Bachelor and master projects
Exploring the properties of Quark-Gluon Plasma with anisotropic flow measurements at the Large Hadron Collider
Supervisor: Ante Bilandzic
Level: Bachelor and Master
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 through 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 framework written in C++ programming language, and it is used at the moment as a default software in high-energy physics by all major collaborations worldwide, while AliROOT is the more specific analysis framework developed by the ALICE experiment, and which is based on ROOT.)
We wind up the project by letting the student do an independent anisotropic 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.
Study of the strong nuclear force between D mesons and light hadrons
Supervisor: Daniel Battistini
Level: Master
In the last years, several exotic states (hadrons made of more than 3 quarks) were observed in the charm sector; such particles cannot be interpreted as regular baryons or mesons and are thought to be either quark bags or molecular states. To unveil their nature, it is crucial to experimentally constrain the strong force that governs the interaction between the charm hadrons and other hadrons, for instance, via the measurement of the scattering parameters. Despite the importance of constraining the charm-hadron interactions, the available experimental knowledge is very poor: so far, only the D-proton system was investigated, by using the femtoscopy technique in proton-proton collisions at the LHC. This measurement indicates that the interaction between protons and D mesons is attractive but the statistical precision was not sufficient to discriminate between the available theoretical models, some of which predict a bound state, some other simply attractive interaction.
The goal of this thesis work is to repeat the measurement with the Run 3 data. This dataset is much larger than the one of Run 2, and was collected with an upgraded detector, therefore, a significant improvement of the size and quality of the data is expected, and it should be possible to confirm or exclude the presence of Dp bound states. Another option is to study the DK0S interaction, which is instead unmeasured. The student will learn how to use the online-offline analysis framework of the ALICE collaboration (based on ROOT/C++) in order to perform heavy-flavor and femtoscopic analysis and the post-processing framework for femtoscopy (based on python). Depending on when the student starts, a significant part of the work may be dedicated to the study and selection of the D meson candidates.
Exploring the particle emission source in proton-proton collisions via collective expansion
Supervisor: Farid Taghavi
Level: Master and Bachelor
The proposed research aims to investigate if quark-gluon plasma (QGP) is produced in the collision of protons and, if yes, what is its spatial size. The source size in proton-proton collisions plays an essential role in femtoscopy, an outstanding tool to study the strong interaction among particles emitted during the collision process. The research aims to examine the role of collective expansion in relation to the size of QGP.
The so-called mT scaling is assumed to be one of the signatures of the collective effect in large system collisions in which particles with larger transverse mass are emitted from smaller regions in a collision. The same behavior has been observed in the proton-proton collisions in ALICE collaboration. It has yet to be apparent whether the exact mechanism is behind the observed mT scaling in such a small system.
We start the project by studying the concept of collective expansion and hydrodynamics and its application in heavy-ion physics. We learn how to use state-of-the-art hydrodynamic models to simulate realistic events and apply them to proton-proton collisions. We analyze the simulation outcomes and find the hydrodynamics prediction for the source size in the proton-proton collisions. We compare our prediction with that obtained in the experiment in the ALICE collaboration.
The significance of this study lies in the potential to enhance our understanding of the properties of QGP, particularly in small system collision and the mechanisms behind its formation, its evolusion and also the interaction of particles after the emission.
Unlocking the secrets of collective evolution in proton-proton collisions with kinetic theory
Supervisor: Farid Taghavi
Level: Master
In heavy-ion collisions, there have been observations of signatures of collective expansion, particularly anisotropic flow. These observations provide strong evidence of collectivity in the produced quark-gluon plasma (QGP), at least in some stages of the process. In large systems, the collective effects are mostly modeled via relativistic hydrodynamics. However, it has been remained uncertain whether relativistic hydrodynamics can be applied to small systems such as proton-proton collisions where similar flow-like signals have been observed. This leads to a long-standing question, whether the flow-like signals are the signature of the collective effect in small systems.
The objective of this project is to investigate the effect of collective expansion in small system collisions using an alternative framework based on kinetic theory, which has a broader range of applicability compared to hydrodynamics. The project will have a theoretical part to advance the existing framework and a computational part, which consists of developing code primarily based on the C++ programming language. Through this approach, we hope to gain a deeper understanding of the true origin of observed flow-like signals and contribute to the ongoing research in this field.
Investigation of baryon production mechanism in hadronic collisions in ALICE with multiparticle correlations
Supervisor: Igor Altsybeev
Level: Master
In 2016, the ALICE collaboration revealed an unexpected lack of baryon pairs produced in proton-proton collisions at the LHC and emitted in a similar direction (https://arxiv.org/abs/1612.08975). The physical origin of such depletion is still unclear, since the effect is observed not only for identical baryons (e.g. pairs of protons), but also for non-identical ones (like proton-Lambda pairs), so that the “blocking” of the identical fermions due to Fermi-Dirac statistics at the hadronic level can’t serve as an explanation. It is not clear also, whether this depletion is just a pair-wise effect (for example, it would be the case if baryons are produced in a “linear” process of quark-gluon string fragmentation), or the effect is intrinsically multi-particle, like it would be if baryons are emitted during the hadronisation process of the quark-gluon plasma.
In this project, the ALICE proton-proton and Pb-Pb collisions data will be analyzed with the special type of 3-particle correlation observable (with an attempt to go to the higher, 4th order), which possess two useful properties: (1) the lower-order correlations are subtracted to reveal only the genuine multi-particle interactions, and (2) the observable is constructed in a special way to suppress fluctuations of the system size from one collision to another, which eliminates trivial (non-dynamical) correlations between particles. Results will be compared with predictions from various models, to make conclusions about the physics origin of the depletion effect in the angular correlations between baryons.
Study of the emission source for three baryons in pp collisions at the Large Hadron Collider
Supervisors: Raffaele Del Grande, Dimitar Mihaylov
Level: Bachelor and Master
Performing scattering experiments with three unbound hadrons in the initial state is extremely challenging and not yet feasible with standard methods. This goal can be achieved at the high-energy facilities using alternative techniques. The matter created in ultra relativistic collisions at the Large Hadron Collider (LHC) can be exploited as a source of particles, emitted as a result of the collisions. These particles are typically produced at relative distances of 1 femtometer, therefore, they are close enough to scatter after the emission. This procedure allows studying the scattering of three particles and the first measurement for p-p-p was performed by the ALICE Collaboration at the LHC. The experimental observable is the correlation function. To interpret this measurement two ingredients are necessary: 1) the source function which incorporates the properties of the particle emission; 2) the p-p-p scattering wave function which includes information on the interaction.
In this project the student will characterize the three particle emission source for a triplet of protons. This will be done by employing the existing hypothesis of a common source for all baryons, which can be factorized into single particle emission probability. The student will evaluate the expected three-particle source by using the previous results of the ALICE Collaboration on the two-body emission. The ultimate goal is to use the theoretical predictions on the three-body wave function, together with the newly obtained source function, in order to be able to model the p-p-p correlation function. This will enable correlation techniques as an effective method to perform three-body scattering, representing an extremely novel methodology in experimental physics.
This project will allow the student to familiarize themselves with the analysis method related to correlation techniques at the LHC, as well as the related physics topics. There are no strict prerequisites, nevertheless good programming skills, or a desire to obtain these during the project, will be helpful.
General information
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Write a bachelor thesis at the physics department
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