Review of Top Cited HEP Articles
Based on data from the SPIRES-HEP Literature database, SLAC Library
One of the most popular features of the SLAC SPIRES-HEP Literature database is the citation search, which identifies how many subsequent papers have cited a particular journal article or an e-print archive paper. Such a search can be used to identify influential contributions to high-energy physics and related fields. In this document, we present the articles which have received the most citations.
These lists reflect the standings in the SPIRES-HEP database as of December 31, 2002.
Top-Cited Papers of 2002
Here we present the list of the 40 high energy physics articles that have collected the most citations in calendar year 2002. We know of no better indicator of which are the "hot" topics in the field today. In the remainder of this section, we will describe these 40 articles in groups corresponding to their subject matter. The comments on the beauty or technical merit of these papers, as opposed to their quantifiable popularity, are the personal responsibility of the reviewer.
Particle Data Group - PDG
The number 1 cited article, with citation counts off the normal scale, is the Review of Particle Physics, compiled by the Particle Data Group (PDG). The most recent two editions (2002 and 2000) have collected 1414 citations in the past year, with a few more to the older editions. For better or worse, it has become a standard practice, especially in the theoretical literature, to cite this very useful compilation of data rather than the original experimental sources. The PDG does a service to the community which is more than just bibliographic. It produces well-thought-out averages and analyses of the data, making use of the opinions of leading experts. The PDG averages are intentionally conservative and are meant to reflect community consensus. I like to quote the PDG values for basic input quantities that I hope will not be controversial. For experimental values crucial to a given analysis, there is more to be learned by going back to the original sources. But you must judge whether this new information is signal or noise.
NeutrinosWhereas all of my previous editions of the topcite list began with superstring theory, it is appropriate this year to begin with neutrino physics. Though papers on neutrinos do not occupy the very highest level of the topcites, they begin with paper #7 and include 10 entries--a quarter of the entire list. Further, in contrast to my lament in the 1997 on the small number of experimental papers in the topcites, all of these papers represent new experimental results. This year's topcite list also contains 7 experimental papers in cosmology, which I will discuss in the next section. In all, 40% of this year's topcite list consists of papers that bring new information about the universe that exists outside of our heads and our mathematics.
Neutrino physics made a major appearance on the 1999 topcite list with the result from the Super-Kamiokande experiment (#9) that gave definitive evidence for flavor oscillation in atmospheric neutrinos. The paper reported on observations of muons and electrons from neutrinos produced in cosmic ray interactions in the atmosphere. The downward-going leptons, from neutrinos produced just above the detector, showed the predicted ratio of muon to electron neutrinos. However, upward-going leptons, from neutrinos produced on the other side of the earth, showed a deficit of muons, indicating conversion of the muon neutrinos to another species. This result is confirmed by observations from other atmospheric neutrino detectors (summarized in hep-ex/9912007) and by the result of the K2K long-baseline neutrino experiment (hep-ex/0212007) that sent a few-GeV beam of neutrinos from the KEK proton synchrotron to the Super-Kamiokande detector. The topcite list also includes a paper giving evidence from Super-Kamiokande that this flavor oscillation converts muon neutrinos to tau neutrinos (#23). The Super-Kamiokande paper has become the most highly cited experimental paper in the SPIRES-HEP database (#37 on the all-time list), with 1625 total citations, to be compared with 1265 and 1183 citations, respectively, for the papers that announced the discovery of the J (#63 on that list) and the psi (#81).
The discovery of flavor oscillations for atmospheric neutrinos gave new impetus to the idea that the deficit of solar neutrinos, observed over several decades by a chlorine detector in the Homestake Mine, could also be the result of neutrino flavor mixing. The experimental results from the Homestake Mine appeared belatedly in the topcites last year and appear this year as #29. The current list also includes observations of the solar neutrino deficit at higher energies than Homestake in the Super-Kamiokande experiment (#18, #19, #26) and at lower energies in the gallium detector GALLEX (#37). However, the most important news comes from a new detector SNO, a water Cherenkov detector with heavy water as the active medium. This detector allowed independent measurements of the solar neutrino flux in charged-current and neutral-current scattering from nuclei and from neutrino-electron scattering. The neutral-current cross section is independent of the neutrino flavor (but is, of course, zero for sterile neutrinos). The charged-current reaction comes only from electron neutrinos. The cross-section for neutrino-electron scattering is larger for electron neutrinos but is sensitive to all three lepton neutrino species. As the SNO collaboration reports in #14, the three measurements consistently determine two fluxes, the flux of electron-type neutrinos and the total flux of muon and tau neutrinos. SNO found that the sum of these fluxes is just equal to the prediction of the electron neutrino flux from solar models. Thus, the deficit of solar neutrinos is explained--and, indeed, completed accounted for--by the flavor oscillation of electron neutrinos to other active lepton neutrino types! An earlier paper of SNO appears as the most highly cited neutrino physics paper at #7. The oscillation from electron to muon and tau neutrinos has now also been observed in a reactor neutrino experiment, KamLAND (hep-ex/0212021). I expect to include a discussion of that experiment in next year's topcite report.
Additional ingredients in the phenomenology of neutrino mixing remain to be discovered. The observed oscillations of atmospheric and solar neutrinos constrain two mixing angles of the neutrino system. In principle, a third mixing angle should exist, parametrizing, for example, the oscillation of muon neutrinos into electron neutrinos at the atmospheric oscillation rate. This angle is strongly constrained by data from the Chooz reactor experiment (#16). If this angle should prove to be nonzero, there may be observable effects of CP violation in neutrino mixing. It is quite possible that the CP violation responsible for cosmic baryogenesis and thus for our existence arises in the neutrino sector.
Additional information on neutrino mixing can be found in the 1999 topcites review. More recently, Gonzalez-Garcia and Nir have written a comprehensive review article on this subject for Reviews of Modern Physics (hep-ph/0202058).
CosmologyThe other recent source of remarkable new information about the universe has come from cosmology. It has been clear for a long time that the total energy content of the universe is dominated by invisible sources. Possible candidates included massive neutrinos, heavy weakly-interacting particles, and a vacuum energy or cosmological constant. In the past few years, neutrinos have gone out of favor. The familiar neutrinos are now known to be too light to be of cosmological importance and, in any event, correspond to "hot dark matter" that smears out the formation of galaxies and larger structures. On the other hand, the evidence for "cold dark matter", heavy particles of an unknown type, has improved over time. And, in 1998, two groups observing distant supernovae announced that the expansion of the universe has recently been accelerating, giving evidence for a cosmological constant (or a more general entity, "dark energy") as the dominant component of energy in the universe.
The supernova observations, from the Supernova Cosmology Project and the High-Z Supernova Search Team, immediately appeared in the topcites and this year occupy positions #4 and #8, respectively, on the list. The topcites also include observations relevant to cosmology from the fluctuations of the cosmic microwave background and from large-scale galaxy surveys. The resulting new picture of cosmology, with 70% of the energy of the universe in dark energy and 30% in matter (25% in dark matter), was discussed at some length in last year's topcite report. The material in this section supplements that discussion.
The most recent improvements of our understanding of cosmology have come from measurements of the flucutation spectrum of the cosmic microwave background. The photons of the microwave background originate from the cosmological event called "recombination", the moment at which electrons and protons combined to form atoms and the universe made a transition from an ionized plasma to a transparent gas. Early in the history of the universe, radiation dominated the energy density, and matter has dominated only more recently as the temperature of the universe has decreased. The cross-over between radiation- and matter-dominance occured at a temperature of a few eV, very close to the moment of recombination. In a radiation-dominated universe, structures cannot aggregate under gravity; in a matter-dominated universe, this becomes possible and galaxies begin to condense. The cosmic microwave background, then, gives a window into the nature of the universe at just the era that galaxies began to form. The fluctuation spectrum of the microwave background contains peaks that reflect the waves in the local matter density beginning to evolve and grow. The early data on the microwave background fluctuations showed evidence for the first of these "acoustic peaks" (see, for example, Fig. 5 of the 1999 review by Dodelson, hep-ph/9912470). In 2000, however, the leading peak was clearly observed by the BOOMERANG (#20, #25) and MAXIMA (#60) balloon experiments, and the BOOMERANG experiment also showed clearly the presence of subsidiary acoustic peaks. The angular size of fluctuations corresponding to the leading peak directly reflects the geometrical shape of the universe through which the microwaves have propagated from the recombination event to our detectors. The observed location of the leading peak at an angular size of about 10 mrad provides evidence that the universe is flat (more exactly, that the total energy content of the universe is within 10% of the critical density). The accuracy of these measurements continues to improve. The most recent advances reflected in the topcites come from the DASI experiment, a ground-based interferometer at the South Pole ( #31, #35). More recently, the WMAP satellite experiment has brought the measurement of the total energy content of the universe to within 1% of the critical density and has dramatically improved measurements of other parameters of the early universe (astro-ph/0302207). I will include a full discussion of the WMAP results in next year's topcites report.
The topcites also include other papers that are more specialized in astrophysics. The appearance of these papers reflects a notable change in the SPIRES-HEP database. The database now indexes the astro-ph eprint archive along with the archives that it has more traditionally covered in high-energy and nuclear physics. Since the community of high-energy physicists is larger and its interests are more parochial, I expect that papers in high-energy physics will continue to dominate the citation counts. But from now on you should also expect very prominent papers in astrophysics to be included.
This being so, the current topcites list includes two very significant contributions to cosmology from the more astrophysical side. The first is a 1996 paper of Navarro, Frenk, and White (#24) that includes a deep numerical study of galaxy clustering through N-body simulations and a set of very useful parametrizations of the associated correlation functions. The second is the landmark determination of the Hubble constant from the Space Telescope observation of Cepheid variable stars in the Virgo cluster (#36).
The list also includes a paper of great methodological interest in astrophysics. At #39, we find a 1989 paper of Cardelli, Clayton, and Mathis that present what has become the standard method to correct for foreground dust in observations of distant galaxies. This paper has an amazing 791 total citations in SPIRES. This paper is often cited together with the all-sky map of intergalactic dust compiled by Schlegel, Finkbeiner, and Davis (astro-ph/9710327). I expect to see the citation count of this paper also increase as SPIRES' coverage of astrophysics becomes more complete.
Extra Space DimensionsI now return to the papers in the traditionally high-scoring areas of theoretical physics. Papers #3, #5, and #6 on the topcites list are the influential papers of Randall and Sundrum and Arkani-Hamed, Dimopoulos, and Dvali that proposed models including new space dimensions that address the `hierarchy problem' of particle physics, the large ratio of the Planck scale to the natural scales of particle physics.
Arkani-Hamed, Dimopoulos, and Dvali proposed in #5 to cure the hierarchy problem by adding extra space dimensions that were approximately flat and imagining that we lived on a 3-dimensional hypersurface or "3-brane" in this larger space. The bosons that mediate the strong, weak, and electromagnetic interactions would also be confined to this 3-brane, while gravitons would move in the full higher-dimensional space. If the extra dimensions are sufficiently large, gravity would naturally be weaker than the interactions of particle physics even with a quantum gravity mass scale as low as 1 TeV. The idea that the interactions of particle physics are confined to a brane fits together nicely with the picture of Horava and Witten of the strong-coupling dynamics of the heterotic string theory (#38). The phenomenological implications of this extra-dimensional model were pursued by Antoniadis, Arkani-Hamed, Dimopoulos, and Dvali in #12 and #33. The form and implications of this model were discussed in some detail in the 1999 topcites report.
Randall and Sundrum (#3) formulated a different model of the hierarchy between particle physics and gravitational interactions by modelling the universe as a volume with two 3-branes enclosing a slice of negatively curved anti-de Sitter space. The curvature of anti-de Sitter space causes the scale of distances to change dramatically from side of the slice to the other. Thus, one brane can naturally see an energy scale up to the Planck scale while the other sees physical distance scales only up to 1 TeV. This Randall-Sundrum model and another with an open 5-dimensional space but effectively 4-dimensional gravity (#6) were discussed extensively in the 2000 topcites report.
Both models of particle physics with extra dimensions continue to attract the interest of model-builders and experimenters. These developments have recently been reviewed by Hewett and Spiropulu in a paper written for the Annual Review of Nuclear and Particle Science (hep-ph/0205106).
String TheoryFinally, we come to the subject of string theory. This field does claim the #2 paper on the topcite list, the seminal paper of Maldacena that proposed a duality between conformally invariant quantum field theories in d dimensions and theories of gravity in anti-de Sitter space in d+1 dimensions. This paper first appeared on the 1998 topcite list, and its content was reviewed at some length in that year's topcite report. It occupied the #2 position, next after the Particle Data Group, in its first three years on the list and rebounds to that ranking this year. By now, Maldacena's paper has reached the position #11 on the all-time citation list, with a total of 2504 citations. To put this in perspective, this is double the number of citations reported by SPIRES-HEP for the 1985 paper of Gross, Harvey, Martinec, and Rohm that introduced the heterotic string. The other leading papers in string theory on this year's list are the papers of Witten (#10) and Gubser, Klebanov, and Polyakov (#11) that expanded on Maldacena's ideas and the review paper on Maldacena's duality by Aharony, Gubser, Maldacena, Ooguri, and Oz (#17). Regular readers of this column are by now thoroughly familiar with these papers. Another familiar paper is the Seiberg-Witten paper on noncommutative geometry (#13) that was discussed in the 2000 edition of the topcite report.
In addition, a new activity in string theory has surfaced this year. This is the study of string theory in time-dependent background space-times. Almost all work in string theory up to now has concerned static space-times which could be thought of as the vacuum configurations for string models of particle physics and gravity. Considerable progress has been made in understanding the microscopic structure of black holes and other static gravitational singularities in string theory. For example, look back at the discussion in the 1997 topcite report of papers of Strominger and Vafa (#11 on that list, hep-th/9601029) and Callan and Maldacena (#37 on that list, hep-th/9602043) in which the black hole entropy is precisely accounted for in a class of black hole solutions by computing the open string partition function in an associated configuration of branes. It would be wonderful if we could extend this understanding to time-dependent singularities, including the greatest of them all, the Big Bang.
The new papers on this year's list take a small step in that direction, by considering string theory in the space-time background of a gravitational wave. It is a fact that is remarkable, though perhaps not obviously significant, that it is possible to have gravitational waves solutions that retain the maximal supersymmetry of eleven-dimensional or ten-dimensional Type II supergravity. These solutions were constructed by Blau, Figueroa-O'Farrill, Hull, and Papadopoulos (#22,#30). These authors also noted the simplifications that arise from going to light-cone coordinates oriented with the direction of motion of the plane wave. To achieve the highest degree of supersymmetry, it is necessary to put into the background solution nonzero fluxes of the high-spin gauge fields of maximal supergravity. One might suspect that this would be a problem for the transition to string theory, since in string theory these fluxes are strings with two spinor indices (Ramond-Ramond states), which are the most problemmatic to quantize. However, for these gravitational waves, the inclusion of all of the ingredients I have mentioned has a beautiful synergy that allows the quantization of string theory in this background to be almost as simple as in flat space. When Metsaev and Tseytlin (#21,#32) and Berenstein, Maldacena, and Nastase (#15) discovered these simplifications, they set up an explosion of interest in the string community. The second group also demonstrated beautiful relations of the quantum theory to other models through the various dual relations of M-theory (discussed, for example, in the 1997 and 1998 editions of this review). They showed how to use Maldacena's famous duality to connect the quantum theory of strings in this background to a sector of N=4 super-Yang-Mills theory with a large charge under one of its global symmetries, and also how to represent the theory as a quantum theory of matrix variables using the matrix description of M-theory. The subject of dualities of string theory in plane wave backgrounds has recently been reviewed by Sadri and Sheikh-Jabbari, in a paper written for Reviews of Modern Physics (hep-th/0310119).
High Energy Physics ResourcesThe final places on the topcites list include two papers intended to help experiments describe and simulate processes of the Standard Model, the description of the event generator PYTHIA (#28) and the most recent set of parton distributions produced by the CTEQ collaboration (#34).
In addition, the list includes descriptions of two important theoretical proposals that may represent the next steps in our understanding of particle physics. The first of these is the original paper of Kobayashi and Maskawa that proposed the 6-quark model of CP violation (#27). This theory has recently received an enormous boost from the observation of time-dependent CP-violation in B meson decays by the BaBar and BELLE experiments. For a pedagogical review of all of the new developments in this area, see the proceedings of the 2002 SLAC Summer Institute. The second is the theory of supersymmetry, represented by the review papers of Nilles (#40) and Haber and Kane (#43). (In addition to these papers, I strongly recommend the review article by Martin (hep-ph/9709356).) Here will still wait for glimpses of experimental confirmation. Perhaps next year's report will bring some more promising news.
Here we present the list of all-time favorite articles in the HEP database. The list contains the 105 journal articles with more than 1,100 citations recorded since 1974 in the HEP database. Number 1 is again the `Review of Particle Properties'. The list following reads like a Who's Who of theoretical high-energy physics. 21 of the listed papers were published in Physical Review, 27 in Nuclear Physics, 17 in Physical Review Letters, 10 in Physics Letters, 5 in Physics Reports, and 25 in other journals. Although our counting of only one year's collection of citations in the annual Top-40 list works against the inclusion of these classic papers, still 31 of these papers also appear among the most highly cited articles of 2002.
The number one position in citations goes again to the Particle Data Group, accumulating over 18,000 citations to the various editions of their review. The next five papers in terms of total citations are all classic theoretical papers on the structure of the Standard Model. The original papers on the unified theory of weak and electromagnetic interactions by Weinberg and Glashow appear as #2 and #6. We regret that, because Salam's original paper on this model was published in a conference proceeding, its citations are not registered in the database. The paper #3 on the list is the model of CP violation of Kobayashi and Maskawa. In the new era of B-factories, this proposal might soon be on an equally strong footing. The prototype for this model, the theory of quark mixing in weak interactions of Glashow, Iliopoulos, and Maiani, appears as #4. Another extremely influential theoretical idea that is yet to be confirmed is the concept of grand unification of elementary particle interactions. The original papers on this topic, by Georgi and Glashow and Pati and Salam appear at #10 and #15, respectively. The next group of papers contains the leading works on the structure of the strong interactions. Here we find the classic papers of Altarelli and Parisi on the evolution of parton distributions (#5), of Shifman, Vainshtein, and Zakharov on QCD sum rules (#7), of Wilson on the mechanism of quark confinement (#8), and of 't Hooft on the physics of instantons (#13, #19). The paper of Nambu and Jona-Lasinio that introduced the idea of chiral symmetry breaking in the strong interactions appears at #14. Finally, the original papers by Politzer and Gross and Wilczek that announced the discovery of asymptotic freedom appear as #22 and #23, respectively. The reviews of supersymmetry by Nilles and by Haber and Kane, discussed at the end of the previous section, appear at #9 and #12, respectively.
The remainng papers among the top 25 are classic works on quantum field theory and its applications. Guth's paper that proposed the inflationary universe appears at #16. The foundational paper on two-dimensional conformal field theory by Belavin, Polyakov, and Zamolodchikov appear at #17. After this, we find Wolfenstein's paper on neutrino oscillations in matter (#18), Coleman and Weinberg's paper on the effective potential (#20), 't Hooft and Veltman's paper on dimensional regularization (#21), and 't Hooft's paper on the large N expansion in QCD (#24). At the head of this list, at #11, is Maldacena's paper on string duality discussed above. At the end of the list at #25, but still as relevant as ever after half a century, is Julian Schwinger's 1951 classic `On Gauge Invariance and Vacuum Polarization' (1850 citations after 1974!).
For those who wonder where the experimental papers are, I should point out that, while seminal theoretical papers have a long life on the citation lists, experimental papers tend to make a splash which is relatively short-lived and then to have their results incorporated into the PDG compendium. To reach 1000 citations, the splash has to be gargantuan. For a long time, only one experimental discovery stirred the waters enough--the 1974 discovery of the J/psi at Brookhaven (#63) and SLAC(#81). Recently, both of these papers were overtaken by the paper from the Super-Kamiokande group announcing the discovery of atmospheric neutrino oscillations (#37; see above).
The complete list shows titles, authors, publication information, and the exact number of citations on December 31, 2002.
Do not be disappointed if the papers that guide your work do not appear on any of the lists. The citation lists do display certain systematic biases. The most important is that experimental papers are grossly undercited, partially because experimenters surrender their citations to the PDG, and partially because theorists often look more at perceived trends than at the actual data. In addition, the citation lists, viewed on any short term, reflect the latest fashions as much as any linear progress in understanding. It is important to recall that both the unified electroweak model and superstring theory spent many years in the cellar of the citation counts before coming to prominence. Both, in their dark years, had proponents of vision who continued to study these models and eventually proved their worth to the community. Perhaps your favorite idea will also have this history, and perhaps you can even ride it to fame. In any case, we hope that you find the citation lists an instructive snapshot of the most popular trends in present day high-energy physics.