For Students
Bachelor thesis
Title: Modelling of the associated production of the top-quark pair and Z boson at the ATLAS experiment
Supervisor: Mgr. Lucia Keszeghová, PhD.
Consultant: Mgr. Michal Dubovský, PhD.
Annotation:
Monte Carlo simulations are essential for the analysis of data measured at the Large Hadron Collider (LHC) at CERN. The simulated data are used not only to optimize individual analysis steps, but also to determine the values of the measured quantities. The preparation of simulated data consists of two main parts. First, it is necessary to model the processes that occur in proton-proton collisions using Monte Carlo generators. For sufficiently stable generated particles, the detector response is then simulated. Accurate modeling is also crucial for measurements of the associated production of the top-quark pair and the Z boson at the ATLAS experiment. ttZ measurements test the Standard Model, since a significant deviation of the measured effective cross section for the ttZ process from the Standard Model prediction would mean that the interaction between the top quark and the Z boson is given by a new physics.
Only one Monte Carlo generator has been used to model the ttZ process so far. Recently, the ttZ process has been implemented in two other generators. These generators are expected to improve the modeling of the ttZ process. In addition, these generators also provide internal variations that allow for a more accurate estimation of systematic uncertainties.
Goal:
The main goal of this thesis is to compare the modeling of the ttZ process by different Monte Carlo generators and to determine which generator is the best for description of the measured data.
Title: Impact of higher center-of-mass energy of proton-proton collisions on charge asymmetry in top-quark pair production in the ATLAS experiment
Supervisor: Mgr. Barbora Eckerová, PhD.
Consultant: doc. Mgr. Pavol Bartoš, PhD.
Annotation:
CERN is the largest European laboratory for particle research situated on the borders of Switzerland and France. The largest particle accelerator, the Large Hadron Collider (LHC), built at CERN is used to accelerate particles (either protons or heavy ions) to high energies. The particles traveling in 27 km long accelerating tube are reaching almost the speed of light after accelerating process. Collisions are happening at 4 different places, which are surrounded by detectors. The largest is the ATLAS detector, which is used to study behavior and attributes of elementary particles. One of them, which is particularly interesting, is the top quark. This particle is unique among other quarks due to its large mass. As a consequence of top-quark mass, its lifetime is quite short, which results in no top-flavored hadrons. It allows us to study top-quark properties directly from its decay products. Specific attribute of top-quark pair production is an effect called charge asymmetry. Basically, it describes different behaviour of top quark and top antiquark produced by collisions of protons. By studying this phenomenon, some new intermediate boson may be found or new way of quark interaction may be spotted.
In 2022, 3-year-long shutdown has come to an end and the new data collection period has already started (Run 3). Upgrades installed during the break enable proton collisions at higher center-of-mass energies (from 13 TeV the energy increased to 13.6 TeV). The change in energy brings unique opportunity to observe even heavier new particles. The goal of this thesis is to study the effects of higher center-of-mass energy on the charge asymmetry in top-quark pair production in the ATLAS Experiment.
Goal:
The aim of the thesis is to explore the work of particle physicist. Particularly, a candidate should learn how to work with basic analysis software used for analysis of measured data or Monte Carlo (MC) simulations (includes mainly ROOT, codes in C++ and python). A desired outcome is student's own script for data analysis. Main goal of such script is to compare distributions of various observables in MC simulations of 13 TeV and 13.6 TeV data. According to the comparison, the impact of higher center-of-mass energy of collisions on charge asymmetry may be concluded.
Title: Bose-Einstein correlations in high energy proton-proton collisions in the ATLAS experiment
Supervisor: Mgr. Róbert Astaloš, PhD.
Annotation:
The thesis studies the effect of Bose-Einstein correlations among identical pions, created in proton-proton collisions in the ATLAS experiment. It is manifested by an increase of the number of the identical bosons close to each other in the phase space, which is quantified by a Lorentz-invariant variable Q, representing their four-momentum difference modulus. For experimental evaluation, a ratio of Q distributions is calculated for pairs of identical particles and for reference sample created the way that it does not contain neither Bose-Einstein correlations nor any other correlations not present in the signal sample. The Bose-Einstein correlations effect is observed as an increase of this ratio for small Q values. The goal of the thesis is to use the Bose-Einstein correlations effect to determine the time-space characteristics of the pion source (and thus the effective radius of the hadronization region created in the proton collisions at the LHC). The Bose-Einstein correlations effect can be described by a symetrized two-particle wave functions as well as by some more complex models like quantum optics model and Tau model explaining also the origin of anticorrelations, which are also observed in the experimental data.
Goal:
To analyse Bose-Einstein correlations among identical particles (bosons) using simulated and reconstructed experimental data obtained in proton-proton collisions in the ATLAS experiment at LHC in CERN.
Note:
The work includes computer reconstruction of events recorded by the ATLAS experiment and their physics analysis using standard programming. The work
involves travel to particle physics research centres (CERN).
Diploma thesis
Title: Study of the associated production of top-quark pair and Z boson at the ATLAS experiment in the four-lepton channel at LHC Run III
Supervisor: Mgr. Michal Dubovský, PhD.
Consultant: prof. RNDr. Stanislav Tokár, DrSc.
Annotation:
Reaching a center of mass energy of 13 TeV at Large Hadron Collider (LHC) at CERN allowed us to study processes, which were not experimentally accessible before. One of such processes is the associated production of the top-quark pair and the Z boson, shortly “ttZ process”, which was observed for the first time using the 13 TeV data. This process is important for precision testing of the Standard Model, since it is sensitive to the coupling constant of the Z boson to the top quark. It is also an important background processes for some of the Standard Model measurements, as well as for searches for a physics beyond the Standard Model.
In 2022 the LHC Run III started, reaching center of mass energy of 13.6 TeV. The planned size of the dataset collected at this energy is 250 fb-1. In the previous measurements of the ttZ cross section, the channels with two, three and four leptons in the final state have been used. The uncertainty on the measurements in two- and three-lepton channels was dominated by systematic uncertainty, while the four-lepton channel was dominated by statistical uncertainty, which suggests that a significant improvement in the sensitivity of this channel can be expected when using the full dataset from Run III, which is expected to have approximately two times higher luminosity compared to Run II dataset. Because of a low sensitivity on the systematic uncertainties, the four-lepton channel can become the most sensitive decay channel at the higher luminosity.
The aim of the diploma thesis will be to estimate the expected uncertainty on the ttZ cross-section obtained from the fit in four-lepton channel, using the Monte Carlo events simulated for 13.6 TeV ATLAS sample. The student is expected to reprocess the samples, apply a basic selection, calculate the values of the discriminating variables, which allow us to distinguish a background from the ttZ signal. These variables will be then used to train a neural network, or a similar machine learning (ML) algorithm. The output of the ML algorithm will be then used to obtain histogram templates, which will be fitted using profile likelihood fit with the aim of obtaining the expected uncertainty of the ttZ cross-section.
Goal:
Estimate the uncertainty of the ttZ cross-section measured in four-lepton channel using the Monte Carlo events corresponding to 13.6 TeV ATLAS dataset, scaled to the expected Run III luminosity of 250 fb-1.
Note:
The topic is devoted to current issues in particle physics at the ATLAS detector at the CERN. During the study, the student will become familiar with modern trends in particle physics as well as programming in C++ and ROOT. He will also gain the knowledge necessary to use modern machine learning algorithms. The visit in the CERN is assumed during the studies, where the student will also work on this analysis.
Title: Optimization of the charge asymmetry measurement in top-quark pair production in the ATLAS Experiment using combination of data with different center-of-mass collision energies
Supervisor: Mgr. Barbora Eckerová, PhD.
Consultant: doc. Mgr. Pavol Bartoš, PhD.
Annotation:
CERN is the largest European laboratory for particle research situated on the borders of Switzerland and France. The largest particle accelerator, the Large Hadron Collider (LHC), built at CERN is used to accelerate particles (either protons or heavy ions) to high energies. The particles traveling in 27 km long accelerating tube are reaching almost the speed of light after accelerating process. Collisions are happening at 4 different places, which are surrounded by detectors. The largest is the ATLAS detector, which is used to study behavior and attributes of elementary particles. One of them, which is particularly interesting, is the top quark. This particle is unique among other quarks due to its large mass. As a consequence of top-quark mass, its lifetime is quite short, which results in no top-flavored hadrons. It allows us to study top-quark properties directly from its decay products. Specific attribute of top-quark pair production is an effect called charge asymmetry. Basically, it describes different behaviour of top quark and top antiquark produced by collisions of protons. By studying this phenomenon, some new intermediate boson may be found or new way of quark interaction may be spotted.
Currently, the data taking period called Run3 is happening at the LHC. The data collected during this period has a great potential to squeeze down the uncertainty of charge asymmetry. The center-of-mass energy in Run 3 has increased to 13.6 TeV from previous 13 TeV. The change in energy brings unique opportunity to observe even heavier new particles. The goal of this thesis is to optimize fit parameters and event selection used in the analysis of Run 2 data which will help to effectively combine Run 2 and Run 3 data and measure the charge asymmetry in top-quark pair production in the ATLAS Experiment with the highest precision ever achieved.
Goal:
Through this work you'll become a member of international analysis team, whose goal is to obtain the most accurate value of the charge asymmetry by combining data from Run 2 and Run 3. The main task of this work is to optimize fit parameters and selection criteria in the analysis of Run 2 data, which will help to effectively combine Run 2 and Run 3 data to measure the charge asymmetry with the highest precision ever achieved.
Note:
Knowledge of English is important, as well as positive attitude towards programming. Visiting CERN during working on the thesis is possible.
Title: Study of the reference distributions for measurement of Bose-Einstein correlations in high energy proton-proton collisions in the ATLAS experiment
Supervisor: Mgr. Róbert Astaloš, PhD.
Annotation:
The thesis studies the effect of Bose-Einstein correlations among identical pions, created in proton-proton collisions in the ATLAS experiment. It is manifested by an increase of the number of the identical bosons close to each other in the phase space, which is quantified by a Lorentz-invariant variable Q, representing their four-momentum difference modulus. For experimental evaluation, a ratio of Q distributions is calculated for pairs of identical particles and for reference sample created the way that it does not contain neither Bose-Einstein correlations nor any other correlations not present in the signal sample.
The Bose-Einstein correlations effect is observed as an increase of this ratio for small Q values. The goal of the thesis is to analyze various reference distributions with aim to find an optimal distribution for the measurement of the Bose-Einstein correlations. The Bose-Einstein correlations effect can be used to determine the time-space characteristics of the pion source (and thus the effective radius of the hadronization region created in the proton collisions at the LHC).
Goal:
To analyse various reference distributions especially mixing particles from different events and use them in measurement of Bose-Einstein correlations among identical particles (bosons) using simulated and reconstructed experimental data obtained in proton-proton collisions in the ATLAS experiment at LHC in CERN.
Note:
The work includes computer reconstruction of events recorded by the ATLAS experiment and their physics analysis using standard programming. The work involves travel to particle physics research centres (CERN).
PhD. thesis
Title: Top quark charge asymmetry in the Run 3 data collected by the ATLAS experiment.
Student: Mgr. Matúš Kosťov
Supervisor: doc. Mgr. Pavol Bartoš, PhD.
Consultant: prof. RNDr. Stanislav Tokár, DrSc.
Goal: Study of inclusive and differential charge asymmetry in the top-quark pair production using Run 3 data.
Title: Simultaneous calibration of the jet energy scale and resolution of small-radius jets in the ATLAS detector using single-lepton top-quark pair production events
Student: Mgr. Adriana Dohnalová
Supervisor: Mgr. Róbert Astaloš, PhD.
Consultant: prof. RNDr. Stanislav Tokár, DrSc.
Goal: To provide in-situ calibration that would be used on small-radius jets measured with the ATLAS detector at LHC in CERN in order to restore their particle level energy that gets distorted due to several effects.
Title: Bose-Einstein correlations in 3D approach in high energy proton-proton collisions in the ATLAS experiment
Student: Mgr. Daniel Suchý
Supervisor: Mgr. Róbert Astaloš, PhD.
Consultant: prof. RNDr. Stanislav Tokár, DrSc.
Goal: To analyze Bose-Einstein correlations among identical particles (bosons) using simulated and reconstructed experimental data obtained in proton-proton collisions in the ATLAS experiment at LHC in CERN. The analysis should contain also a three-dimensional approach.