The 1999 DESY Theory Workshop concentrated on the growing symbiosis between particle physics (particularly neutrinos) and cosmology.
The latest round of the traditional annual theory workshop at DESY focused on the interplay between particle physics and cosmology, a theme of increasing interest to specialists in both domains. Two experiments and observations highlighted topical interest: Y Totsuka reported the strong case for the observation of neutrino (n) oscillations at the Superkamiokande underground experiment in Japan, while B Leibundgut overviewed the recent results on the “Hubble diagram” for 1a type supernovae, which point towards a cosmological constant (L).
Both topics are both very immediate and fundamental – they affect the question of what the universe is made of, the possible need for modifications of standard cosmology, the generation of masses and scales in grand unified field theories or even deeper theories, and the issue of hidden generation symmetries that may explain the observed patterns of elementary particle masses. And both issues are not yet understood.
Results that began to emerge in 1998 make it clear that not all muon-neutrinos produced in the atmosphere arrive in underground detectors – in transit they appear to switch to another type of neutrino. Superkamiokande has reported a dependence of this neutrino “extinction” on the path length between production and detection. Such a dependence is characteristic of neutrino oscillations and implies that such neutrinos must have mass.
At the DESY meeting, theorists tried to understand the resulting neutrino mass and mixing implications. The debate centred on a consistent determination of the neutrino masses and mixing angles from the various experimental observations. New effects and experiments were proposed to resolve the issue.
On the more theoretical side, most specialists agreed that we understand why the neutrino masses are so much smaller than the electron or quark masses. In the standard model, renormalizability forbids neutrino masses – having no mass is consistent with the gauge symmetries. To introduce masses means going beyond the standard model. After years of searching for “physics beyond the standard model”, it has now arrived.
Grand unification
Neutrino masses are most likely due to the violation of lepton conservation in an extension of the standard model, which could happen in the vicinity of the remote grand unification scale. The results are effective non renormalizable couplings between neutrinos and Higgs scalars. The Higgs mechanism then relates a typical neutrino mass to the Fermi scale – a mere fraction of an electronvolt.
A concrete manifestation of these general aspects is the “see-saw mechanism”, in which the non-renormalizable interaction is generated by the exchange of superheavy singlet neutrinos. Another is the induced vacuum expectation value of a superheavy scalar triplet.
The need for neutrino masses thus gives direct experimental evidence that the standard model needs to be extended, hinting towards grand unification or similar ideas. Even though less spectacular, the theoretical implications of the neutrino oscillations may turn out to be of comparable importance to proton decay – a process long thought to be inevitable but yet to be observed.
Particularly intriguing are the possible consequences of these effects for the generation of matter asymmetry in the early universe. Our very existence demands processes that produce more matter than antimatter.
The precise mass and mixing pattern for the neutrinos is not well understood. It may be as rich as for the quarks, but with a completely different generation structure. Why is the mixing angle for the muon neutrino maximal? Generation symmetries and their spontaneous breaking are the prominent candidates for possible explanations of the mass patterns.
Hubble diagrams
The other basic question debated at the workshop hinged on basic cosmology. A “Hubble diagram” of brightness versus redshift (related to velocity of recession) of very distant type 1a supernovae suggests that the expansion of the universe is accelerating. This could be the effect of a cosmological constant (L), proposed long ago by Einstein. Some doubts remain in the interpretation of the data. The main uncertainty is the lack of understanding of how the average brightness of supernovae has evolved. These are explosions that happened in quite early stages of the evolution of the universe.
On the other hand, K Gorski and J Silk compared the anisotropy of the cosmic microwave background radiation with structure formation in the universe. This indicates that the background energy density seems to participate in the formation of the structure. There seems to be some homogeneous component, and the cosmological constant would be a candidate.
At DESY, specialists admitted to being quite puzzled by these findings. Theorists still have no good explanation of why the cosmological constant should vanish, and even less why it should have a tiny non-zero value. The energy density in radiation or matter decreases with the second inverse power of time, so a true constant L that influences today’s evolution of the universe would have been completely negligible at early stages of the universe.
A significant role for L “today” – and neither earlier nor later in the history of the universe – seems to require an unacceptable matching or “fine-tuning” of numbers. Some say that the “natural guess” for the value of L is off by 120 orders of magnitude – probably the worst failure of an educated guess ever made. Already Einstein was worried about his constant, and we still are.
Popular alternatives are models with a cosmological evolution of a scalar field, often named “quintessence” today. In these models the homogeneous part of the energy density varies with time in such a way that it is relevant today.
In a class of these models, “cosmic attractor” solutions avoid the fine-tuning problem. And some of them mimic the effects of a cosmological constant on the supernovae Hubble diagram. One of these models could finally lead to a cosmology consistent with observation. Nevertheless, a satisfactory explanation from fundamental particle physics or string theories is still missing. Much remains to be done and understood.
