Prof. Steffen Schumann
II. Physikalisches Institut
Universitaet Goettingen

Monte Carlo event generators are an indispensable tool for the analysis and interpretation of experimental data from high-energy collider experiments, like the Fermilab Tevatron or the CERN LHC. We are concerned with the development of one such generator, called Sherpa.

Sherpa is a general-purpose tool for the simulation of particle collisions at high-energy colliders. It accounts for all aspects of individual scattering events including the hard-process generation, a parton-cascade simulation and phenomenological models for describing the parton-to-hadron transition. As a result Sherpa can be used to generate realistic, i.e. fully exclusive, pseudo-data for example for LHC collision events.
 
Meanwhile Sherpa represents one the standard tools to describe and interprete Tevatron and LHC collision data. It is widely used by the big Fermilab and CERN experimental collaborations as well as for state-of-the-art theoretical and phenomenological studies.

Link

• http://www.sherpa-mc.de/

Software

• C++, Sherpa, Rivet, Root

Publications

• A. Buckley et al., "General-purpose event generators for LHC physics", Phys. Rept. 504 (2011) 145. online
• T. Gleisberg, S. Hoeche, F. Krauss, M. Schonherr, S. Schumann, F. Siegert and J. Winter, "Event generation with SHERPA 1.1", JHEP 0902 (2009) 007. online
• S. Hoeche, F. Krauss, S. Schumann and F. Siegert, "QCD matrix elements and truncated showers", JHEP 0905 (2009) 053. online
• S. Hoeche, S. Schumann and F. Siegert, "Hard photon production and matrix-element parton-shower merging", Phys. Rev. D 81 (2010) 034026. online
• C. Englert, T. Plehn, P. Schichtel and S. Schumann, "Jets plus Missing Energy with an Autofocus", Phys. Rev. D 83 (2011) 095009. online
• S. Schumann, A. Renaud and D. Zerwas, "Hadronically decaying color-adjoint scalars at the LHC", JHEP 1109 (2011) 074. online
• E. Gerwick, T. Plehn and S. Schumann, "Understanding Jet Scaling and Jet Vetos in Higgs Searches", Phys. Rev. Lett. 108 (2012) 032003. online
• C. Englert, T. Plehn, P. Schichtel and S. Schumann, "Establishing Jet Scaling Patterns with a Photon", JHEP 1202 (2012) 030. online

Christoph Englert
IPPP
University Durham

Tilman Plehn
Institut für Theoretische Physik
Universität Heidelberg

Peter Schichtel
Institut für Theoretische Physik
Universität Heidelberg

Steffen Schumann
2. Physikalisches Institut
Universität Göttingen

We study the number of jets in LHC collisions to develop powerful tools in searches for new physics as well as to achieve a better understanding of QCD. In particular we observe two distinct patterns in QCD radiation: Staircase and Poisson scaling. Poisson processes are well understood as independent statistical processes, also known from QED. In contrast to that we can link the occurrence of staircase patterns to the gluon self interaction.

We use an analytical formalism to understand the jet radiation patterns in electron as well as proton colliders. These calculation we like to cross check against Monte Carlo simulations, which we know describe the data well. We are interested in how to translate our findings into the language of commonly used jet algorithms at the LHC.

The photon plus jets channel is one possibility to study the radiation patterns also in experimental data. It provides a clear standard model signature with a high cross section and is thus suitable even for early LHC studies. We identify kinematic regimes, where we expect either staircase or Poisson scaling to occur. The numbers we find translate to Z plus jets an important background to Higgs and new physics searches.

Once we understand the radiation pattern, e.g. the number of jets distribution we use it for physic searches. In Higgs searches the usual Weak Boson Fusion cuts drive the backgrounds into the Poisson regime, while the signal processes stay in the staircase regime. In this way we can understand the jet veto probabilities, which are important in those search strategies. In new physics searches we are interested in heavy colored states, which decay to dark matter candidates. These introduce two interesting new features: additional jets from their decay, as well as a mass scale ink the event. In understanding the jet radiation patterns we are able to get information about the color structure of those particles and we can restrict the uncertainties of other multi-jet observables, which are usually very notorious. The so called effective mass then yields the information about the mass scale. Thus we use these two observables to perform a log-likelihood analysis. As we use only very generic cuts the analysis automatically focuses on the interesting regions in phase space, giving information about the color structure and the mass scale.

Software

• Sherpa, MadGraph, Pythia, HepMC, Fastjet, Root

Publications

• Jets plus Missing Energy with an Autofocus. Christoph Englert, Tilman Plehn, Peter Schichtel, Steffen Schumann. Published in Phys.Rev. D83 (2011) 095009, e-Print: arXiv:1102.4615 [hep-ph]
• Establishing Jet Scaling Patterns with a Photon. Christoph Englert, Tilman Plehn, Peter Schichtel, Steffen Schumann. Published in JHEP 1202 (2012) 030, e-Print: arXiv:1108.5473 [hep-ph]

Riccardo Catena
Institut für Theoretische Physik
Universität Heidelberg

The dark matter (DM) direct detection technique has played a major role in the quest for the identification of the DM component of the Universe. The goal is measure the recoil energy due to the scatterings on a target material of the particles forming the Milky Way DM halo. The interpretation of a count rate or an annual modulation effect in a given experiment in terms of properties of DM particles depend on a number of assumptions implemented  in the analysis of the data. Besides issues regarding understanding the target material, such as nuclear form factors or whether channeling occurs, and the performance of the detector,  such as determining the energy threshold, the quenching factors and background rejection/contamination, there are uncertainties related to the DM signal itself. First of all there is an uncertainty in the normalization of the incident DM particle flux, which scales with the local halo density, often quoted to be unknown within a factor of 2 or so. Large uncertainties also affect the energy spectrum of the DM particles in the detector frame, in turn connected to their velocity distribution in the Galactic frame and to the proper motion of the Sun/Earth system. Concerning these, the vast majority of the analyses adopt a standard paradigm in which the velocity distribution is assumed to be Maxwell-Boltzmann  truncated to the value assumed for the escape velocity. The Maxwellian distribution is the configuration maximizing the entropy for a self-gravitating collisionless system and is associated to the spherical isothermal sphere density profile, which declines as r to the −2 at large radii and hence supports a flat rotation curve. It is a well-motivated form but unlikely a fair description of the Milky Way DM halo. In particular cosmological N-body simulations find that DM halos have density profiles falling more rapidly at large radii, as r to the −3, and velocity distributions showing significant departures from the Maxwell-Boltzmann shape.

Our project aims at reducing the present uncertainties in the local DM density and velocity distribution function (the product of which is defined as the DM phase-space density) in order to be able to more clearly interpret the results of a next generation of DM direct detection experiments. In a recent series of works, we presented a new determination of the local dark matter phase-space density. This result has been obtained implementing, in the limit of isotropic velocity distribution and spherical symmetry, Eddington’s inversion formula, which links univocally the DM distribution function to the DM density profile. This analyses required the implementation, within a Bayesian framework, of a Markov Chain Monte Carlo algorithm to sample mass models for the Milky Way against a broad and variegated sample of dynamical constraints. The Figure shows the mean dark matter phase-space density as a function of the detector rest frame velocity. The four curves correspond to our results (three colored lines) obtained for three different choices of DM profile and to the usual Maxwell-Boltzmann approximation (black line) associated with a local circular velocity equal to 220 km/s, local escape velocity equal to 544 km/s and local dark matter density equal to 0.3 GeV per cubic cm.


Software

• MPI, C, MKL

Publications

• R. Catena and P. Ullio; A novel determination of the local dark matter density; JCAP 1008 (2010) 004.
• R. Catena and P. Ullio; The local dark matter phase-space density and impact on WIMP direct detection; arXiv:1111.3556, to be published in JCAP.

Kay-Uwe Giering
Institut für Theoretische Physik
Universität Heidelberg

The two-dimensional repulsive Hubbard model is frequently considered as an effective low-energy model for cuprate high-temperature superconductors. We study this model with focus on Van Hove particle filling by means of the functional renormalisation group, in its formulation for irreducible vertices and in the level-two-truncation. Our main objective is to use the power of the recently devised exchange parametrisation for the interaction vertex to go beyond previous studies and investigate in detail the flowing self-energy and the frequency-dependent interaction vertex. Whereas the Fermi surface deformation due to electronic interactions remains small and has limited impact, the frequency-dependent self-energy can have drastic effects because of strongly reduced quasi-particle weights at the Van Hove points. We detect the same landscape of Fermi-liquid instabilities as in the stationary setup. In the region of competing ferromagnetic and d-wave pairing instability our results speak in favour of a quantum critical point. In this region we also extract a non-Fermi-liquid frequency dependence of the self-energy at the Van Hove point. Based on our discrete RG data we furthermore propose an effective functional form for the transfer frequency dependence with a much larger regime of validity than conventional Lorentz curves.

Software

• C, Scilab

Publications

• C. Husemann, K.-U. Giering, and M. Salmhofer; Frequency-dependent vertex functions of the (t, t') Hubbard model at weak coupling; Phys. Rev. B, 85:075121, 2012; online

Tarek Elsayed
Institut für Theoretische Physik
Universität Heidelberg

Boris Fine
Institut für Theoretische Physik
Universität Heidelberg

We study the correlation functions of different classical and quantum spin systems at high temperature. We aim to find effective numerical techniques to calculate the quantum correlation functions for system sizes not accessible to exact diagonalization techniques. Problems of diffusion and localization in disordered spin systems are being investigated in the course of this project.

Software

• C++, MKL, Eigen

Astrid de Wijn
Institute for Molecules and Materials
Radboud University Nijmegen, The Netherlands

Boris Fine
Institut für Theoretische Physik
Universität Heidelberg

We are studying numerically the chaotic dynamics of systems consisting of large systems of interacting classical spins through their Lyapunov spectra and Kolmogorov-Sinai entropy.  We are particularly interested in the behaviour of systems with coupling parameters close integrable cases and in the behaviour near phase transitions.

Conrad Albrecht
Institut für Theoretische Physik
Universität Heidelberg

Due to P.W. Anderson's famous paper Phys. Rev. 109, 1492 (1958), the impact of disorder on quantum systems has attracted theoretical physicist's attention and a lot of effort has been spent on gaining insight to the phenomenon of localization. Apart from rigorous statements or completeness Anderson localization may be scetched as follows: Investigating a single quantum particle subjected to a (sufficiently long) wire exhibiting a random potential, its probability to reach the wire's ends practically vanishes due to the exponential decay of the corresponding wave function. This observation is in total contrast to Bloch's theorem one would arrive at when dropping the disordered potential.

Nowadays, experiments with ultra-cold atoms (e.g. Nature 453, 891 (2008)) allow for the direct access of the quantum system's probability distribution in a setup with easily tunable model parameters, and therefore, a rich playground has opened up to address issues on localization. Topics that we focus on especially deal with the crossover from disorder to order, i.e. we study the behavior of localized quantum states when successively introducing correlation into the noisy environment up to the emergence of Bloch states. The figure shows an energy spectrum (vertical axis) when tuning a quantum system from disorder to order (horizontal axis, left to right). Moreover, our research aims on the understanding of quantum localization in many-particle systems. Since Anderson localization was originally formulated for the single particle setup, questions towards the impact of interaction on localization is a challenging subject.

Besides conceptional considerations to detect multi-particle localization, we employ (exact) numerical diagonalization of the (time-independent) Hamiltonian to gain access to the energy spectrum as well as corresponding wave functions. Therefore extensive usage of computer resources are necessary and we greatfully acknowledge support by the bwGRiD. Due to the numerical approach we are independent from perturbative methods which enables us to investigate the problem ab initio.

Link

• http://www.thphys.uni-heidelberg.de/~albrecht/

Software

• LAPACK

Publications

• C. Albrecht and S. Wimberger; Induced delocalization by correlation and interaction in the one-dimensional Anderson model; Phys. Rev. B 85, 045107 (2012)

Alexander Schug
Steinbuch Centre for Computing
Karlsruhe Institute of Technology (KIT), Germany

Abhinav Verma
Steinbuch Centre for Computing
Karlsruhe Institute of Technology (KIT), Germany

Benjamin Lutz
Steinbuch Centre for Computing
Karlsruhe Institute of Technology (KIT), Germany

Our research aims at quantitatively understanding the structural and dynamical mechanism of biomolecular folding and function in computer simulations. We focus on systems of high biological relevance in the context of genetic regulation, among them Two-Component Signal Transduction Systems and regulatory ncRNA like Riboswitches. A typical challenge in these simulations is reaching sufficiently long timescales while maintaining a realistic description of the system. We therefore develop novel computational tools in a multi-scaling approach and combine efficient sampling techniques at a coarse-grained level of description with computationally more expensive detailed models.

Links

http://www.scc.kit.edu/personen/jrg-mbs.php

Software

• GROMACS, python

Publications

in progress