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

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Feasibility study of performinghigh precision gamma spectroscopyof ΛΛ hypernucleiin thePANDA experimentDoktorarbeitvonAlicia Sanchez-Lorentegeboren in Murcia (Spanien)Institut fur¨ KernphysikJohannes Gutenberg-Universit¨at MainzAugust 20102Erster Berichterstatter:Zweiter Berichterstatter:Dekan des Fachbereichs Physik:Datum der mundlic¨ hen Prufung:¨ 30. September 2010ContentsIntroduction 71 Topics on hypernuclear physics 131.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.2 General Aspects on Λ–Hypernuclei . . . . . . . . . . . . . . . . 141.2.1 The Spin–dependent ΛN interaction . . . . . . . . . . . 151.2.2 Specific production reactions . . . . . . . . . . . . . . . . 161.2.3 Decay modes of Λ–Hypernuclei . . . . . . . . . . . . . . 211.3 Spectroscopy of Λ Hypernuclei . . . . . . . . . . . . . . . . . . . 271.3.1 Historical background . . . . . . . . . . . . . . . . . . . 282 S = -2 System and Double Λ hypernuclei 352.1 Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.2 Principles of Double–Λ–Hypernuclei Formation. . . . . . . . . . 362.2.1 Decay of double Λ Hypernuclei . . . . . . . . . . . . . . 392.2.2 Spectroscopy of double Lambda Hypernuclei . . . . . . . 392.2.3 Status of identified ΛΛ–hypernuclei . . . . . . . . . . . . 403 The PANDA experiment at FAIR 493.1 Physics program of PANDA . . . . . . . . . . . . . . . . . . . . 493.2 Detector Overview . . . . . . . . . . . . . . . . . . .

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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 FAIR [7]. The FAIR complex will include the High Energy Storage
Ring (HESR) to store antiprotons between 0.8 and 14.4MeV energy. Intense
32 −2 −1and high quality beams with luminosities up to 10 cm s and momentum
−5resolutions down to 10 are expected. The PANDA hypernuclear programme
shall reveal the strength of Λ–Λ interaction via the high resolution γ spec-
troscopy of double Λ hypernuclei. Contrary to past hypernuclear experiments,
where only a few double hypernuclei events were found, the challenge of the
PANDA experiment will be to produce statistics of five orders of magnitude
larger. Thereasonresidesinthefactthatgermaniumdetectorshavepresently
an efficiency of only a few percent.
Nevertheless, in combination with high luminosity of the antiproton beam
atHESR,ahighproductionrateofsingleanddoublehypernucleiunderunique
experimental conditions will be possible for the first time.
−TheproductionoflowmomentumΞ hyperonsandtheircaptureinatomic
levelsisthereforeessentialfortheexperiment. Initiallyhyperons, inparticular
− ¯Ξ + Ξ, are produced on a primary nuclear target via the following reactions,
+−p+p→Ξ Ξ (1)
0−p+n→Ξ Ξ (2)
The slowing down of the Ξ proceeds through a sequence of nuclear elastic
scattering (rescattering) processes inside the residual nucleus in which the
antiproton annihilation has occurred, and by energy loss during the passage
through an active absorber. If decelerated to rest before decaying, the particle
canbecapturedinanucleus, eventuallyreleasingtwoΛhyperonsandforming
−a double hypernucleus via the strong interaction Ξ +p → ΛΛ + 28MeV.
This process leaves the system in an high excited intermediate state which
can lead to the emission of slightly excited nuclear or hypernuclear fragments.
In the next stage, those hyperfragments which are in a excited nuclear level
decayfirsttothegroundstateviaelectromagnetictransitions,andthenviathe
weak decay ( mesonic and non–mesonic decay modes). The identification of
double hypernuclei will be performed by using the information obtained from
the electromagnetic transitions, and from the detection of weak decay charged
products. Additionally, theassociatedantihyperonsΞ(orevenpositivekaons)
produced as a result of their annihilation with nucleons of the target nucleus,10 CONTENTS
canbeusedfortaggingthedoublehypernucleiproductionoverthebackground
reactions.
In the present work the feasibilty of performing γ spectroscopy of double
Λ hypernuclei at the PANDA experiment will be investigated by means of a
Monte Carlo simulation.
The major challenges that the simulation has to deal with are on the one
hand the optimization of a dedicated detector layout for accurate measure-
ments in a high rate environment and on the other hand, to provide effective
analysis strategies for the hypernuclei spectroscopy. These two major items
will be treated separately in the present work. Indeed, it must show that
excited double hypernuclei can be produced significantly in order to perform
high resolution gamma spectroscopy with germanium detectors. In addition,
analysis strategies must also be optimized to ensure a unique identification of
the single and double hypernuclei produced.
Accordingly,thestructureofthepresentworkwillbeintroducedinthenext
paragraph. In Chapters 1 and 2, the general aspects of hypernuclear physics
will be treated in detail, giving an overview of the single and double Λ hyper-
nucleiproductionmechanism,andoftheexperimentaldevelopmentsofthelast
decades. These two first chapters will serve as motivation to introduce the hy-
pernuclear physics program at the PANDA experiment. The main parts of the
PANDA detector will be presented in Chapter 3. However, due to the comple-
xityofthehypernuclearproductionmechanism,thePANDAdetectorwillneed
to be extended to accomodate a devoted detector setup for the hypernuclear
project. This devoted detector setup consisting of a primary nuclear target,
a secondary active target and a germanium array detector, will be introduced
in Chapter 4. In addition, the requirements of these three components will be
studiedtooptimizethefirststepofthehypernuclearproductionprocess,which
− ¯comprises of the production of Ξ + Ξ pairs in the primary target, the decel-
−eration and capture of Ξ in the secondary active target, and the evaluation
of possible radiation damage on the germanium detectors due to the hadronic
background interactions. In Chapter 5, the second step of the hypernuclear
production mechanism will be studied. This is related to the production of
excited double Λ hypernuclei via a statistical decay model. In this chapter,
thepopulationofparticlesstableexcitedstatesindoublehypernucleiafterthe
−capture of a Ξ will be investigated. This new technique offers the possibility
of studying the feasibility to perform gamma spectroscopy of double hypernu-
clei at PANDA. Accordingly a full hypernuclear event will be simulated. The
different steps of this simulation will be presented in Chapter 6. The analysis
of this event will be performed by using a strategy which consists of using
the photons emitted from the electromagnetic deexcitations of the produced
double hypernuclei, and the momenta correlation of the negative pions from
their sequential mesonic decay. With this technique, some of the electromag-
netic transitions can be uniquely assigned to the corresponding hypernuclide