Review of Top Cited HEP Articles of 2001

Review of Top Cited HEP Articles

2001 Edition

Reviewer is Michael Peskin, with earlier editions also available.

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, 2001.

Top-Cited Papers of 2001

Here we present the list of the 40 high energy physics articles that have collected the most citations in calendar year 2001. 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 (2000 and 1998) have collected 1323 citations in the past year. 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.

String Theory and M-Theory

Readers of these articles over the past few years will remember that the next highest places in the citation count are traditionally taken by developments in string theory and related areas of mathematical physics. This year, however, three of the top ten places after the PDG are taken by experimental papers, and four more are taken by works that frankly attempt to build models relevant to experiment. However, a glance at the titles of these papers will reveal the persistent influence of the revolution in string theory. One of the three experimental papers concerns cosmology, and all four of the papers on model-building deal with models in which the universe has more than four dimensions. Objects whose importance was discovered in studies of string theory play a central role in theorists' attempts to understand the universe, either on extremely large scales or in its microstructure. These include higher-spin gauge fields and "branes", submanifolds of the higher-dimensional space that might hold their own gauge fields, fermions, and scalars. For example, many of the new theoretical ideas generated in the past year have been developed by exploring the assumption that the Standard Model fields live on a 3-dimensional brane in a higher-dimensional universe. In this way, the abstract and mathematical progress of string theory has changed the thinking of the particle physics community as a whole. Personally, I feel that these new ideas from string theory will lead to connections that will be verified experimentally. But this is yet to be demonstrated.

For 2001, all of the highest cited papers in string theory proper are holdovers from previous years. The paper of Maldacena that has stood as the #2 paper in each of the past three years appears this year in the #5 position. Maldacena's idea was very beautiful one: A quantum gravity theory has a very large gauge invariance--local coordinate invariance--so that the number of physical degrees of freedom in such a theory scales roughly as the size of its boundary. So perhaps there could be an exact duality between a quantum gravity theory in (d+1) dimensions and an ordinary quantum field theory in d dimensions that might be thought to live on its boundary. Maldacena suggested such exact correspondences between string theories in anti-de Sitter space, a geometry with a naturally-defined boundary, and N=4 super-Yang-Mills theories. The precise correspondence of operators between the two theories was then found by Witten (#10) and Gubser, Klebanov, and Polyakov (#13). The many ramifications of this insight have been traced out in a useful review paper by Aharony, Gubser, Maldacena, Ooguri, and Oz (#17). More information about Maldacena's physical picture and its implications can be found in the 1998 edition of these reviews.

The other string theory topic that appeared prominently last year--the study of non-commutative gauge and gravitational theories, is represented on the top-40 list this year only by the paper of Connes, Douglas, and Schwarz (#15) that demonstrated the appearance of non-ommutative geometry on a D-brane immersed in a background higher-spin gauge field, and by the paper Seiberg and Witten (#6) that showed how non-commutative Yang-Mills theory appears in certain string compactifications. The physical interest in non-commutative field theory was described at some length in last year's topcite review. Another major area of investigation in string theory, the Matrix formulation of M-theory, is represented this year only by the original paper of Banks, Fischler, Shenker, and Susskind (#35) that introduced this approach. This was the #2 paper of 1997 and is reviewed in that year's topcite review.

A final string theory paper new to the topcites appears at #38. This is a 1999 paper by Myers introducing a dielectric effect of branes in external higher-spin gauge fields, a phenomenon now sometimes called the `Myers effect'. Essentially, the interaction of the brane with the external field can cause the brane to blow up transversely. In Myers' original example, a 0-brane, which looks like a pointlike (0-dimensional) singular configuration, expands into a spherical form which is interpreted as 0-branes bound to a 2-brane. The Myers effect connects at a deep level to many phenomena in M-theory that have been discovered in the last few years; it is found to regulate singular gravitational configurations (see, e.g., Polchinski and Strassler, hep-th/0003136) and to remove pointlike degrees of freedom from theories of quantum gravity (see, e.g., McGreevy, Susskind, and Toumbas, hep-th/0003075). The dielectrically blown up d-branes also typically have noncommutative field theories living on their surfaces. All of these phenomena smooth chaotic short-distance quantum gravity fluctuations. Their common origin suggests that the Myers effect is a piece of the understanding of string quantum gravity that is still waiting to be found.

Extra Space Dimensions

The topic that has replaced string theory at the top of the topcites, with papers #2, #3, and #4 on the current list, is the idea that Nature might contain additional space dimensions with observable consequences for models of elementary particles. The idea of extra space dimensions was first taken seriously as a part of string theory, which requires 10 or more space-time dimensions for mathematical consistency. In 1990, Antoniadis #27) put forward the idea that some of these dimensions could be large enough to be probed by accelerator experiments. Few people took this idea seriously at that time. But the discovery of string dualities in the mid-90's forced people to reconsider the effects of large space dimensions and their connection to the strength of gravity. Horava and Witten (#20, #34) pointed out that the weakness of gravity at the scale of grand unification could be explained by an enlarged extra dimension; they used this idea to present new embeddings of grand unification in string theory. Arkani-Hamed, Dimopoulos, and Dvali (#4) took this idea to its natural extreme, suggesting that the weakness of gravity relative to the other interactions of particle physics is explained if extra dimensions are large enough to be macroscopic---larger than an atomic nucleus, even as large as a millimter. Their papers #7, with Antoniadis, and #14 explore the relation of this idea to string theory and propose a number of possible experimental tests in particle physics and in astrophysics. Further aspects of the physics of extra dimensions are discussed in 1999 edition of this review.

The models of Arkani-Hamed, Dimopoulos, and Dvali assumed that the extra dimensions have a flat geometry. But there is another simple paradigm for the extra dimensions that allows different interesting avenues for model-building. This is the idea of "warped" extra dimensions, discovered by Randall and Sundrum (#2, #3), Gogberashvili (hep-ph/9812296, hep-ph/9908347), and others. (For additional bibliography, see the 2000 edition of this review.) In this construction, the extra dimensions have the form of anti-de Sitter space, the homogeneous space of constant negative curvature. A brane can form a wall that terminates a region of anti-de Sitter space. Randall and Sundrum considered geometries in which a single brane is immersed in anti-de Sitter space (#2) and in which two separated branes act as the boundaries of a finite-thickness slab of anti-de Sitter space (#3). If the brane has positive surface tension, anti-de Sitter space curves out away from the brane, and so waves have longer wavelength or lower energy. In the model with a single positive-tension brane, gravity is in a sense bound to the brane; the Newton potential for matter on the brane is 1/r even in this 5-dimensional situation. In the second model, which requires a brane of positive tension and a brane of negative tension, the negative-tension brane has a very small gravitational interaction. A model in which the Standard Model lives on the negative-tension brane has a naturally large hierarchy between the gravitational and the electroweak interactions.

Much of the interest in this theory has been in its relation to Maldacena's duality between quantum gravity in 5 dimensions and a non-gravitational field theory in 4 dimensions. Under this duality, the two-brane Randall-Sundrum model is mapped into a 4-dimensional model in which gravity couples to a strongly-coupled matter sector that contains the particles of the Standard Model as composites. This makes it interesting both to attempt to create realistic Randall-Sundrum models from string theory and to attempt to connect features of the Randall-Sundrum theory to phenomenological models with a composite Higgs sector. Some papers that illustrate these research directions were cited in the 2000 edition of this review.


Last year, experimental papers in observational cosmology pushed their way for the first time onto the list of top-cited papers in high-energy physics. In part, this is the result of a convergence of the goals of particle physics and cosmology: Models of electroweak symmetry breaking and the Higgs boson are interesting in part because they provide explanations for the "dark matter" that dominates the mass density in the universe, and models of supersymmetry and quantum gravity are interesting in part because of insights they might give into the small value of the cosmological constant Lamda. But the most important reason for the appearance of papers on cosmology is a real revolution in our understanding of the universe on large scales.

For decades, cosmologists have debated the question of whether the universe is open or closed, permanently expanding or destined to recollapse. Given the uncertain state of the data, their opinions have often been shaped by intuition or prejudice. However, the 1973 textbook of Misner, Thorne, and Wheeler makes the following statement (p. 750): "Today's view of cosmology, as dominated by Einstein's boundary condition of closure (k = +1) and his belief in Lambda = 0, need not be accepted on faith forever. Einstein's predictions are clear and definite. They expose themselves to destruction. Observational cosmology will ultimately confirm or destroy them, as decisively as it has already destroyed the 1920 belief in a static universe and the 1948 steady-state models."

In the past few years, two cosmological measurements have changed our thinking about the universe in this decisive way. First, two groups based in Berkeley have measured the rate of cosmological expansion using Type IIa supernovae as standard candles (#9, #12). The measurement required the discovery of hundreds of new supernovae, and this was done using automated systems to make pixel-by-pixel comparison of telescope images. Second, a number of groups, of which the data of the BOOMERANG (#19, #24) and MAXIMA (#44) collaborations is most decisive, have measured the fluctuations in the cosmic microwave background radiation. The data include both information about fluctations in the primordial cosmic matter density and about the propagation through the universe of the temperature fluctuations that they produce. The microwave background measurements support the idea that the universe is flat (k = 0). Under this constraint, the supernova observations support a picture in which roughly 30% of the energy density of the universe is provided by matter, while 70% is provided by vacuum energy or some more general source (called "dark energy") which has an equation of state with negative pressure. The new perspective is reviewed in an illuminating way by Bahcall, Ostriker, Perlmutter, and Steinhardt (astro-ph/9906463); a more up-to-date review is given by Turner ( astro-ph/0202007). For some well-considered reservations about the new picture, see the review of Peebles and Ratra (astro-ph/0207347).

What is this dark energy? The solution most ready at hand is that it is a positive cosmological constant. Our current understanding of quantum field theory allows a cosmological constant; in fact, it is profound mystery that the universe does not have a cosmological constant a factor 10^60 larger than the observed value. It is possible that whatever unknown mechanism zeroes the cosmological constant misses by a small amount. However, symmetry or duality arguments for a zero value of the cosmological constant might truly predict zero, and many physicists still believe that this is the true value. (Krauss and Starkman have given a philosophical argument against a positive cosmological constant in astro-ph/9902189.) If one insists that the cosmological constant is zero, it is possible to explain the dark energy contribution from the vacuum energy of a scalar field that is not in the minimum of its potential and will eventually relax to a state of zero vacuum energy. This idea was explored some time ago by Ratra and Peebles (#33) and was recently revived, and given the vivid name "quintessence", by Caldwell, Dave, and Steinhardt (#29). Whether the observed vacuum energy is fundamental or emphemeral, it is a pressing problem to provide a physical picture that explains its value. Many of the models proposed involve extra dimensions and branes. Among many works on this subject, the paper of Binetruy, Deffayet, Ellwanger, and Langlois (#36) provides a refreshingly clear and compact analysis of the cosmological evolution of an extra-dimensional universe containing branes.

Muon g-2

After more than a decade of effort, the muon (g-2) experiment at Brookhaven announced its first result with significant statistics on the (g-2) value of the mu+ (#8). The measurement used a dedicated muon storage ring with magnetic fields controlled to fractions of a part per million and observed 3 billion muon decays. Their result for the anomalous magnetic moment of the muon, a = (g-2)/2, had an uncertainty of 15 X 10^{-10} (henceforth, 1.5 ppb). This dramatically improved the class CERN (g-2) measurement, precise to 2 ppm. And, the result differed by 4 ppb from the expectation in the Standard Model. Small wonder that the paper set off a stream of discussion and controversy.

If one accepts the correctness of the deviation, it is not hard to find an explanation for it. Any new heavy charged particle with mass about 100 GeV that couples to the muon can potentially produce an anomaly of this order. The whole catalogue of beyond-the-Standard-Model theories--supersymmetry, compositeness, anomalous W interactions--can be raided to give a suitable effect. Many papers appeared along these lines, but the first of them, by Czarnecki and Marciano (#28) played a special role. This paper illuminating the variety of models of new physics that could potentially contribute to the muon (g-2) and gave precise estimates of the requirements across a broad range of cases.

But did the muon (g-2) group indeed find a deviation from the Standard Model? Their paper reported a deviation of 2.6 sigma. Deviations of this order often appear and disappear in particle physics, being caused by a statistical fluctuation plus some misestimation of systematics. For the muon (g-2), one could immediately raise serious questions about the accuracy of the contributions to the theoretical Standard Model value from diagrams containing hadronic vacuum polarization amplitudes. The contribution to a from these diagrams was 69 ppb, with a claimed error of 0.6 ppb. The value is, however, not unambiguous; it must be extracted from low-energy hadron data, either from e+e- annihilation or from tau decays (in the latter case, correcting for isospin symmetry breaking). Estimates given in the literature vary among themselves by 1 ppb. Worse still, in November, Knecht and Nyffeler (hep-ph/0111058) discovered an overall sign error in another contribution from light hadrons, the effect of hadronic light-by-light scattering (0.5-1.0 ppb). Flipping this sign substantially decreased the significance of the effect. In addition, this work cast into relief the uncertainties in the the light-by-light scattering contribution, which is not under good theoretical control. For a useful summary of these controversies, see K. Melnikov, hep-ph/0105267, updated at this link.

Finally, this year, the muon (g-2) collaboration released a new experimental result with decreased errors (now 0.9 ppb) and essentially the same central value (hep-ex/0208001). A final important cross-check on the experimental result will be the analysis of mu- data taken in 2001. At this moment, the anomaly is still present at a significance somewhere between 1 and 3 sigma--tantalizing, but unproven.


The 1998 discovery of atmospheric neutrino oscillations by the Super-Kamiokande collaboration (#11) has led to a tremendous wave of interest in the study of neutrino mass. The subject of neutrino mass leapt onto the topcites in 1999, and there is an extensive review of neutrino oscillation experiments in the 1999 report. This included a discussion of the Super-Kamiokande paper (#11) and the constraint on electron neutrino mixing from the Chooz reactor experiment (#22). One results mentioned only briefly in this discussion, because it did not contribute a paper to the topcites list, was the work of the original solar neutrino experiment, performed by Ray Davis and collaborators in the Homestake Mine. This experiment first established that the flux of solar electron neutrinos reaching the earth was substantially lower than that predicted by solar models. In retrospect, we might see this as the first evidence for neutrino mass. I am pleased that the 1998 summary paper from the Homestake Mine experiment, published in the Astrophysical Journal, has been swept up into the SPIRES database and appears this year in the topcites as #32.

This year's topcite list also includes a major new experimental result on solar neutrinos, from the SNO collaboration (#18). Using a heavy water target, SNO supplemented the measurement of neutrino-electron scattering reaction rate from high-energy solar neutrinos, done by Super-Kamiokande, with a measurement of the charged-current reaction rate in the same energy region. The charged-current rate measures the flux of electron neutrinos; the nu-e scattering rate receives most of its contribution from electron neutrinos but has significant contributions from the other neutrino flavors. With both numbers in hand, it is possible to solve for the contribution of electron and, separately, muon and tau neutrinos. From these values, one can determine the total neutrino flux. The SNO collaboration found that the total neutrino flux determined by this method is in good agreement with the electron neutrino flux predicted by solar models. This result was strengthened this year by a new paper from SNO (nucl-ex/0204008) announcing the collaboration's measurement of the rate of neutral-current scattering of solar neutrinos. This quantity receives equal contributions from the three neutrino flavors and so fixes the mu and tau neutrino fluxes more accurately. The result implies that every neutrino predicted by the solar models is accounted for, either as an electron neutrino reaching earth or as a neutrino that has oscillated to another flavor. Finally, after more than 40 years of controversy, the solar neutrino problem is definitively ascribed to neutrino oscillations. The correct set of mixing parameters has yet to be determined, though new experiments are expected to address this question in the next few years.

Also new to this year's topcite list is a paper by Super-Kamiokande on the identity of the species into which the atmospheric muon neutrinos oscillate (#40). Using the effect of the matter on neutrino oscillations, first discussed by Wolfenstein (#25)---specifically, the matter effect on muon neutrinos passing through the earth, measured as a function of energy---the Super-Kamiokande collaboration claims to distinguish oscillation to a tau neutrino from oscillation to a neutrino without neutral current interactions at the 3 sigma level. If confirmed, this observation narrows the range of mixing schemes that could describe the original Super-Kamiokande result.

High Energy Physics Resources

The final places on the topcites list include a number of papers which help experimenters describe and simulate processes of the Standard Model. These include the description of the event generators PYTHIA (#16) and the latest set of parton distributions produced by the CTEQ collaboration (#23). In addition, there are classic theoretical papers that are of continuing relevance either to experimental analyses or to theory. These include the Altarelli-Parisi paper on parton evolution (#26), the reviews of supersymmetry in particle physics by Haber and Kane (#30) and Nilles (#41), Hawking's paper on radiation from black holes (#31), the original paper of Shifman, Vainshtein, and Zakharov on QCD sum rules (#37), and Gasser and Leutweyler's paper on chiral perturbation theory (#39).

The final work on the list is the paper of Kobayashi and Maskawa that introduced the model of CP violation based on six quarks in the standard electroweak model (#21). In the summer of 2001, the measurements of CP asymmetries in B decays by the BaBar and Belle collaborations finally became accurate enough to give tangible evidence that the six-quark model explains the magnitude of CP violation in the neutral kaon system. Whereas only a few years ago it might have been said that we had no experimental clue to the origin of CP violation, it now seems that Kobayashi-Maskawa model has the inside track. As BaBar and Belle refine their measurements over the next few years, and as the Tevatron experiments come into play also in B physics, we will learn whether this model is truly the correct one or just one part of a more complex picture. I hope very much to report surprises in this study in future editions of the topcite report.

All-Time Favorites

Here we present the list of all-time favorite articles in the HEP database. The list contains the 107 journal articles with more than 1,000 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. 22 of the listed papers were published in Physical Review, 26 in Nuclear Physics, 19 in Physical Review Letters, 10 in Physics Letters, 5 in Physics Reports, and 24 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 16 of these papers also appear among the most highly cited articles of 2001.

The number one position in citations goes again to the Particle Data Group, accumulating over 16,000 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 stand 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 model 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 the grand unification of elementary particle interactions. The original papers on this topic by Georgi and Glashow and Pati and Salam stand at #9 and #14, respectively.

The next group of papers contains the leading works on the structure of the strong interactions. The first is the paper of Altarelli and Parisi (#5) on the evolution of parton distribution functions. Though, truly, Gribov and Lipatov should get prior credit for this formalism, Altarelli and Parisi's paper paper made the story clear to everyone and is still one of the best expositions of the QCD theory of structure functions. Wilson's paper which demonstrated the confinement of quarks in QCD appears as #7. The paper #8 presents applications of the ITEP QCD sum rules by Shifman, Vainshtein, and Zakharov. The paper of Nambu and Jona-Lasinio that introduced the idea of chiral symmetry breaking in the strong interactions appears at #13. Finally, the original papers by Politzer and Gross and Wilczek which which announced the discovery of asymptotic freedom appear as #20 and #21 (inexplicably differing by 8 citations).

Almost all of the other top 25 papers are classic works in the formalism of quantum field theory. A first group includes 't Hooft's papers on instantons in quantum field theory (#12 and #17), the foundational paper on conformal quantum field theory by Belavin, Polyakov, and Zamolodchikov (#16) and the Coleman-Weinberg paper on the effective potential (#18). In the middle of this group is a new classic, Maldacena's paper on the duality between gravitational and conformal field theories (#15; see above). Below, we find Guth's paper introducing the inflationary universe (#19), the classic but newly relevant paper of Wolfenstein on neutrino oscillations in matter (#22; see above), and paper of `t Hooft and Veltman that introduced dimensional regularization (#23), Adler's paper on the axial-vector anomaly (#24), and `t Hooft's paper on the 1/N expansion in QCD (#25).

The last two of the top 25 papers are the classic review articles on supersymmetry by Nilles (#10) and Haber and Kane (#11). These are excellent and useful papers, and the renewed interest in supersymmetry as the theory of the next scale in physics has brought these papers back into the current years' topcite list (see above). However, though supersymmetry has not yet been discovered, there has been a great deal of theoretical work on the formalism and observational consequences of supersymmetry; in this sense, these two reviews are now out of date. The paper I now recommend as a general introduction to supersymmetry and its phenomenology is a beautiful review by Martin (hep-ph/9709356). This deserves to be a topcite; please make it so.

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 (#61) and SLAC (#72). However, this year, both of these papers have been overtaken by the paper from the Super-Kamiokande group announcing the discovery of atomospheric neutrino oscillations (#60, and see above).

The complete list shows titles, authors, publication information, and the exact number of citations on December 31, 2001.


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. An update should follow a year from now. See the page on most cited HEP articles for references to previous years.