Particle Physics and Cosmology
Particle Cosmology is an interdisciplinary field of fundamental research, which very fruitfully interfaces to high energy physics, astroparticle physics, general relativity and early Universe physics. It currently addresses very distinct and fundamental problems of our present understanding of the structure of the Universe: the nature of Dark Matter and its embedding in models beyond the Standard Model of particle physics, the creation of a matter-antimatter asymmetry, the explanation of the presence of Dark Energy and the implementation of an early inflationary phase. These questions are of course related to the high energy completion of the Standard Model and so to string theory on one side, and to particle physics phenomenology on the other.
While there is solid gravitational evidence for the existence of dark matter (DM), its particle physics properties are unknown.
In many beyond the Standard Model theories, such as in the simplest supersymmetric models, the dark matter candidate corresponds to a weakly interacting massice particle (WIMP) with only a negligible self scattering. Such collisionless cold DM successfully explains cosmological structure formation at large scales. In recent years however there has been an increasing interest in self-interacting DM, which has been invoked to ameliorate tensions between N-body simulations and astrophysical observations on small scales. Within the DESY Cosmology group models for self-interacting DM as well as potential astrophysical signatures are developed. In addition more classical signatures of dark matter at underground direct detection experiments as well as at colliders are studied, creating strong links to the Collider Phenomenology group at DESY.
In the DESY theory group different mechanisms for the generation of the cosmological matter-antimatter asymmetry are studied: electroweak baryogenesis at the scale of electroweak symmetry breaking, and leptogenesis, which is based on the properties of Majorana neutrinos. In both cases techniques from nonequilibrium quantum field theory are used to obtain a description of baryogenesis beyond the classical Boltzmann equations. A relatively new developement is hereby the attempt to link baryogenesis to flavor model building.
Large Scale Structure of the Universe
The large scale structure of the Universe are formed by the gravitational collapse of dark and baryonic matter. While this highly non-linear problem is mostly studied by simulations, the largest scales are eventually accessible to analytic methods. These are often based on the resummation of perturbative results or consist of relations that are based symmetry arguments.
Dark Energy, Inflation and Gravitational Waves
Inflation, a likely phase of exponential expansion of the universe, provides a successful description of the initial conditions for the present state and of the cosmic microwave background data. However many conceptual problems are still unsolved, especially in the embedding of inflation in wider particle physics models beyond the Standard Model. Particularly promising appear string theories with its "landscape" of vacua. An important prediction of inflation and cosmological phase transitions are primordial gravitational waves. These are observable in the polarization data of the CMB or with space-based interferometers.
Axion-like Particles, WISPs and the Low Energy Frontier
Axion-like particles and WISPs (Weakly-Interacting Sub-eV Particles) are predicted in many extentions of the Standard Model and in particular in String Theory. They have the common feature of being very light particles, so they do not need a high energy to be produced, but they are nevertheless very difficult to produce and detect due to their very weak couplings. The main door to investigate this type of particles is via their coupling to the photons, as in light-shining-through-the-wall experiments like ALPS, and via their cosmological consequences.
Grand Unified Theories
Supersymmetry and grand unification in four and more dimensions are interesting concepts for embedding the Standard Model of particle physics into a more fundamental theory. These ideas are pursued in the context of heterotic string compactifications, supergravity models in six dimensions and F-theory compactifications.
The group is supported by the German Science Foundation (DFG) through a Collaborative Research Center Sonderforschungsbereich 676 on Particles, Strings and the Early Universe.
Some suggested papers for further reading:
- A First-Order Electroweak Phase Transition in the Standard Model from Varying Yukawas.
I. Baldes, T. Konstandin and G. Servant, (submitted to PRL), arXiv:1604.04526
- Unifying inflation with the axion, dark matter, baryogenesis and the seesaw mechanism.
G. Ballesteros, J. Redondo, A. Ringwald and C. Tamarit, (submitted to PRL), arXiv:1608.05414
- Structure formation with massive neutrinos: going beyond linear theory.
D. Blas, M. Garny, T. Konstandin and J. Lesgourgues, JCAP 1411 039 (2014) no.11, arXiv:1408.2995
- Lattice QCD for Cosmology.
S. Borsanyi et al., (accepted for publication in Nature), arXiv:1606.07494
- Starobinsky-Type Inflation from alpha'-Corrections.
B. J. Broy, D. Ciupke, F. G. Pedro and A. Westphal, JCAP 1601, 001 (2016), arXiv:1509.00024
- de Sitter vacua and supersymmetry breaking in six-dimensional flux compactifications.
W. Buchmuller, M. Dierigl, F. Ruehle and J. Schweizer, Phys. Rev. D 94 (2016) no.2, arXiv:1606.05653
- Split symmetries.
W. Buchmuller, M. Dierigl, F. Ruehle and J. Schweizer, Phys. Lett. B 750 (2015) 615, arXiv:1507.06819
- Science with the space-based interferometer eLISA. II: Gravitational waves from cosmological phase transitions.
C. Caprini et al., Mon.Not.Roy.Astron.Soc. 437 (2014) arXiv:1512.06239
- Cool WISPs for stellar cooling excesses.
M. Giannotti, I. Irastorza, J. Redondo and A. Ringwald, JCAP 1605 (2016) no.05, 057, arXiv:1512.08108
- Axion Monodromy and the Weak Gravity Conjecture.
A. Hebecker, F. Rompineve and A. Westphal, JHEP 1604, 157 (2016), arXiv:1512.03768
- On the interpretation of dark matter self-interactions in Abell 3827.
F. Kahlhoefer, K. Schmidt-Hoberg, J. Kummer, S. Sarkar, Mon.Not.Roy.Astron.Soc. 452 (2015) arXiv:1504.06576
- Colliding clusters and dark matter self-interactions.
F. Kahlhoefer, K. Schmidt-Hoberg, M. T. Frandsen, S Sarkar, Mon.Not.Roy.Astron.Soc. 437 (2014) arXiv:1308.3419
- String cosmology — Large-field inflation in string theory.
A. Westphal, Int. J. Mod. Phys. A 30, no. 09, 1530024 (2015), arXiv:1409.5350