Neutrino oscillations refer to a natural phenomenon in which a beam of neutrinos of one flavor can switch to another flavor. For example, a source of muon neutrinos might swing into being electron neutrinos. If there is a probability of oscillating, the flavor mix will depend on distance from the source. Theories of the neutrino extending beyond the standard electroweak theory almost always predict a neutrino mass. Neutrino oscillations are by far the most sensitive probe for neutrino mass and could be relevant for particle physics, astrophysics, and cosmology. Oscillations would, for example, completely alter the interpretation of the solar neutrino experiments, and neutrino masses would be an important component of the mass of the universe.

   Clearly, then, there is a lot at stake in #8, and the lack of confirmation elsewhere increases the importance of this short paper, which reports the defection of 51 antineutrinos from muon flavor to electron flavor. The flight path from source to detector is 30m. Following an upgrade and further data acquisition, the LSND apparatus has now recorded 83 ± 24 muon to electron events above the background. They have seen decays at rest and in flight, and have used two different targets. The implied neutrino mass is 0.4 eV. The latest news from Karmen is that conflict with LSND is looming!

   Another hot topic on the list is represented by the string trio at #5, #7 and #10. The high priests of superstring theory incant strange words, such as D-branes, p-branes, or M-theory, as they puzzle out how physics might work in up to 11 dimensions. Their goal is to reconcile quantum physics with general relativity. This fusion through superstring theory of the two great frameworks of 20th century physics occurs at the Planck length, which is 10-³³ cm. A D-brane is the topological entity on which a string is allowed to end.

   For Science Watch, Michael R. Douglas, Rutgers University, Piscataway, New Jersey, describes the highlights of #5. "Our basic result is that the D-branes really are substringy, and that their interactions have a different character at short distances, on length scales corresponding to the 11-dimensional Planck scale. They are described entirely by quantum effects of gauge theories, similar to the gauge theories of the Standard Model. This includes their gravitational interactions: gravity at short distances is a quantum effect of gauge theory. Special cases were discovered in which these quantum effects agree precisely with general relativity. However, in other cases they disagree.

   "At present the greatest impact of the paper has been through the conjectures it led to for complete definitions of M theory in certain regimes, such the Matrix theory proposal in your Hot Paper #1. The current thrust is to identify situations in which the quantum effects of the gauge theory can exactly agree with conventional general relativity, allowing numerous and fruitful comparisons between these very different formalisms. These conjectures and the general idea that gravity is ultimately a quantum effect are now the focus of intensive research. Many physicists believe that mysteries such as the information loss paradox of black hole evaporation will soon be understood along these lines."

   Concerning black hole evaporation, old timer #3 has been with us over a year, logging 176 citations. In a demonstration that involves wrapping several different types of string soliton around a compactified space to produce a heavy localized object, Strominger and Vafa produced an interpretation of the entropy of extremal black holes. The amazing thing was to produce from string theory a result on Bekenstein-Hawking entropy already known from completely independent results on the area of the horizon of extremal black holes. (A black hole is extremal if it cannot degrade to a lower-energy state.)

   Spurred on by #3, Curtis Callan and Juan Maldacena, Princeton University, New Jersey, looked at not-quite-extremal black holes. As Callan notes, "We were able to produce a mechanism for Hawking radiation: the extremal black hole has zero Hawking temperature and no radiation, while the non-extremal one has temperature and radiates. The radiation occurred because string excitations could collide. Then via standard string theory rules, the excitations create a graviton (say) that is no longer bound to the solitons. We found that the energy distribution of the emitted particles was thermal with the correct Hawking temperature." More recent work has found many cases of agreement between this mechanistic string theory picture of Hawking radiation and what was inferred by classical arguments from general relativity.

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