The Large Hadron Collider (LHC) at CERN in Geneva is the largest particle accelerator in the world. It collides protons at a centre-of-mass energy of 13.6 TeV, probing physics at the highest energy scales ever achieved in collider experiments. The biggest success of the LHC so far was the discovery of the Higgs boson by the ATLAS and CMS experiments in 2012, an elementary particle whose study is crucial for the understanding of the origin of elementary particle masses. Since 2022, the LHC has resumed operations at even higher collision rates, which provides plenty of opportunities to improve precision measurements of elementary particles as well as to extend the reach of searches for new particles beyond the standard model (SM) of particle physics.
The experimental particle physics group at UGent is involved in data analysis with the Compact Muon Solenoid (CMS) detector, covering a wide range of topics including precision measurements of SM processes, new physics searches, and calibration of detectors and reconstruction methods. All data analysis projects make use of the Python programming language and employ specific high-energy-physics software packages. Some basic knowledge of Python (or another programming language) is required, while more advanced programming skills will be taught during the project. After the completion of one of the data analysis projects, the students will have – Gained insight in the design, technology, and software of one of the most advanced particle detectors in the world; – Improved their programming skills in the field of data analysis and visualization, possibly including machine-learning techniques; – Got experience in taking part in an international environment at the frontier of fundamental research.
We offer the possibility of staying at CERN in Geneva/Switzerland for a summer project to all master students that join the CMS analysis group.
Learning the language of top quarks: Machine learning for precision measurements in associated top quark production
The production of a top quark pair in association with a Z boson (tt̄Z) is a key process for testing the electroweak interactions of the top quark. Precise measurements of this process improve our understanding of rare standard model events and provide sensitivity to potential new-physics effects. This project explores the use of machine learning to construct optimized observables for tt̄Z production. Initially, we will train machine-learning algorithms to reconstruct key kinematic quantities—such as the invariant mass of the tt̄Z system—directly from reconstructed jets and leptons at the detector level. A more advanced approach will involve jointly optimizing observables at both the parton and detector levels to enhance sensitivity to new-physics deviations. By leveraging modern machine-learning techniques, this project aims to refine measurement strategies and maximize the discovery potential of the LHC.
Event distribution as a function of the invariant mass of the top quark pair in three bins of a spin correlation observable chel. Shown is the standard model prediction for tt̄Z production (red), as well as two predictions for new heavy bosons decaying to tt̄Z (red and green). The new-physics scenarios differ by the mass ordering of the new heavy scalar (H) and pseudoscalar (A) boson. Whether the lighter boson is a scalar or pseudoscalar impacts the spin correlations, resulting in one or the other case being dominant in the chel bins. Neither of the two observables used here for new-physics searches has been measured experimentally in tt̄Z production so far. From: F. Arco et al., 2502.03443.
Promoter: Prof. Didar Dobur Supervisors: Jules Vandenbroeck, Dr. Joscha Knolle Contact | Topic 44828 on PLATO
Shining light on top quarks: Measuring the interplay between the heaviest particle and the massless photon with the highest-energy LHC data
The production of top quark-antiquark pairs (tt̄) is one of the most common processes at the LHC. Top quark production is so abundant that the LHC is even nicknamed “top quark factory”, and tt̄ measurements are among the most precise cross section measurements ever performed by the CMS Collaboration. To study the electroweak interactions of the top quark, measurements of the production in association with a vector boson are a crucial tool. In this project, the CMS data collected in 2022–2024 will be analysed to identify tt̄ production events that contain additional photons (tt̄+γ). This will include careful studies of systematic effects in the identification of electrons, muons, and photons, as well as in the reconstruction of jets and tagging of jets originating from b quarks. The goal is to perform the first cross section measurement of top quark pair production as a function of the number of additional photons, and to constrain modifications introduced by physics beyond the standard model to the coupling between photons and top quarks.
Feynman diagrams for tt̄+γ production, where the photon originates either from a top quark (left), an incoming quark (middle), or a charged lepton in the top quark decay (right). From: JHEP 05 (2022) 091.The distribution of the photon transverse momentum in tt̄+γ events, shown for standard model (SM) couplings between photon and top quark (yellow filled area) and for different scenarios with modified couplings induced by new-physics at a higher energy scale (red and blue lines). From: JHEP 05 (2022) 091.
Promoter: Prof. Didar Dobur Supervisors: Jules Vandenbroeck, Dr. Joscha Knolle Contact | Topic 44825 on PLATO
Top quarks as proton probes: Enhancing PDF determinations with differential cross section measurements
What’s in a name, Julliet once asked. In case of the LHC, it is large hadron collider. This refers to the protons it collides most of the time. One typically thinks of protons as 2 up and 1 down quark, but in reality the proton is more like a sea of many quarks and gluons (collectively called partons) popping up and disappearing continuously. This sea is described with parton density functions or PDFs: distributions of each type of parton inside the proton as a function of fraction of the total proton momentum they carry. They are essential ingredients to predict cross sections in proton collisions, but unfortunately, these PDFs are so far not calculable from first principles. Instead, they have to be determined from experiments. The LHC provides a large dataset of top quarks, which can provide valuable information about the pdfs at high energies.
In this project we will study the production of single top quarks, which is interesting for PDF determinations. We will focus on differential measurements of kinematic quantities in single top quark production, since these are particularly sensitive to the momentum of the initial state partons. The main goal of this project is to study different distributions and how they can contribute to global PDF fits by running the PDF fitting tools ourselves. We will identify the most promising candidates for differential measurements and implement them for data analysis in an ongoing CMS measurement.
Cross section measurements used to determine the proton PDFs by the NNPDF Collaboration as a function of momentum fraction of the proton and collision energy. From: http://nnpdf.mi.infn.it.Proton PDFs at a collision energy of 3.2 GeV. From: http://data.nnpdf.science.
Promoter: Prof. Didar Dobur Supervisors: Maarten de Coen, Dr. Jan van der Linden Contact | Topic 44831 on PLATO
How to count pp collisions: Using Z boson events as a standard candle to integrate the 2024 luminosity
A crucial ingredient to precision cross section measurements at a collider experiment is the precise knowledge of the integrated luminosity, which quantifies the total number of collisions from which experimental data was recorded. At the CMS experiment, the luminosity measurement is calibrated with data recorded during one special day, and then integrated over the full data-taking period. Uncertainties arise from the extrapolation of the calibration over time, since e.g. changes in the detector conditions from aging, radiation damage, or operational changes can result in a reduced response of the detectors. In this project, we will implement a measurement of Z→μ⁺μ⁻ in short time intervals of about 30 minutes for the data to be recorded in 2024. Since this process is well known and occurs at a high rate, the “tag-and-probe method” can be applied separately in each time interval to measure the efficiency of the muon reconstruction and selection. In the tag-and-probe method, the event yields are compared for two classes of events: where both muons pass a certain reconstruction step, and where only one muon passes this reconstruction step. The difference in yields corresponds to the reconstruction efficiency. By measuring the efficiencies separately in each time interval, no time extrapolation needs to be made, and the “Z counting” rate can be measured precisely over the whole data-taking period. This can then be used to obtain a precise integration of the luminosity measurement.
Example fit results for the tag-and-probe measurement of the trigger (”HLT”) efficiencies. In the left plot, only one of the two muons has passed the trigger selection, whereas in the right plot, both muons have passed the trigger selection. The number of events is shown as a function of the invariant mass of the dimuon system. From the yields in the ”passing” and ”failing” categories, the trigger efficiency can be extracted. From: Eur. Phys. J. C 84 (2024) 26.Extracted luminosity measurement (left) and efficiencies of the different selection steps (right) during 10 hours of data-taking in 2022. The efficiencies are not constant but change as a function of time, and it is thus important to measure the efficiencies separately in each time interval rather than assuming a constant efficiency over the whole 10 hours. From: CMS-DP-2023-003.
Promoter: Prof. Didar Dobur Supervisors: Maarten de Coen, Dr. Joscha Knolle Contact | Topic 44827 on PLATO
Tracing jet origins: Boosted object identification for precision Higgs searches
The production of a top quark pair in association with a Higgs boson (tt̄H) is an important process to probe the coupling of the Higgs boson to other particles. The coupling of the Higgs boson to the particles of the third generation (top quarks, bottom quarks, tau leptons), and the gauge bosons (W and Z boson) are well-known and precisely measured, while the elusive couplings of the Higgs boson to second-generation particles such as the charm quark are still an active field of research. The predicted probability of Higgs bosons decaying to charm quark pairs is very low (~2%) and the identification of charm quark jets is a difficult task which only in the last few years was made possible with the improvements in machine learning techniques used for jet flavor tagging. Previous measurements of the Higgs-charm couplings, performed at UGent, have been able to set strong limits on this interesting coupling parameter of the standard model. In order to observe it with certainty, more options will have to be explored.
In this project, we will target the H→cc decay where the decay products are closely collimated and produce a single particle jet, which we will attempt to identify with state-of-the-art jet flavor tagging algorithms. We will explore whether this approach is promising and if it can be used as a complementary approach to the other measurements of the Higgs-charm coupling, thereby contributing significantly to its discovery. A key aspect of this project is to identify an optimal boundary between this collimated regime of Higgs boson decays and the resolved regime, in order to maximize the sensitivity of both channels.
Summary of all CMS measurements of Higgs boson couplings to other particles. From: Nature Phys. 607 (2022) 60.Illustration of tt̄H production and subsequent decay of the top quark pair and the Higgs boson into a pair of charm quarks.
Promoter: Prof. Didar Dobur Supervisors: David Kavtaradze, Dr. Jan van der Linden Contact | Topic 44832 on PLATO
At the energy frontier: Searching for hidden new-physics effects in four-top quark events
Four top quark production (tt̄tt̄) is a rare but powerful probe of physics beyond the standard model. As the rarest standard model process observed at the LHC so far, in an analysis performed at UGent, it is very sensitive to possible modifications from new heavy particles beyond the standard model at masses just outside of the reach of direct searches. In this project, we will use the framework of effective field theory (EFT) to study new-physics effects that modify four-fermion interactions, leading to deviations in the high-energy tails of kinematic distributions of tt̄tt̄ production. We will focus on identifying the most sensitive observables for constraining such effects, using both current CMS data and projections for the High-Luminosity LHC. A key aspect of the study is exploring whether machine-learning techniques can enhance the construction of optimal observables, improving sensitivity to subtle new-physics signatures.
Illustration of the principle of effective field theories: assuming that new physics beyond the standard model introduces particles at higher energies than can be probed at the LHC, the peak cannot be seen experimentally. However, virtual contributions can be parameterized as energy-suppressed modifications of the couplings between the standard model particles, which are visible in the tails of energy-sensitive distributions.Predictions for the differential cross section of tt̄tt̄ production as a function of the invariant mass of the tt̄tt̄ system. Shown is the standard model prediction in green, and predictions for various modified four-fermion couplings in other colours. It can be seen that modified couplings enhance especially events with high values of this invariant mass. From: C. Degrande et al., JHEP 07 (2024) 114.
Promoter: Prof. Didar Dobur Supervisors: Amber Cauwels, Dr. Joscha Knolle Contact | Topic 44829 on PLATO
From remnants to reality: Reconstructing top quarks to probe the limits of the standard model
Top quarks are key players in precision tests of the standard model, and their kinematic properties provide crucial insights into fundamental interactions. They are, however, unstable particles and decay almost instantly, leaving behind a cascade of lighter particles that are detected in the experiment. Reconstructing the properties of the original heavy particles from their decay products is a key challenge in CMS data analysis. In this project, we want to develop kinematic reconstruction algorithms that can be used, e.g., for tt̄Z and tt̄tt̄ production measurements. Depending on the decay mode, a top quark can be reconstructed from either three jets (hadronic decay) or a combination of one jet, one lepton, and missing transverse momentum (semileptonic decay). Developing accurate reconstruction techniques is essential for defining observables—such as the top quark transverse momentum—that enable precise comparisons between experimental measurements and theoretical predictions. After developing a kinematic fit as a baseline reconstruction algorithm, we will investigate machine-learning methods to further improve the precision of the reconstruction.
Illustration of the decay of a tt̄ pair into a system of three jets (left) or of one jet, one muon, and one neutrino (right). The challenge of the top quark reconstruction is to infer the top quark momentum from the measured properties of the decay products, taking detector resolution effects into account. From: http://tikz.net.Prediction for the differential cross section of tt̄Z production as a function of the transverse momentum of the top quark. The three lower panels show the ratio of the highest-order prediction to the baseline prediction, separately for three different choices of the energy scale used in the cross section calculation. The fact that these ratios are not constants indicate that measurements of the top quark transverse momentum provide insight to the high-energy interactions of top quarks. From: A. Kulesza et al., Eur. Phys. J. C 80 (2020) 428.
Promoter: Prof. Didar Dobur Supervisors: David Marckx, Dr. Joscha Knolle Contact | Topic 44830 on PLATO
Towards quantum entanglement with qutrits: Measuring diboson spin correlations
Quantum state tomography is the determination of the spin density matrix of a quantum mechanical system from an ensemble of measurements of similarly prepared states. The production of a specific final state at the LHC provides such an ensemble of similarly prepared states, and can thus be used to measure the spin density matrix of a system of elementary particles. So far, spin correlations have been measured only for top quark-antiquark pair (tt̄) production, where the spin-½ particles with two possible polarization states can be treated as “qubits”. Recently, also quantum entanglement in tt̄ events has been observed for the first time. In this project, we will study the spin correlations in the production of WZ diboson events. Massive spin-1 particles have three possible polarization states and are thus treated as “qutrits”. Leptonic decays of the W and Z boson will lead to events with three charged leptons, and we will devise analysis strategies to reconstruct distributions of angular separations between the charged leptons and to extract the spin correlation coefficients from these distributions.
Angle of the charged lepton from the W boson decay in WZ production events, shown separately for longitudinal (”0”), left-handed (”L”), and right-handed (”R”) polarization. From a measurement of this angular distribution, the fractions of the different polarization states can be measured. Other angular distributions will be investigated for the measurement of spin correlations. From: JHEP 07 (2022) 032.Lower bound on the concurrence as a function of the production angle and the invariant mass of the WZ system in WZ production events. A state is entangled if the concurrence is larger than zero. In the standard model, WZ production events are most entangled (yellow areas) for large production angles and small invariant masses, and for smaller production angles but much larger invariant masses. Entanglement can be measured from spin correlation observables. From: R. Aoude et al., JHEP 12 (2023) 017.
Promoter: Prof. Didar Dobur Supervisor: Dr. Joscha Knolle Contact | Topic 44826 on PLATO
DisplacedCMS: Search for long-lived particles with the CMS experiment at CERN
Many BSM theories predict the existence of new particles, such as heavy neutral leptons, that can spend a significant time inside of the CMS detector before decaying, leading to the production of the so-called displaced vertices in the CMS Tracker. Such long-lived particle decay signatures are extensively explored at the LHC, providing new constraints on the BSM models.
A representative Feynman diagram of the process with the production of a heavy neutral lepton (N) and its decay. From: JHEP 02 (2025) 036.The corresponding detector signature of the N production process with a lepton produced at the interaction point of the colliding beams and a displaced hadronic object originating from the CMS Tracker volume.
The project includes the first experimental study of the CMS sensitivity to the N production via the right-handed W boson decay predicted within the Left-Right Symmetric Model (LRSM). A feasibility study will involve different production channels and N decay modes. An essential part of the study will be devoted to the efficiency measurement for the identification of the displaced vertices in the CMS Tracker volume. Potential background contributions will be studied using CMS simulated events.
Promoter: Prof. Didar Dobur Supervisor: Adina Maria Tomaru, Dr. Kirill Skovpen Contact | Topic 44842 on PLATO
Particle physics detector topics
MuonEye: Gaseous detectors for muon tomography
A non-destructive imaging technique involving naturally produced atmospheric muons has become a well-established method to perform studies of various internal structures. The internal density is reconstructed by measuring the muon fluxes before and after passing through the target object. The muon tomography methods have been successfully used to study the interiors of active volcanoes, pyramids, industrial buildings, nuclear waste surveys, homeland security, among many other applications. Muon detection methods based on gaseous technologies are widely used for muon tomography providing a large coverage area with high muon detection efficiency at a relatively low cost. The Resistive Plate Chambers (RPCs) and Gas Electron Multipliers (GEMs) are often used as the muon detectors.
A magma area of an active volcano using photography and muon tomography methods.Cosmic rays as a tool for cargo screening at the border customs control.
A small RPC prototype will be built using glass as the main material for resistive electrodes. Additionally, a small prototype involving GEMs with multiplication stages based on PCBs with thick holes will be fabricated. Both technologies will be studied in terms of their detection performances, spatial and timing accuracies. A simulation package will be developed to create a model of a muon detector to optimize its design towards the best performance.
References: – L. Bonechi et al., Atmospheric muons as an imaging tool, Reviews in Physics 5 (2020) 100038. – M. Abbrescia et al., Resistive Gaseous Detectors: Design, Performance, Perspectives, ISBN 9783527340767, 2018. – F. Sauli, Micro-Pattern Gaseous Detectors, ISBN 9811222215, 2020.
Promoter: Prof. Didar Dobur Supervisor: Karam Kaspar, Dr. Kirill Skovpen Contact | Topic 44836 on PLATO
FastRead: Fast readout electronics for particle detectors
Particle detectors used in high-energy physics (HEP) collider experiments often face an important challenge of establishing an efficient analog signal processing chain capable of operating at a high collision rate. A circuitry design defines at large the timing and the energy resolutions in the detection of various particles produced in the collisions, and therefore, it has a direct implication on the final performance of a detector. The Silicon Photomultipliers (SiPMs) are used for detection of the scintillation light created by particles interacting with the active medium in the detector and can be found in almost any collider experiment.
Various types of Silicon Photomultiplier (SiPM) detectors.An example of a multichannel readout board for amplification and digitization of analog signals from the SiPM detectors.
A prototype of a readout board will be designed for the fast processing of the SiPM signals. The analog part of the circuitry will involve an amplification and a pulse shaping to allow for the optimal time and energy measurements. The amplified signals will be digitized using integrated circuits based on the Field Programmable Gate Arrays (FPGAs). The performance of the prototype will be compared to various off-shelf commercial readout systems that are used in HEP applications.
References: – S. Gundacker and A. Heering, The silicon photomultiplier: fundamentals and applications of a modern solid-state photon detector, Phys. Med. Biol. 65 (2020) 17TR01. – S. H. Hesari et al., A comprehensive survey of readout strategies for SiPMs used in nuclear imaging systems, Photonics 8 (2021) 266.
Promoter: Prof. Didar Dobur Supervisor: Dr. Kirill Skovpen, Guy Janssens, Bart Vancauteren Contact | Topic 44837 on PLATO
TileCal: Calorimeter design using plastic scintillators for the SHiP experiment at CERN
Sampling calorimeters are used in many high-energy physics experiments for the energy measurement. In such calorimeters, plastic scintillators often represent the active material to measure the energy that is released in electromagnetic and hadronic showers created by the incoming particle in the absorber layers. A new sampling calorimeter is currently built at UGent for the Scattering and Neutrino Detector (SND), which is part of the SHiP experiment at CERN. The main building blocks of this calorimeter are plastic scintillator tiles with 2D optical fiber readout based on silicon photomultipliers (SiPMs).
Plastic scintillator tiles with optical fiber grooves manufactured at UGent.Simulation of an interaction of an energetic electron in a prototype of the sampling calorimeter for SND@SHiP.
Multiple test beam studies for the sampling calorimeter are foreseen at CERN to measure the energy and time resolution of this new detector. For this purpose, a small-scale prototype will be built and commissioned at UGent using plastic scintillator tiles, optical fibers, and SiPMs. A student will be involved in the conceptualization process of the calorimeter design and commissioning of a cosmic muon test bench to measure its main characteristics. This work will be also complemented by simulation studies to derive the expected performance of the calorimeter and to further optimize its design towards the final implementation and integration in SHiP.
Promoter: Prof. Didar Dobur Supervisor: Dr. Kirill Skovpen Contact | Topic 44838 on PLATO
SoilCube: Measuring soil moisture with cosmic rays
The scarcity of water resources is naturally connected to the process of climate change. The project explores the feasibility of measuring the volumetric water content in soil through application of the innovative technology of cosmic-ray neutron sensing (CRNS) detectors. Cosmic-ray thermal neutron fluxes, which are inversely correlated with the presence of water above the ground, can be measured using radiation detectors capable of thermal neutron identification.
Illustration of the main concept of probing soil moisture through detection of cosmic-ray neutrons. A fraction of thermal neutrons is absorbed by the water present in the soil, while the rest is detected by the cosmic ray neutron sensor (CRNS).A CRNS probe installed at Proeftuin campus.
A prototype of the soil moisture sensor will be developed to measure cosmic-ray neutrons above the ground with wireless data transmission and powered by solar panels. This probe will be built using plastic scintillators and lithium-6-enriched materials for neutron identification. The performance of the prototype will be compared to the state-of-art sensor that is currently collecting data at Proeftuin campus. The study will also make use of various simulation techniques to model the atmospheric flux of neutrons, their interactions in the soil, and detection efficiencies. The neutron fluxes will be studied as a function of altitude, soil type, and environmental conditions.
References: – S. Gianessi et al., Testing a novel sensor design to jointly measure cosmic-ray neutrons, muons and gamma rays for non-invasive soil moisture estimation, Geosci. Instrum. Method. Data Syst. 13 (2024) 9. – L. Stevanato et al., A Novel Cosmic-Ray Neutron Sensor for Soil Moisture Estimation over Large Areas, Agriculture. 2019; 9(9):202.
Promoter: Prof. Didar Dobur Supervisor: Dr. Kirill Skovpen, Bart Vancauteren, Guy Janssens Contact | Topic 44839 on PLATO
GemShip: Gaseous detectors for particle identification in the SHiP experiment at CERN
A particle identification (PID) system is an integral part of almost any high-energy particle physics experiment, allowing for the robust identification of different types of particles entering the detection volume. A new SHiP experiment at CERN will study neutrino interactions as well as search for the production and decay of hypothetical particles predicted in various beyond the standard model theories. This experiment is currently being built and commissioned at CERN. The SHiP’s PID system includes electromagnetic and hadronic calorimeters for the energy measurement. A precise localization of particle trajectories in the PID system relies on the application of the gaseous technologies using Gas Electron Multiplier (GEM) detectors.
A schematic illustration of the operation principle of a Gas Electron Multiplier (GEM) detector.A simulated distribution of charges in the GEM hole amplification region using Garfield++ package.
Understanding the fundamentals of the GEM detection methods requires an in-depth study of the operation of these detectors for various incoming particle energies and fluxes. A prototype of a small triple-GEM detector will be built at UGent based on the expertise of the already commissioned GEM modules for the CMS experiment at CERN. The performance of the GEM prototype will be studied in the dedicated test beams at CERN, as well as using simulation toolkits based on Garfield++ and GEANT4. As part of the project, the proposed GEM high-precision layers will be also integrated in the simulation of the SHiP detector.
Promoter: Prof. Didar Dobur Supervisor: Karam Kaspar, Dr. Kirill Skovpen Contact | Topic 44840 on PLATO
Future experiments & collider topics
DisplacedSHIP: Search for feebly interacting particles with the SHiP experiment at CERN
Numerous particle physics experiments around the world have been extensively searching for new particles to construct a more complete theory of nature. The SHiP experiment at CERN aims at filling an important gap in the current phase space of sensitivities probed by various experiments by employing a high-intensity Super Proton Synchrotron (SPS) beam-on-the-target collisions and a large decay volume to search for the decays of new light particles (N), such as heavy neutral leptons, predicted in many beyond the standard model theories.
A schematic illustration of a process with the production of heavy neutral lepton (N) in the target area and its subsequent decay in the hidden sector decay vessel of the SHiP experiment.The expected sensitivity of SHiP to the mass and interaction couplings of N, compared to the latest CMS results at the Large Hadron Collider.
A comprehensive characterization of the dominant production and decay channels of N particles produced on the target is needed to assess the sensitivity of the SHiP experiment. The project will explore the simulation methods to generate N events, calculating expected cross sections and potential backgrounds. A simulation study will employ the hard-level process generation events as well as the fully simulated events inside of the SHiP detector to account for various detector-level effects.
Promoter: Prof. Didar Dobur Supervisor: Adina Maria Tomaru, Dr. Kirill Skovpen Contact | Topic 44844 on PLATO
TopFuture: Top quarks at future electron-positron colliders
A precise study of the properties of the heaviest elementary particle of the standard model (SM) of particle physics, the top quark, is among the foremost goals of the ongoing experiments at the Large Hadron Collider at CERN. Future collider projects aim at further improving the precision of these measurements, setting more stringent constraints on the new physics phenomena. The Future Circular Collider (FCC) factory is a proposed international project at CERN that will significantly extend our knowledge about this particle through a detailed energy scan of the production cross section of top quark pairs near the threshold. The FCC will allow for the first study of the top quark production in electron-positron collisions.
An aerial view of the site with the proposed Future Circular Collider (FCC) experiment at CERN.A dependence of the production cross section of top quark pairs near the production threshold in electron-positron collisions at FCC.
A kinematic reconstruction of the top quark decay products allows to define a set of kinematic observables highly sensitive to potential contributions from new particles predicted in various extensions of the SM. In addition to the top quark mass reconstruction with potential observation of a new bound state of top quarks, the toponium, the study will involve the measurement of the forward-backward asymmetry of top quark pairs and spin correlations through a set of dedicated angular observables. The derived kinematic variables will be used to apply an effective field theory interpretation to the measured cross sections across all observables. The analysis will be implemented using simulated events involving the full detector simulation for a realistic description of the detector-level effects.
Promoter: Prof. Didar Dobur Supervisor: Jules Vandenbroeck, Dr. Kirill Skovpen Contact | Topic 44845 on PLATO
Gravitational waves data analysis topics
Overcoming observational science challenges of the Einstein Telescope
The Einstein Telescope (ET) will have more than ten times the sensitivity of the current LIGO-Virgo-KAGRA detector network. As a result we will see merging black holes which are much farther away (out to the edge of the known universe), as well as heavier black holes, (thousands of times the mass of the sun), which merge at lower frequencies made accessible by the ET. Typical signals observed by the ET will also be much longer – minutes to hours or even longer (as opposed to a fraction of a second for the current detector network). This is exciting. However it brings forth several computational challenges. How will we carry out data analysis for such long signals? Signals will certainly overlap – how will we disentangle them from each other? The ET Collaboration Observational Science Board was recently established to answer such questions.
In this project, you will develop prospective techniques for ET data analysis. In particular you will attempt to bring together traditional methods such as Bayesian Markov Chain Monte Carlo (MCMC) together with more recently developed Convolutional Neural Network (CNN) algorithms with the goal of efficiently analyzing multiple signals present simultaneously in the data.
The work involved will be computational. You will learn about statistical methods and get exposed to state-of-the-art data analysis techniques. You will be involved in a large international collaboration in the forefront field of GW science.
Promoter: Prof. Archisman Ghosh Contact | Topic 44524 on PLATO
Measuring the Hubble constant with LIGO-Virgo-KAGRA data
The fourth observing run (O4) of the LIGO-Virgo-KAGRA (LVK) gravitational-wave (GW) detector network began in May 2023 and is expected to run until December 2024.
After interesting candidate events have been identified and their parameters have been measured, it will be time to obtain science results out of the observations. A central role played by researchers in UGent is in the Cosmology working group of the LVK. A short-term goal of this group is to measure the Hubble constant, H0, the local expansion rate of the universe. One uses the GW distance measurement together with complementary redshift information (from possible electromagnetic counterparts, host galaxies, or galaxy clusters) to infer the cosmological parameters such as H0. Due to a tantalizing discrepancy between the local and early-universe measurements of H0, now dubbed as the “Hubble tension,” such an independent measurement of H0 from the GW sector can prove to be invaluable.
The latest measurement of the Hubble constant by the LIGO-Virgo-KAGRA Collaboration, from B. P. Abbott et al. “Constraints on the cosmic expansion history from GWTC-3,” arXiv: 2111.03604 [astro-ph.CO].
In this project you will work with researchers who have developed the LVK codebase gwcosmo+ for cosmology inference and have put together the upGLADE galaxy catalogue to go along with it. You will be a part of the team that carries out the first cosmology analyses on O4 data. If we are lucky, we may observe a multimessenger signal in O4 and get to analyze the data from it. Until now, we have seen only one such multimessenger signal, namely GW170817.
The work involved will be computational. You will learn about statistical methods and get exposed to state-of-the-art data analysis techniques. You will be involved in a large international collaboration in the forefront field of GW science.
Promoter: Prof. Archisman Ghosh Contact | Topic 44525 on PLATO
Gravitational waves instrumentation topics
The direct detection of gravitational waves (GWs) is a breakthrough discovery of recent years. The several GW detections by the Advanced LIGO and Virgo detectors, since they were first discovered in 2015, have opened up a new window to the observable universe. The current “second generation” detectors are Michelson interferometers with km-scale arms. The principle behind the detector is based on the fact that when a GW passes through, the arms of the interferometer are stretched and squeezed in opposite ways in the two directions.
Effect of a gravitational wave (propagating in the direction perpendicular to the plane of the paper) on the arms of a Michelson interferometer.
The current detectors will eventually be replaced by the next generation of GW detectors of significantly higher sensitivity and distance reach. One of these “third generation” detectors is the Einstein Telescope (ET), which is planned to be built in Europe in the 2030s. Fundamentally new technology will be required to go beyond the limitations of the current detectors. Extensive work and study is required for each of the new techniques to be implemented. ETpathfinder is an R&D facility, located in Maastricht, the Netherlands, with the aim to test these new techniques and to give important inputs for the design and the construction of third generation GW detectors like ET. The ETpathfinder is a Fabry-Perot Michelson interferometer with 10 m arm cavities working at cryogenic temperatures. It will mainly focus on developing prototypes and testing cryogenic temperatures (120 K, 15 K), new mirror material (Silicon), new laser wavelengths (1550 nm, 2090 nm), and advanced quantum noise reduction techniques.
Calibrating GW detectors using scattered light
With the increase of the sensitivity, gravitational-wave detectors will require calibration with better accuracy and precision. Currently, gravitational-wave detectors are calibrated using two independent methods, in order to be able to cross-check the results. The two methods are the Photon Calibrator (PCal) and Newtonian calibration method (NCal) and for both of them the calibration is done inducing a displacement on the mirrors.
A new independent technique which can use scattered light as a signal for the detector calibration was recently proposed [M. Wąs et al., 2021, Class. Quantum Grav.38 075020]. The scattered light is injected in the interferometer from the back of the end mirrors using a scattering element that can be modulated in amplitude. Unlike the other two, this technique does not involve the movement of the mirrors and thus it can be very useful to cross-check the results and be sure that the mirror suspensions do not add any error in the model.
The Ghent Gravity Group, in collaboration with the University of Antwerp, is planning to develop for the Advanced Virgo detector this new calibration technique. This is a very innovative work, since this technique has not yet been used in any other GW detector so far. The work involved will be hands-on instrumentational, with focus on optics and mechanics. The project will be under joint supervision with Dr. Daniela Pascucci (UGent) and Prof. Hans Van Haevermaet (UAntwerpen). For some parts of your research, you may need to work at the lab in UAntwerpen. You will also get a chance to work at the ETpathfinder facility in Maastricht and be a part of the exciting activity in the Belgian-Dutch region centred around the Einstein Telescope.
Layout of Advanced Virgo. The scattered light used for the calibration will come from the Suspended West and North End Benches (SWEB and SNEB) behind the West End (WE) and North End (NE) mirrors.
Promoters: Prof. Archisman Ghosh, Prof. Hans Van Haevermaet (UAntwerpen) Supervisor: Dr. Daniela Pascucci Contact | Topic 44514 on PLATO
Optical characterization of 2μm photodetectors
Next generation GW observatories such as the Einstein Telescope (ET) will use silicon mirrors as test masses to detect low frequency GW with unprecedented sensitivities. This implies the use of new laser wavelengths at 1550nm or 2090nm. The performance of such optics will be first tested at the ETpathfinder facility in Maastricht. To detect the main interferometer signal, coming from the GW strain, photodiodes with a high quantum efficiency (HQE) and a large dynamic range for these new wavelengths are needed. At 1550nm HQE InGaAs sensors exist, however at 2090nm more development is needed and custom made extended InGaAs photodiodes with optimized quantum efficiency, especially for 2090 nm, will need to be acquired and tested. In this project we will contribute to the development of such novel 2090nm HQE photodiodes by performing optical characterizations of available photodiodes.
UGent has recently set up an optical lab for characterization of optical components for 2090nm. You will set up a measurement system at UGent optical lab with existing readout electronics (available in UAntwerpen) to test the sensor for e.g. dark noise, efficiency, and optical power levels. The work involved will be hands-on instrumentational, with focus on optics and mechanics. The project will be under joint supervision with Prof. Hans Van Haevermaet (UAntwerpen). You will also get a chance to work at the ETpathfinder facility in Maastricht and be a part of the exciting activity in the Belgian-Dutch region centred around the Einstein Telescope.
Promoters: Prof. Archisman Ghosh, Prof. Hans Van Haevermaet (UAntwerpen) Supervisor: Dr. Daniela Pascucci Contact | Topic 44519 on PLATO
Optical layout of the ETpathfinder
Ghent University has been involved in the ETpathfinder project since its beginning. The facility in Maastricht is currently still under construction, with the aim to have the interferometer up and running by the end of this year.
Although the layout of ETpathfinder is broadly defined, optical simulations are needed to define position and specifications of all auxiliary optics used to have the proper beam parameters at each step of the path. You will be actively involved in the development of ETpathfinder, working on optical simulations to define the final layout.
You will be a member of the Einstein Telescope Collaboration. You will get involved with the technology and challenges of interferometric detectors in the active field of gravitational-wave science. The project will be done in close coordination with the ETpathfinder community. Travel to Maastricht, where the facility is located, may be required.
Simplified layout of ETpathfinder in its first phase.Picture of the ETpathfinder hall with all the vacuum towers assembled.
Promoter: Prof. Archisman Ghosh Supervisor: Dr. Daniela Pascucci Contact | Topic 44520 on PLATO
Second harmonic generators for 2µm wavelength
Second harmonic generator (SHG), also known as frequency doubling, is a mechanism happening when a light beam passes through a material with non‐linear dielectric coefficient.
Using non-linear crystals as SHG to create green light from an infrared light source has been widely used in the past in a wide range of applications, like display technology, biomedicine, etc. Also in GW instrumentation this technique is not new. In fact, it is used in Advanced Virgo to generate from a 1064nm input source the green light of auxiliary lasers needed for the cavity control system and the squeezing system. However, it has never been used for 2 μm wavelength. Furthermore, since one of the biggest challenges for next generation detectors is the development of high‐efficiency photodetectors, like cameras and photodiodes, and this study can be a first step to develop a method can be used to overcome the problem, changing the wavelength of the laser beam before it reaches the photodetector. Currently the most used materials (from IR to green) are the Periodically-poled Lithium Niobate (LiNbO3 or PPLN) and the Periodically Poled-Potassium Titanyl Phosphate (KTiOPO4 or PPKTP). However, other materials will be studied and taken into account.
The main objective of the proposed research is to study possible SHG materials for 2 μm wavelength, analysing if they are suitable for GW detectors.
Schematic representation of how a non-linear optical medium acts as a second harmonic generator.Periodically-poled Lithium Niobate (LiNbO3 or PPLN).Periodically-poled Potassium Titanyl Phosphate (LiNbO3 or PPKTP).Pictures of two possible materials suitable for 2µm wavelength.
Promoter: Prof. Archisman Ghosh Supervisor: Dr. Daniela Pascucci Contact | Topic 44521 on PLATO
Modelling the effect of Newtonian noise for the Einstein Telescope
Newtonian noise or gravity gradient noise is an effect of the change of the Newtonian gravitational potential due to fluctuations of displacements on the surface of the Earth. Although it is a small effect for current GW detectors, it can become a major nuisance for future detectors such as the ET. Newtonian noise couples to a test mass in a manner very similar to GWs, and it is not possible, even in principle, to remove this noise source (except via active subtraction). It is therefore important to understand and quantify the impact of Newtonian noise. While estimation of Newtonian noise from homogeneous media has been well explored [1], a lot remains to be known in cases of complex, heterogeneous media.
(a) A schematic of Newtonian-noise coupling to the test-mass. (b) Contribution of different fundamental noises to ET’s sensitivity with Newtonian noise dominating the low-frequency contributions.
In this project, you will analyze the generation mechanism of Newtonian noise and simulate scenarios that will best represent the contribution of Newtonian noise to ET’s low-frequency sensitivity. Simulations of elastic waves in a heterogeneous medium will be performed using a spectral finite element solver SPECFEM3D. A software library will be developed to compute the gravitational acceleration on the interferometer’s test-masses, based on the simulated displacement of the medium’s elements. In particular, focus will be given to the understanding of contributions from different parts of the medium like interfaces, and cavern walls that host the vertices of ET.
You will work in collaboration with researchers in the University of Liège, including Dr. Soumen Koley, who is one of the leading experts in modeling Newtonian noise for GW detectors.
Your results may have a significant impact in bringing the ET to Belgium!
Promoter: Prof. Archisman Ghosh Supervisor: Dr. Daniela Pascucci Contact | Topic 44797 on PLATO
Neutrino experiment topics
Towards the neutrino mass measurement with Project 8
Project 8 is a next-generation direct neutrino mass experiment using the tritium endpoint method. To reach the target sensitivity of 40 meV, new technologies are required. Among them is Cyclotron Radiation Emission Spectroscopy (CRES), a new non-destructive technique to measure electron energies via the frequency of their cyclotron radiation. CRES has been successfully demonstrated on a small scale in a waveguide section. To be able to scale to large volumes of tritium gas, we are exploring resonant cavities as detectors. The first demonstrator of CRES in cavities, the Cavity CRES Apparatus, is being commissioned at the University of Washington in Seattle and expected to start data taking in Fall 2025. In this project, you will assist with data taking remotely (such as by monitoring data quality and performing initial reconstructions), refine reconstruction algorithms, and study the electron energy resolution in the cavity. Experience with Python and basic knowledge of C++ is helpful. See https://doi.org/10.5281/zenodo.13385713 for a recent conference presentation.
Promoter: Prof. Juliana Stachurska Contact | Topic 44857 on PLATO
Cavity design for a large-volume demonstrator for the Project 8 neutrino mass experiment
Project 8 is a next-generation direct neutrino mass experiment using the tritium endpoint method. To reach the target sensitivity of 40 meV, new technologies are required. Among them is Cyclotron Radiation Emission Spectroscopy (CRES), a new non-destructive technique to measure electron energies via the frequency of their cyclotron radiation. CRES has been successfully demonstrated on a small scale in a waveguide section. Scaling to large gas volumes requires large, cubic-meter scale detectors. You will work on refining the design of a resonant cavity that can serve as the tritium holding vessel and electron detector and the associated readout probe. Of particular interest is the mode structure and achievable bandwidth. Mode-filtering techniques can be explored to better isolate the readout mode. This is mostly a simulation project employing commercial and custom software. Depending on the features of the best-performing design, the project may culminate with the design of a smaller prototype for testing in an extant setup at the University of Washington.
Project 8 cavity concept with cavity (or- ange), multipole magnet for atom trapping and pinch coils for electron trapping (red) and solenoid magnet to induce cyclotron motion (blue). Electrons couple to the TE011 mode (grey), read out from the center. The electron trapping field is shown on the left. From: PoS (TAUP2023) 229.
Promoter: Prof. Juliana Stachurska Contact | Topic 44858 on PLATO
Triggering on neutrino radio signals with RNO-G
With the discovery of a high-energy astrophysical neutrino flux, IceCube has opened a new window into the Universe. The Radio Neutrino Observatory in Greenland (RNO-G) aims to extend our knowledge of the astrophysical neutrino flux into to higher energies. Consisting of 35 autonomous stations, of which several have already been deployed, RNO-G is the first large-scale radio neutrino observatory. The harsh Greenlandic climate and remote location poses challenges to the stations’ design. The focus of this project lies on a trigger board that has been designed for RNO-G within the Brussels group. We are now working together on the next version of the design to improve the performance.
A single RNO-G station consists of three strings of antennas (Hpol and Vpol) plus surface antennas (LPDAs), as well as three calibration pulsers located both deep in the ice and also at the surface. The string containing the phased array trigger is designated as the power string, while the two additional strings are designated as support strings. From: JINST 16 (2021) 03, P03025.
Promoter: Prof. Juliana Stachurska Contact | Topic 44859 on PLATO
