Wochenübersicht

This seminar consists of a journey on the path-integral approach to classical mechanics and a short walk on “dreams in progress” regarding the possible role of the action in the dark matter and dark energy problems.

The pure spinor superfield formalism is a systematic way to construct supersymmetric multiplets from modules over the ring of functions on the nilpotence variety. After a short review of the technique, I present its derived generalization and explain how the derived formalism yields an equivalence of dg categories between multipets and modules over the Chevalley--Eilenberg algebra of supertranslations. This equivalence of categories is closely related to Koszul duality. If time permits, I will comment on applications to six-dimensional supersymmetry.

The inside of black holes is shielded from observations by an event horizon, a virtual one-way membrane through which matter, light and information can enter but never leave. This loss of information, however, contradicts some basic tenets of quantum physics. Does such an event horizon really exist? What are its effects on the ambient light and surrounding matter? How does a black hole really look? Can one see it? Recently we have made the first image of a black hole and detected its dark shadow in the radio galaxy M87 with the global Event Horizon Telescope. Detailed supercomputer simulations faithfully reproduce these observations. Simulations and observations together provide strong support for the notion that we are literally looking into the abyss of the event horizon of a supermassive black hole. The talk will review the results of the Event Horizon Telescope, the nature and meaning of the black hole shadow, its scientific implications and future expansions of the array.

Final states in collider experiments are characterized by correlation functions of the energy flow operator - which plays the roll of an idealised calorimeter. In this talk, I will show that the top quark imprints itself as a peak in the three-point correlator at an angle determined by its mass and transverse momentum. This provides direct access to one of the most important parameters of the Standard Model in one of the simplest field theoretical observables. The analysis I will present provides a new paradigm for a precise top mass determination that is, for the first time, highly insensitive to soft physics and underlying event contamination.

The mass of the W boson, a mediator of the weak force between elementary particles, is tightly constrained by the symmetries of the standard model of particle physics. The Higgs boson was the last missing component of the model. After the observation of the Higgs boson, a measurement of the W boson mass provides a stringent test of the model. We measure the W boson mass using data corresponding to 8.8 inverse femtobarns of integrated luminosity collected in proton-antiproton collisions at a 1.96 TeV center-of-mass energy with the CDF II detector at the Fermilab Tevatron collider.
Slides here...

The negatively-charged nitrogen-vacancy defect (NV–) possesses an interesting combination of spin and optical properties that can potentially be exploited in applications such as solid-state qubits, highly sensitive electric and magnetic field probes and single-photon emitters. Within the diamond bandgap, the NV– centre forms an optically accessible two-level quantum system which consists of a spin-triplet ground state of 3A2 symmetry and a spin-triplet excited state of 3E symmetry. Two more electronic levels, both being spin-singlet states (1E and 1A1), are situated within the bandgap. NV centres can also exist in the neutral charge state (NV0). The predominantly utilized feature of the NV– centre is the spin-triplet 3A2 state that can be manipulated with microwave radiation and its spin state read out via the PL yield of the triplet transition as optically detected magnetic resonance (ODMR). Recently, photoelectric detection of magnetic resonance (PDMR) has been demonstrated as an alternative that utilizes state-selective ionization of NV– to NV0 and photocurrent detection. Despite being one of the best-studied solid-state defects, the non-equilibrium dynamics of NV centres are not yet fully understood, particularly with respect to charge conversion. We present results using time-resolved spectroscopic techniques such as transient absorption spectroscopy, photocurrent spectroscopy and THz time-domain spectroscopy to investigate the dynamics of ensembles of NV centers in bulk diamond after photoexcitation by probing the transient response of its optical signatures. Two separately wavelength-tunable femtosecond pulses (450-1040nm) for excitation, combined with broadband spectral probing (400-1650nm) over timescales reaching from fs to ms enable us to probe all relevant optical transitions in a time-resolved fashion, providing a direct measure of complex processes such as photoionization and charge conversion. Variation of the concentration of single substitutional nitrogen (Ns) in different samples permits us to characterize their influence on NV dynamics. We probe the electronic dynamics of both NV0 and NV– centres. For the latter one, we characterize the whole spin polarization cycle and find two additional localized electronic states. We find that recombination of electrons from the conduction band after photoionization of NV– via 3E proceeds through two distinct relaxation channels. Using photocurrent spectroscopy, we also experimentally determine the photon energy threshold for photoionization from 3E.