Topics:

• Precision Measurements (such as spin and polarisation) in Top Quark Pair Production at 13 and 13.6 TeV
• Top Quark Reconstruction
• Search for ttbar bound states (toponium)
• Search for Axion-Like Particles (ALPs) coupling to Top Quarks and Gluons
• Search for New Physics (such as Dark Matter and Heavy Higgs bosons) using Top Quark Pair Events
• Search for anomalous Effective Field Theory (EFT) couplings in Top Quark Pair Events
• Search for Dark Matter in association with b-quark pairs
• Search for Dark Matter in heavy Higgs decays
• Searches for New Physics in the the Dark Higgs Sector

 

Current job opportunities:

 

Postodctoral Fellows:

• DESY Fellowship in Experimental Particle Physics. Deadline for applications: 31 March 2025

• DESY Fellowship in Experimental Particle Physics. Following deadline for applications: 30 September 2025

 

PhD Students:

• PhD position at DESY as part of the DASHH Interdisciplinary Graduate School on Top Quark Reconstruction with Generative Models for Searches for Heavy Higgs Bosons and Top-Antitop Quark Bound States with the CMS Experiment. Deadline for applications: 20 October 2024 (please contact us, if you see this after the deadline: group leaders)

 

Opening for PhD position at DESY on the CMS experiment in collaboration with the HAW Hamburg as part of the DASHH
Interdisciplinary Graduate School - Data Science in Hamburg - Helmholtz Graduate School for the Structure of Matter. This is German 75% EGR. 13 (TV-AVH) position for three years.

The DESY Exotics group at the CMS experiment is offering a position for a Ph.D. student in high energy physics as part of the DASHH graduate school. In this project, the student will analyse Run3 data and investigate an intriguing excess observed in the production of top-antitop quark pairs. While bound states have been detected for all five lighter quark types, such states, called toponium, have yet to be observed for the heaviest of all elementary particles, the top quark.

To distinguish heavy Higgs particles from toponium or background fluctuations, advanced machine learning methods using regression algorithms are essential for accurately reconstructing top quarks and their kinematics in data. The student will develop and investigate
generative models, such as generative adversarial networks or normalizing flows and graph neural networks, and combine them with disambiguation-free partial label learning techniques. We expect that such an approach will allow us to improve the sensitivity of the analysis and increase the regression accuracy.

Requirements:

• Diploma or Master's degree in physics with excellent grades
• knowledge and interest in particle physics and machine learning
• research experience in particle physics is a plus but not mandatory
• strong software skills (Python, ROOT, C++)
• good teamwork and communication skills
• good English language skills
• ability and eligibility to work in different institutions (HAW and DESY) and to travel to CERN, Geneva, Switzerland

 

More information an be found here.

For more questions please contact: Dr. Alexander Grohsjean, Prof. Dr. Christian Schwanenberger

 

• For the topics above there are many more possible projects for a PhD. Please also look into the descriptions of the various research projects. For more information please contact the DESY Exotics group leaders.

Master Students:

• For the topics above there are many possible projects for master students. Please check the topic list for bachelor students, which in principle can be extended to a master thesis. Please also look into the descriptions of the various research projects. For more information please contact the DESY Exotics group leaders.

Bachelor Students:

For the topics above there are many possible projects for bachelor students. A suggestion of possible topics is given below. Please also look into the descriptions of the various research projects. For more information please contact the DESY Exotics group leaders.

• Studies on the FCNC production of same sign Top quark pairs in the di-lepton final state: A study to asses the viability and sensitivity of the search for same-sing top quarks in the dileptonic final state with the full data collected by the CMS detector during Run 2 (2016-2018) will be carried out. The analysis targets exotic scenarios where there could exist a neutral bosonic state that mediate a Flavor Changing (FC) interaction between the top quark and another light quark. This neutral state could also decay invisibly to another Beyond the Standard Model (BSM) particles, constituting a possibility for a mediation between the Dark Matter and SM sectors. The study uses the same-sign production of top quark pairs decaying into a final state containing two same-sign leptons to investigate the viability of a search like this within the LHC context. Modern algorithms dedicated to top quark kinematic reconstruction will be used to separate the expected signal from the main background. The main source of background is expected to come from QCD multi-jet production, therefore a simple statistical analysis will be used to assess the sensitivity of this search. The use of Monte Carlo generators like Madgraph5_aMC@NLO to produce the signal is expected.
• Study of the cross section behaviour for searches for Heavy Higgs bosons in 2-Higgs-Doublett Models with an additional Pseudoscalar (2HDM+a): These type of processes have been studied in the context of simplified Dark Matter models, where simple s-channel production of (pseudo)scalar mediators has been considered. However, the scenario of a more complex and ultraviolet-complete model like the the Two Higgs Doublet Model plus and additional Pseudoscalar (2HDMa) involves more scalars in the particle content, in particular two pseudscalars, which could lead to interference effects on the total production cross section, as well as kinematic changes with respect to the single-mediator process. The study aims to investigate these effects in view of estimating the bias incurred when considering only simplified models. The project relies on the use of the modern Monte Carlo generator Madgraph5_aMC@NLO to perform the signal modeling and thus analyze the above mentioned effects.
• Parameterized Machine Learning in Searches for Heavy Higgs bosons (2HDM+a) in bbZa final states: in this project a novel type of neural network using parameterizations of the signal, called Parameterized Neural Network, shall be utilized in our search for new physics in bbZa final states (see research topics). The parameters of the signal model, the mass of the heavy scalar H and the pseudoscalar a, are utilized as input to train the neural network using all signal points together, instead of training the neural network to each signal point separately. This will be a large improvement in our quest to find new physics in bbZa final states.
• Search for Colorons in Top-Anti-top Quark Pair Production: So-called “4321” renormalizable models allow for sizeable flavour violation in quark-lepton currents, while suppressing flavour changing neutral currents in quark-quark and lepton-lepton sectors. This mechanism can accommodate the current experimental anomalies in B-meson decays, both in charged and neutral currents, while remaining consistent with all other indirect flavour and electroweak precision measurements and direct searches at high-pT . As part of the model heavy g' vector particles exist which are colour octets and are called colorons. There is no tree-level coupling between a single g' and a gg pair. Due to the flavour structure of the model, the couplings to light quarks are suppressed, however the PDF enhancement of valence quarks relative to third generation quarks in the proton ensures that coloron production to 3rd generation quark-anti-quark pairs is nevertheless dominant. The interesting regimes of the model are when the width is rather large (but still calculable) or the resonance is narrow but rather heavy. In this project, for the tt production process, using the Madgraph5_aMC@NLO Monte Carlo program which contains a coloron implementation, the coloron signature shall be investigated. It should be studied how a coloron signal can be separated from the SM background, and what the expected sensitivity will be to discover colorons at the LHC.
• Search for Axion-Like Particles (ALPs) associated with Top-Anti-top Quark Pair Production: Axions might explain the so-called "Strong CP-Problem" and are therefore very interesting particles to search for. At the LHC, in proton-proton collisions we can search for similar particles, so-called Axion-Like Particles (ALPs). In the DESY CMS Exotics Group we search for Heavy Scalar and Pseudoscalar particles associated with the production of a top-antitop quark pair (see research topics). ALPs will be pseudoscalar particles, however, with different couplings to top quarks and gluons compared to pseudoscalar Higgs bosons. In this project it should be studied using Monte-Carlo simulations, how energy/momentum but also angular distributions might look different between heavy pseudoscalar Higgs bosons and ALPs particles. It should then be investigated which observables are best suited to separate a possible ALPs signal from ttbar production in the Standard Model. Depending on the progress, those variables shall be combined in a Machine Learning method, e. g. by creating a neural network. The expected sensitivity in a search using LHC data collected by the CMS experiments shall be inferred. This study might be important for future searches for ALPs in our DESY CMS Exotics group.
• Silicon Sensor Qualification for the BCM1F Detector at CMS: see poster attached