Physics Top Ten: Higgs Bosons, Neutrinos, Polymer Solar Cells, and Stellar Surveys Vie for Attention

January 2014
Cross section of an LHC dipole in the tunnel. | Photo: Maximilien Brice | Courtesy of CERN

The pair of papers heading the current Top Ten reports the observation of the long-awaited Higgs boson. The Nobel Prize in Physics 2013 was recently awarded jointly to François Englert and Peter W. Higgs for their contributions to understanding the origin of mass. (And just prior to the Nobel announcement, both scientists were selected as Thomson Reuters Citation Laureates, demonstrably worthy of science’s highest award.) The citation for the Prize refers directly to the discoveries reported in #1 and #2, specifying the scientists’ “theoretical discovery of a mechanism…which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider” (LHC). According to citations recorded in the Web of Science during a recent two-month period, these are currently the most-cited physics papers published in the last two years.

 

What’s Hot in Physics

Rank Paper Citations This Period (May-Jun 13) Rank Last Period (Mar-Apr 13)
1 ATLAS Collaboration, “Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC,” Phys. Lett. B, 716(1): 1-29, 17 September 2012. [267 institutions worldwide] 132 1
2 CMS Collaboration, “Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC,” Phys. Lett. B, 716(1): 30-61, 17 September 2012.  [228 institutions worldwide] 126 2
3 L.T. Dou, et al., “Tandem polymer solar cells featuring a spectrally matched low-bandgap polymer,” Nature Photonics, 6(3): 180-5, March 2012.  [U. Calif., Los Angeles; Natl. Renew. Energy Lab., Golden, CO] 48 3
4 J.K. Ahn, et al., “Observation of reactor electron antineutrinos disappearance in the RENO experiment,” Phys. Rev. Lett., 108(19): No. 191802, 11 May 2012. [12 South Korean institutions] 33 5
5 H.P. Zhao, et al., “Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells,” Optics Express, 19(4): A991-1007, 4 July 2011. [Lehigh U., Bethlehem, PA; Case Western Reserve U., Cleveland, OH] 32 10
6 E.F. Schlafly, et al., “Measuring reddening with Sloan Digital Sky Survey stellar spectra and recalibrating SFD,” Astrophys. J., 737(2): No. 103, 20 August 2011. [Harvard U., Cambridge, MA; Harvard Smithsonian Ctr. Astrophys., Cambridge, MA] 32 +
7 K. Abe, et al., “Indication of electron neutrino appearance from an accelerator-produced off-axis muon neutrino beam,” Phys. Rev. Lett., 107(4): No. 041801, 18 July 2011. [67 institutions worldwide] 31 9
8 V. Mourik, et al., “Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices,” Science, 336(6084): 1003-7, 25 May 2012. [Delft U. Technol., Netherlands; Eindhoven U. Technol., Netherlands] 30 +
9 D.J. Eisenstein, et al., “SDSS-III: Massive spectroscopic surveys of the distant universe, the Milky Way, and extra-solar planetary systems,” Astronom. J., 142(3): No. 72, September 2011. [84 institutions worldwide] 30 +
10 C.E. Small, et al., “High-efficiency inverted dithienogermole-thienopyrrolodione-based polymer solar cells,” Nature Photonics, 6(2): 115-20, February 2012. [U. Florida, Gainesville] 30 +

SOURCE: Thomson Reuters Web of Science
NB. Only papers indexed by Thomson Reuters since July 2011 are tracked. The “+” sign indicates that the paper was not ranked in the Top Ten during the last period. In the event that two or more papers collected the same number of citations in the most recent bimonthly period, total citations to date determine the rankings

 

According to the Standard Model (SM) of particle physics, the mean lifetime of a Higgs boson is 1.56 x 10-22 s and the mass is 126 GeV/c2. Such a short-lived particle cannot be identified directly, so the search methods analyze the decay products, hoping to identify remnants that could only be created by the decay of the Higgs particle. That’s much harder than the proverbial search for a needle in a haystack. The results announced in #1 and #2 required an analysis by the Worldwide LHC Computing Grid of 3 x 1014 proton-proton collisions.

At the LHC, the ATLAS and Compact Muon Solenoid (CMS) collaborations had already reported in 2011 excesses of events in proton-proton collisions with a center-of-mass energy of 7 TeV. Although the events were compatible with the production and decay of the Standard Model Higgs boson in the mass range 124–126 GeV, the standard deviations were too low to be convincing.

With the collision energy at the LHC ramped up to 8 GeV the statistics improved significantly. The ATLAS collaboration conducted its searches in three different modes through which a Higgs boson decays into two photons, or into two Z bosons, or into two W bosons. The sister experiment CMS obtained its best statistics for decay modes to two photons or two Z bosons. The ATLAS and CMS teams announced these findings at a seminar held at CERN on 4 July 2012, and publication followed on 17 September 2012.

Full confirmation came on 14 March 2013 with an announcement from CERN that the new particle had zero spin and positive parity, as predicted for a Higgs boson by the SM. This was the culmination of a lifetime of effort, and the beginning of an entirely new era in particle physics. In November 2013 ATLAS issued exciting news of evidence that the Higgs can decay into tau fermions, a property strongly favored by the SM. Alternative theories going beyond SM physics predict several bosons, and eliminating them will require much more data.

LHC is currently shut down for maintenance and an upgrade of the beam energy to 6.5 GeV. Experiments will resume in 2015, when the collision energy will be raised to 13 TeV. The LHC teams will learn more about the basic forces that have shaped the universe since the beginning of time. Among the possible unknowns that still lie in the future, include extra dimensions of space, microscopic black holes, and a search for dark-matter candidates.

 

Neutrinos Appear and Disappear

Elsewhere in the Top Ten there’s a pair of papers, #4 and #7, on neutrino oscillations. The oscillation phenomenon shows that neutrinos are not massless. The scale and phase has implications for the imbalance of matter and antimatter that we observe in the universe. Both papers report measurement of the mixing angle q13 , a parameter that describes how neutrinos oscillate between the electron flavor and the tau flavor. This was thought to be very small, maybe even zero.

In Japan the T2K (Tokai-to-Kamioka) experiment finds evidence of muon neutrino to electron neutrino oscillation. The T2K collaboration fired a muon neutrino beam to the SuperKamiokande underground detector 295 km away, where the detector looked for signs of electron neutrinos appearing in the beam. Just six events were found. Although the statistics were not robust enough to hit the 3-sigma confidence level, they did spark a global race to measure q13 ,  because T2K found that the muon-electron mixing angle is surprisingly large. This paper is highly cited because it launched neutrino physics on a new path, along which the possibility of testing CP violation by neutrinos beckons.

Paper #4 describes the disappearance over short baselines (294 and 1383m) of reactor-generated electron antineutrinos. The experiment gave a large value for sin22q13 , with a confidence level of 4.9-sigma. Disappearance experiments are considered more reliable than appearance experiments because they have a relatively simple oscillation formula. The citation rate of #4 has overtaken #7 in the ScienceWatch analysis because of the improvement in the confidence level.

 

Making Solar Cells More Efficient

In applied physics, two papers, #3 and #10, report improvement in the power conversion efficiency (PCE) of polymer solar cells. Huge efforts are being made to design and improve heterojunction solar cells because polymers are transparent, light in weight, and suitable for roll-to-roll processing. The solar panels of our dreams could be made in carpet factories and become mass-market consumer products.

A neat feature of the set-up outlined in #3 is that harvesting a broader spectrum of solar radiation is achieved by stacking solar cells with complimentary photoactive properties in a multi-layer device. Paper #3 gives the recipe for making a polymer that has delivered a PCE of 8.62%, which is considered quite an advance on the single-layer PCE of around 6%. The big breakthrough reported by #3 is the design of a new type of conjugated polymer.

Paper #10 likewise reports on improvements in polymer chemistry, leading to a PCE of 7.4%. In this paper, the reported breakthrough is a technical advance in prepared low-defect and uniform ZnO films for the electron-transporting layer.

 

Sloan Digital Sky Survey

The Sloan Digital Sky Survey (SDSS) contributes two papers, #6 and #9, to the Top Ten in this session. SDSS has been superlative as a facility for undertaking spectrometry and photometry of huge numbers of stars and galaxies.

Paper #9 provides an overview of the six-year structured program known as SDSS-III. It commenced in July 2008, and it builds on the success of SDSS-I and SDSS-II to produce four wide-field spectroscopic surveys devoted to three themes. The three strands are: dark energy and the cosmological parameters; the history and structure of the Milky Way; and the population of giant exoplanets around other stars. SDSS has always provided regular public releases of its data, and #9 is being cited by the numerous groups making use of the SDSS-III made public from January 2011.

Paper #6 is a splendid example of the exploitation of SDSS-III data. Edward Schlafly and Douglas Finkbeiner (Harvard University) used the spectra of 500,000 stars in our galaxy. Interstellar dust scatters and absorbs starlight, particularly in the ultraviolet through infrared. The authors of #6 focused on the two-dimensional distribution of dust by deriving the amount of reddening in the individual spectra of the stars. Mapping the three-dimensional distribution of dust is an important future goal that will require data mining of the Pan-STARRS (Hawaii) and LAMOST (China) sky surveys in order to build on the SDSS-III results.


Dr. Simon Mitton is in the Department of the History and Philosophy of Science, University of Cambridge, UK.

The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.