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The work presented in this thesis addresses a key issue of the CBM experiment at FAIR, which aims to study charm production in heavy ion collisions at energies ranging from 10 to 40 AGeV . For the first time in this kinematical range, open charm mesons will be used as a probe of the nuclear fireball. Despite of their short decay length, which is typically in the order of few 100 µm in the laboratory frame, those mesons will be identified by reconstructing their decay vertex.
The study of the electromagnetic structure of hadrons plays an important role in understanding the nature of matter. In particular the emission of lepton pairs out of the hot and dense collision zone in heavy-ion reactions is a promising probe to investigate in-medium properties of hadrons and in general the properties of matter under such extreme conditions. The first experimental observation of an enhanced di-electron yield in the invariant-mass region 0:3 - 0:7 GeV/c2 in p+Be collisions at 4:9 GeV/u beam energy [2] was announced by the DLS collaboration [1]. Recent results of the HADES collaboration show a moderate enhancement above n Dalitz decay contributions for 12C+12C at 1 and 2 GeV/u [3, 4] confirming the DLS results. There are several theoretical explanations of this observation, most of them focusing on possible in-medium modifications of the properties of vector mesons. At low beam energies the question whether the observed excess is related to any in-medium effects remains open because of uncertainties in the description of elementary di-electron sources. In this work the di-electron production in p+p and d+p reactions at a kinetic beam energy of 1:25 GeV/u measured by the HADES spectrometer is discussed. At Ekin = 1:25 GeV/u, i.e. below the n meson production threshold in proton-proton reactions, the delta Dalitz decay is expected to be the most abundant source above the pi 0 Dalitz decay region. The observed large difference in di-electron production in p+p and d+p collisions suggests that di-electron production in the d+p system is dominated by the n+p interaction. In order to separate delta Dalitz decays and np bremsstrahlung the di-electron yield observed in p+p and n+p reactions, both measured at the same beam energy, has been compared. The main interest here is the investigation of iso-spin effects in baryonic resonance excitations and the off-shell production of vector mesons [5]. We indeed observe a large difference in di-electron production in p+p and n+p reactions. Results of these studies will be compared to recent calculations. We will also present our experimentally defined cocktail for heavy-ion data. At much higher beam energies experimental results of the CERES [6] and NA60 [7] collaborations also show an enhancement in the invariant mass region 0:3 - 0:7 GeV/c2, in principle similar to the situation in DLS. A strong excess of lepton pairs observed by recent high energy heavy-ion dilepton experiments hint to a strong influence of baryons, however no data exist at highly compressed baryonic matter, achievable in heavy-ion collisions from 8 - 45 GeV/u beam energy. These conditions would allow to study the expected restoration of chiral symmetry by measuring in-medium modifications of hadronic properties, an experimental program which is foreseen by the future CBM experiment at FAIR. The experimental challenge is to suppress the large physical background on the one hand and to provide a clean identification of electrons on the other hand. In this work, strategies to reduce the combinatorial background in electron pair measurements with the CBM detector are discussed. The main goal is to study the feasibility of effectively reducing combinatorial background with the currently foreseen experimental setup, which does not provide electron identification in front of the magnetic field.
This work presents the study on the suitability of single-crystal CVD diamond for particle-detection systems in present and future hadron physics experiments. Different characterization methods of the electrical and the structural properties were applied to gain a deeper understanding of the crystal quality and the charge transport properties of this novel semiconductor material. First measurements regarding the radiation tolerance of diamond were performed with sensors heavily irradiated with protons and neutrons. Finally, detector prototypes were fabricated and successfully tested in various experiments as time detectors for minimum ionizing particles as well as for spectroscopy of heavy ions at the energy ranges available at the SIS and the UNILAC facilities of GSI. ...
The HADES (High Acceptance DiElectron Spectrometer) is an experimental
apparatus installed at the heavy-ion synchrotron SIS-18 at GSI, Darmstadt.
The main physics motivation of the HADES experiment is the measurement
of e+e− pairs in the invariant-mass range up to 1 GeV/c2 in heavy-ion collisions
as well as in pion and proton-induced reactions.
The HADES physics program is focused on in-medium properties of the light
vector mesons ρ(770), ω(783) and φ(1020), which decay with a small branching
ratio into dileptons. Dileptons are penetrating probes which allow to study
the in-medium properties of hadrons. However, in heavy-ion collisions, the
measurement of such lepton pairs is difficult because they are rare and have a
very large combinatorial background.
Recently, HADES has been upgraded with new detectors and new electronics
in order to handle higher intensity beams and reactions with heavy nuclei up
to Au.
HADES will continue for a few more years its rich physics program at its
current place at SIS-18 and then move to the upcoming international Facility
for Antiproton and Ion Research (FAIR) accelerator complex. In this context
the physics results presented in this work are important prerequisites for the investigation
of in-medium vector meson properties in p + A and A+A collisions.
This work consists of five chapters. The first chapter introduces the physics
motivation and a review of recent physics results. In the second chapter, the
HADES spectrometer is described and its sub-detectors are presented. Chapter
three deals with the issue of lepton identification and the reconstruction of
the dielectron spectra in p + p collisions is presented. Here, two reactions
are characterized: inclusive and exclusive dilepton production reactions. From
the spectra obtained, the corresponding cross sections are presented with the
respective statistical and systematical errors. A comparison with theoretical
models is included as well. Conclusions are given in chapter four.
The final part of this work is dedicated to the HADES upgrade, whose goal
is among others the achievement of a reliable and fast data acquisition of the
Multiwire Drift Chambers (MDCs). Chapter five presents my contribution to
this successful project during the three years of my stay at GSI.
Ein wesentlicher Forschungsgegenstand der Kernphysik ist die Untersuchung der Eigenschaften von Kernmaterie. Das Verständnis darüber gibt in Teilen Aufschluss über die Erscheinungsweise und Wechselwirkung von Materie. Ein Schlüssel liegt dabei in der Untersuchung der Modifikation der Eigenschaften von Hadronen in dem Medium Kernmaterie, das durch Parameter wie Dichte und Temperatur gekennzeichnet werden kann. Man hofft damit unter anderem Einblick in die Mechanismen zu bekommen, welche zur Massenbildung der Hadronen beitragen. Zur Untersuchung solcher Modifikationen eignen sich insbesondere Vektormesonen, die in e+e- Paare zerfallen. Die Leptonen dieser Paare wechselwirken nicht mehr stark mit der Materie innerhalb der Reaktionszone, und tragen somit wichtige Informationen ungestört nach außen. Das HADES-Spektrometer bei GSI wird dazu verwendet die leichten bei SIS-Energien produzierten Vektormesonen rho, omega und phi zu vermessen. Hierzu wurde zum erste mal das mittelschwere Stoßsystem Ar+KCl bei einer Strahlenergie von 1,76 AGeV gemessen. Die im Vergleich zum früher untersuchten System C+C höhere Spurmultiplizität innerhalb der Spektrometerakzeptanz verlangte eine Anpassung der bisher verwendeten Datenanalyse. Das bisher verwendete Verfahren, mehrere scharfe Schnitte auf verschiedene Observablen seriell anzuwenden, um einzelne Leptonspuren als solche zu identifizieren, wurde durch eine neu entwickelte multivariate Analyse ersetzt. Dabei werden die Informationen aller beteiligten Observablen mit Hilfe eines Algorithmus zeitgleich zusammengeführt, damit Elektronen und Positronen vom hadronischen Untergrund getrennt werden können. Durch Untersuchung mehrerer Klassifizierer konnte ein mehrschichtiges künstliches neuronalen Netz als am besten geeigneter Algorithmus identifiziert werden. Diese Art der Analyse hat den Vorteil, dass sie viel robuster gegenüber Fluktuationen in einzelnen Observablen ist, und sich somit die Effizienz bei gleicher Reinheit steigern lässt. Die Rekonstruktion von Teilchenspuren im HADES-Spektrometer basiert nur auf wenigen Ortsinformationen. Daher können einzelne vollständige Spuren a priori nicht als solche gleich erkannt werden. Vielmehr werden durch verschiedene Kombinationen innerhalb derselben Mannigfaltigkeit von Positionspunkten mehr Spuren zusammengesetzt, als ursprünglich produziert wurden. Zur Identifikation des maximalen Satzes eindeutiger Spuren eines Ereignisses wurde eine neue Methode der Spurselektion entwickelt. Während dieser Prozedur werden Informationen gewonnen, die im weiteren Verlauf der Analyse zur Detektion von Konversions- und pi0-Dalitz-Paaren genutzt werden, die einen großen Beitrag zum kombinatorischen Untergrund darstellen. Als Ergebnis wird das effizienzkorrigierte, und auf die mittlere Zahl der Pionen pro Ereignis normierte, Spektrum der invarianten Elektronpaarmasse präsentiert. Erste Vergleiche mit der konventionellen Analysemethode zeigen dabei eine um etwa 30% erhöhte Rekonstruktionseffizienz. Das Massenspektrum setzt sich aus mehr als 114.000 Paaren zusammen -- über 16.000 davon mit einer Masse größer als 150 MeV. Ein erster Vergleich mit einem einfachen thermischen Modell, welches durch den Ereignisgenerator Pluto dargestellt wird, eröffnet die Möglichkeit, die hier gefundenen Produktionsraten des omega- und phi-Mesons durch m_T-Skalierung an die durch andere Experimente ermittelten Raten des eta zu koppeln. In diesem Zusammenhang findet sich weiterhin ein von der Einschussenergie abhängiger Produktionsüberschluss von F(1,76) = Y_total/Y_PLUTO = 5,3 im Massenbereich M = 0,15...0,5 GeV/c^2. Die theoretische Erklärung dieses Überschusses birgt neue Erkenntnisse zu den in-Medium Eigenschaften von Hadronen.
Das Compressed Baryonic Matter Experiment (CBM) wird im Rahmen der Facility for Antiproton and Ion Research (FAIR) entwickelt, um das Phasendiagramm stark wechselwirkender Materie vorwiegend im Bereich hoher Dichte ausgiebig zu studieren. Dazu sollen Kollisionen schwerer Ionen durchgeführt werden und die Reaktionsprodukte mit hoher Präzision in Teilchendetektoren gemessen und identfiziert werden. Eine wichtige Aufgabe besteht in der Unterscheidung von Elektronen und Pionen, zu der ein Übergangsstrahlungszähler (Transition Radiation Detector) beiträgt. Übergangsstrahlung wird im relevanten Impulsbereich dieser Teilchen nur von Elektronen emittiert und soll im Detektor registriert werden.
In der vorliegenden Arbeit wird die Entwicklung dieses Detektors auf Basis von Vieldrahtproportionalkammern (Multiwire Proportional Chamber ) hauptsächlich anhand von Simulationen diskutiert, aber auch erste Testmessungen eines Prototypen vorgestellt. Der Schwerpunkt der Simulationen eines einzelnen Detektors liegt in der Untersuchung der Effiienz in Abhängigkeit seiner Dicke.
Der Übergangsstrahlungszähler für CBM wird aus mehreren Detektorlagen bestehen. Daher wird außerdem die Effizienz des Gesamtsystems analysiert, indem verschiedene Methoden zur Kombination der einzelnen Signale angewendet werden. Darüber hinaus wird die Effizienz des verfolgten Detektorkonzepts in Abhängigkeit des Radiators, der Anzahl der Detektorlagen, sowie des Teilchenimpulses präsentiert.
The first measurement of the fluctuation of the kaon-to-proton ratio in relativistic heavy-ion collisions is presented. This thesis details the analysis procedure for identifying kaons and protons using the NA49 experiment at CERN-SPS and discusses the results in the context of the current state of the field.
In April and May 2012 data on Au+Au collisions at beam energies of Ekin = 1.23A GeV were recorded with the High Acceptance Di-Electron Spectrometer, which is located at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. At this beam energy all hadrons containing strangeness are produced below their elementary production threshold. The required energy is not available in binary NN collisions but must be provided by the system e.g. through multi-particle interactions or medium effects like a modified in-medium potential (e.g. KN/ΛN potential). Thus, a high sensitivity to these medium effects is expected in the investigated system.
The baryon-dominated systems created in relativistic heavy-ion collisions (HIC) at SIS18 energies reach densities of about 2-3 times ground state density p0 and may be similar to the properties of matter expected in the inner core of neutron stars. It is in particular the behavior of hadrons containing strangeness, i.e. kaons and hyperons, and their potentials in the dense medium which may have severe implications on astrophysical objects and processes. As ab-initio calculations of quantum chromodynamics (QCD) cannot be performed rigorously on the lattice at finite baryo-chemical potentials due to the fermion sign problem, effective descriptions have to be used in order to model properties of dense systems and the involved particles. The only way to access the in-medium potential of strange hadrons above nuclear ground state density p0 is by comparing data from relativistic HIC to such effective microscopic models. Up to now, not much data on neutral kaons and Λ hyperons are available from heavy collision systems close to their NN production threshold. These two electromagnetically uncharged strange hadrons are in particular well suited to study their potential in a dense nucleon-dominated environment as their kinematic spectra are not affected by Coulomb interactions.
In this thesis, the production of charged kaons and Φ mesons in Au+Au collisions at sqrt sAuAu = 2.4 GeV is studied. At this energy, all particles carrying open and hidden strangeness are produced below their respective free nucleon-nucleon threshold with the corresponding so-called excess energies: sqrt sK+ exc = -0.15 GeV, sqrt sK- exc = -0.46 GeV, sqrt sΦ exc = -0.49 GeVGeV. As a consequence, the production cross sections are very sensitive to medium effects like momentum distributions, two- or multistep collisions, and modification of the in-medium spectral distribution of the produced states [1]. K+ and K- mesons exhibit different properties in baryon dominated matter, since only K- can be resonantly absorbed by nucleons. Although strangeness exchange reactions have been proposed to be the dominant channel for K- production in the analyzed energy regime, the production yield and kinematic distributions could also be explained in smaller systems based on statistical hadronization model fits to the measured particle yields, including a canonical strangeness suppression radius RC, and taking the Φ feed-down to kaons into account [2, 3]. For the first time in central Au+Au collisions at such low energies, it is possible to reconstruct and do a multi differential analysis of K- and Φ mesons. In principle, this should be the ideal environment for strangeness exchange reactions to occur, as the particles are produced deeply sub-threshold in a large and long-living system. Therefore, it is the ultimate test to differentiate between the different sources for K- production in HIC.
In total 7.3x10exp9 of the 40% most central Au(1.23 GeV per nucleon)+Au collisions are analyzed. The data has been recorded with the High Acceptance DiElectron Spectrometer HADES located at Helmholtzzentrum für Schwerionenforschung GSI in April/May 2012. A substantially improved reconstruction method has been employed to reconstruct the hadrons with high purity in a wide phase space region.
The estimated particle multiplicities follow a clear hierarchy of the excess energy: 41.5 ± 2.1|sys protons at mid-rapidity per unit in rapidity, 11.1 ± 0.6|sys ± 0.4|extrapol π-, (3.01 ± 0.03|stat ± 0.15|sys ± 0.30|extrapól) x10 exp -2 K+, (1.94 ± 0.09|stat ± 0.10|sys ± 0.10|extrapol)x10 exp -4 K- and (0.99 ± 0.24|stat ± 0.10|sys ± 0.05|extrapol)x10 exp -4 Φ per event. The multiplicities of the strange hadrons increase more than linear with the mean number of participating nucleons hAparti, supporting the assumption that the necessary energy to overcome the elementary production threshold is accumulated in multi-particle interactions. Transport models predict such an increase, but are overestimating the measured particle yield and are not able to describe the kinematic distributions of K+ mesons perfectly. However, the best description is given by the IQMD model with a density dependent kaonnucleon potential of 40 MeV at nuclear ground state density.
The K-=K+ multiplicity ratio is constant as a function of centrality and follows with (6.45 ± 0.77)x10 exp -3 the trend of increasing with beam energy indicated from previous experiments [4]. The effective temperature of K- TK+eff = (84 ± 6) MeV is found to be systematically lower than the one of K+ TK+eff = (104 ± 1) MeV, which has also been observed by the other experiments.
The Φ=K- ratio is with a value of 0.52 ± 0.16 higher than the one obtained at higher center-of-mass energies and smaller systems. This behavior is predicted from a tuned version of the UrQMD transport model [5], when including higher mass baryonic resonances which can decay into Φ mesons and from statistical hadronization models when suppressing open strangeness canonically. The found ratio is constant as a function of centrality and results with a branching ratio of 48.9%, that ~ 25% of all measured K- originate from Φ feed-down decays. A two component PLUTO simulation, consisting of a pure thermal and a K- contribution originating from Φ decays, can fully explain the observed lower effective temperature in comparison to K+ and the shape of the measured rapidity distribution of K-. As a result, we find no indication for strangeness exchange reactions being the dominant mechanism for K- production in the SIS18 energy regime, if taking the contribution from Φ feed-down decays into account.
The hadron yields for the 20% most central collisions can be described by a statistical hadronization model fit with the chemical freeze-out temperature of Tchem = (68 ± 2) MeV and baryochemical potential of μB = (883 ± 25) MeV, which is higher than expected from previous parameterizations. The analysis of the transverse mass spectra of protons indicate a kinetic freeze-out temperature of Tkin = (70 ± 4) MeV and radial flow velocity of βr = 0.43 ± 0.01, which is in agreement with the parameters obtained from the linear dependence of the effective temperatures on the particle mass Tkin = (71.5 ± 4.2) MeV and βr = 0.28 ± 0.09.
HADES (High Acceptance DiElectron Spectrometer), located at GSI, is a versatile detector for precise spectroscopy of e+ e- pairs and charged hadrons produced on a fixed target in a 1 to 3.5 AGeV kinetic beam energy region. The main experimental goal is to investigate properties of dense nuclear matter created in heavy ion collisions and learn about in-medium hadron properties.
In the HADES set-up 24 Mini Drift Chambers (MDC) allow for track reconstruction and determining the particle momentum by exploiting charged particle deflection in a magnetic field. In addition, the drift chambers contribute to particle identification by measuring the energy loss. The read-out concept foresees each sensing wire to be equipped with a preamplifier, analog pulse shaper and discriminator. In the current front-end electronics, the ASD-8 ASIC comprises the above modules. Due to limitations of the current on-board time to digital converters (TDC), especially regarding higher reaction rates expected at the future FAIR facility (HADES at SIS-100), the electronics need to be replaced by new board featuring multi-hit TDCs. Whereas ASD-8 chips cannot be procured anymore, a promising replacement candidate is the PASTTREC ASIC, developed by JU Krakow, which was tested w.r.t. suitability for MDC read-out in a variety of set-ups and, where possible, in direct comparison to ASD-8.
The timing precision, being the most crucial performance parameter of the joint system of detector and read-out electronics, was assessed in two different set-ups, i.e. a cosmic muon tracking set-up and a beam test at the COSY accelerator at Juelich using a minimum ionizing proton beam.
The beam test results were reproduced and can thus be quantitatively explained in a three dimensional GARFIELD simulation of a HADES MDC drift cell. In particular, the simulation is able to describe the characteristic dependence of the time precision on the track position within the cell.
A circuit simulation (SPICE) was used to closely model the time development of a raw drift chamber pulse, measured as a response to X-rays from a 55 Fe source. The insights gained from this model were used for attributing realistic charge values to the time over threshold values measured with the read-out ASICs in a charge calibration set-up. Furthermore, a high-level circuit simulation of the PASTTREC shaper is implemented to serve as a demonstration of the effect of the individual shaping and tail cancellation stages which are present in both ASICs.
