# 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 go-to 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 X-ray wavelengths. The observation was then followed up by a 3,500+ author paper on multi-wavelength 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.) Gravity-wave 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 Peccei-Quinn 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 LHC-related 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 next-to-leading 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 anti-kT 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 off-scale citation count--456 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 now-classic 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

INSPIRE Topcites homepage

### References

 [a] Planck 2018 results. VI. Cosmological parameters Planck Collaboration (N. Aghanim (Orsay, IAS) et al.). Jul 17, 2018. 71 pp. e-Print: arXiv:1807.06209 [astro-ph.CO] | PDF References | BibTeX | LaTeX(US) | LaTeX(EU) | Harvmac | EndNote ADS Abstract Service [b] Multi-messenger Observations of a Binary Neutron Star Merger LIGO Scientific and Virgo and Fermi GBM and INTEGRAL and IceCube and IPN and Insight-Hxmt and ANTARES and Swift and Dark Energy Camera GW-EM and DES and DLT40 and GRAWITA and Fermi-LAT and ATCA and ASKAP and OzGrav and DWF (Deeper Wider Faster Program) and AST3 and CAASTRO and VINROUGE and MASTER and J-GEM and GROWTH and JAGWAR and CaltechNRAO and TTU-NRAO and NuSTAR and Pan-STARRS and KU and Nordic Optical Telescope and ePESSTO and GROND and Texas Tech University and TOROS and BOOTES and MWA and CALET and IKI-GW Follow-up and H.E.S.S. and LOFAR and LWA and HAWC and Pierre Auger and ALMA and Pi of Sky and DFN and ATLAS Telescopes and High Time Resolution Universe Survey and RIMAS and RATIR and SKA South Africa/MeerKAT Collaborations and AstroSat Cadmium Zinc Telluride Imager Team and AGILE Team and 1M2H Team and Las Cumbres Observatory Group and MAXI Team and TZAC Consortium and SALT Group and Euro VLBI Team and Chandra Team at McGill University (B.P. Abbott (LIGO Lab., Caltech) et al.). Oct 16, 2017. 59 pp. Published in Astrophys.J. 848 (2017) no.2, L12 LIGO-P1700294, VIR-0802A-17, FERMILAB-PUB-17-478-A-AE-CD DOI: 10.3847/2041-8213/aa91c9 e-Print: arXiv:1710.05833 [astro-ph.HE] | PDF [c] Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant Adam G. Riess et al.. Apr 27, 2018. 13 pp. Published in Astrophys.J. 861 (2018) no.2, 126 DOI: 10.3847/1538-4357/aac82e e-Print: arXiv:1804.10655 [astro-ph.CO] | PDF References | BibTeX | LaTeX(US) | LaTeX(EU) | Harvmac | EndNote ADS Abstract Service [d] First measurement of the Hubble constant from a dark standard siren using the Dark Energy Survey galaxies and the LIGO/Virgo binary-black-hole merger GW170814 DES and LIGO Scientific and Virgo Collaborations (M. Soares-Santos (Brandeis U.) et al.). Jan 6, 2019. 19 pp. Published in Submitted to: Astrophys.J. FERMILAB-PUB-18-629-AE e-Print: arXiv:1901.01540 [astro-ph.CO] | PDF [e] Observation of $H \rightarrow b\bar{b}$ decays and $VH$ production with the ATLAS detector ATLAS Collaboration (Morad Aaboud (Oujda U.) et al.). Aug 24, 2018. 28 pp. Published in Phys.Lett. B786 (2018) 59-86 CERN-EP-2018-215 DOI: 10.1016/j.physletb.2018.09.013 e-Print: arXiv:1808.08238 [hep-ex] | PDF [f] Combined measurements of Higgs boson couplings in proton-proton collisions at $\sqrt{s}=$ 13 TeV CMS Collaboration (Albert M Sirunyan (Yerevan Phys. Inst.) et al.). Sep 27, 2018. 67 pp. Published in Submitted to: Eur.Phys.J. CMS-HIG-17-031, CERN-EP-2018-263 e-Print: arXiv:1809.10733 [hep-ex] | PDF [g] Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector ATLAS Collaboration (M. Aaboud et al.). Jun 1, 2018. 19 pp. Published in Phys.Lett. B784 (2018) 173-191 CERN-EP-2018-138 DOI: 10.1016/j.physletb.2018.07.035 e-Print: arXiv:1806.00425 [hep-ex] | PDF [h] Observation of $\mathrm{t\overline{t}}$H production CMS Collaboration (Albert M Sirunyan (Yerevan Phys. Inst.) et al.). Apr 7, 2018. 17 pp. Published in Phys.Rev.Lett. 120 (2018) no.23, 231801 CMS-HIG-17-035, CERN-EP-2018-064 DOI: 10.1103/PhysRevLett.120.231801 e-Print: arXiv:1804.02610 [hep-ex] | PDF