he Physics Top Ten this period is once again a prism that refracts the stream of papers on neutrino physics, now dominating our table. Half the papers are on solar neutrinos (#2, #3, #7, #9) and atmospheric neutrinos (#4). This intense activity is being driven by the new-found ability of physicists to capture and analyze neutrinos in worthwhile numbers, compared to a trickle of events a decade ago. Furthermore, modern experiments are revealing spectacular new physics, with neutrinos spontaneously transforming type while in flight.

Paper #9 is a classic in solar neutrino astronomy. This paper asks,  What is the present rate of neutrino production in the solar core? For decades researchers could only detect the solar neutrinos produced in small side reactions to the main proton-proton fusion reactions. The Gallex experiment changed all that, with the direct detection of proton-proton reactions. In Gallex, the solar neutrinos cause inverse beta decay in ⁷¹Ga to ⁷¹Ge, and the number of ⁷¹Ge atoms produced gives the solar neutrino flux. Gallium detection was a huge leap forward because 93% of solar neutrinos are only detectable through this channel.

The Gallex experiments ceased in 1997. A year later the Gallium Neutrino Observatory (GNO) was approved, as a major overhaul and modernization of the experimental set-up. Paper #9 reports the first 19 months of observations. For Science Watch, team member Till Kirsten (Max Planck Institute for Nuclear Physics, Heidelberg, Germany) explained the importance of the results. "The gallium technique is still the only way to detect solar neutrinos with energies below 0.5 MeV, where more than 90% of all solar neutrinos are. Our experiment is complementary to Super-Kamiokande (#3, #4) and the Sudbury Neutrino Observatory (#2, #7). What all these experiments are aiming for is an exact determination of the mass and mixing parameters for neutrinos. These papers already add up to extremely important proof of neutrino mass, which is new physics. The high citation rates are because of the significance of the results for new physics, and the interdependence of the papers themselves. Neutrino physics is still some way from being able to conduct real-time observations of pp-neutrino reactions. That’s still at least five years away."

The Super-Kamiokande (SK) results in paper #3 describe the precise measurement of the solar neutrino flux from ⁸B. These neutrinos are not detectable by gallium techniques. The 1,258-day dataset has 18,464 detection events, whose time sequence beautifully demonstrates the annual variation of the detected flux, due to the eccentricity of the earth’s orbit. The focus of these SK data is evidence for neutrino oscillations. That’s because the Sudbury Neutrino Observatory (SNO) cannot see electron neutrinos that have transmuted to muon neutrinos, whereas SK is sensitive to all flavors. The comparison of SNO and SK data is important for determining the extent of neutrino oscillations.

Lying just below the Top Ten there’s a completely different kind of physics paper: not neutrino physics, not cosmology, and not superconductivity. Paper #11, with 17 citations this period, is H. Jeong, et al., "The large-scale organization of metabolic networks," (Nature, 407[6804]:651-4, 5 October 2000), an interdisciplinary report from a team of authors based in departments of physics (University of  Notre Dame, South Bend, Indiana) and pathology (Northwestern University Medical School, Chicago, Illinois). One of the five authors,  Zoltan Oltvai of Northwestern, informed Science Watch that "the motivation to our work was that the systematic organization of living cells is an area where we lacked, and still lack, a fundamental understanding. That, of course,  is changing, and systems biology by now is a very lively field, attracting the attention of a large number of interdisciplinary groups. Our paper is the first attempt to understand the global organization of a particular cellular function, in this case metabolism, and as such it has sparked several new lines of investigations."

The nub of #11 is an attempt to derive a systematic mathematical analysis of the metabolic networks of 43 organisms. Many individual characteristics of a cell may be well known, in the sense of specifying the energy flows, information transfer, and interactions between proteins, DNA, RNA, and small molecules. But how do all these components link up so that organism functions? The case made here is that viewing a microorganism as a network of genes and proteins offers a promising strategy for understanding the complexity of living systems. A startling conclusion is that metabolic networks in widely different systems show the same topological scaling properties, with similarities to the network organization of inanimate systems such as the World Wide Web and social networks. In a nutshell, successful complex networks, both animate and inanimate, have an inherent design that is scale-free and therefore robust and error-tolerant.