decay model . . . . . . . . . 89
5.1.1 The double Λ compound nucleus model . . . . . . . . . . 90
5.1.2 Population of Excited States in Double Hypernuclei . . . 93
5.1.3 Comparison with E906 experiment . . . . . . . . . . . . 96
5.2 Simulation of hypernuclei production . . . . . . . . . . . . . . . 103
5.2.1 Hypernuclei event generator . . . . . . . . . . . . . . . . 103
5.2.2 Hypernuclei Definition . . . . . . . . . . . . . . . . . . . 104
5.2.3 Decay modes of Hypernuclei . . . . . . . . . . . . . . . . 104
6 Hypernuclear Event Simulation 107
6.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.2 Event Generation . . . . . . . . . . . . . . . . . . . . . . . . . . 108
6.3 Digitization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
6.4 Reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
6.4.1 Momentum resolution and low momentum particles . . . 117
6.4.2 Charged Particle Identification. . . . . . . . . . . . . . . 121
6.5 Analysis of a Hypernuclear Event . . . . . . . . . . . . . . . . . 128
6.5.1 Background influence . . . . . . . . . . . . . . . . . . . . 133
6.5.2 Comparison with UrQMD+SMM calculations . . . . . . 136
6.5.3 Kaon identification based on timing measurements . . . 138
6.5.4 Radiation damage studies . . . . . . . . . . . . . . . . . 146
7 HPGe Detector operating in magnetic field 151
7.1 Germanium Detectors in magnetic fields . . . . . . . . . . . . . 151
7.2 Experimental Details . . . . . . . . . . . . . . . . . . . . . . . . 152
7.2.1 The Euroball Cluster Detector . . . . . . . . . . . . . . . 153
7.2.2 The segmented clover detector, VEGA . . . . . . . . . . 153
7.2.3 Experimental set–up . . . . . . . . . . . . . . . . . . . . 153
7.3 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
7.3.1 High Rates measurements . . . . . . . . . . . . . . . . . 159
7.3.2 Angular dependence . . . . . . . . . . . . . . . . . . . . 160
7.4 Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . . 161
8 Conclusions and Outlook 169
A Appendix 175
A.1 The Track Model . . . . . . . . . . . . . . . . . . . . . . . . . . 175
A.2 Simulated Detector Setup . . . . . . . . . . . . . . . . . . . . . 176
A.3 Digital Pulse Shape Analysis . . . . . . . . . . . . . . . . . . . . 181
A.4 Single and Double hypernuclei spectroscopy . . . . . . . . . . . 184
A.5 Production probability of double hypernuclei . . . . . . . . . . . 185
A.5.1 Hypernuclei definition . . . . . . . . . . . . . . . . . . . 186CONTENTS 5
Bibliography 2136 CONTENTSIntroduction
Quantum Chromo Dynamics (QCD) is the theory of the force responsible for
the confinement of quarks and gluons in hadrons, but it is also responsible
for the binding of nucleons in nuclei and thus of the appearances of ordinary
matterinouruniverse. Whiletheinternalstructureofhadronsandthespectra
of their excited states are important aspects of QCD, it is also important to
understand the origin of the nuclear force in a more rigorous way out of QCD
andhownuclearstructures-nucleionthesmallscaleanddensestellarobjects
on the large scale - are formed [1].
Furthemore, strangeness has played important roles in nuclear physics.
Introducing strangeness into nuclei extends our understanding of matter as
hadronic many-body systems, rather than nucleon many-body systems, and
provides new clues to their nature that can hardly be understood directly
through QCD.Indeed, ahyperonboundinanucleus offers aselective probe of
the hadronic many-body problem as it is not restricted by the Pauli principle
in populating all possible nuclear states, in contrast to neutrons and protons.
On one hand a strange baryon embedded in a nuclear system may serve as a
sensitive probe for the nuclear structure and its possible modification due to
the presence of the hyperon. On the other hand properties of hyperons may
change dramatically if implanted inside of a nucleus. Therefore a nucleus may
serve as a laboratory offering a unique possibility to study basic properties of
hyperonsandstrangeexoticobjects. Thus,hypernuclearphysicsrepresentsan
interdisciplinarysciencelinkingmanyfieldsofparticle,nuclearandmany-body
physics.
Detailed information on hyperon–nucleon and hyperon–hyperon interac-
tions is indispensable to the present understanding of high-density nuclear
matter inside neutron stars, where hyperons are possibly mixed and play
crucial roles(e.g. ref. [2]). Indeed, when the strangeness number increases,
strangeness hadronic matter made of equal numbers of protons, neutrons, Λs,
− 0Ξ s and Ξ s with neutral charge is expected to be stable [3]. It is argued that
in the core of neutron stars, where hyperons appear due to the large Fermi
energy of neutrons, the fractions of various baryons as a function of density
are very sensitive to the hyperon-nucleon (YN) and hyperon-hyperon (YY) in-
teractions,ofwhichrealisticdatahavepartlybeenobtainedfromhypernuclear
78 CONTENTS
studies in recent years as reviewed in Ref. [4].
Furthemore, it is also clear that a detailed and consistent understanding of
the quark aspect of the baryon-baryon forces in the SU(3) space will not be
possible as long as experimental information on the hyperon-hyperon channel
is not available.
Since scattering experiments between two hyperons are impractical, the
precise spectroscopy of multi-strange hypernuclei will provide a unique a-
pproach to explore the hyperon-hyperon interaction. Nuclei with strangeness
Figure 1: Present knowledge on hypernuclei. Experimental data are limited to
only 39 Λ hypernuclei in the S = 1 plane (blue) and only very few individual events
of double hypernuclei (yellow) have been detected and identified so far [5].
can be plotted in the three-dimensional nuclear chart, where the strangeness
number (S) increases along the third axis. Experimental data are limited to
only 39 Λ hypernuclei in the S = 1 plane (blue) and a few ΛΛ hypernuclei
(yellow) in the S = 2 plane (Fig. 1).
Acommondifficultytoalltheoreticalinvestigationsofmulti-strangehyper-
nuclei(e.gdouble–Λhypernuclei)isthelackofhigh–resolutionandsystematic
data on multi–hypernuclei and their level structure. Although the analysis of
emulsion stacks by automated means (e.g, by scanning the film with a CCD
camera) has been found possible in some circumstances, these studies cannot
provide high production rates. Furthermore, whether particle unstable statesCONTENTS 9
(whose decay photons are not detected) are populated, or whether neutrons
are being emitted, this method gives ambiguous results. The possibility to
trigger via kaon detection on potentially interesting events clearly calls for ex-
perimentsbasedonelectronicdetectors. Moreover,sincethekineticenergiesof
the nuclear fragments are very low, these experiments have to rely on specific
decay channels.
Hypernuclear research will be one of the main topics addressed by the
PANDA experiment [6] at the planned Facility for Antiproton and Ion Re-
search