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Topic 1: "Fundamental Particles and Forces"

Introduction to the topic

 

Elementary particle physics

Research in this topic aims at an understanding of the fundamental building blocks of matter and their interactions which, over the past decades, have been extremely successfully described by the Standard Model (SM) of elementary particle physics. While the recent spectacular discovery of a Higgs boson is a confirmation of the Higgs mechanism and the associated electroweak scale as predicted by the SM, this model is nevertheless known to be incomplete. Since it does not include gravity, it describes only three of the known four fundamental interactions and must break down at the Planck scale, where the gravitational interaction becomes comparable in strength to the other three interactions. The fact that two such vastly different scales as the Planck scale and the weak scale coexist in nature is highly problematic. Quantum corrections should lead to the destabilisation of the weak scale; that this does not appear to occur defies any explanation within the SM. New physics — physics effects beyond the SM — at the TeV scale could stabilise this huge hierarchy of scales.

 

The SM also fails to explain other physical observations. For example, the mechanisms of mass generation and of electroweak symmetry breaking are not yet understood — important insights are expected from the detailed investigation of the recently discovered Higgs particle. The SM does not provide a candidate particle to account for Dark Matter, which represents about 25% of of the total energy density of the Universe. The quest for Dark Matter is addressed both by the programme topics Fundamental Particles and Forces and Matter and Radiation from the Universe using complementary approaches. Furthermore, the nature of the force that results in the constantly accelerated expansion of the Universe, "dark energy", remains without explanation in the SM. The same is true for the observed matter-antimatter asymmetry in the Universe. Answering these and other questions will require New Physics. Such New Physics might eventually also lead to a "grand unification" of interactions.

 

This programme Matter and the Universe addresses the questions raised above, which are truly the most fundamental questions of physics research today

 

The major experimental tools that particle physicists will use in this research are accelerators such as the Large Hadron Collider (LHC) at CERN, SuperKEKB at KEK or the planned International Linear e+e Collider (ILC). Operating at the highest possible energies or the highest luminosity, these machines resolve the structures of matter at the smallest attainable scales. They are capable of creating energy densities comparable to those in the early Universe, thus recreating the short-lived particles and uncovering the forces and symmetries that existed in the earliest Universe. Accordingly, accelerator-based particle physics simultaneously probes the structure of matter and it provides a means to understand the large-scale structure of the cosmos, which carries the fingerprints of its origin, the big bang.

 

 

The special role of the LHC

Recently, progress in accelerator-based particle physics has been dominated by the LHC. Today the LHC defines the forefront of research in elementary particle physics. Its location in CERN, the European particle physics laboratory, ensures Europe's leading role in particle physics; however it is a truly global endeavour driven by a community of thousands of physicists world-wide. With a centre-of-mass energy of 8 TeV reached during its first phase of operation, the LHC has given access to previously uncharted territory and has explored physics at the energy scale of electroweak symmetry breaking.

 

The discovery of a Higgs particle at CERN in 2012 with the ATLAS and CMS experiments has resulted in the award of the Nobel prize in 2013 to Englert and Higgs and initiated a new era of particle physics. The Higgs boson has been the object of theoretical and experimental study for almost half a century. Within the framework of the SM, all its couplings and properties are entirely predicted. These properties now need to be measured accurately — an experimental programme that has only just begun. The LHC has also provided first glimpses of the TeV scale, which will soon be explored more deeply when the centre-of-mass energy is increased to about 14 TeV. New Physics may well emerge directly as resonances in the observed mass distributions. Alternatively, it can show up in deviations between measurements and precise theory predictions based on the SM. The performance of the experiments has already met or exceeded expectations in performing precision SM measurements and in disentangling the complexity of the final state at a hadron collider.

 

The ATLAS and CMS experiments also contribute competitively to the investigation of heavyion physics and of matter at the highest densities — "extreme matter", which is also the focus of the dedicated experiment ALICE. In this context, the topic Fundamental Particles and Forces connects to the topic Cosmic Matter in the Laboratory, an aspect that has been specifically addressed in the cross-topic activities.

 

 

Particle physics strategy beyond the LHC

Current results from the LHC and extrapolations of its future performance indicate physics topics that require further study at other facilities, especially at lepton colliders with their inherently superior measurement precision. In this sense, the Belle II experiment at SuperKEKB, to which DESY contributes significantly is an important component of the experimental programme and a complement to the LHC experiments. Similarly, precision is also the compelling reason for the long-standing effort towards the construction of a linear e+e collider. The discovery of a Higgs particle at a mass of 126 GeV, precisely the region in which the pattern of Higgs decay modes is most complex, has further strengthened the physics case for such a collider. Consequently, a clear consensus on the future directions of the field of elementary particle physics has emerged. German particle physicists updated their roadmap in November 2012, emphasising the necessity to fully exploit the LHC and expressing enthusiastic support for the idea of constructing the ILC in Japan. In addition a strong engagement in flavour physics in Japan at SuperKEKB is favoured. Recently, the CERN Council, representing European particle physics, has adopted the updated European roadmap for particle physics. The roadmap emphasises the need for operation and full exploitation of the LHC, including its upgrades until around 2030, encourages vigorous R&D for future colliders at the highest-energy frontier and welcomes the Japanese initiative for an e+e linear collider. The recent "Snowmass" process, part of the ongoing strategic process in the USA, has also given strong support to the ILC. This is supposed to conclude by mid 2014.

 

 

DESY and its role in particle physics

DESY physicists are active in all aspects of the LHC physics programme: they are engaged in the design and construction of the experiments, in the operation of the accelerator and the detectors and in the analysis and publication of the data. They explore viable physics models and interpret the data. They bring to bear DESY's tremendous experience in building and running large-scale facilities and its competence in large-scale analysis endeavours. The theory group provides the necessary precision calculations for interpreting the experimental data, explores the theory space of new models and guides the experimenters at DESY and elsewhere. DESY forms a knowledge springboard for other national and also international efforts.

 

This role is exemplified by the Helmholtz Alliance Physics at the Terascale: the Alliance — a network of DESY, KIT, the MPI for Physics and 18 German universities working in the field of elementary particle physics — proved vital both in defining a common strategy for all German institutes and in optimally combining the complementary expertise of Helmholtz centres and university institutes. The close collaboration of physicists from different institutes, different experiments and between experiment and theory significantly increased the visibility of German research and its impact internationally.

 

An integral part of the Alliance is the Analysis Centre at DESY — a platform for education, training and exchange. The Analysis Centre was instrumental in the creation of topical working groups and the organisation of an intense training and workshop programme of the Alliance, attracting many hundred participants at all career levels every year. Physicists from all Alliance institutes — notably from the theory community — contributed their knowledge to these events, making the programme one of the biggest successes of the Alliance.

 

The prominent role of DESY is enhanced by the yearly summer student programme, which brings some hundred young researchers to the laboratory to acquaint them with cutting-edge research. About 60 students each year engage in elementary particle physics. Many of these students — selected from about 500 applications from all over the world — continue their careers in science and often refer to their experience and to the contacts established in the laboratory. Similarly, the approximately 100 Ph.D. students concurrently working at the laboratory contribute substantially to the pool of excellent young scientists from which the field selects its future staff. Finally, the two renowned and extremely competitive DESY fellowship programmes in experimental and theoretical physics attract leading physicists world-wide. To give a quantitative indication of the competitiveness: of the roughly 350 applications per year to the theory fellowship programme only about 10 candidates are selected. The impact of the DESY theory group is underlined by the fact that currently, more than 50% of all permanent staff in theoretical particle physics at German universities had at some time been a Ph.D. or a fellow in DESY's theory department.

 

In addition to its function of promoting the best young people for the German and international community, DESY is a key partner in numerous collaborations and networks. The availability of a wide range of facilities, including test beams, computing, etc. and the other characteristics mentioned above mean that DESY is clearly the central hub for particle physics in Germany and a leading institute on an international scale.

 

 

DESY as a Helmholtz centre

DESY is internationally renowned for successfully operating a host of large facilities: formerly these included DORIS, PETRA and HERA, as well as the current FLASH facility, launched as the TESLA test facility. Soon the European X-ray Free Electron Laser (European XFEL) will go into operation as a photon user facility. DESY also provides the infrastructure for large-scale detector developments and construction. Test beams are a tremendous asset and are in high demand for detector development. Furthermore DESY significantly contributes to developing the computing models and tools for physics analyses. The demands of the LHC physics analyses in terms of CPU and storage are enormous, and the provision of Tier-1 (GridKa at KIT) as well as of Tier-2 facilities and the National Analysis Facility (NAF) (DESY Grid and Cloud Centre) are central to the success of German data analyses at the LHC.

 

The provision of project-independent funds for small and medium-size experiments originally not embedded in the larger project landscape provides the necessary flexibility to react to new developments and exciting innovative ideas in an exploratory fashion. Such projects are an excellent opportunity to fully exploit the existing infrastructure and expertise at DESY without large upfront investments. Examples for projects from the previous funding period are OLYMPUS and Belle. The latter is a long-term project that has now been integrated into the topic Fundamental Particles and Forces. ALPS-II is a project that is planned to be supported by project-independent funding during this period. It profits from the availability of the large superconducting HERA dipole magnets which enable high-sensitivity experiments at moderate project cost.

 

Given the infrastructure and the physical and engineering talents in the Helmholtz centres, the German particle physics community is well positioned to address the fundamental questions of the field. Their long-standing engagement and scientific successes of German physicists have placed them at the forefront of research, resulting in a strong position to shape the strategy of the field. Given the nature of the research in Fundamental Particles and Forces, such engagements are large and typically long term. DESY and KIT assume a vital role in rendering this successful engagement sustainable, a success that is founded on sharing of responsibilities between universities and Helmholtz centres.