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Home > INSPIRE  Topcites 2018 Edition Review 
The top ranks of the 2018 Topcite list are again dominated by astrophysics. The top spot on the list (now numbered 0) is always held by the central reference work for our field, the Review of Particle Properties [0]. Below this, the #1 paper is again the 2015 paper on cosmological parameters from measurements of the cosmic microwave background radiation by the Planck Collaboration [1]. The Planck results are not likely to be displaced soon as the goto reference for the shape and contents of the universe. The 2015 results in [1] were updated in August 2018 [a] but that paper has not yet reached Topcite status.
Just below, the Topcite list is full of gravitational wave detections. Paper [3] is the original observation of the merger of two black holes by the LIGO and Virgo collaborations, released in February 2016. More observations of mergers are found at [8, 15, 17, and 27]. Paper [8] is the report by LIGO and Virgo in October 2017 of a merger of two neutron stars that was discussed prominently in last year's report. A neutron star coalescence spews out hot matter, including heavy elements newly produced in the encounter, and produces a spectacular signal observable at visible and Xray wavelengths. The observation was then followed up by a 3,500+ author paper on multiwavelength observations of the event, named the 2017 breakthrough of the year by Physics World [b]. (This latter paper, however, missed the cutoff for the 2018 Top Forty on INSPIRE's list.) Gravitywave astronomy has arrived, and we expect that it will provide many more important discoveries over the coming years.
We should note that there are still unresolved issues in the area of cosmological measurements. The most controversial point today is the value of the Hubble constant. The 2018 Planck analysis [a] reports $H_0 = 67.4±0.5$ in units of km/sec/Mpc; this is not in agreement (by $3.6\sigma$) with the latest results based on measurements by the Hubble Space Telescope, which give $H_0 = 73.52 ± 1.62$ [c]. The Hubble analysis uses Cepheid stars as "standard candles" whose intrinsic luminosity is known a priori. There is a new analysis by LIGO, Virgo, and the Dark Energy Survey that uses a black hole merger as a "standard siren", whose (gravitational) noise is known a priori, and achieves $H_0 = 75.2 ± 35$ [d]. While this is not yet impressive in itself, this method is capable of achieving an accuracy of a few percent in the next five years as gravitational wave events accumulate. The most exciting possibility is that the value of Hubble constant measured in the present era is not compatible with the value extrapolated from the time that the cosmic microwave background was created. This would indicate that there are more light species in the universe than the photon and neutrinos called for in the Standard Model.
Other astrophysical topics prominent on the Topcite list are measurements of the accelerating expansion of the universe [22,26, and, of course, 1], inflation ([21] for the Planck measurements, [24,25] for the original papers of Guth and Starobinsky), and searches for dark matter ([32,33] for the most recent XENON1T and LUX results). The possibility of axions as a dark matter candidate returned the 1977 PecceiQuinn paper on the strong CP problem [28] to the Top Forty in 2017 after a thirty four year hiatus. Adding the original papers on black hole radiation by Hawking and Bekenstein [19,37], 16 of this year's Top Forty Topcite papers link to astrophysics and cosmology.
Papers related to LHC are also prominent on the Topcite list. The ATLAS and CMS papers on the discovery of the Higgs boson continue to hold a high rank [6,7]. The JINST papers describing the CMS and ATLAS detectors also appear prominently [14,18]. So far, these experiments have not reported any major deviations of the properties of the Higgs boson from the predictions of the Standard Model. Still, the 2016 ATLAS/CMS summary of the properties of the Higgs boson appears as [30]. In 2018, ATLAS and CMS confirmed the presence of the last two major couplings of the Higgs boson to be discovered, the couplings to the bottom quark [e,f] and top quark [g,h]. These challenging analyses deserve applause  and citations.
Another category of LHCrelated papers are those on software tools used in the analysis of the LHC data. Remarkably, the top position after Planck is held by the authors of the program MadGraph5_aMC@NLO, which automates the computation of nexttoleading order QCD corrections to literally any relevant LHC process [2]. This remarkable development, which makes the recent breakthroughs in precision theory available to the graduate student on the street, deserves its 1100 citations. Also very high on the list, with 1039 citations, is the GEANT4 software package for detector simulation [4]. Going down the Topcite list, we find the antikT jet algorithm [9], the event generator PYTHIA [10,12,13], the FastJet package for jet clustering and analysis [11], the NNPDF parton distributions [20], the POWHEG Box strategy for computing QCD corrections [34], the DELPHES 3 package for simplified detector modelling [39], and two papers on statistical methods [29,38]. In all, 11 papers, a quarter of the list, fall into this category. Keep citing these papers, and keep applying these very useful tools!
Among papers in theoretical theory, the classic papers on the discovery of the AdS/CFT correspondence retain their high standing. The breakthrough paper of Maldacena appears at [5], with 1035 citations. We described Maldacena's paper in our 1998 Topcites report, beginning with the comment "The paper #2, by Juan Maldacena, has an offscale citation count456 citations in a year, a number comparable to the total size of the string theory community (including wannabees)." Twenty years later, it is still providing insight into strongly coupled field theory and quantum gravity, and many other topics that link to these fields. The papers by Witten and Gubser, Klebanov, and Polyakov that clarified this discovery appear at [16,23]. The nowclassic 2006 paper of Ryu and Takayanagi that applies Maldacena's idea to compute entanglement entropies appears at [31].
Rounding out the list are the classic theory papers on neutrino masses [35,36], discussed in detail in last year's review.
Heath O'Connell and Michael Peskin