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Collider PhenomenologyThe central themes in the collider phenomenology group at DESY are studies of physics at HERA, at the LHC and at a future electron-positron linear collider, the connection to cosmology and also the embedding of the Standard Model into grand unification and supersymmetry. The collider phenomenology group provides theory support for the Analysis Centre of the Terascale Alliance and keeps close contact with the experimentalist community. LHC physicsWhile the high energy reach of the LHC will allow the exploration of TeV scale physics, the LHC experiments are significantly more complex than any previous particle physics experiment. Identifying the nature of physics at the TeV scale will require intense collaborative efforts between experimentalists and theorists. The correct identification of signals of new physics at the LHC requires precise theoretical predictions for both signal and background processes. While quantum effects of the strong interaction have been intensively studied for signal processes in some beyond the Standard Model (BSM) scenarios, electroweak quantum corrections can also be very important. Here at DESY we are working on the computation of precise predictions involving both types of corrections, the estimation of the remaining theoretical uncertainties for the relevant processes and the modelling of both new physics and Standard Model (SM) contributions. In the first phase of LHC data taking, the primary goal will be to understand SM physics before any discoveries of new effects can be made. Precise preditions will be worked out for processes that will ultimately be used as "standard candles" for the luminosity determination at the LHC as well as for processes that can serve to determine the jet energy scale and, at a later stage, to measure the parton distribution functions directly at the LHC. For these first phase, the knowledge of the structure of hadrons gained at HERA will play a key role. Structure and interactions of hadronsQuantum chromodynamics is the part of the Standard Model describing the strong force. An outstanding task is to understand the relation between the observed physical states, hadrons, and the fundamental degrees of freedom in the theory, quarks and gluons. As a characteristic feature of a strongly bound field theory, this relationship is of interest in its own right. Moreover, theoretical control over the associated phenomena is a prerequisite for understanding most reactions at high-energy colliders. The concept of factorization provides a key for separating short-distance dynamics (which can be evaluated in perturbation theory) from long-distance physics (which needs to be addressed by non-perturbative methods or to be extracted from experiment). Detailed studies of the structure and interactions of hadrons have in particular been possible in electron-proton collisions at HERA. To transfer their results to proton-proton collisions at LHC is an ongoing enterprise, addressed in a dedicated workshop series. Generalized parton distributionsA wealth of aspects in the study of hadron structure can be described and interconnected using the framework of generalized parton distributions. A topic of special interest is the spatial distribution of quarks and gluons inside a hadron, and its correlation with the longitudinal parton momentum. Generalized parton distributions also encode rather unique information about spin structure, in particular about orbital angular momentum and about correlations between polarization and the spatial distribution of partons. Recent activities in the theory group have addressed several theoretical and phenomenological questions, ranging from the exclusive scattering processes where generalized parton distributions can be measured to the physical interpretation of the distributions and to their calculation in lattice QCD. Transverse momentum of partonsA basic degree of freedom of partons is their transverse momentum inside a hadron—an aspect that is in particular quantified by unintegrated parton distributions and fragmentation functions, which explicitly depend on this kinematic variable. How exactly factorization works when transverse parton momentum is retained is an active area of research, given that the importance and intricacies of gluon exchange have only been fully realized in the past few years. A variety of polarization phenomena can be explored in this context, with time-reversal-odd observables playing a special role because they exhibit non-trivial features already at low orders in perturbation theory. Recent work in the group has focused on a systematic description of transverse momentum dependence for semi-inclusive lepton-nucleon scattering and for related processes in electron-positron annihilation or in hadron-hadron collisions. Electroweak symmetry breakingA particular focus of the investigations at the LHC will be on possible manifestations of the electroweak-symmetry breaking mechanism, This involves predictions for relevant observables that probe the Higgs sector of the SM and its extensions and that could ultimately be used to distinguish between the different mechanisms. Particular importance in this context are the theoretical predictions for the Higgs boson production processes, decay rates and transverse momentum distributions, including strong and electroweak corrections and CP violating effects. We plan to study also the phenomenology of more exotic models of new physics and the interplay of the Higgs sector with other sector of the new physics scenarios. Flavour physicsTheoretical work done at DESY has also played a key role for the determination of the CKM matrix, through state of the art predictions of higher order contributions to flavour changing neutral current processes (rare B-decays, neutral meson mixings). Regarding BSM physics, the scenario of minimal flavour violation was studied. Flavour data bring complementary informations, which can strongly constrain BSM physics. Selected publications:
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