Central themes in the collider phenomenology group at DESY are studies of physics at present and future colliders, in particular the LHC and a future electron-positron linear collider, in the quest to uncover the underlying physics responsible for electroweak symmetry breaking and for stabilising the huge hierarchy between the weak scale and the Planck scale. Models of physics beyond the Standard Model (SM) often also predict the existence of possible candidates for cold dark matter in the Universe. Information from HERA and other colliders as well as from electroweak precision physics, the quark and lepton flavour sectors and from cosmology provides important input for physics analyses at the LHC.
The correct identification of signals of new physics at the LHC will require accurate theoretical predictions for both signal and background processes. Theoretical work at DESY is carried out to obtain precise predictions for relevant processes at the LHC, involving quantum effects of the strong and the electroweak interaction.
An intense interplay between theory and experiment will be necessary to reveal the nature of physics at the TeV scale. The collider physics group at DESY provides theory support for the Analysis Centre of the Terascale Alliance and keeps close contact with the experimentalist community.
Physics of electroweak symmetry breaking
In order to explore the mechanism of electroweak symmetry breaking (EWSB), one must in particular determine whether one or more Higgs bosons exist in nature, and—if this is the case—whether the properties of the new state(s) agree with model predictions. Now that a Higgs-like signal has been observed at the LHC the task is to establish whether the particle is indeed the relic of the EWSB mechanism, and whether it has the properties predicted by the SM or extensions of it. Practically all theories of physics beyond the SM predict either an extended Higgs sector with more physical states or new physics affecting electroweak symmetry breaking. This may imply that a SM-like Higgs boson is supplemented with further Higgs states or that Higgs phenomenology differs drastically from the SM case. If no clear signal of a particle with properties consistent with a Higgs boson emerges, this implies far-reaching consequences about the origin of EWSB.
The DESY theory group is pursuing work in deriving precise predictions for possible manifestations of the EWSB mechanism at the LHC. In particular, predictions for Higgs boson masses and couplings, Higgs production processes, decay rates and transverse momentum distributions are studied in different models. Furthermore the phenomenology of more exotic models of new physics is investigated, and the interplay of the Higgs sector with other sectors of the new physics scenarios is analysed. Scenarios where no Higgs candiadate would be detected in the early LHC data are also systematically studied.
The investigations of non-standard sectors of EWSB comprise in particular supersymmetric models, strongly interacting models, Little Higgs models, so-called Moose Models, and model-independent search strategies such as vector boson scattering at colliders. Little Higgs models are an extension of global and gauge symmetries of the SM which allow a weakly coupled model at the TeV scale, where new strong interactions do not set in below the scale of 10 TeV. Here, model building aspects, constraints and limits from past and present experimental data, as well as collider searches at the LHC and a possible future lepton collider are studied.
The experimental results obtained at the LHC will we exploited to constrain the possible structure of the EWSB sector. In this context, improved methods for determining the properties of possible Higgs states at the LHC will be developed, and the impact of uncertainties from the theoretical (on signal and background processes) as well as from the the experimental (statistical and systematic) side will be analysed. Based on those results it will be investigated how signals of different kinds of new physics can be distinguished by making optimal use of LHC data as well as input from precision measurements at lower energies, measurements in the flavour sector and cosmological data.
Quantum chromodynamics is the part of the Standard Model that describes the strong force. Strong interaction dynamics in high-energy collisions always involves a wide range of scales, from the collision energy down to the scale where quarks and gluons are confined within hadrons. Theoretical control over the associated phenomena is a prerequisite for understanding most reactions at colliders. The concepts of factorization and of effective field theories provide keys for separating the dynamics at different scales, from the perturbative to the non-perturbative domain.
The production of hadronic jets is one of the most basic processes at hadron colliders, and initial- and final-state QCD radiation provide important corrections to low-order perturbation theory in many processes of interest. Effective field theory methods can be used for the systematic resummation of the most important corrections to all orders in perturbation theory, as well as for the study of nonperturbative effects.
In proton-proton collisions at high energies, several partons in one proton can undergo a hard scattering with partons in the other proton, each hard scatter producing particles with large mass or transverse momentum. The effects of such multiparton interactions average out in sufficiently inclusive observables but can have important consequences for the details of the final state. Their theoretical description raises a number of interesting questions, ranging from conceptual issues of factorization to nontrivial aspects of hadron structure and to the phenomenology of pp collisions.
Monte-Carlo event generators have become essential tools in high-energy physics. They provide fully exclusive predictions for the final state, incorporating QCD effects at all relevant energy scales. The group is involved in the systematic study and improvement of the theoretical precision of Monte-Carlo generators. One aspect is the development of the GENEVA Monte-Carlo framework, which combines the theoretical control and precision of higher-order resummed predictions with the versatility of Monte-Carlo generators.
Physics beyond the Standard Model
TeV scale physics is expected to shed light on the mechanism responsible for stabilising the huge hierarchy between the weak scale and the Planck scale. Supersymmetry (SUSY), which is the most thoroughly studied extension of the SM, can manifest itself at the LHC in many different ways. Other scenarios of physics beyond the SM, such as the presence of extra dimensions of space, which is related to string theory and motivated for instance by "fine-tuning" and "little hierarchy" problems of conventional supersymmetric models, can give rise to even more exotic signatures at the TeV scale. The discovery of a fermion-boson symmetry or of extra spatial dimensions would clearly revolutionise the current picture of the fundamental principles of nature. A close collaboration between theory and experiment will be necessary to identify the underlying physics, taking into account the possibility that new physics could manifest itself in unexpected ways that go beyond our present imagination.
Physics studies of new phenomena at the LHC have mainly concentrated in the past on establishing a non-SM like signal. However, this will only be a first step, leading to the more involved task of identifying the nature of the new physics. Extracting properties of new states, for instance couplings, spin and CP properties, from the LHC data will often require certain theoretical assumptions concerning the properties of some of the new states, their mass hierarchies, etc. One of the research topics in the DESY theory group is to improve the theoretical predictions in different scenarios of new physics and to develop strategies for determining the underlying structure of new physics with as little model dependence as possible.
Within specific models, on the other hand, global fits are performed making use of all experimental data from colliders, electroweak precision measurements, flavour physics and cosmology. In this way it is possible to assess how well different model describe the data and to identify which parameter regions of the models are preferred or excluded.
Results obtained at the LHC are incorporated as they arise. The ultimate goal of these analyses will be to establish a certain scenario of new physics, to decipher its underlying structure (in the case of SUSY, for instance, to determine the SUSY-breaking mechanism) and to reveal the possible nature of candidates for dark matter in the Universe.
Particular classes of models studied in the DESY theory group are for instance supersymmetric Grand Unified Theories based on exceptional gauge symmetries, broken down at the GUT scale both by Higgs mechanisms as well as orbifold boundary conditions in higher dimensions. This makes close contact to the field of string theory phenomenology by investigating embeddings into the heterotic string setup or F theory. Furthermore, the parameter space of these models and collider implications are being studied.
Another branch of activities of the DESY theory group is to provide theoretical predictions in different models of new physics in the form of tools that can be used by theorists for phenomenological studies and by experimentalists for their analyses of the data. In this context the DESY theory group is also involved in the active development of all kinds of facets of Monte Carlo event generators for collider experiments, in particular in the context of the WHIZARD generator. Topics studied include BSM setups, NLO development, multi-leg matrix elements, parton showers, efficient phase space algorithms and extend to more IT-based research like massive parallel computing and virtual machines.
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