## Review of Top Cited HEP Articles

1998 Edition## reviewer: Michael Peskin

Earlier editions are available.

The series was originated by Hrvoje Galic.

## Based on data from the HEP (PREPRINT) database, SLAC Library

One of the most popular features of the SLAC SPIRES-HEP (PREPRINT) 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, 1998.

## How Citations are Collected

The SPIRES database system at SLAC is a treasure-chest of information. The flagship database is HEP (PREPRINT), a joint project of the SLAC Library and the DESY Library. HEP contains more than 389,000 entries with extensive bibliographic descriptions of high-energy physics preprints, of journal articles, and of papers from the Los Alamos e-print archives. 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 (PREPRINT) database now also includes citations to Los Alamos e-prints. When an e-print is published, citations from the e-print version are added to the total citation count for the journal version.

There are some important limitations to the citation count. Citations 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 citations indexed. This means, for example, that papers published only as articles in conference proceedings or festschrift do not have their citations indexed. For further explanation, see a more detailed note on the collection of citations. We are hoping to receive additional funding to improve and expand the citation collection in the HEP database.

## Top-Cited Papers of 1998

Here we present the list of the 40 high energy physics articles that have collected the most citations in calendar year 1998. 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 (1994 and 1996) have collected more than 1098 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. 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.

## Strings, Branes, and M-theory

The next four top-cited papers give new developments in the connection between string theory, supersymmetry, and the theory of gravity. In fact, 20 of the top 40 papers either deal directly with some aspect of this relation or provide foundational material for that study. The paper #2, by Juan Maldacena, has an off-scale citation count--456 citations in a year, a number comparable to the total size of the string theory community (including wannabees). To explain what all the fuss is about, I will start with some of the papers lower on the list and build up the context for Maldacena's remarkable idea.

A good place to begin is paper #20, by Witten, which was last year's paper #3. Since the original formulation of superstring theory in the 1970's, it has been understood that this theory lives fundamentally in a ten-dimensional space-time. String theory can be formulated in space-times of lower dimension (four, for example) by taking the extra dimensions to form some small compact manifold. Witten studied, in the ten-dimensional theory, some massive states which resemble charged black holes. He showed that the charge of these objects can be reinterpreted as a quantized momentum in an extra dimension, in such a way that this extra space dimension opens up as the coupling constant of the string theory is taken to infinity. Thus, he found, superstring theory is really eleven-dimensional and incorporates the `mother of all supergravity theories', the eleven-dimensional supergravity of Cremmer, Julia, and Scherk. Witten, Hull and Townsend, and others showed how to use the symmetries of this eleven-dimensional theory to relate previously disjoint formulations of string theory to one another. So, apparently, it is the eleven-dimensional theory which is the most fundamental. Schwarz gave this the name `M-theory'. There is still no clear idea of what M-theory is. A proposal for the basic equations of M-theory in terms of particles moving in a noncommutative geometry (paper #5, by Banks, Fischler, Shenker, and Susskind), was the hottest topic in the field last year and still survives all tests that have been applied to it. But there is not yet a striking proof of this proposal, and so string theorists have begun to look for more insight along other routes.

One approach that has led to a great deal of insight has been to study the classical solutions of string theory, of which the charged black hole mentioned above provides an example. It turns out that string theory contains not only localized, particle-like solutions, but also solutions that resemble sheets of various dimensionalities. The technical name for a fundamental state which is a p-dimensional surface is a `p-brane', generalizing the notion of point particles (0-branes), strings (1-branes), and membranes (2-branes). (The abominable nomenclature is due to Townsend.) In paper #12, Polchinski identified the sheet-like states in string theory as `Dirichlet-p-branes', or `D-branes', a fancy term which denotes hypersurfaces on which strings can end. With this identification, many of the properties of D-branes can be worked out from string theory. Polchinski has written a beautiful set of lecture notes on D-branes (#16), which provides a basic sourcebook for this area of study.

D-branes have many unexpected properties, uncovered by Witten (#13), among others. The basic D-p-brane has a p-dimensional U(1) supersymmetric gauge theory living on its surface. For a D-brane of charge N this symmetry is extended, not to N U(1)'s, but to SU(N), an indication of the non-commutative nature of the D-brane geometry. Properties of vacuum states of supersymmetric SU(N) gauge theory can be read from the geometrical form of the D-branes. In the most remarkable example, Witten (#34) analyzed a system of two 5-branes in 11-dimensional M-theory. Including time, the 5-brane is a 6-dimensional object; Witten considered configurations with 4-dimensional translation symmetry. He found that the shape of the curved surface in the other 2 dimensions is exactly that of the space of vacuum states in the celebrated exact solution to SU(2) super-Yang-Mills theory that he and Seiberg discovered in 1994 (#9, #19).

With this understanding, we can look at a D-brane in two ways. Macroscopically, the D-brane is a thin surface with a supersymmetric Yang-Mills theory living on the surface. Microscopically, the D-brane is a solution to the string theory equations of motion, which become the Einstein equations of supergravity for slowly varying fields. A way to make the D-brane configuration slowly varying is to pile many D-branes on top of one another. Then, microscopically, we have a field configuration that, in the transverse dimensions, resembles a black hole solution. In the limit as we approache the horizon, the background space is approximately anti-de Sitter space (a cosmology of constant negative curvature). Macroscopically, we have a surface with a U(N) super-Yang-Mills theory living on it, considered in the limit of large N. This limit of Yang-Mills theory has its own special simplifications, pointed out long ago by 't Hooft (#24)

In paper #2, Maldacena conjectured that these two limiting theories are identical. Let me be more specific. Consider a compactification of 10-dimensional string theory in which 5 dimensions form an anti-de Sitter space while the other 5 dimensions form a sphere. The symmetries of 5-dimensional anti-de Sitter space form the large group SO(4,2); in the limit of zero curvature this includes both translations and the Lorentz group SO(3,1). The string theory compactification also has a high degree of supersymmetry, corresponding to N=8 supersymmetry in 4 dimensions. Anti-de Sitter space has the peculiar property that light can propagate to spatial infinity in finite time. Thus, the boundary at spatial infinity plays an essential role in the dynamics. Maldacena conjectured that an N=4 super-Yang-Mills theory on the 4-dimensional boundary completely describes the dynamics of this supergravity theory. Since the super-Yang-Mills theory is conformally invariant, it also has an SO(4,2) symmetry and doubled supersymmetry. In addition, as a consequence of the supersymmetry, it has a bosonic SO(6) global symmetry--just the symmetry of 5 spherical dimensions of the supergravity problem. At first sight, it seems peculiar that a theory in d dimensions can have all of the degrees of freedom of a theory in d+1 dimensions. But 't Hooft and Susskind have pointed out that, in order for the number of physical states in quantum gravity to reproduce the Bekenstein-Hawking black hole entropy, the number of states associated with any region of space should grow only as its surface rather than its volume. Maldacena's association of a local quantum field theory in d dimensions with a quantum gravity theory in d+1 dimensions gets this just right. More concrete formulae for this correspondence, and operator relations between the two theories, were proposed by Witten in #3 and by Gubser, Klebanov, and Polyakov in #4.

In Maldacena's correspondence, the classical limit of the supergravity theory is connected to the large-N limit of the super-Yang-Mills theory. This means that problems of large-N gauge theories--intrinsically quantum strong-coupling problems--can be attacked by classical gravity computations. Even before Maldacena, Gubser, Klebanov, Peet, and Tseytlin had computed the density of states of large-N super-Yang-Mills theory, and certain 2-point functions in this theory, using classical gravity. In #26, Witten proposed a Maldacena-type correspondence for nonsupersymmetric gauge theories that has given hope for `practical' applications of this formalism to QCD. Kachru and Silverstein used aspects of the correspondence to identify nonsupersymmtric gauge theories with some of the magical diagram cancellations associated with supersymmetry (#39).

At the same time, Maldacena's conjecture has sparked a new look at quantum gravity. At this moment, this is refleced in the citation lists only in the large numbers of citations for the black hole solution in anti-de Sitter space (found by Banados, Teitelboim, and Zanelli (#23) in 1992), and for the D-brane picture due to Strominger and Vafa (#30) of the black hole entropy and Hawking radiation (#31). I expect that we will hear more about this direction in next year's top citation listings.

The final M-theory topic which appears on the top citation list is the approach suggested by 11-dimensional models to phenomenologically reasonable grand unified theories. The foundational papers on this approach are those of Horava and Witten (#25, #37).

As a coda to this discussion, I would like to point out three surprising sociological features of Maldacena's paper (#2). The third is the very high citation count, already noted above. The second is that this paper was published in the journal

Advances in Theoretical and Mathematical Physics, which claims to have a print version but mainly exists on the internet only, as an overlay of the Los Alamos e-print depository. Thus, this paper, and also Witten's paper #3, might be seen to signal a shift away from conventional paper publication. This is overshadowed, though by the first surprise, that, according to my poll of SLAC and Stanford theorists, relatively few people realize that these papers are officially published at all, and everyone who had read them had gotten them directly from the Los Alamos archive. This indicates the arrival of a new mode of scientific publication, which is also illustrated in the following remark, overheard at lunch at SLAC: `He didn't publish it; he just sent the paper to thePhysical Review.'## Neutrinos

In experimental high-energy physics, the big news of 1998 was the announcement of the observation of neutrino oscillations by the SuperKamiokande experiment (#10). The idea of neutrino oscillations is an old one, going back to papers of Pontecorvo from the 1950's and of Sakata from the early 60's. To understand why neutrino oscillation should occur, consider first the flavor violation in the weak interactions of quarks. It is well established that, while the weak interactions mainly link the up quarks in each generation (u,c,t) with the corresponding down quarks (d,s,b), there are also weak interaction amplitudes that connect quarks in different generations. The quarks are identified by their mass, so one way of expressing this is to say that the quark mass matrices and the weak interaction couplings are diagonalized in different bases. If it this is true for quarks, shouldn't it also be true for leptons? (In grand unified theories, this is even required.) Since neutrino masses are very small, it makes the most sense, for leptons, to diagonalize the (e,mu,tau) mass matrix and define the various neutrinos to be the weak-interaction partners of the corresponding charged leptons. But then, if there are neutrino masses, and if the mass matrix is not diagonal in the same basis as the weak-interaction couplings, the neutrino states of definite mass will involve mixtures of the partners of the three leptons. In weak interaction process, then, a lepton will be accompanied by a coherent mixture of the three neutrinos in a potentially time-dependent wavefunction.

Potential oscillations due to large neutrino masses have been excluded for a long time. The neutrino experiments rely on special features to achieve sensitivity to very low masses, low-energy production of neutrinos and long travel distances from the source to the detector. For terrestrial neutrino sources, the best current limits come from the CHOOZ (#17) and LSND (#28) experiments. The second of these claims a signal corresponding to a neutrino mass of a few eV, the first claims exclusion of the effect in an almost overlapping region.

With water Cerenkov detectors and other large-mass neutrino detectors, it is possible to gain more sensitivity by using neutrinos of astrophysical origin--from cosmics rays or from the sun. It is a longstanding problem that neutrinos in the 10 MeV energy range are observed coming from the sun than ought to be produced in a reasonable model of the solar luminosity. Wolfenstein (#11) and Mikheev and Smirnov (#22) have presented an elegant mechanism of resonant flavor conversion in the sun to explain this deficit using neutrino oscillations. Another indication, dating from the 1980's, is the observed deficit of muon versus electron neutrinos in cosmic rays. This anomaly, first discovered by the IMB proton decay experiment, was confirmed by the Kamiokande experiment and by others; the most recent Kamiokande paper is #27. The important progress of the recent SuperKamiokande results was the ability to detect an azimuthal dependence to the muon neutrino deficit--only a small deficit for downgoing neutrinos, a large deficit for upgoing neutrinos which originated on the other side of the earth. All previous evidence for neutrino oscillations depended on knowledge of the production and detection processes and could be questioned on this basis. For the superKamiokande results, this azimuthal varying has no apparent explanation except for neutrino mass and the oscillations those mass terms should produce.

## High-Energy Physics Resources

In truly high-energy experimental physics the only highly cited papers are those describing simulations of the Standard Model background processes above which we eventually hope to see signals of new physics. The description of the popular event generator PYTHIA ranks as #6 on the citation list. Two parametrizations of the parton structure functions of hadrons, those of Gluck, Reya, and Vogt and of the CTEQ collaboration, rank as #18 and #21, respectively. Positions #7 and #8 on the list are held by the two leading reviews of supersymmetry as a candidate for physics beyond the Standard Model, those of Nilles and Haber and Kane. This is an indication that experimenters are searching assiduously for those exotic signals. Perhaps, next year, Nature will be kinder in showing her hand.

The final positions on the top-40 citation list are held by classic theory papers. The top positions belong to the original paper of Kobyashi and Maskawa on CP violation by flavor mixing (#14) and the explication by Altarelli and Parisi of parton evolution in hard scattering processes (#15). The list also includes the paper of Shifman, Vainshtein, and Zakharov which first applied QCD sum rules to light hadrons (#29), the paper of Balitskii and Lipatov on high-energy scattering in perturbative QCD which introduced the `BFKL pomeron' (#32), the two original papers of Gasser and Leutwyler on chiral perturbation theory (#33, #35), Weinberg's 1967 paper on the electroweak theory (#38), and the 1960 paper of Nambu and Jona-Lasinio which introduced chiral symmetry breaking into the phenomenology of hadrons (#40). And, amazingly for a field such as ours that tends to devour its past, the ethereal 1951 paper of Julian Schwinger which explained how gauge invariance is manifested in QED is still a top-cite, almost 50 years later, at #36.

## All-Time Favorites

Here we present the list of all-time favorite articles in the HEP database. The list contains the 62 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. Seventeen of the listed papers were published inPhysical Review, seventeen inNuclear Physics, ten inPhysical Review Letters, five inPhysics Reports, five inPhysics Letters, and seven 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 twelve of these papers also appear among the most highly cited articles of 1998.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. We hope that, in the coming era of B-factories, this proposal can be put 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. 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 #7 and Pati and Salam #10.

The next group of papers contains the leading works on the structure of the strong interactions. Next comes 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 #8. The paper #9 presents applications of the ITEP QCD sum rules by Shifman, Vainstein, and Zakharov. Finally, the original papers by Politzer and Gross and Wilczek which announced the discovery of asymptotic freedom appear as #17 and #19 (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 paper on quantum field theory in the instanton field (#11) and 't Hooft's original letter on instanton effects #18, Nambu and Jona-Lasinio's paper on chiral symmetry breaking (#13), the foundational paper of Belavin, Polyakov, and Zamolodchikov on conformal field theory (#15), and the Coleman-Weinberg paper on the effective potential (#16). Below, we find Adler's paper on the axial vector anomaly (#20), the paper of `t Hooft and Veltman on dimensional regularization (#21), Guth's proposal of inflationary cosmology (#22), Polyakov's paper on the functional integral formulation of string theory (#23), and the paper of Candelas, Horowitz, Strominger, and Witten (#24) which introduced string compactifications on Calabi-Yau manifolds. Any particle physicist who has not read these papers is not an educated person.

The last three of the top 25 papers are classic review articles on phenomenological topics, the reviews of supersymmetry by Nilles and Haber and Kane (#12 and #14, differing by 84 citations) and the review of `supercollider physics' (still relevant under the title `LHC physics') by Eichten, Hinchliffe, Lane, and Quigg (#25). Ken and Chris will be disappointed to note that the supersymmetrists have overtaken them by more than 300 citations, and that the gap is growing every year. On the other hand, this is only the court of public opinion.

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 (#44) and SLAC (#52).

The complete list shows titles, authors, publication information, and the exact number of citations on December 31, 1998.

## 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.

Original list by H. Galic

1998 Edition by Michael Peskin

Work performed at Stanford Linear Accelerator Center (SLAC)

Copyright

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