Dense and Strange Hadronic Matter

Course with Participations of Group Members

Offers for Theses in the Group

Absorption of antinuclei in ALICE Time Projection Chamber using machine learning algorithms

Dark Matter (DM) is believed to account for roughly 27% of the mass-energy of our Universe, and its nature remains one of the most intriguing unsolved questions of modern physics. Multiple balloon- and space-borne experiments are searching for the traces of DM using the idea of possible annihilation or decay of DM particles into ordinary (anti)particles, including light (anti)nuclei. The latter (such as antideuterons and antihelium nuclei) are considered as especially promising probe for such indirect DM searches, as the background stemming from ordinary collisions between cosmic rays and the interstellar medium is expected to be very low with respect to the DM signal. In order to reliably estimate the fluxes of antinuclei near Earth stemming from DM and from background, it is necessary to know the probability for antinuclei to interact inelastically with ordinary matter on their way to the detectors (e.g. with interstellar medium and Earth's atmosphere). This probability is driven by the inelastic cross section of corresponding processes, which for antinuclei are still poorly (or not) known. This fact hinders precise calculations of antinuclei fluxes near Earth and forces existing estimates to rely on extrapolations and modelling. The here advertised master project will deal with the analysis of inelastic interactions of antinuclei inside the gas volume of the Time Projection Chamber of the ALICE detector. Such interactions typically create a bunch of secondary (charged) particles with low momentum with a characteristic topology of secondary vertex inside the TPC volume. The pattern can be recognised by machine learning algorithms trained on simulated events, in which such annihilation processes happen in a controlled environment. After the validation of algorithms with simulated events, one can analyse real experimental data and tag the annihilation events of interest, which in turn can be used to evaluate the effective antinuclei + A inelastic cross section. This project will be structured in the following way: - simulation of the inelastic interactions of antinuclei with the TPC gas using Geant4 toolkit - training and validation of neural networks to reliably recognise antinuclei annihilation events - Analysis of the ALICE experimental data from pp collisions at sqrt(s) = 13 TeV - Evaluation of the effective antinuclei + A inelastic cross sections

suitable as

• Master’s Thesis Nuclear, Particle, and Astrophysics

Supervisor: Laura Fabbietti

Building and characterising a hybrid gaseous particle detector

Micropattern gas detectors (MPGD) are a type of particle detector used in many large high energy physics experiments (Atlas, CMS, ALICE). MPGDs work by amplifying the signals generated by charged particles traversing a gas volume. This is made possible by using amplification structures with micrometre scale patterns. There are different designs of such detectors, which include Gas Electron Multiplier (GEM) and Micro-Mesh Gaseous Structure (Micromegas). These are both highly scalable and offer good performance on their own but by combining both designs in a hybrid detector the performance can be pushed even further. The Hydra (Hypernuclei Decay R3B Apparatus) TPC project in GSI plans to make use of the high radiation hardness and tracking capabilities of a hybrid Micromegas+GEM detector to study the mesonic decay of hypernuclei into nuclei and pions. In the scope of this thesis project, the building and characterization of a prototype Micromegas+GEM detector will be conducted. For this task, a new dedicated detector chamber will be built, and performance tests will be conducted looking at the achievable signal amplification, energy & spatial resolution, and ion back flow suppression. In addition to these, novel exciting studies investigating electric field distortions under an applied magnetic field using a strong electromagnet will also performed. The results obtained from this research and development effort will be used for the final design of the Hydra TPC detector!

suitable as

• Master’s Thesis Nuclear, Particle, and Astrophysics

Supervisor: Laura Fabbietti

Characterizing CsI coated THGEMs for photon detection

Traditional devices for photon detection like the Photomultiplier Tube or more recent technologies such as Silicon Photomultipliers are very cost-intensive. Therefore, especially with large area experiments in mind it is very interesting to investigate new ways of detecting photons. In this project we are taking the approach of combining a photosensitive material with a Thick GEM (THGEM) in a gaseous detector. THGEMs are robust, low-cost devices, which can be used for electron multiplication. The THGEM is coated with a photosensitive material and placed within an electrical field. When a photon releases an electron from the material the photo electron drifts in the THGEM hole and undergoes avalanche amplification due to a high electric field that is preset inside the holes. Below the THGEM an anode is used for electron readout. Depending on the gain of the THGEM this could enable single photon detection. In the scope of the thesis CsI coated THGEMs, which are useful in the UV light range, will be characterized and their quantum efficiency will be studied.

suitable as

• Master’s Thesis Applied and Engineering Physics

Supervisor: Laura Fabbietti

Characterizing CsI coated THGEMs for photon detection

Traditional devices for photon detection like the Photomultiplier Tube or more recent technologies such as Silicon Photomultipliers are very cost-intensive. Therefore, especially with large area experiments in mind it is very interesting to investigate new ways of detecting photons. In this project we are taking the approach of combining a photosensitive material with a Thick GEM (THGEM) in a gaseous detector. THGEMs are robust, low-cost devices, which can be used for electron multiplication. The THGEM is coated with a photosensitive material and placed within an electrical field. When a photon releases an electron from the material the photo electron drifts in the THGEM hole and undergoes avalanche amplification due to a high electric field that is preset inside the holes. Below the THGEM an anode is used for electron readout. Depending on the gain of the THGEM this could enable single photon detection. In the scope of the thesis CsI coated THGEMs, which are useful in the UV light range, will be characterized and their quantum efficiency will be studied.

suitable as

• Bachelor’s Thesis Physics

Supervisor: Laura Fabbietti

Development of new approach to study particle correlations using femtoscopy

One of the most precise and most powerful approaches to probe the strong interaction between different particle pairs is the femtoscopy method [1] which is based on measuring the correlation in the momentum space between hadrons produced at particle colliders such as the LHC. The correlation function is obtained by using the same and mixed event particle pair distributions as functions of their relative momenta. The former carries the information about the interaction while the latter provides information on the available phase-space of the produced pair. Two particles produced in one collision makes the same event pair while the mixed event pairs are constructed by taking two particles from two di erent collisions. The technique of event mixing is a very well working approximation, however, more sophisticated solutions are of increasing interest as the femtoscopic method is used in more and more complicated studies and is being extended to probe the multi-hadron interactions. In this work, a new approach to account for the particle phase space will be elaborated by using marginalized probability distribution functions. Toy Monte Carlo simulations will be used to test the mathematical basis of the method which will be then applied on the proton-proton collision data collected by the ALICE detector to study its feasibility in real experimental conditions. [1] Hadron-hadron interactions measured by ALICE at the LHC, L. Fabbietti, V. Mantovani Sarti, O. V azquez Doce, arXiv: 2012.09806 (2020)

suitable as

• Bachelor’s Thesis Physics

Supervisor: Laura Fabbietti

Exploring the properties of Quark-Gluon Plasma with anisotropic flow measurements at the Large Hadron Collider

The matter produced in ultra-relativistic heavy-ion collisions resembles the Quark-Gluon Plasma (QGP), which is an extreme state of nuclear matter consisting of deconfined quarks and gluons. Such a state existed in the early Universe, just a few microseconds after the Big Bang. Its properties can be experimentally accessed by measuring the azimuthal anisotropies in the momentum distribution of produced particles in heavy-ion collisions, for instance, in lead-lead collisions reconstructed with the ALICE experiment at CERN's Large Hadron Collider (LHC). Of particular interest in this context is the anisotropic flow phenomenon, which is an observable directly sensitive to the properties of QGP. In this project, we introduce the basics of anisotropic flow and corresponding analyses techniques, and we guide a student throughout all steps needed for its final measurement, in the large-scale LHC datasets distributed on Grid. We start a project by briefly introducing a theoretical framework within which an anisotropic flow phenomenon can be defined and quantified. Next, we introduce sophisticated multi-particle correlation techniques, which were developed recently by experimentalists particularly for anisotropic flow measurements. We go in detail through the practical implementation of multi- particle correlations, students are expected at this point to perform some simple analytic calculations, and to learn and perform programming tasks both in ROOT and AliROOT. ROOT is the object-oriented analysis frame- work written in C++ programming language, and it is used at the moment as a default software in high-energy physics by all major collaborations world- wide, while AliROOT is the more speci c analysis framework developed by ALICE experiment, and which is based on ROOT. We wind up the project by letting the student do an independent ani- sotropic flow analysis with his/her own newly developed code in AliROOT, utilizing multi-particle correlation techniques, over real heavy-ion collisions collected by ALICE at LHC, and stored on Grid.

suitable as

• Master’s Thesis Nuclear, Particle, and Astrophysics

Supervisor: Laura Fabbietti

Exploring the properties of Quark-Gluon Plasma with anisotropic flow measurements at the Large Hadron Collider The

The matter produced in ultra-relativistic heavy-ion collisions resembles the Quark-Gluon Plasma (QGP), which is an extreme state of nuclear matter consisting of deconfined quarks and gluons. Such a state existed in the early Universe, just a few microseconds after the Big Bang. Its properties can be experimentally accessed by measuring the azimuthal anisotropies in the momentum distribution of produced particles in heavy-ion collisions, for instance, in lead-lead collisions reconstructed with the ALICE experiment at CERN's Large Hadron Collider (LHC). Of particular interest in this context is the anisotropic flow phenomenon, which is an observable directly sensitive to the properties of QGP. In this project, we introduce the basics of anisotropic flow and corresponding analyses techniques, and we guide a student throughout all steps needed for its final measurement, in the large-scale LHC datasets distributed on Grid. We start a project by briefly introducing a theoretical framework within which an anisotropic flow phenomenon can be defined and quantified. Next, we introduce sophisticated multi-particle correlation techniques, which were developed recently by experimentalists particularly for anisotropic flow measurements. We go in detail through the practical implementation of multi- particle correlations, students are expected at this point to perform some simple analytic calculations, and to learn and perform programming tasks both in ROOT and AliROOT. ROOT is the object-oriented analysis frame- work written in C++ programming language, and it is used at the moment as a default software in high-energy physics by all major collaborations world- wide, while AliROOT is the more speci c analysis framework developed by ALICE experiment, and which is based on ROOT. We wind up the project by letting the student do an independent ani- sotropic flow analysis with his/her own newly developed code in AliROOT, utilizing multi-particle correlation techniques, over real heavy-ion collisions collected by ALICE at LHC, and stored on Grid.

suitable as

• Bachelor’s Thesis Physics

Supervisor: Laura Fabbietti

How quark-gluon plasma flows in ALICE experiment

It is predicted that in its early stage, our Universe was made of deconfined quarks and gluons, a state known as the quark-gluon plasma (QGP). Understanding its properties is therefore a crucial element in the description of the first instants after the Big Bang. In order to deepen our current knowledge of the QGP, high-energy nuclear physicists turn to the study of ultrarelativistic collisions in the Large Hadron Collider (LHC) at CERN. QGP formation is indeed a well-known stage in the evolution of the collision of two heavy ions, like lead, while signs of its possible creations have been seen in smaller system sizes, like p-Pb or even pp. The estimation of the anisotropies in the azimuthal distribution of the produced particles is one of the probes used by experimentalists to access the collective properties of the QGP. Sophisticated analysis techniques, gathered under the name of multiparticle correlation techniques, have been developed in recent years to measure the anisotropic flow phenomenon at the origin of the observed asymmetric distributions and provided great help in constraining the parametrization of the initial state and collective evolution of the system. Another important development towards this goal is the introduction of Bayesian methods, where the model parameters can be inferred from the experimental observables. This leads to a better understanding of the underlying theoretical models. Nevertheless, despite these formidable advancements in the evaluation of the QGP properties, the uncertainties remain still large, and the introduction of novel observables and new measurements is needed to fix this issue. After an introduction to the phenomenon of anisotropic flow, and its analysis techniques, the candidate students will have the possibility to work on various research topics: ● Experimental measurement of newly developed observables in Pb-Pb collisions recorded by the ALICE detector at the LHC, during Run1 and Run2, and their comparisons with the predictions from different tunings of Bayesian analyses. The determination of new tunings and consecutive generations of simulated events by the student can also be considered. ● Investigations on the dependence of the various flow observables on the collision system size. ALICE has already collected events for pp, p-Pb, Xe-Xe and Pb-Pb collisions, and O-O data are planned for the future Run3. Studying the anisotropic flow in such a wide range of system sizes helps advance our understanding of QGP formation. Moreover, such measurements would provide great insights into the supposed robustness of the parameter tuning obtained with Bayesian studies. ● Developments of new theoretical observables, complementary to the traditional techniques, and feasibility studies of their experimental measurements. Through these projects, the candidate student will be able to develop data analysis skills using some of the most used programming languages in high-energy particle physics (e.g. ROOT, bash, Mathematica, C++, ...), while experiencing the atmosphere of an international collaboration.

suitable as

• Master’s Thesis Nuclear, Particle, and Astrophysics

Supervisor: Laura Fabbietti

Performance evaluation of humidified gaseous particle detectors

Micropattern gas detectors (MPGD) are a type of particle detector used in many large high energy physics experiments (Atlas, CMS, ALICE). MPGDs work by amplifying the signals generated by charged particles traversing a gas volume. This is made possible by using amplification structures with micrometre scale patterns. There are different designs of such detectors, which include Gas Electron Multiplier (GEM) and Micro-Mesh Gaseous Structure (Micromegas). These are both highly scalable and offer good performance. Still, one of the main limiting factors for the long-term stable operation of MPGDs is the formation of electrical discharges inside the amplification structures. Due to this, there has been an extensive effort over the years to understand and develop methods to mitigate these discharges. One of the remaining questions however is how the humidity (the water content) of the used gas mixture affects the formation of discharges in MPDS. This project aims to conduct the first comprehensive studies investigating the correlation between gas humidity and MPGD discharge stability. For this task, a dedicated detector chamber will be used with modifications to the gas system which enables the precise control of the ambient humidity levels. This venture is a great entry point for anyone interested in particle detector hardware research and development. The achieved results of which might end up shaping the next generation of cutting-edge high energy physics experiments!

suitable as

• Bachelor’s Thesis Physics

Supervisor: Laura Fabbietti

Performance evaluation of humidified gaseous particle detectors

Micropattern gas detectors (MPGD) are a type of particle detector used in many large high energy physics experiments (Atlas, CMS, ALICE). MPGDs work by amplifying the signals generated by charged particles traversing a gas volume. This is made possible by using amplification structures with micrometre scale patterns. There are different designs of such detectors, which include Gas Electron Multiplier (GEM) and Micro-Mesh Gaseous Structure (Micromegas). These are both highly scalable and offer good performance. Still, one of the main limiting factors for the long-term stable operation of MPGDs is the formation of electrical discharges inside the amplification structures. Due to this, there has been an extensive effort over the years to understand and develop methods to mitigate these discharges. One of the remaining questions however is how the humidity (the water content) of the used gas mixture affects the formation of discharges in MPDS. This project aims to conduct the first comprehensive studies investigating the correlation between gas humidity and MPGD discharge stability. For this task, a dedicated detector chamber will be used with modifications to the gas system which enables the precise control of the ambient humidity levels. This venture is a great entry point for anyone interested in particle detector hardware research and development. The achieved results of which might end up shaping the next generation of cutting-edge high energy physics experiments!

suitable as

• Master’s Thesis Applied and Engineering Physics

Supervisor: Laura Fabbietti

Production of Hypertriton and 3He via Coalescence in Event Generators

In high-energy hadron-hadron/hadron-nuclei collisions at the LHC, the production of light (anti)nuclei and more complex multi-baryon bound states is observed. Their production yields have been measured as a function of transverse momentum (pT). Such measurements play key role in important astrophysical value as they provide input for the background estimates in indirect dark matter searches in space. However the current theoretical description of the production mechanism of light(anti)nuclei and hyper(anti)nuclei is still a big challenge. There are two phenomenological models are typically used to describe the production of multi-baryon bound states: the statistical hadronisation model (SHM) and the coalescence model. The former assumes that the light nuclei are emitted from a source in local thermal and hadrochemical equilibrium and their abundances are fixed at chemical freeze-out. Whereas in the later, light nuclei are assumed to be formed by the coalescence of protons and neutrons which are close in phase space at kinetic freeze-out. In the recent developments of coalescence model, the quantum-mechanical properties of nucleons, and nuclei are taken into account and the coalescence probability is calculated from the overlap between the source function of the emitted protons neutrons and hyperons which are mapped on the Wigner density of the nuclei and hypernuclei. Goal: Simulation of the yield-spectra as a function of transverse momentum (pT) using a wigner density approach in coalescence model and compare with the measurements in hadronhadron collisions for 3He and  He . The wigner densities will be constructed using the three-body wave functions. The student will first be introduced to the ALICE software based on the C++ programming language. Over the course of time all the necessary concepts related to theory will be taught in close supervision of two PhD students.

suitable as

• Master’s Thesis Nuclear, Particle, and Astrophysics

Supervisor: Laura Fabbietti

Studying the spacial and kinematic properties of the particle emission in pp collisions at the LHC

In recent years, the method of femtoscopy has been successfully applied to make use of two-particle momentum correlations to investigate the strong force acting upon a pair of hadrons [Nature 588 (2020) 232-238]. This has been achieved by creating, at TUM, a numerical framework capable of modeling the pair-wise particle emission profile [Phys.Lett.B 811 (2020) 135849]. Nevertheless, the required input is model dependent, as it requires knowledge on the spacial and momentum correlations of the particles at the moment of their production. The task of the participant is a continuation of the existing work, with the specific duty of studying the possibility of applying more generic methods to obtain information on the initial properties of the emission region. Finally, the bachelor candidate will implement the procedure within a universal framework, that is to be used in future N-body femtoscopic studies. The requirements are: 1) Basic knowledge and interest in KTA. 2) Basic knowledge and interest in programming. The gained experience is: 1) The basics of the statistical thermal model analysis of particle production at LHC. 2) Understanding of the femtoscopy method, with a stress on the particle emission in high-energy proton-proton collisions. 3) Working with numerical Monte-Carlo models. 4) Improvement of the programming skills, in particular C++ and the ROOT framework, both of which are essential in the field of data analysis. In addition, basic knowledge related to Python and Linux will be gained.

suitable as

• Bachelor’s Thesis Physics

Supervisor: Laura Fabbietti

Studying the spacial and kinematic properties of the particle emission in pp collisions at the LHC

In recent years, the method of femtoscopy has been successfully applied to make use of two-particle momentum correlations to investigate the strong force acting upon a pair of hadrons [Nature 588 (2020) 232-238]. This has been achieved by creating, at TUM, a numerical framework capable of modeling the pair-wise particle emission profile [Phys.Lett.B 811 (2020) 135849]. Nevertheless, the required input is model dependent, as it requires knowledge on the spacial and momentum correlations of the particles at the moment of their production. The task of the participant is a continuation of the existing work, with the specific duty of studying the possibility of applying more generic methods to obtain information on the initial properties of the emission region. Finally, the bachelor candidate will implement the procedure within a universal framework, that is to be used in future N-body femtoscopic studies. The requirements are: 1) Basic knowledge and interest in KTA. 2) Basic knowledge and interest in programming. The gained experience is: 1) The basics of the statistical thermal model analysis of particle production at LHC. 2) Understanding of the femtoscopy method, with a stress on the particle emission in high-energy proton-proton collisions. 3) Working with numerical Monte-Carlo models. 4) Improvement of the programming skills, in particular C++ and the ROOT framework, both of which are essential in the field of data analysis. In addition, basic knowledge related to Python and Linux will be gained.

suitable as

• Master’s Thesis Nuclear, Particle, and Astrophysics

Supervisor: Laura Fabbietti

Study the absorption of antihelium-4 in ALICE experiment at CERN LHC

Dark Matter (DM) is believed to account for roughly 27% of the mass-energy of our Universe, and its nature remains one of the most intriguing unsolved questions of modern physics. This massive hole in our knowledge drives multiple experimental searches for DM, and one of the indirect ways to search for DM is to look for the annihilation or decay of DM particles into ordinary (anti)particles such as light (anti)nuclei as employed by several balloon- and space-borne experiments. Low-energy light antinuclei (e.g. antihelium-4) are particularly promising signals for these indirect DM searches, since the background stemming from ordinary collisions between cosmic rays and the interstellar medium is expected to be low with respect to the DM signal. In order to reliably estimate the detection probability of interesting events such as DM -> helium-4 + antihelium-4 + ..., the interaction probability of antihelium-4 with ordinary matter (like interstellar medium, Earth's atmosphere) must be measured, since it defines the amount of antihelium-4 particles lost on the way to detector. However, nuclear inelastic cross sections of antihelium-4 + A processes are completely unknown, forcing current estimates of expected antihelium fluxes near Earth to rely on extrapolations and modelling. The topic of the here advertised master project deals with the measurement of these interactions using the ALICE detector. In heavy-ion collisions at LHC energies (anti)helium-4 nuclei are produced in significant amounts, and unique tracking and PID capabilities of the ALICE experiment make it possible to reliably detect the (anti)helium-4 nuclei in different sub-detector systems. This allows us to quantify the inelastic interaction probability of (anti)helium-4 with the ALICE detector material. This project will be structured in the following way: Analysis of the inclusive spectra of helium-4 and antihelium-4 nuclei in PbPb collisions at TeV Evaluation of the effective antihelium-4 + A inelastic cross sections Estimation of the antihelium-4 rates expected for different DM models in current and future satellite experiments

suitable as

• Bachelor’s Thesis Physics

Supervisor: Laura Fabbietti

Three-body strong interaction studied using femtoscopic correlation functions

One of the main challenges of the modern era in particle physics is to obtain a quantitative description of the three-body strong interaction among particles, such as nucleons and hyperons. Three-body forces are indeed crucial for the understanding of the nuclear structure as well as the inner composition of compact stellar objects like the neutron stars. Nevertheless, an accurate microscopic description of the three-body problem is still far from being achieved and precise experimental data to test the existing models are strongly demanded. In recent years, the ALICE Collaboration at the Large Hadron Collider (LHC) provided unprecedented experimental information on the two-body strong interaction among particle pairs using the femtoscopy technique (for a complete review see [1]). The interaction models are constrained by studying the particle correlation in the momentum space. The extension of such a powerful technique to the case of three-particle systems is currently under development, in view of the future LHC Run-3 data-taking which will allow to probe the three-body interactions. The student will contribute to the development of the three-body femtoscopy by studying the shape of the correlation functions in the case of attractive and repulsive three-body potentials. To this end, the student will perform analytical calculations at the second order of the perturbation theory in Quantum Mechanics using the available potential models and will make use of toy Monte Carlo simulations to produce the expected correlation functions. [1] L. Fabbietti, V. Mantovani Sarti, O. Vazquez Doce, arXiv:2012.09806 [nucl-ex]

suitable as

• Master’s Thesis Nuclear, Particle, and Astrophysics

Supervisor: Laura Fabbietti

Current and Finished Theses in the Group