In paper #2, Steven Gubser, Igor Klebanov, and Alexander Polyakov establish a duality between gauge theory in four dimensions and string theory in five dimensions. Duality expresses the idea that two superficially different theories can have the same physical content. Gauge theory is a special case of quantum field theory (QFT), where pointlike particles are treated as excitations of some field. Gauge theories have special symmetries which enforce the conservation of physical quantities like electric charge. Unfortunately there is no place for gravity in QFT. However, by giving up the pointlike description of particles and imagining instead that the fundamental objects are one-dimensional extended objects, we arrive at string theory.

All string theories include a massless particle of spin two, which generates gravitational interactions via its coupling to all other string states. So an advantage of string theory over QFT is that it includes gravity. The relation between gauge theories and string theories has fascinated theorists for over three decades. String theory itself was invented to describe the strong interactions but it was later realized that the gauge theory called quantum chromodynamics (QCD) was better suited for the high-energy behavior of the strong interactions.

For Science Watch, Steve Gubser describes the material in #2 as "a step in the direction of the long-standing effort to cast QCD as a string theory. In this program, strings are realized as tubes of electric flux. Previous work by Alexander Polyakov had indicated the need for a curved space of higher dimension to adequately describe a QCD string." The paper formulates a general method for calculating correlation functions of the gauge theory from the response of string theory to boundary conditions in anti-de Sitter space (the maximally symmetric solution of Einstein’s field equations with a negative cosmological constant). Gubser adds: "We gave substance to the remarkable claim that a theory with gravity (in particular, string theory) can be physically equivalent to a quantum field theory in a lower dimension."

The high impact of #2 is reinforced by an additional 600 citations in the high-energy physics electronic archive of papers submitted but not yet published. The mathematical statement of the correspondence between the theories has set the stage for an enormous range of calculations, and its effect on the future evolution of string theory should be impressive.

New entrant #9 is from the Liquid Scintillator Neutrino Detector (LSND) collaboration at the Los Alamos Meson Physics Facility. This group had previously reported observation of muon to electron oscillations in antineutrinos. Paper #9 extends the concept to neutrinos. The LSND apparatus consists of 167 tons of liquid scintillator surrounded by 1,220 photomultiplier tubes. A source of muon neutrinos is generated by the decay of pions. To capture any electron neutrinos resulting from oscillations, the apparatus detects Cerenkov light resulting from the high energy electron which is released in the interaction of an electron neutrino with a carbon nucleus. The experimental physics is very demanding: interactions of neutrinos with ordinary matter are very rare, and there is a large background of Cerenkov events from cosmic rays.

The results reported are of just 40 events consistent with reactions involving electron neutrinos. This was double the count expected from contamination and background events. Reassuringly, the implied oscillation probability precisely matches the earlier result from antineutrinos. The neutrino oscillation scene is being followed with great interest because of the implications for cosmology and solar physics.