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 total decay width of the Higgs boson

The measurement of the total decay width of the Higgs boson at the ILC is possible because the special Higgsstrahlung production mode gives model-independent access to the absolute coupling of the Higgs to the Z boson. This gives an absolute no rmalisation to all cross-section x branching ratio measurements and thus allows to extract the total width, eg. from the measurement of the WW-fusion pro duction cross-section and the partial decay width for the decay of the Higgs into two W bosons. The full power of the ILC at a center-of-mass energy of 500 GeV has not yet been evaluated in full detector simulation. Preliminary studies showed that t his analysis is very sensitive to the reconstruction of the visible mass in the events, as well as to the flavour tag. Therefore this important measureme nt is an interesting testbed for further optimisation of the ILD detector concept.

Bachelor thesis or extendable to Master thesis

Systematic uncertainties on total decay width of the Higgs boson

Sofar, 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

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 silison 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