Review of Top Cited HEP Articles of 1999

Review of Top Cited HEP Articles

1999 Edition

Reviewer is Michael Peskin, with earlier editions by Hrvoje Galic.

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, 1999.

Top-Cited Papers of 1999

Here we present the list of the 40 high energy physics articles that have collected the most citations in calendar year 1999. 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 (1998 and 1996) have collected 1269 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 and Branes

The next three papers in the citation count were also papers # 2, 3, and 4 in last year's topcites list. These are the basic papers on the magical correspondence, discovered by Maldacena [2], between quantum gravity in (d+1)-dimensional anti-de Sitter space and scale-invariant nongravitational field theories in d dimensions. The physics of this correspondence was explained at some length in last year's topcite review. In fact, all of the top-cited papers in this category, with the exception of the review paper [17], appeared also on last year's list. Thus, I will be brief in this year's report. You can find an extended explanation of the physics of Maldacena's duality in last year's topcites review.

Last year, I also remarked on the very high citation count of Maldacena's paper, a count comparable to the total number of physicists working in string theory. Apparently, all of these people are still working on Maldacena's duality, because in 1999 this paper acquired an additional 625 citations. In fact, this paper is the fastest to reach the 1000-citation plateau in the history of the SPIRES-HEP database.

Let me now briefly summarize the recent progress in this area. The work of Witten [33] and Hull and Townsend, pointing out duality relations between strong and weak coupling in string theory, set off a furious exploration of nonperturbative string physics. The most important result of this work was Polchinski's realization [14] that `D-branes' (or `Dp-branes' with p= 0,1,2,..., (p+1)-dimensional hypersurfaces in space-time on which strings are permitted to end) are also classical solutions of nonperturbative string theory characterized as sources of certain gauge fields. D-branes have unexpected properties, worked out by Polchinski, Witten [35], and many others. Most notably, D-branes carry gauge fields confined to their surfaces, and, for a collection of N coincident D-branes, the gauge symmetry is U(N), not N copies of U(1). In supersymmetric theories, the U(N) gauge bosons are accompanied by multiplets of fermions and scalars, all confined to the brane surface. Separating a collection of N D-branes into groups of K and L is equivalent to spontaneously breaking U(N) to U(K) X U(L) by the Higgs mechanism. More complicated arrays of stretched D-branes exhibit geometric counterparts to the nonperturbative effects of strongly-coupled supersymmetric Yang-Mills theory discovered by Seiberg and Witten [19]. The physics of D-branes is lucidly reviewed by Polchinski in [24]. The amazing properties of D-branes have motivated Banks, Fischler, Susskind, and Shenker to propose that all of string theory can be derived from a picture in which the brane point particles, D0-branes, are the fundamental objects [15]. This work, which was paper #2 of 1997, continues to suggest new and unexpected properties of string theory at strong coupling.

Maldacena triggered the most recent stage of development of this field with his conjecture [2] that two pictures derived from a collection of N coincident D-branes in different limits are, in fact, identical quantum theories. Viewed as a soliton of a gravity theory, the region near a Dp-brane has a (p+2)-dimensional anti-de Sitter geometry with gravity and string excitations living in it. Viewed as a surface on which strings end, the D-brane can be viewed as a quantum field theory containing a scale-invariant Yang-Mills theory. In this view, however, it is (p+1)-dimensional. Nevertheless, one can imagine matching Green's functions in the lower-dimensional space to S-matrix elements for the high-dimensional space. Witten [3] and Gubser, Klebanov, and Polyakov [4] wrote an explicit operator relation for the connection. The analysis is becoming well understood for maximally supersymmetric cases, but the correspondence has also been applied, by Witten [30]and others, to study strongly coupled theories with neither supersymmetry nor scale invariance, for example, ordinary QCD. Several of the leading figures in this study--Aharony, Gubser, Maldacena, Ooguri, and Oz--have written a lengthy review article [17] which takes the dedicated reader from the foundations of Maldacena's duality to these enticing applications.


Earlier editions of the top-40 citation list have included at most four experimental papers. This year's list includes six, and all concentrated in one area, the search for neutrino oscillations. What is the news?

The idea of neutrino oscillations was conceived in work of Pontecorvo from the 1950's and of Sakata and his group in the 1960's. In the description of the weak interactions based on the electroweak gauge theory, neutrino oscillations are the natural manifestation of neutrino mass. Up and down quarks, charged leptons, and neutrinos all come in three flavors. In general, the couplings of these particles might be off-diagonal in this flavor quantum number. In the electroweak theory, there are two possible couplings that could be off-diagonal, the gauge coupling which links up- and down-type species to the W boson, and the Higgs couplings, which are also responsible for the fermion masses. Typically, we distinguish quarks by mass. Thus, it makes sense to diagonalize the matrix of Higgs couplings. This leads to off-diagonal gauge couplings, parametrized by the Cabibbo-Kobayashi-Maskawa (CKM) mixing angles. Similarly, it makes sense to diagonalize the matrix of Higgs couplings to the charged leptons. But the masses of neutrinos are very small, and neutrinos are typically produced in weak interactions with energies large compared to any reasonable upper bound on their masses. In this circumstance, it is most useful to diagonalize the gauge coupling. Thus, we refer to the electron-, muon-, and tau-type neutrinos as those produced in weak interactions in association with the corresponding charged lepton. If the gauge and Higgs couplings are not simultaneously diagonalizable, the neutrino mass matrix will have off-diagonal terms in this weak-interaction basis, and thus the various species of neutrino will mix as neutrinos propagate. Typically, models for the fermion masses which allow nonzero CKM angles also predict that, if the neutrino mass matrix is nonzero, it should have off-diagonal terms. In grand unified theories with nonzero neutrino masses, such neutrino mixing is even required.

There are two ways to observe neutrino mixing. In an `appearance' experiment, one creates a neutrino in association with a lepton of one flavor and observes a charged-current reaction giving a lepton of a different flavor. In a `disappearance' experiment, one creats a flux of neutrinos in association with leptons of one flavor and then measures a smaller flux in the inverse charged-current process some distance away. The ideal, controlled experiment would use neutrino beams produced in a well-defined way at an accelerator. Neutrino mixing has also been searched for with a nuclear reactor as the source of (anti-)neutrinos. In that case, the low energy dictates that the neutrino must have been produced with an electron. However, the only convincing positive evidence of neutrino oscillations to date involves astrophysical neutrino sources, that is, neutrinos from the sun and neutrinos produced in the atmosphere by cosmic rays. The observations involve the disappearance of expected neutrino flux. For these experiments, it is wise to be very skeptical that the neutrino sources are well enough understood that the calculated fluxes should be believed.

The crucial breakthrough in the study of neutrino oscillations was the observation by the Super-Kamiokande experiment in 1998 of a difference in the amount of disappearance for upward- versus downward-going atmospheric muon neutrinos [5, 32]. It had been known for a long time that the overall rate of muon charged current events in underground detectors was lower than what was predicted from models of cosmic ray neutrino production, while the corresponding estimates for electron neutrinos were in reasonable agreement. This result was first presented in 1986 by the IMB collaboration and was established with increasing confidence by the Kamiokande collaboration and others. The Super-Kamiokande high-statistics measurement of this effect appears on the topcite list as [28]. However, one always had to wonder whether the Monte Carlo predictions of the atmospheric neutrino fluxes could be flawed. The huge size of the Super-Kamiokande detector (50 kT) allowed the experimenters to collect a large sample of higher-energy atmospheric neutrinos, for which the direction could be reliably determined. A deficit was found only in the sample of muon neutrinos that originate on the other side of the earth. This effect cannot be explained by a defect in the atmospheric neutrino production model but only by a flavor-changing property of the neutrinos themselves. There is corroborating evidence for the Super-Kamiokande result from the MACRO and Soudan 2 experiments; the current situation is reviewed in Mann's talk at the 1999 Lepton-Photon Symposium (hep-ex/9912007).

In principle, the muon neutrinos which disappear from the Super-Kamiokande experiment could be oscillating to electron or tau neutrinos, or to neutrinos of a new type. Since only three neutrino species appear in Z decays, any new light neutrino must be `sterile', that is, uncharged under the weak interaction gauge symmetry. The possibility that the muon neutrinos oscillate to electron neutrinos is excluded by the fact that neutrino oscillations are not observed in a powerful antineutrino disappearance experiment at the Chooz nuclear reactor [25]. So the interpretation of the Super-Kamiokande results is restricted, though several possibilities are still open.

A second setting in which there is considerable evidence for neutrino mixing is in the study of solar neutrinos. It has been a persistent problem for measurements of the solar neutrino flux, beginning with the original experiment of Davis in the 1960's, that the measured flux was much lower than the theoretical prediction. For a long time, it seemed reasonable to attribute the discrepancy to problems with the model of solar structure. However, the structure predicted by Bahcall's `standard solar model' has been remarkably confirmed over the past decade by helioseimology measurements. Another important theoretical development was the realization by Mikheev and Smirnov [20] that the effect of a nonzero matter density on the neutrino mass matrix, first studied by Wolfenstein [10], could lead to a substantial conversion of electron- to muon-neutrinos in the sun. Taking account of this MSW effect, one finds four distinct solutions which account for the solar neutrino deficits observed by different detectors---two solutions relying on the MSW effect and having large and small mixing angles, a solution mainly relying on oscillations in vacuum which average out to the observed deficit over variations in the earth-sun distance, and a so-called `just-so' solution in which the oscillation length is of the order of the earth-sun separation. Bahcall's recent review of the current situation appears on the topcite list as [16] The various possible solutions can be distinguished by the Super-Kamiokande experiment and by other new solar neutrino experiments through their prediction of characteristic perturbations of the measured flux, a day/night effect for the large-angle MSW solution, a distortion of the energy spectrum for the small-angle MSW solution, and an annual modulation for the just-so solution. None of these effects could be explained by modification of the solar model; thus, the observation of one of these effects would not only select a particular set of parameters of neutrino mixing but also comfirm that the overall deficit is due to neutrino mixing rather than solar structure. Super-Kamiokande has presented suggestive but statistically unpersuasive evidence concerning these effects. Stay tuned!

The most straightforward type of neutrino mixing to observe at an accelerator is the oscillation of a muon neutrino, produced from an intense pion beam, into an electron neutrino. There have been many searches over the years, almost all with negative results. However, in 1996 the LSND experiment at Los Alamos reported an observation of muon- to electron-neutrino oscillations [38, 40]. For various reasons, this result has not been generally accepted by the community. An example of the skeptical response is given in Richter's summary talk at the 1999 Lepton-Photon conference [hep-ex/0001012]. It does not help that there is a competing experiment at the Rutherford Laboratory, KARMEN, which excludes neutrino oscillations in a region of the mixing parameter space that almost, but not quite, overlaps the region claimed by LSND. The KARMEN experiment is a very beautiful one, making use of precision timing to remove almost all sources of background. It is tempting to say that this experiment did not make the topcites list because it reported a negative result, but real reason seems to be that the results have not actually been published. A substantial part of the LSND region is also excluded by the constraint from the Chooz reactor experiment [25] mentioned above. In the next two years, a new experiment at Fermilab, called mini-BooNE (following Fermilab's official capitalization conventions), should definitively confirm or exclude the LSND result. A summary of all of the current constraints on neutrino masses and mixings is given in this figure prepared by Hitoshi Murayama.

It is noteworthy that there is no theoretical paper on the topcite list giving an interpretation of the Super-Kamiokande data. The large mixing angle required to explain the data took everyone by surprise. Though there are now many theoretical models which incorporate this result, there is not yet a particularly compelling model that has won wide acceptance. For those who would like to break into the topcites, this question is still open.

Extra Space Dimensions

There have long been speculations that the universe contains more than four dimensions. In fact, additional, extremely small space dimensions are required in string theory and give this subject is geometrical flavor. However, it is assumed in the standard viewpoint that these new dimensions are so small that they can never be observed. In the past few years, a number of new developments in string theory and in particle phenomenology have shaken the community out of this complacency. In particular, in 1998, a paper by Arkani-Hamed, Dimopoulos, and Dvali [6] made the striking claim that there might be unobserved new dimensions of size as large as a millimeter. This year, nine papers on the topcites list discuss the experimental implications of the presence of new space dimensions.

In principle, new dimensions could be present at any size between the current observational limits and the Planck scale. However, there are specific choices that are most well motivated. To describe these, I should first remind you that, if there exist new space dimensions larger than the Planck size, then the fundamental mass scale of quantum gravity is lower than the Planck mass. To see this, note that, in 4 dimensions, the gravitational potential is 1/r, while in (4+n) dimensions, the gravitational potential varies as 1/r^{1+n}. The Planck mass is defined from the dimensionful coefficient of the measured gravitational potential. If, as we move to distances smaller than the size of some extra dimensions, the gravitational potential increases faster than 1/r, we will arrive sooner at the scale at which gravity becomes strong.

There are four choices for the size of extra dimensions which have been given the most attention in the literature. These can be classified as micro-, mini-, midi-, and maxi-size extra dimensions. The micro-dimensions are those which are of order the Planck scale. I will consider the alternative possibilities in turn.

Mini-size extra dimensions are those which are of the order of the inverse of the grand unification mass scale, 2 x 10^16 GeV. In such models, the fundamental gravitational scale is also lower than the Planck scale and can actually also be of the order of the grand unification scale. This scenario was first explored by Horava and Witten [12, 18, 37] who showed that the strong-coupling limit of the 10-dimensional heterotic string theory is an 11-dimensional theory in which gauge fields and fermions live on 10-dimensional walls at the boundaries of the 11-dimensional space. Supergravity lives in the interior. By making the 11th dimension slightly larger than other compact dimensions, it is possible to achieve a unification of gravity with the gauge interactions of elementary particle physics. An interesting conjecture is that a supersymmetric version of the Standard Model lives on one wall, while the `hidden sector' responsible for supersymmetry breaking lives on the other wall. Horava, Randall and Sundrum, and others have advanced proposals for the characteristic spectrum of supersymmetric particles that follows from this scenario. There is no agreed-upon result yet, but I am hopeful that we can eventually test this idea by precise measurement of the spectrum of supersymmetric particles at the LHC and at an e+e- linear collider.

Midi-size extra dimensions are those which are of the order of the inverse of the TeV mass scale. Antoniadis [22] first proposed new dimensions of this size to break supersymmetry in string theory with TeV-scale superparticle masses. Dienes, Dudas, and Gherghetta [23] have studied the possibility of grand unification in this scenario. Necessarily, unification must occur at a lower scale than usual; this might lie between 10^5 and 10^11 GeV. The scenario has dramatic predictions for the LHC and the linear collider. If the photon, Z, and gluon fields live in the full higher-dimensional space, these fields will have Fourier components with nonzero momentum in the compact directions. The magnitude of momentum in the extra dimensions will appear from a 4-dimensional point of view as a contribution to the mass. These components will thus appear in high-energy experiments as new spin 1 fields with the gauge couplings of the familiar photon, Z, and gluon, but with TeV-scale masses. By observing the spectrum of these heavy states, we would experimentally reconstruct the Fourier transform of the new dimensions. Randall and Sundrum [41] have proposed a model in which the Planck scale and the TeV scale are related by the effect of space-time curvature in the extra dimensions. This model has similar experimental signatures, except that the new resonances visible in experiments are modes of the graviton field.

Maxi-size extra dimensions were introduced by Arkani-Hamed, Dimopoulos, and Dvali in the paper mentioned at the beginning of this section [6]. The idea is the result of taking the notion of lowering the fundamental gravitational scale to its logical extreme and moving this scale all the way down to the TeV scale. This requires extra dimensions of size 1 fermi (for n=6) to 1 millimeter (for n=2). For a particle physicist, these length scales are not only macroscopic but enormous. The validity of Standard Model for high-energy scattering tells us that quarks, leptons, photons, and gluons cannot move into these large extra dimensions. So it is necessary to assume that the Standard Model lives on a 4-dimensional surface inside the large (4+n)-dimensional space. It is possible that this surface could be a D3-brane (see above), in which case the scale of string theory would also be the TeV scale and string amplitudes would be visible at high-energy colliders. This possibility was explored by Lykken in [31]. The idea of maxi-size extra dimensions has a number of striking consequences, first explored by Antoniadis, Arkani-Hamed, Dimopoulos, and Dvali in [8] and [9]. Experiments at high energy should show missing-energy events from the radiation of gravitons into the new dimensions. Standard Model scattering amplitudes should be modified. Many aspects of unified model-building and many features of the evolution of the early universe must be rethough. I find this opening of new possibilities exhilarating. In future years, we will see where it leads.

The previous three paragraphs give three different scenarios in which experiments at the next generation of accelerators might reveal that Nature contains more than four dimensions. Those who believe that the Standard Model is close to a final theory of particle physics should take note that Nature could have more--and wilder--surprises in store for us.

CP Violation

The year 1999 saw the confirmation of the existence of direct CP violation and the overthrow of the superweak model by the NA48 and KTeV experiments, the first evidence for CP violation outside the neutral kaon system in the CDF experiment's study of B -> psi + K_S, and the startup of major new experimental programs in CP violation at SLAC and KEK. However, the only paper on CP violation that appears on the topcites list is the classic paper of Kobayashi and Maskawa [27] which presented the mechanism by which of CP violation could appear in the Standard Model. However much I would like to see Kobayashi and Maskawa celebrated, I would like even more to see their theory overturned. Perhaps this will happen in the year 2000.

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 [7] and HERWIG [26], and the parametrizations of parton distributions by the CTEQ collaboration [21], by Martin, Roberts, Stirling, and Thorne [34], and by Gluck, Reya, and Vogt [39]. Completing the list of the top-40 cited papers, we find the classic reviews of supersymmetry by Nilles [11] and Haber and Kane [13], and two classic theoretical papers, Hawking's original paper on Hawking radiation [29] and Gasser and Leutwyler's description of chiral perturbation theory [36].

This past year, I had the opportunity to review the major topics in high-energy physics for one of the summer conferences. You might be interested to compare the reference list of that review [hep-ph/0002041] to the list of the top-40 cited papers. It is amusing to note the extent of the overlap (or, perhaps, the lack thereof).

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 Who of theoretical high-energy physics. Nineteen of the listed papers were published in Physical Review, eighteen in Nuclear Physics, eleven in Physical Review Letters, six in Physics Letters, five in Physics 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 1999.

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 be on an equally strong footing (see above). 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 [11].

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 [9] and [25] present 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 [18] and [19] (inexplicably differing by 10 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 [17], 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 [20], the paper of `t Hooft and Veltman on dimensional regularization [21], Adler's paper on the axial vector anomaly [22], Polyakov's paper on the functional integral formulation of string theory [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 two of the top 25 papers are the classic review articles on supersymmetry by Nilles and Haber and Kane [10] and [13], (differing by 90 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 [26]. Ken and Chris will be disappointed to note that the supersymmetrists have overtaken them by more than 480 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 [47] and SLAC [59]. In fact, the citations of the SLAC paper announcing the discovery of the psi have already been overtaken by those of Maldacena's paper [58] ([2] this year) discussed at the beginning of this article.

The complete list shows titles, authors, publication information, and the exact number of citations on December 31, 1999.


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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Top Cited HEP Articles, 1999 edition by Heath O'Connell, SLAC. Reviewer is Michael Peskin, SLAC. Original edition by H. Galic. Work performed at Stanford Linear Accelerator Center (SLAC)