FLC - Forschung mit Lepton Collidern

Thesis offers

The FLC group is looking continuously for excellent Bachelor, Master and PhD students. DESY collaborates with the University of Hamburg and other universities in Germany and abroad, from which you will obtain your degree. Within the ILC Physics subgroup we offer theses from fully physics case driven topics close to phenomenology to topics concerning the optimisation of certain detector aspects or the development of new reconstruction methods. PhD students are offered the option to combined detector development work (c.f. TPC, HCAL, Polarimetry subgroups) with a physics case study.

Below we list a few example topics.

Please contact Jenny List or Ties Behnke for any further information.

Higgs

After the discovery of a Higgs boson at the LHC in summer 2012, the precise characterisation of this particle is among the prime goals of the ILC. Only the subpercent precision achievable at Lepton Colliders can reveil any possible deviation from the properties of the Higgs boson of the Standard Model of particle physics.

Measuring the self-interaction of the Higgs boson

The coupling of the Higgs boson to itself determines the shape of the Higgs potential and its evolution in the early universe. At Lepton Collider with center-of-mass energies of at least 500 GeV, it can be measured from di-Higgs production. The first study of ZHH production in full detector simulation and including all relevant backgrounds was performed in our group and showed that if some of the limiting factors in the event reconstruction could be overcome, the measurement precision could be improved drastically, by up to a factor of two. Several of the identified limiting factors have been addressed since, and should now be applied to the Higgs self-coupling measurement.

PhD thesis

Detector and Reconstruction Techniques important for Higgs Physics

Most detector concepts for future lepton colliders do not yet exploit the possibility to identify different types of hadrons in the detector. An expection is the ILD detector concept with its Time Projection Chamber, which allows to identify protons and kaons based on the specific energy loss dE/dx. In addition, or, in case of detectors with all-silicon tracking, instead, time-of-flight information could be used to identify hadrons if the outer tracker or inner calorimeter layers could offer time measurements with a resolution of ~100 ns or better. Goal of this project is to evaluate the potential benefit of time-of-flight information for Higgs observables and to derive requirements and R&D goals for the detectors.

Master thesis or part of a PhD thesis, single aspects can be covered in Bachelor theses

Systematic uncertainties on precision Higgs boson measurements

So far, most studies evaluated the statistical uncertainty. In the Higgs sector, many analyses aim for subpercent precision, requiring a careful understanding of the involved systematic uncertainties. This includes various experimental uncertainties, but in the special case of the total decay width also the theoretical uncertainties which enter when extracting the width from the observable cross-sections and branching ratios.

Master thesis or part of a PhD thesis, single aspects can be covered in Bachelor theses

Searches for exotic decays of the Higgs boson

The Higgs boson could have additional decay modes beyond those predicted by the Standard Model of particle physics. The clean environment of a Lepton Collider is ideally suited to discover such decay modes. The sensitivities to different exotic modes have to be evaluated, and can be combined with information from the SM decay modes in order to test how well specific BSM models can be distinguished from the SM.

Master thesis or part of a PhD thesis, single aspects can be covered in Bachelor theses

CP properties of the tth coupling

For the Detector volume of the ILC Technical Design Report, the achievable precision on the top Yukawa coupling has been evaluated, showing that at ECM = 1 TeV a statistical precision of 4.3% can be reached on the absolute value of this coupling, assuming that the radiated Higgs state is a pure CP-even scalar like in the Standard Model. In extended Higgs sectors, there could however be a CP-odd admixture! By combining the measurement of the cross-section at difference center-of-mass energies, the polarisation asymmetry and the up-down asymmetry, such admixtures could be discovered or severely constrained.

PhD thesis

Supersymmetry

Light Higgsinos

A key prediction of natural SUSY models are light, nearly mass-degenerate Higgsinos. Due to their small mass differences, their soft decay products provide a very interesting test case for many aspects of detector performance. While an initial feasibility study based on fast detector simulation and very few decay modes has been published recently, many aspects remain to studied. This includes the reconstruction of exclusive hadronic decay modes using particle identification information, measurement of the lifetimes of the charged and neutral Higgsinos, the impact of beam background, reconstruction algorithms for very low energetic particles and so on.

Master thesis or part of an PhD thesis, single aspects can be covered in Bachelor theses.

SUSY with extra Gauge Groups

The extension of the MSSM with an extra U(1) gauge group is well motivated (eg by explaining neutrino masses) and can change the collider signatures significantlty. We would like to investigate strategies how to distinguish such scenarios from plain MSSM, in both the Higgs and SUSY sectors, and test them in realistic detector simulations.

PhD thesis, single aspects can be covered in a Master thesis.

Detector aspects important for Physics Beyond the SM

New particles with near-degenerate mass spectra produce very soft decay products. Their reconstruction is challenging even at the ILC due to material effects and beam background.

Reconstruction of sub-GeV photons in the ILD Detector

Eg. in case of light Higgsinos (s.a.), the reconstruction of sub- to few-GeV photons is crucial. The efficiency and purity achievable with the current design of the ILD detector concept should be evaluated. This includes the development of dedicated reconstruction algorithms. Based on these, potential critical aspects of the detector design should be identified and realistic options to optimise the design should be evaluated.

Master thesis, single aspects can be covered in Bachelor theses.

Reconstruction of sub-GeV charged particles in the ILD Detector

The reconstruction of sub-GeV tracks suffers beam background, which creates a lot of additional hits in the inner detectors. In particular for particles which do not reach the TPC, the stand-alone pattern recognition in the inner silicon detectors in the presence of beam background is a not yet fully solved problem, depending on the assumed time-stamping capabilities of the silicon detectors. In this project, the stand-alone silicon pattern recognition should be advanced, using as a test case eg. light Higgsinos (s.a.) or other new particles with small mass differences. The physics impact of the number of bunch-crossings integrated by the different layers of the inner tracking system should be evaluated and the overall design of the silicon tracking system optimized for stand-alone tracking in the presence of background.

PhD thesis, single aspects can be covered in Master or Bachelor theses.

Non-pointing photons from New Physics

Another special signature of new physics could be meta-stable particles decaying into a photon and an invisible particle. In such events, it is crucial to b able to reconstruct the angle of the photon from the cluster shape in the calorimeter. Currently, two different technologies are being discussed for the electromagnetic calorimeter of ILD, or even a hybrid of the two. The performance of all options w.r.t. cluster angle reconstruction should be investigated and the physics impact evaluated.

Master thesis or part of an PhD thesis

Supervisor: Jenny List