Quarks sind die elementaren Bestandteile, aus denen Hadronen (Baryonen und Mesonen) aufgebaut sind. Zusammen mit den Leptonen und den Eichbosonen gelten sie heute als die fundamentalen Bausteine, aus denen alle Materie aufgebaut ist. Im Standardmodell der Teilchenphysik werden diese Ergebnisse zusammengefasst. Es gibt sechs verschiedene Quark-Arten (flavours): up, down, strange, charm, bottom und top. In der Natur kommen keine isolierten Quarks vor, sondern nur Kombinationen aus z.B. einem Quark-Antiquark Paar (Meson) oder aus drei Quarks (Baryon). Die Quantenchromodynamik (QCD) beschreibt die starke Wechselwirkung zwischen Quarks und Gluonen. Quarks bauen unter anderem Protonen und Neutronen auf. Gluonen vermitteln die Wechselwirkung zwischen den Quarks. Konzeptionell ist die QCD an die Quantenelektrodynamik (QED) angelehnt, die die Wechselwirkung elektrisch geladener Teilchen (z.B. Elektron oder Positron) durch den Austausch von Photonen beschreibt. Analog wirkt die Kraft, die durch den Austausch von Gluonen beschrieben wird, zwischen Teilchen, die eine Farbladung (rot, grün, blau) tragen. Im Vergleich zur QED, wo das Photon neutral ist, trägt das Gluon selbst Farbe und wechselwirkt daher mit anderen Gluonen. Bei kleinen Quarkabständen und hohen Energien bzw. hohen Impulsüberträgen, fällt die Kopplungskonstante der starken Wechselwirkung (αs) ab. Bei kleinem αs sind Quarks und Gluonen schwach gebunden (Asymptotische Freiheit). Bei grossen Abständen bzw. kleinen Impulsüberträgen ist αs gross. Die Zunahme von αs bewirkt, dass unendlich viel Energie benötigt wird, um Quarks aus Hadronen herauszulösen. Dies hat die Folge, dass es günstiger ist ein neues Quark-Antiquark Paar zu erzeugen. Das erklärt, warum Quarks immer in Hadronen (Mesonen und Baryonen) gebunden sind und nie isoliert beobachtet werden können (Confinement).Wenn Kernmaterie stark komprimiert wird, steigen Energiedichte und Temperatur, und möglicherweise erfährt die Kernmaterie einen Phasenübergang zu einem Zustand der als Quark Gluon Plasma (QGP) bezeichnet wird. Das QGP ist ein Zustand der Materie, in dem das Confinement der Quarks und Gluonen aufgehoben ist (Deconfinement). Dieser Zustand ist gekennzeichnet durch ein quasi-freies Verhalten der Quarks und Gluonen. Quarks und Gluonen in Hadronen ein. Im heutigen Universum existiert das QGP höchstens noch im Zentrum von Neutronensternen und explodierenden schwarzen Löchern.
Table of Contents
1. Introduction
1.1. Hadronic Matter
1.1.1. Quark Combinations
1.1.2. Color Charge
1.2. The Strong Interaction: Confinement
1.2.1. Chiral Symmetry Restoration
1.3. The Phase Diagram of Strongly Interacting Matter
1.4. Relativistic Heavy Ion Collisions
1.5. Strangeness
1.5.1. Strangeness Production in a Hadronic Gas
1.5.2. Strangeness Production in a Quark Gluon Plasma
2. The NA49 Experiment at CERN SPS
2.1. The NA49 Detector Layout
2.2. Time Projection Chambers
2.2.1. The NA49 TPCs
2.3. Event Reconstruction
2.3.1. V0 Reconstruction
2.3.2. V0 Finding
2.3.3. V0 Fitting
2.3.4. Multi-Strange Hyperon Reconstruction
2.3.5. Ξ Finding
3. Data Analysis
3.1. Data sets
3.2. Event Cuts
3.2.1. Central Pb+Pb
3.2.2. Minimum Bias Pb+Pb
3.2.3. Semi-Central Si+Si
3.3. Analysis Cuts
3.3.1. Cuts on the Ξ Candidate
3.3.2. Cuts on the Daughter π of the Ξ Candidate
3.3.3. Cuts on the Daughter Λ Candidate
3.4. Invariant mass method
3.5. Correction
3.5.1. Geometrical Acceptance
3.5.2. Reconstruction Efficiency
3.5.3. Centrality Bin Size Effect
3.5.4. Influence of δ Electrons
4. Extraction of Spectra, Yields and Systematic Error
4.1. Transverse Momentum and Transverse Mass Spectra
4.1.1. Extrapolation of the Transverse Momentum Spectra
4.2. Rapidity Spectra and 4π Yields
4.3. Stability Checks of the Results and the Systematic Error
4.4. Lifetime
4.5. Comparision with another Ξ Analysis at 158 AGeV
5. Discussion
5.1. Comparison with Other Experiments
5.2. Energy Dependence
5.2.1. Inverse Slope Parameter and Mean Transverse Mass of the Ξ Hyperon
5.2.2. Strange Hadron Yield Enhancement
5.2.3. Excitation Function of Ξ production
5.2.4. Antibaryon/Baryon Ratio
5.3. Theoretical Models
5.3.1. Spectator-Participant Model
5.3.2. RQMD v2.3
5.3.3. UrQMD v1.3
5.3.4. Statistical Hadron Gas Models
5.3.5. Comparison to Models
6. Summary and Conclusion
Objectives and Topics
The primary objective of this dissertation is the experimental analysis of the energy and system-size dependence of Ξ-hyperon production in heavy-ion collisions at the CERN Super Proton Synchrotron (SPS). The work investigates whether the production rates of these strange particles provide evidence for the formation of a Quark Gluon Plasma (QGP) and examines how these findings compare to predictions from statistical and microscopic theoretical models.
- Investigation of the energy dependence of Ξ- production in central and minimum-bias Pb+Pb collisions.
- Study of the impact of system size on hyperon production, including data from Si+Si collisions.
- Evaluation of strangeness enhancement as a diagnostic signature for the deconfinement phase transition.
- Comprehensive comparison of experimental results with established theoretical frameworks such as UrQMD, RQMD, and Statistical Hadron Gas models.
Excerpt from the Book
1.2. The Strong Interaction: Confinement
The strong nuclear force is described by Quantum Chromodynamics (QCD), the parallel field theory to Quantum Electrodynamics (QED) that describes the electromagnetic force. It is propagated by gluons analogously to photons in the electromagnetic force, but unlike photons, which do not carry electric charge, gluons carry color, and they can self-interact. The fact that gluons are not color neutral is an important difference between the strong and electromagnetic forces, which is manifested in the behaviour of the strong force potential.
The potential between a quark and antiquark with a distance r apart is of the form V(r) ~ -4/3 * alpha_s(r)/r + kr, where alpha_s(r) is the strong coupling constant, k is a constant of the order of 1 GeV/fm and r is the separation of the quarks. The 1/r term determines the potential at short distances, where the gluon distribution from a quark is radial. Between any two separating quarks, for r >= 1 fm, the second term in equation 1.1 dominates and V(r) -> infinity. Here, the constant k can be thought of as a spring constant providing the tension in the string. The self coupling of gluons causes the color field lines between the quarks to form a tube. Therefore, the potential at large distances increases linearly with the separation of the quarks as the density of field lines remains constant.
One implication of this is that an infinite amount of energy is required to separate two color charges. However, in practice, if the color flux tube is stretched enough, it becomes energetically favorable to rupture the tube and terminate the field lines with a qq-bar pair created out of the QCD vacuum. Therefore it is not possible to separate two quarks on a large distance scale.
Summary of Chapters
1. Introduction: Summarizes the fundamental concepts of hadronic matter, the role of QCD, and the motivation for studying strangeness in relativistic heavy-ion collisions as a signature for the QGP.
2. The NA49 Experiment at CERN SPS: Describes the experimental apparatus, including the Time Projection Chambers, and the data reconstruction procedures used to identify strange hadrons.
3. Data Analysis: Details the systematic data selection process, including event and track cuts, the invariant mass method for particle identification, and the necessary efficiency corrections.
4. Extraction of Spectra, Yields and Systematic Error: Explains the methodologies for calculating transverse momentum and rapidity spectra, as well as the estimation of systematic uncertainties in the measurement.
5. Discussion: Compares the experimental results with other experiments and theoretical models, focusing on energy and system-size dependencies and the interpretation of strangeness enhancement.
6. Summary and Conclusion: Consolidates the findings regarding Ξ-production, re-evaluates the validity of strangeness enhancement as a QGP indicator, and discusses the implications for phase transition physics.
Keywords
Quark Gluon Plasma, QGP, Relativistic Heavy-Ion Collisions, CERN SPS, NA49 Experiment, Strangeness Production, Ξ-Hyperons, Quantum Chromodynamics, QCD, Confinement, Deconfinement, Hadronic Matter, Transverse Momentum Spectra, Rapidity Spectra, Statistical Models
Frequently Asked Questions
What is the core subject of this thesis?
This thesis focuses on the production of strange baryons, specifically Ξ- and Ξ-bar hyperons, in relativistic heavy-ion collisions at the CERN SPS accelerator.
What are the primary research themes?
The work examines the energy and system-size dependence of strangeness production, the role of hadronic versus deconfined (QGP) phases, and the validity of strangeness enhancement as a signal for the QGP.
What is the main objective?
The objective is to analyze experimental data from the NA49 detector to test if the production of multi-strange hyperons provides consistent evidence for the deconfinement phase transition and to compare these results with existing theoretical predictions.
Which scientific methods are employed?
The analysis utilizes the "bin-by-bin" correction method to account for detector acceptance and efficiency, applies geometric and kinematic analysis cuts, and uses invariant mass reconstruction to extract yields.
What does the main body cover?
It covers the detector setup, the data reconstruction chain, the specific cut criteria for Ξ- identification, the extraction of spectra and systematic errors, and a discussion comparing the data with models like UrQMD and Statistical Hadron Gas models.
Which keywords best characterize this work?
Key terms include QGP, NA49, Ξ-Hyperons, strangeness enhancement, heavy-ion collisions, and QCD.
Why are Ξ-hyperons significant for this research?
Ξ-hyperons are multi-strange particles; their production is expected to be significantly higher in a deconfined QGP compared to a standard hadronic gas, making them effective probes for the phase of the system.
What is the conclusion regarding the NA57 comparison?
The thesis identifies a discrepancy between its results and those reported by the NA57 collaboration, which remains an open issue in the field of high-energy physics.
- Citar trabajo
- Michael Mitrovski (Autor), 2007, Energy and System Size Dependence of Xi− and Xi+ Production in Relativistic Heavy-Ion Collisions at the CERN SPS, Múnich, GRIN Verlag, https://www.grin.com/document/287093
