Standard Model of Particle Physics
The Standard Model of particle physics is a remarkably successful theory of particle interactions at highest energies. However, multiple observations such as dark matter and neutrino oscillations as well as theoretical considerations such as the flavour puzzle and the strong CP problem imply that the Standard Model should be extended in order to stay valid at energies which will be accessible at future experiments. Such extensions usually introduce additional symmetries and so far undetected particles. These particles can have evaded detection by being either too heavy or too feebly coupled to the Standard Model.
The construction of the Large Hadron Collider represents the latest major increase in energy reach in a series of successful high energy physics accelerator programs. It promptly lead to the discovery of the Higgs particle, predicted by the Standard Model. However, until the construction of an even larger collider planned for the future, the discovery potential at the energy frontier will only slowly increase beyond the current reach. In the meantime current, e.g. NA62, and future, e.g. SHiP, fixed target experiments explore the intensity frontier by searching for feebly interacting messengers to hidden sectors. During the high luminosity run (from 2026) the Large Hadron Collider will also shift its focus towards this goal.
Currently I am exploring the detection possibility of right-handed neutrinos, axion-like particles and other feebly interacting particles at fixed target and colliders experiments [Searching for Long-Lived Particles beyond the Standard Model at the Large Hadron Collider]. I have explored the potential of various experiments to search for displaced signatures caused by the very small interaction strength of these particles.
Heavy Neutrinos at NA62
I have shown that comparable light right-handed neutrinos of the neutrino minimal standard model can not only be detected in NA62 but that NA62 in its dump mode is at the moment the leading experiment for the discovery of such particles for masses between the Kaon and the D-meson thresholds [NA62 sensitivity to heavy neutral leptons in the low scale seesaw model]. Additionally, I have shown how strongly the mixing angles of the right-handed neutrinos of the neutrino minimal standard model are constrained by neutrino oscillation data.
Heavy neutrinos at the Large Hadron Collider
I have explored the prospects of the CMS, ATLAS and LHCb experiments to discover light right-handed neutrinos with masses of a few GeV via displaced vertex searches [Heavy Neutrinos in displaced vertex searches at the LHC and HL-LHC]. This study includes the prospects of the high luminosity LHC and a comparisons of all right-handed neutrino flavours.
Long lived particles in heavy ion collisions
I have shown that it is possible to search for long lived particles exploiting the heavy ion runs of the Large Hadron Collider [A Heavy Metal Path to New Physics], [ Long Lived Particles Searches in Heavy Ion Collisions at the LHC]. Fascinatedly, heavy ion collisions can lead, under the assumption of equal running time, to better detection possibilities than proton collisions for such particles. This study comprises a foundational work for the very young field of searches beyond the Standard Model in heavy ion collisions [New physics searches with heavy-ion collisions at the LHC].
The Energy Frontier
Deviations from the predicted Higgs couplings can contain valuable hints about extensions of the Standard Model. At the same time, the question which mechanism stabilizes the electroweak scale remains unanswered. Any physics at higher scale can easily destabilize the Higgs mass via loop corrections.
Heavy Higgs Particles in Two Higgs Doublet Models
I have studied the detectability of heavy Higgs bosons appearing in extensions of the Standard Model featuring two Higgs doublets such as the minimal supersymmetric Standard Model. For the first time I have shown that the complete tan β range can be probed using associated production of heavy Higgs bosons up to masses of 1 and 10 TeV at the Large Hadron Collider and a potential future 100 TeV collider, respectively [Heavy Higgs Bosons at 14 TeV and 100 TeV], [Heavy Higgs bosons at low tan β: from the LHC to 100 TeV]. Parts of this proposal have been picked up by the CMS collaboration.
Scalar and fermionic top partners appearing in supersymmetric and composite extensions to the Standard Model are the most promising candidates to stabilize the Higgs mass scale. I have analyzed the constraint imposed by the Large Hadron Collider on little Higgs and supersymmetric models. I have discovered an extremely simple relation between top sector parameters that must hold after electroweak symmetry breaking in order for models to be natural. Additionally I have investigated how well the couplings relevant for naturalness considerations can be constrained at the Large Hadron Collider and future hadron colliders [Testing naturalness at 100 TeV].
For a long time models with low scale supersymmetry were the preferred solution to the hierarchy problem. While the majority of models feature exact R-parity conservation and therefore have a stable dark matter candidate I have investigated the implications of R-parity violation favored by models with gravitino dark matter [Long-lived neutralino as probes of gravitino dark matter].
Gravitino Dark Matter
I studied experimental predictions of a cosmologically consistent model with gravitino dark matter. A high reheating temperature motivated i.e. by leptogenesis, leads to the gravitino problem in early universe cosmology. If the gravitino is the lightest supersymmetric particle, the next-to-lightest supersymmetric particle has a rather large lifetime and can spoil successful predictions of big bang nucleosynthesis. A small amount of R-parity breaking causes the next-to-lightest supersymmetric particle to decay before the nucleosynthesis and renders supersymmetry models consistent [Broken R-Parity in the Sky and at the LHC].
I investigated the case of bilinear R-parity breaking, which is achieved at the supersymmetry breaking scale and stays consistent at loop level [Gravitino and scalar τ-lepton decays in supersymmetric models with broken R-parity]. I discussed the predictions of this model in a basis where bilinear mass mixing terms are traded for R-parity breaking Yukawa couplings. This procedure generated a new coupling of right-handed up-quarks previously not considered in the literature. Furthermore, R-parity breaking gravitino and neutralino decays turned out to be governed by the same parameter. This fact allows to obtain a bound on the neutralino decay length at the Large Hadron Collider from the Fermi-LAT constraints on decaying dark matter.
Neutralino decays at the LHC
I predicted possible signatures for bino and higgsino next-to-lightest supersymmetric particles at the Large Hadron Collider [Quasi-stable neutralinos at the LHC], [Long-lived higgsinos as probes of gravitino dark matter at the LHC]. I found that depending on the superparticle mass spectrum the Large Hadron Collider is in both cases able to probe the strength of R-parity violation beyond the present upper bounds from astrophysics and cosmology. Furthermore, even for rather low values of R-parity breaking parameters, the distribution of missing transverse energy is altered in comparison to usual supersymmetry models such that the signal can be missed if the search strategy heavily relies upon it. Therefore, such models can be well hidden despite the Large Hadron Collider data.
Higgsino lightest supersymmetric particle
I have analyzed a more concealed supersymmetric model resulting from grand unification with extra dimensions at the scale of Grand Unfied Theories. A hybrid mediation pattern of supersymmetry breaking leads in this model to an unusual spectrum with light and degenerate higgsinos, very heavy squarks and gluinos and possibly intermediate third generation squarks. I investigated the prospects to discover this model at the Large Hadron Collider and tried to answer the question by which means one can distinguish it from constrained minimal supersymmetric Standard Model-like models with a heavy spectrum [Searching for light higgsinos with b-jets and missing leptons].
I have conducted detailed collider studies for the Large Hadron Collider and a future 100 TeV collider [CEPC-SPPC Preliminary Conceptual Design Report. 1. Physics and Detector], [Searches for non-SM heavy Higgses at a 100 TeV pp collider]. Thereby, I have developed and published the software “Boosted Collider Analysis” which can be used to search for collider signatures using multiple boosted decision trees. This software uses substructure information such as subjets, pull, dipolarity and subjettiness in order to tag Standard Model particles, such as bottom and top jets, as well as signatures beyond the Standard Model.
I have shown, that deep neuronal networks are able to identify new physics signals after being trained only on Standard Model signals. I have also addressed the problem, that Monte Carlo background distributions of the Standard Model have very long tails in any given parameter space, which a naive algorithm could falsely identify as signals of new physics [Novelty Detection Meets Collider Physics].