## Review of Top Cited HEP Articles

1997 Edition## 1997 Edition by: Michael Peskin

Earlier editions 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, 1997.

## 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 359,000 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 festschift 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 1997

Here we present the list of the 40 high energy physics articles that have collected the most citations in calendar year 1997. 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. Please note that, while previous versions of the Top-40 list have integrated over a three-year period, this version looks at citations only for the past year. We have also tried to give this version of the Top-40 list a new personality, which we hope at least adds entertainment value.

## 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.

## M-theory

The next four top-cited papers, and 19 of the top 40, deal with aspects of string theory. This reflects a ferment in this area which has bubbled up from the remarkable discoveries made in 1995 by Hull and Townsend (#14) and Witten (#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'. As for what M-theory is, that is the question of the hour. Schwarz' lucid review of the duality relations implied by M-theory appears on our list as #31. Vafa's proposal of a 12-dimensional fundamental theory is #29. (Vafa is coy about the reality of the second time dimension in this proposal.)

M-theory is not a string theory. Instead, it contains excitations that can be described as higher-dimensional closed surfaces. 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.) The 2-brane of the 11-dimensional theory was studied already in 1987 by Bergschoeff, Sezgin, and Townsend, and was found at that time to lead to strings after compactification of one dimension. M-theory also contains 5-branes, and this and the 2-brane generate a variety of surface-like states in string theory. In the new developments, Polchinski (#7) identified these as `Dirichlet-p-branes' or `Dp-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. This study has become an industry, and it even has spin-offs, to be described in a moment. Polchinski's beautifully clear lecture notes on D-branes (#5 and #36) deserve their high rank on our list.

What about M-theory itself? Here there is no certain knowledge. The paper #2 on our list is the conjecture that has kicked up the most dust during the past year. This is the `M(atrix)-theory' proposal of Banks, Fischler, Shenker, and Susskind #2, who claim that the D0-branes are the fundamental objects from which all other states can be constructed. These branes do have remarkable properties. They resolve distances in space-time smaller than the sizes of strings (Douglas, Kabat, Pouliot, and Shenker, #26), they couple to a non-commutative space-time (Witten, #12, Polchinski, #5, and Douglas et al.), and, after compactification, they sprout extra degrees of freedom (Taylor, #38). Whether the theory of D0-branes will turn out to be fundamental description of M-theory or just a useful heuristic principle will be a major topic in next year's Top-40 list.

In the meantime, the calculus of branes has many important applications. Strominger and Vafa (#11) and Callan and Maldacena (#37) have shown that black holes solutions of string theory can be thought to contain D-branes in their compactified dimensions. The ensemble of strings which end on these D-branes has an entropy equal to the Bekenstein-Hawking entropy, thus giving a physical statistical mechanics picture of this quantity. In another domain, Dp-branes contain supersymmetric gauge theories in p space dimensions living on their surfaces. This suggests that there is a connection between the geometry of branes and the exact properties of these gauge theories. A pioneering paper in this direction, by Hanany and Witten, appears as #25. In #39, Witten uses brane dynamics to give a new derivation of the celebrated exact solutions of four-dimensional N=2 super-Yang-Mills theory of Seiberg and Witten (still popular as #4 and #9). Seiberg's qualitative picture of N=1 super-Yang-Mills theory (#34) can also be understood in this way. There will be more developments along this line in next year's Top-40 list.

## Experimental Developments

With the Standard Model triumphant, there have been only a few experimental articles that have captured the wide attention in the theoretical community needed to win a place on the Top-40 list. The only pure experimental papers on the list this year are the announcements by the CDF and D0 experiments at Fermilab of the discovery of the top quark (#15 and #19, differing by 9 citations) and the announcements by the H1 and ZEUS experiments at HERA of an anomalous excess of events at large momentum transfer (#16 and #17, differing by 1 citation). The top quark is for real. It is now observed in several different channels, and the experimenters' concern has turned to precision mass measurements. The HERA anomaly, which made a big splash in 1997, was probably a fluctuation. Though in the early reports it seemed that these events appeared at a single value of Feynmann x and so could be caused by a new particle, further results announced in the summer did not corroborate that conclusion. More data is expected in the coming year.

The Standard Model is so successful that Monte-Carlo programs which simulate Standard-Model physics are cited as highly as experiments. The most popular program, PYTHIA, by Anderson, Sjostrand, and their collaborators at Lund, appears as #6. HERWIG, by Webber and collaborators at Cambridge, appears as #23. Recent fits to the proton structure functions also appear in the Top-40 list. Before the HERA experiments were done, it was anticipated that the parton distributions would show interesting effects at small Feynman x due to the saturation of the cascade of quark and gluon production which appears in Altarelli-Parisi parton evolution. One of the other big disappointments of HERA is that the observed parton distributions are fit by a simple Altarelli-Parisi picture down to the smallest values of x observed. This is documented by Gluck, Reya, and Vogt in paper #13. Papers #21 and #22 give the state-of-the-art parton distribution fits of the CTEQ collaboration. Curiously, the pedagogical article by the CTEQ group that explains how to fit for parton distributions has only 40 citations in the HEP database; it deserves to be on the all-time list.

## Supersymmetry Phenomenology

While we wait for the discovery of physics beyond the Standard Model, it still pays to look for new ideas about what is out there. Over the past year, all of the most popular approaches have involved the assumption of supersymmetry. This is indicated by the high standing of the review papers on supersymmetry by Nilles and Haber and Kane (#8 and #10, differing by 5 citations). These articles are both quite dated. Nilles' paper is still valuable for its discussion of the theoretical principles of supersymmetry model construction. The paper of Haber and Kane, on the other hand, is now useful only as a source for the Feynman rules for the supersymmetric Standard Model. Haber and Kane also include a section on how to avoid minus sign errors in computations with Majorana fermions, but these pages do not seem to have had the beneficial effects that the authors anticipated. A new pedagogical review of supersymmetry, which includes the new ideas in model-building developed since 1984 and an updated treatment of the phenomenology of superparticle, would be a valuable contribution, and it would immediately generate a bushel of citations.

A new strategy for the construction of realistic models with supersymmetry, due to Dine, Nelson, Nir, and Shirman, appears as #32 on the list, accompanied by a preceding work at #30. This idea of 'gauge mediation' moves supersymmetry breaking down to lower energies and gives a characteristic prediction for the superspectrum. It also opens the door to models which include fermion compositeness and other strong-coupling effects, though these have not appeared on our list yet. Another new route to model-building involves the use of the new extra dimension from M-theory. The original paper on this idea is that of Horava and Witten (#33).

## Old Friends

The remaining papers on the list are classic theoretical works. These have continuing value as touchstones to various lines of research, but they are also very instructive papers, and I hope that people are continuing to read them. Our list includes the original paper by Kobayashi and Maskawa on CP violation in the Standard Model (#18), the original paper on chiral symmetry breaking and the hadron mass spectrum by Nambu and Jona-Lasinio (#28), and Hawking's original paper on black hole radiation (#35). All three of these papers are of Nobel-Prize quality and feel fresh and interesting today. Finally, the list includes four very important contributions to theoretical methodology, the paper of Altarelli and Parisi on parton evolution (#20), the paper of Shifman, Vainstein, and Zakharov on the ITEP QCD sum rules (#24), the second paper on chiral perturbation theory by Gasser and Leutwyler (#27) and `t Hooft's paper on the quantum field theory in the field of an instanton (#40).

These, then are the Top 40 papers of 1997 in terms of their citations in the HEP database. The complete list shows titles, authors, publication information, and the exact number of citations on December 31, 1997.

## All-Time Favorites

Here we present the list of all-time favorite articles in the HEP database. The list contains the 58 journal articles with more than 1,000 citations recorded since 1974 in the HEP database. This list reads like a

Who's Whoin high-energy physics. Seventeen of the listed papers were published inPhysical Review, sixteen inNuclear Physics, ten inPhysical Review Letters, five inPhysics Reports, four inPhysics Letters, and six 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 nine of these papers also appear among the most highly cited articles of 1997.The number one position in citations goes again to the Particle Data Group, accumulating nearly 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 #6 and Pati and Salam #10.

The next group of papers contains the leading works on the structure of the strong interactions. Wilson's paper which demonstrated the confinement of quarks in QCD appears as #7. Next comes the paper of Altarelli and Parisi (#8) 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. 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 #16 and #17 (inexplicably differing by 9 citations).

Almost all of the other top 20 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, the foundational paper of Belavin, Polyakov, and Zamolodchikov on conformal field theory (#12), the Coleman-Weinberg paper on the effective potential (#13), and Nambu and Jona-Lasinio's paper on chiral symmetry breaking (#14). Below, we find Adler's paper on the axial vector anomaly (#20), Polyakov's paper on the formalism of string theory (#21), `t Hooft and Veltman's paper on dimensional regularization (#22), the paper of Candelas, Horowitz, Strominger, and Witten (#23) which introduced string compactifications on Calabi-Yau manifolds, and Guth's proposal for inflationary cosmology (#24). 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 (#15 and #19, differing by 76 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 about 200 citations. On the other hand, this is only the court of public opinion.

The complete list shows titles, authors, publication information, and the exact number of citations on December 31, 1997.

## 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

1997/1998 Edition by Michael Peskin

Work performed at Stanford Linear Accelerator Center (SLAC)

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