Directions beyond the Standard Model

25 February 1999

The annual theory workshop at the German DESY laboratory in Hamburg traditionally focuses on a burning physics issue. The latest event, exploring “Directions beyond the Standard Model”, tried to peer beyond the physics horizon.


While the Standard Model (SM) of strong, weak and electromagnetic interactions is healthy and keeps surviving experimental tests, motivations to go beyond it are primarily based on theoretical considerations in the quest for a unified theory of all fundamental forces. Leading candidates at the moment are supersymmetric field theories and string theories.

On the experimental side, traditional high-energy physics experiments have tested the SM to a high degree of precision and strongly constrain many new theoretical ideas. However, the 1998 harvest of new results on atmospheric neutrinos, together with the accumulated data on solar particles, might provide the first sign of physics beyond the Standard Model. Other more indirect arguments for physics beyond the SM might come from cosmological considerations, like the question of the baryon asymmetry or the nature of dark matter in the universe.

The status of the “Search for new particles at high-energy colliders” was covered by P Zerwas of DESY. Most notably the searches at LEP at CERN and the Tevatron at Fermilab have pushed the mass threshold for new particles to higher and higher values. He then went on to explain how CERN’s LHC proton collider or, looking further ahead, an electron­positron linear collider (TESLA, NLC, JLC) might help shed new light on the physics beyond the Standard Model. This will not only give information about particle spectroscopy, but will also allow us to study the Higgs mechanism and distinguish between specific models. The design parameters of such a linear collider might play a crucial role for these future precision experiments.



S Pokorski of Warsaw reviewed the supersymmetric extension of the Standard Model and its comparison with electroweak precision experiments. While many of the superpartners have to be quite heavy, the prediction of a light Higgs boson in the supersymmetric models seems to be consistent with available data.

On the theoretical side, string theory (a generalization of quantum field theory to one-dimensionally extended objects) has been revolutionized in the past few years. Various unexpected symmetries, called “dualities”, relate different string theories to each other. This leads to the conjecture that all string theories are interconnected via dualities, and unified in what is called M-theory. Apart from strings, such theories would then also contain higher dimensional objects (“p-branes”), as was explained by S Theisen (LMU Munich).

P Mayr from CERN reported on “Insights into field theory from string theory”. He explained how many results in quantum field theory can be derived by embedding these theories in higher dimensional string theories. Parameters in field theory, like gauge coupling constants, then become geometric objects with extra dimensions in string theory. S Yankielowicz (Tel Aviv) explained the origins of the new Maldacena-conjecture which suggests a deep conceptual connection between string and field theory.

Black holes

H Verlinde of Amsterdam linked these results to progress in the understanding of black holes and quantum gravity, including the celebrated “holographic principle”, originally introduced in an attempt to unify general relativity and quantum mechanics.

That such developments could have an impact on possible generalizations of the SM was emphasized by I Antoniadis (Paris) and L Ibanez (Madrid). “New aspects in string phenomenology” appear as a result of string dualities, including some results concerning non-perturbative aspects in the effective low-energy supergravity theories, as Ibanez explained. Antoniadis concentrated on currently popular models where the string scale (usually identified with the Planck scale, 1018 GeV) is lowered to the TeV range. Such models would be tested at future collider experiments, as well as experiments probing the structure of gravity in the sub-millimetre range, which could reveal deviations from Newtonian gravitation at distances smaller than a millimetre.

Generalizations of the Standard Model often link to cosmology and astrophysics, and observations in these fields would then test particle physics theories. M Drees (Sao Paulo) concentrated on supersymmetric candidates for cold dark matter and the experimental efforts to find signals in direct detection of so-called WIMPs (Weakly Interacting Massive Particles). Other “Astronomical probes of new physics”, like ultra-high-energy cosmic rays, the cosmological constant and density perturbations in the cosmic microwave background, were discussed by S Sarkar of Oxford.

C Wagner (CERN) described the possible generation of a baryon asymmetry of the universe during the electroweak phase transition. Such a mechanism (not possible in the SM) might be operative in the MSSM with a relatively light Higgs boson. Q Shafi (Delaware) considered models of an inflationary universe and its predictions for the large-scale structure of the universe to test the nature of nonbaryonic dark matter, including neutrinos. This brought the focus back to neutrino oscillations. The implications of the new experimental results on grand unified theories were analysed by T Yanagida of Tokyo.

The final talk came from M Koshiba (Tokyo) who recounted the landmark history of Kamiokande and Super-Kamiokande and looked forward to new insights in particle physics and neutrino astronomy.

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