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Review of Top Cited HEP Articles

2000 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, 2000.

## How Citations are Collected

The SPIRES database system at SLAC is a treasure-chest of information. The flagship database is the SPIRES-HEP literature database, a joint project of the SLAC Library and the DESY Library. HEP contains more than 470,000 entries with extensive bibliographic descriptions of high-energy physics preprints, of journal articles, and of papers from the arXiv.org. The database also has pointers to viewable versions of many thousands of articles in postscript depositories worldwide. Since 1974, HEP has tracked the number of times a published high-energy physics journal article has been cited by later works. The citations are collected from the preprint version of the paper, received by the SLAC Library in advance of formal journal publication. The library receives more than 12,000 preprints yearly, and each of the preprints is a potential source of many citations.

In earlier years, the library indexed only the citations to articles published in `core' high-energy physics journals. The HEP database now also includes citations to eprints. When an eprint is published, citations from the journal version are added to the total citation count for the e-print version.

There are some important limitations to the citation count. References are taken from the preprint version of a paper. Thus articles that were never pre-printed or e-printed receive a bibliographic entry in HEP but do not have their references recorded. For further explanation, see a more detailed note on the collection of citations.

One of our goals for the future is to automatically harvest citations from the source files of eprints submitted to the Los Alamos archive. You can help us---and at the same time, make your preparation of citations easier---by copying and pasting references from SPIRES using the special LaTeX formats rather than typing in these citations yourself. Please do not omit the last, commented out, line of the format. This is the SPIRES key marking the citation for automatic retrieval.

## Top-Cited Papers of 2000

Here we present the list of the 40 high energy physics articles that have collected the most citations in calendar year 2000. 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

1236citations 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. There is always more information just below the surface, which may be either signal or noise. But if you wish to dig it out, you had better go back to the original sources.## 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. String theorists often speak of their subject as a search for an ultimate 11-dimensional theory called `M-theory' and explain that they do not know what M stands for. But in the topcite list, M certainly stands for Maldacena, whose 1997 paper introducing a magical string theory relation called the `AdS-CFT correspondence' has occupied the [2] position for three straight years. Its total of more than 1600 citations has moved this paper to the top 25 of the all-time citation list and given it the status of an instant classic.Maldacena's remarkable idea was that theories of quantum gravity in (d+1)-dimensional anti-de Sitter space could be identified with d-dimensional conformally-invariant quantum field theories without gravity. The most symmetrical example of this correspondence begins with 10-dimensional superstring theory in a space in which 5 dimensions form an anti-de Sitter space and the other 5 dimensions are compactified into a sphere. It turns out the the global symmetries of this theory are precisely those of 4-dimensional Yang-Mills theory with maximal (N=4) supersymmetry. Anti-de Sitter space has a natural boundary, in the sense that light signals propagate to spacelike infinity in a finite time. One can imagine that the corresponding super-Yang-Mills theory lives on this boundary. Then Maldacena's correspondence identifies supergravity or superstring S-matrix elements for particles propagating in from the boundary with super-Yang-Mills theory correlation functions of operators placed at the corresponding boundary points. Just after Maldacena's paper, Witten [7] and Gubser, Klebanov and Polyakov [9] stated the correspondence more precisely in the form I have just given and began the study of its nontrivial consequences. These have now blossomed into a broad subject linking strongly-coupled gauge theory, string theory, and quantum gravity. The review paper [14], by Aharony, Gubser, Maldacena, Ooguri, and Oz, gives a detailed introduction to this new area of mathematical physics.

An interesting aspect of Maldacena's correspondence is the way that the length scales of the ordinary conformal quantum field theory are laid out in anti-de Sitter space. In the boundary theory, sources and sinks that are very close together involve, in Fourier space, large momentum transfer. Distant sources allow only smaller momentum transfer. Sources that are close on the boundary produce disturbances in the outer region of the anti-de Sitter space, while distant sources exchange disturbances that have propagated through the inner regions of anti-de Sitter space. Thus, the outer region of anti-de Sitter space at large radius corresponds to high momentum in the corresponding conformally-invariant field theory, while the interior of anti-de Sitter space corresponds to low momenta. A d-dimensional surface or `brane' that restricts the volume of anti-de Sitter space then corresponds to a momentum cutoff on the corresponding d-dimensional field theory, a brane at large radius corresponding to an ultraviolet cutoff, and a brane in the interior corresponding to an infrared cutoff.

Other string-theory topics discussed in previous years also maintain a place on the topcites list. `D-branes', classical surfaces in space on which strings can end, remain a major topic of investigation. As a sign of this, Polchinski's original paper on the properties of D-branes remains on the topcite list as [19]. The `Matrix Theory' proposal of Banks, Fischler, Shenker, and Susskind that string theory or M-theory can be alternatively defined as a theory of point particles that are the endpoints of strings (`D0-branes') holds position [20]. For further discussion of D-branes, see the 1997 and 1998 topcite reviews. The paper of Banks et al. was the #2 topcite paper in 1997, and the idea still seems to hold up. Taylor has recently given this approach to M-theory an illuminating review (hep-th/0101126). There has also been considerable activity in studying the instabilities of brane-antibrane systems and the resolution of these instabilities as new classical objects. Most of this literature has not emerged into the topcite lists, but one interesting product, Witten's discussion of brane solutions using K-theory, a scheme for the mathematical classification of vector bundles, appears at [34].

## Non-Commutative Field Theory

As string theory and M-theory have suggested many different lines of investigation into the underlying structure of gravity and space-time, several of these lines have converged on a specific model system that, in the past year, became an object of concerted study. For a long time, string theory has suggested that space-time coordinates are not only dynamical objects but noncommuting operators. In parallel with these ideas, the idea of a quantum theory of gravity seems to require space-time to be described by operators. Yoneya (Mod. Phys. Lett A4, 1587 (1989), hep-th/9611072) has proposed a `space-time uncertainty principle' for quantum gravity in which orthogonal coordinates have a product of uncertainties bounded below by the square of the string scale. There is an intuitive picture that, in quantum gravity an extremely high energy collision (giving very small uncertainty int) cannot probe extremely small distances (small uncertainty inx) because it leads to the formation of a black hole of a large size which shields or removes the details of short distances. Yoneya argued that, in string theory, the nonlocality of string determines the scale at which this uncertainty sets in.The space-time noncommutativity of string theory appears in an especially explicit way in two interesting contexts. First, in Matrix Theory ([20]), the basic quantum-mechanical operators are the space-time coordinates of D0-branes. Second, when strings are compactified in the presence of a nonzero antisymmetric-tensor gauge field B, this background field distorts the compactification geometry in planes where B is nonzero. Connes, Douglas, and Schwarz [11] and Douglas and Hull [16] brought these ideas together to show that theories of D-branes in a background B field have have a natural and very simple limit which encapsulates this noncommutativity. This is the `noncommutative Yang-Mills theory', invented by Connes in 1980, a theory of vector bosons on a rigid space-time in which the space derivatives have a nonzero commutator characterized by a fixed antisymmetric matrix. The Connes theory is nonlocal but has an exact gauge invariance. The appearance of noncommutative Yang-Mills theory as a limit of string theory was recently popularized by Seiberg and Witten [6], who showed that this idea has many nontrivial implications for D-brane systems and string compactifications. The implications of noncommutative Yang-Mills theory for string theory and its relevance as a model for quantum gravity have led to a tremendous interest in the study of this system. As an example, we find at [26] on the topcites list the analysis of weak-coupling perturbation theory in noncommutative Yang-Mills theory by Minwalla, van Raamsdonk, and Seiberg.

## Extra Space Dimensions

A new topic on last year's topcites list was the possibility of observable extra space dimensions in Nature. String theory, as is well known, requires new space dimensions, but for a long time these were assumed to be very small. In the past few years, a large number of models have been proposed in which the new dimensions are as large as other interesting scales in physics, from the grand unification scale up to even millimeter sizes. The various possibilities, classified as micro-, mini-, midi-, and maxi-sized extra dimensions, were reviewed at some length in the 1999 topcite report. Two of these ideas are still strongly represented in this year's topcite list. In addition, I should call attention to the paper of Antoniadis [37], which gave the first serious proposal in which the extra dimensions of string theory would be observably large.Among models of GUT-scale (mini) extra dimensions, there is a specific scenario which arises naturally from M-theory, in which gravity propagates in the space with an 11th dimension of finite size, bounded by walls on which the the matter fields of the Standard Model live. In [15], [18], [24], Horava and Witten have shown that this geometry is realized as the strong-coupling limit of the heterotic string theory, and they have shown how the geometrical construction can lead to theories which unified the elementary particle interactions with gravity.

At the other end of the spectrum, in [5], Arkani-Hamed, Dimopoulos, and Dvali have suggested that postulating new (maxi) dimensions of up to millimeter size allows the fundamental scale of quantum gravity to be as low as 1 TeV, so that quantum gravity effects will appear in particle physics. In this model, quarks, leptons, and gauge bosons must live on a thin 3-dimensional brane. The papers [10], [13] (the first with Antoniadis) develop the many consequences of this idea for gravitational, particle physics, and cosmological observations. Both of these approaches are being actively pursued as routes to the construction of fundamental theories of Nature. (An early speculation on the idea that we live on a brane in a higher-dimensional space, by Rubakov and Shaposhnikov, also appears on the topcite list at [31]. Many other authors, including Akama and Holdom, put forward similar ideas at the same time, though, to my knowledge, everyone at that time missed the connection to TeV-scale quantum gravity.)

There is another set of ideas about extra dimensions which appeared only as a footnote in the previous report but now, in the past year, has become a major research direction. Consider a flat 4-dimensional brane embedded in a 5-dimensional anti-de Sitter space with a negative cosmological constant. For a certain relation between the surface energy of the brane and the cosmological constant, this geometry solves the equations of general relativity and is a consistent space-time. With two such branes at finite separation, one with positive and one with negative surface energy, we have a model with a compact fifth dimension. Randall and Sundrum [4] and Gogberashvili (hep-ph/9812296) observed that, in this scenario, the stretching due to the curvature of the anti-de Sitter space moves quantum fluctuations on the second brane to much lower energy, in fact, to arbitrarily low energy as the separation of the branes is increased. For a particular choice of the separation, one can arrange that the quantum fluctations in the neighborhood of the first brane run up to the Planck scale, while the quantum fluctuations in the neighborhood of the second brane run only up to the TeV scale. This gives an alternative geometrical explanation of large ratio of the Planck scale and the weak-interaction scale. Alternatively, these authors showed ([3], hep-ph/9908347) that a sensible universe results from the limit in which the second brane is pushed off to infinity. (See also the 1985 paper of Visser [hep-th/9910093], which contains additional early references.) In that case, the curvature of space keeps the gravitational flux within a finite distance of the first brane, yielding a 1/

r^{2}Newton's law attraction, as expected in 4 dimensions, between sources on the brane. In this case, gravity is effectively localized to a subspace of the full 5-dimensional space.Randall and Sundrum's work excited a large amount of interest in modelling the observed world as a 4-dimensional brane in anti-de Sitter space. Among the papers that developed this idea are those of Goldberger and Wise [30], who studied explicit models for stabilizing the position of the second brane in two-brane scenarios, and DeWolfe, Freedman, Gubser, and Karch, [38] who developed beautiful and general methods for solving for stable brane configurations in anti-de Sitter space. Part of the interest in the Randall-Sundrum approach is that it links naturally with Maldacena's correspondence between gravitational and ordinary conformal field theories. The Randall-Sundrum geometry, in this dual description, becomes a strongly-coupled quantum field which is approximately scale-invariant between a high-energy domain near the Planck scale and a low-energy regime at the TeV scale. With many mathematical tools available to analyze models of this type, perhaps we soon can expect further progress toward concrete models of the elementary particle forces.

## Cosmology

There is another development that pushes geometrical models of the forces of Nature that has come from the astrophysical observations. For many years, we have known that the universe is almost flat and slowly expanding. However, in the past few years, cosmological observations have given a much more precise and refined knowledge of the cosmological expansion. One of these observations has made enough of a splash to penetrate the topcite list in high-energy physics: Two groups based in Berkeley have applied systematic automated methods to the search for distant supernovae and have collected enough of them to make a precise measurement of the cosmological expansion up to redshiftszof about 1 [17], [27]. The results show that the expansion is accelerating, a sign of a small but cosmologically significant positive cosmological constant. More recent evidence from the large-scale distribution of matter and from the fluctuations in the cosmic microwave background confirms this picture. Some time ago, Bahcall, Ostriker, Perlmutter, and Steinhardt wrote an illuminating review of the new picture of cosmology (astro-ph/9906463). A very recent paper from the MAXIMA and BOOMERANG experiments (astro-ph/0007333) brings the state of these observations up to date.

## Neutrinos

The 1999 topcite review indicated great interest from the community in the subject of neutrino oscillations, stimulated by the observation of the disappearance of cosmic ray muon neutrinos by the Super-Kamiokande experiment. In the past year, the excitement over neutrinos has continued, with three papers from Super-Kamiokande [8], [32], [39], and CHOOZ reactor constraint on the oscillation of electron neutrinos [40], and the fundamental theoretical paper of Wolfenstein [21], again present on the topcite list. For a review of these papers and related developments in the theory of neutrino mass, please see the 1999 report.

## 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 descriptions of the leading event generators PYTHIA [12], and HERWIG (just off the list at [41]) and the parametrizations of parton distributions by Martin, Roberts, Stirling, and Thorne [36]. Completing the list of the top-40 cited papers, we find the classic reviews of supersymmetry by Haber and and Kane [22] and Nilles and [23], the original paper of Kobyashi and Maskawa on the now-standard theory of CP violation [25], the Shifman , -Vainshtein-Zakharov . paper on QCD sum rules [28], the Seiberg-Witten solution of strongly coupled supersymmetric Yang-Mills theory [29], Gasser and Leutwyler's description of chiral perturbation theory [33] and Hawking's original paper on black hole radiation [35].

## All-Time Favorites

Here we present the list of all-time favorite articles in the HEP database. The list contains the 73 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 Whoof theoretical high-energy physics. Nineteen of the listed papers were published inPhysical Review, eighteen inNuclear Physics, eleven inPhysical Review Letters, six inPhysics Letters, five inPhysics Reports, and thirteen in other journals. Although our new policy of including only one year's collection of citations in the annual Top-40 list works against the inclusion of these classic papers, still eight of these papers also appear among the most highly cited articles of 2000.The number one position in citations goes again to the Particle Data Group, accumulating over 10,000 citations to the various editions of their review. The next seven 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 [5] on the all-time list. 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 stand as , Iliopoulos, and Maiani, appears as [4]. Next comes another extremely influential theoretical idea that is yet to be confirmed, the concept of the grand unification of elementary particle interactions put forward by Georgi and Glashow stand as [8] and Pati and that, because Salam's [13].

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 [6] on the evolution of parton distribution functions. Though, truly, Gribov and Lipatov should get prior credit for this formalism, Altarelli and Parisi's 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 [9] presents applications of the ITEP QCD sum rules by Shifman , Vainshtein , and Zakharov . Finally, the original papers by Politzer and Gross and Wilczek which announced the discovery of asymptotic freedom appear as [18] and [19] (inexplicably differing by 8 citation).

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 paper on quantum field theory in the instanton field [12] and 't Hooft's paper on 's original letter on instanton effects [16], Nambu and Jona-Lasinio's paper on chiral symmetry breaking [14], the foundational paper of Belavin, Polyakov, and Zamolodchikov on conformal field theory [15], and the Coleman-Weinberg paper on the effective potential [17]. Below, we find Guth's proposal of inflationary cosmology [20], the paper of ''t Hooft and Veltman on dimensional regularization [21], Adler's paper on the axial vector anomaly [22], and Polyakov's paper on the functional integral formulation of string theory [24]. Any particle physicist who has not read these papers is not an educated person.

So far, all of the papers mentioned appeared on last year's top-25 all-time list, and on most of the preceding all-time lists as well. However, the recent developments in theoretical and experimental physics have elevated two new papers to this Olympus of citation. The first is the foundational paper of Wolfenstein on matter-induced neutrino oscillations [23]. The second is the paper of Maldacena described at the beginning of this review, which stands at [25] on this list, and [2] in the past year.The last two of the top 25 papers are the classic review articles on supersymmetry by Nilles and Haber and Kane [10] and [11], (differing by 78 citations). These are excellent and useful papers, but I am still disappointed that they have not been replaced by new and up-to-date review articles. Maybe this will require the discovery of supersymmetry. The extremely useful review by Eichten, Hinchliffe, Lane, and Quigg of `supercollider physics' (still relevant under the title `LHC Physics') has been pushed off to [31]. Ken and Chris will be disappointed to note that the supersymmetrists have overtaken them by more than 610 citations, and that the gap is growing every year. The community needs a new review of supercollider physics--one with an extensive chapter on supersymmetry--in time for the start of the LHC experiments. It is a big job, but the potential reward is measured in thousands of citations.

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. At the moment, only one experimental discovery has stirred the waters enough--the 1974 discovery of the J/psi at Brookhaven [55] and SLAC [64].

The complete list shows titles, authors, publication information, and the exact number of citations on December 31, 2000.

## Epilogue

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.

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SLAC-authored documents are sponsored by the U.S. Department of Energy under Contract DE-AC03-76-SF00515. Accordingly, the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce these documents, or allow others to do so, for U.S. Government purposes. All documents available from this server may be protected under the U.S. and Foreign Copyright Laws. Permission to reproduce may be required.

Top Cited HEP Articles, 2000 edition by Heath O'Connell, SLAC. Reviewer is Michael Peskin, SLAC. Original edition by H. Galic. Work performed at Stanford Linear Accelerator Center (SLAC)Search HEP | SPIRES Databases | Comments

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