The physicist’s guide to the universe

Teasing out the intricate measurements that separate the smallest components of matter in the universe often involves monumental machines and huge international scientific collaborations. So it’s important that particle physicists are on the same page – or, rather, pages – when it comes to particle-physics results. For the past 60 years, the definitive collection of particle-physics evaluations and reviews has been bound up in a weighty print volume called the Review of Particle Physics, which is published every other year. The latest (2016) edition of what is sometimes referred to as the “bible of particle physics” contains 1808 pages in the complete version published online, and features 117 review articles on topics ranging from the Higgs boson to the Big Bang, statistics and particle detectors. Its Particle Listings include evaluations of 3062 new measurements from 721 papers, in addition to 35,436 measurements from 9843 papers published in earlier editions. The staff behind it carefully evaluate data on around 8000 different quantities to provide averages, fits and best limits.

The Review is the all-time most highly cited publication in particle physics, with recent editions eventually reaching more than 6000 citations. It also has a companion 344 page booklet that is the descendant of “wallet cards” first issued in 1957, with a summary of particle data from the main Review. The PDG website (pdg.lbl.gov) features the complete content of the book as both PDF files and in an interactive version, with downloadable figures and tables, as well as educational materials. The Review continues to grow as we learn more about the basic constituents of matter, and its history reflects a field that is continuously evolving.

Berkeley beginnings

The Review of Particle Physics and its associated publications and website are the products of the international Particle Data Group (PDG), which since its beginnings has been headquartered at the University of California Radiation Laboratory, now the Lawrence Berkeley National Laboratory (Berkeley Lab), in California. More than 200 authors around the globe currently contribute to the contents of the Review, including 3.5 full-time-equivalent physicists in the PDG group at Berkeley Lab who also co-ordinate the effort.

The story began towards the end of 1957 with a paper in the Annual Review of Nuclear Science authored by the late Arthur “Art” Rosenfeld and Murray Gell-Mann. The tables of particle masses and lifetimes associated with that article, which Rosenfeld prepared with Walter Barkas in an unpublished report, “Data for Elementary-Particle Physics,” are credited with PDG’s inception. “The damn thing just grew,” Rosenfeld said of the wallet-card summary of that first report, which now fills a spiral-bound booklet.

Rosenfeld said in 1975 that the motivation for the original 1957 report was to provide particle data for early computer programs that were used to process the data from new particle-physics experiments, including bubble-chamber experiments. The following year the report was revised. The report was next revised in 1961, and during these first few years of the Review Rosenfeld and his colleagues intermittently distributed updates of the report to the particle-physics community, along with the updated wallet cards.

New discoveries in the field led to a growing need for particle-data resources, and Rosenfeld was clear that the 1963 edition should be the last attempted without the help of a computer. A separate effort by Finnish physicist Matts Roos called “Tables of Elementary Particles and Resonant States” also illustrated that it was no longer possible for a single person to compile data critically, reckoned Rosenfeld. So the two separate efforts joined forces, with five Berkeley authors and Roos publishing “Data on Elementary Particles and Resonant States” in 1964. This article, which appeared in the Reviews of Modern Physics journal, comprised 27 pages plus three wallet cards.

The group branded itself as the Particle Data Group in 1968 and published its first data booklet that year. By 1974 the report, by then called Review of Particle Properties, had grown to 200 pages and had 13 authors, several of them based in Europe. An escalation of discoveries in the field during the mid-1970s provided the cornerstones for the Standard Model of particle physics, which described the family of known and theorised particles and their properties. The heavy crush of particle data flowing into PDG during this period led the staff to implement a new media format for distributing some data and additional quality-control measures. In 1973 a microfiche with references and backup material was included in an envelope at the back of the book.

Since then, the population of particle physicists worldwide has exploded and the print version of the Review and related booklets are currently distributed to thousands of physicists. INSPIRE, an information system that tracks published materials and experiments in the field of high-energy physics, now counts more than 1100 active experiments in the field, compared to about 300 in 1975, and the number of particle physicists has also increased from about 7000 in 1975 to an estimated 20,000 today. The print book was getting so big – growing at a rate of about 10% per year – that the PDG dropped its Listings from the print edition in 2016.

“We would have had to print two volumes if we continued to include the Listings. Given that there is likely no single person who wants to read through a major fraction of the Listings, this wasn’t justified,” recalls Juerg Beringer of Berkeley Lab, who became leader of the PDG in 2016. “Looking up data from the Listings online is anyway far more convenient.” The Listings are still available on the PDG website and included in the online journal publication.

Review articles

Many sections in the Listings are accompanied by review articles that provide further information on the data presented. Other review articles summarise major topics in particle physics or cosmology. Review articles can vary from about a page to tens of pages in length, and roughly two-thirds of review articles require updates in each edition. The first PDG review article on the Higgs boson, which appeared in 1988, was two pages long. Today, five years after the Higgs was discovered, the review is about 50 pages long and is the most viewed review on the website, with more than 50,000 downloads each year. PDG even delayed publication in 2012 to accommodate the Higgs boson’s discovery, with staff scrambling to add the Higgs addendum to the Review within eight days of the discovery’s announcement on 4 July 2012.

The scope and importance of PDG has grown substantially, especially during the past 30 years (figure 1). While the size of the PDG group at Berkeley Lab has remained essentially the same, a large number of physicists worldwide were recruited to keep up with the flood of publications in particle physics and write the many PDG review articles that now cover almost every aspect of particle physics. There are now 223 authors who contribute to the review articles or Listings and each will typically write a single review article or handle one Listings section. Collaborators outside Berkeley Lab are volunteers who usually spend only a small fraction of their time on the Review, while PDG group members at Berkeley Lab typically spend half of their time working on the PDG (see image). There is also a European-based PDG “meson team” of about a dozen members, which holds meetings twice a year at CERN, while another PDG sub-group called the baryon team is responsible for the data on baryon resonances.

Michael Barnett, a Berkeley Lab physicist and previous head of PDG having been in the role for 25 years, recalled his first experience working with the Review production when he joined Berkeley Lab in 1984. “It was barely 300 pages long and still put together by hand,” Barnett said. “We used 20 rolls of Scotch Tape to stick pieces together for the camera-ready copy. The section on B mesons was a single page. These days the B-meson section alone is over 120 pages.” In earlier days, the data for the publications were stored on computer punch cards. The print data back then appeared as only uppercase letters, with no mathematical symbols, because the punch cards couldn’t accommodate them. Under Barnett’s watch the design and layout became more reader-friendly. Particle categories multiplied, with properties listed in detail. Many new reviews were added to help explain the content of Listings sections.

Computing era

In the late 1980s a then-modern computing system was developed that served the PDG well for two decades. But a major upgrade eventually became inevitable, and the COMPAS group from the Institute of High Energy Physics in Protvino, Russia, which had been a PDG collaborator for many years, began working on prototypes for a new computing system. Working with COMPAS and experts from Berkeley Lab’s Computational Research Division, Beringer led the development of a new web-based computing platform that was supported by a special grant from the US Department of Energy (DOE). As a result, each collaborator can now directly add data to the PDG database rather than channelling it all through the PDG editor. This platform has made Review updates far more manageable. “The new system allows collaborators to see changes immediately, without waiting for the editor to go through thousands of e-mails with instructions on what to change,” says Piotr Zyla, who succeeded Betty Armstrong as PDG editor in 2003.

As with any large-scale, data-intensive publishing endeavour, there have been a few notable glitches. The 1994 booklet had a ruler with centimetre marks that were shrunk by the publisher so that each centimetre was actually 0.97 of a centimetre. The error was discovered too late to fix, but not too late to insert a disclaimer citing fictitious and comical explanations for why the centimetres fell a bit short: “The PDG feels it has the right to redefine anything it wants”; “The booklets were returned from the printer at 0.25 times the speed of light”; and “A theorist is in charge of the PDG.”

Barnett and his colleagues had considered publishing the Review on the internet since the early days of the World Wide Web – which, of course, was created at CERN in 1989 to more easily share research data around the world. The entire contents of the Review were available on the web in 1995 and its interactive version, pdgLive, appeared with the 2006 edition. An increasingly sophisticated PDG web presence has been influenced by membership surveys asking readers whether in the digital age a printed book is essential, useful, or altogether unnecessary. The first survey, in 2000, got about 2450 responses, half of which found the print version useful and well over a third found it essential. By 2014 the number of responses had tripled. While there was a clear trend in favour of online publications, many respondents still emphasised the importance of the printed book. As one respondent stated in the 2000 survey, “I could live without my right arm, but I don’t want to.”

“We expected older physicists to be the ones who valued the book and the younger ones, who grew up with the internet, not to care,” says Barnett. “We got it backwards. Everybody used the web, but more grad students and postdocs found the printed book essential.” Their comments told why: to those entering physics, the book was not merely a reference but an introduction to the unexplored dimensions of their field. The distribution scheme for the print publications has become fairly sophisticated to minimise shipping costs. There are now four separate distribution channels: in Switzerland, Japan, China and the US. Receiving the print materials is not automatic and recipients must specifically request each new edition. The audience is largely physicists, teachers, students and physics fans, with most mailings going out to high-energy physics centres and academic institutions.

The bulk of the funding for PDG comes from the Office of Science of the DOE and supports the co-ordination and production activities at Berkeley Lab. Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) contributes to these efforts via a US–Japan agreement on co-operative research and development. CERN supports the meson team, and in recent years CERN and the Institute of High Energy Physics of the Chinese Academy of Sciences have paid for most of the printing and shipping costs of books and booklets. Funding agencies in multiple countries, including INFN in Italy, MINECO in Spain, and IHEP in Russia provide travel and other support to PDG collaborators in their countries.

Until recently, the PDG group at Berkeley Lab was able to handle most of the PDG co-ordination tasks. But with the growth of PDG in recent years, combined with a challenging funding environment, even this has become increasingly difficult. Thankfully, INFN recently agreed to help Berkeley Lab in this area. A recent effort to streamline and automate many aspects of PDG’s operations is also providing necessary relief.

Contributing knowledge

The published results collected in the Listings provide best values and limits for a wide range of particle properties. The data can also be used to study how knowledge in particle physics evolves, for example by plotting the evolution of PDG best values over time (figure 2 and table).

Over the decades there have been occasional disputes and discrepancies for PDG staff to resolve. In one instance, discussions escalated to a threatened lawsuit over PDG’s refusal to include one researcher’s particle data in the Review’s Summary Tables. There was also a case in which the experimental measurements of the mass squared of one particle (the electron neutrino) appeared as a negative number in the data: since this is mathematically impossible, the PDG editors adjusted the error margins to account for the problem. Another unusual episode concerned claims of discoveries of pentaquarks about a decade ago and later experiments discounting those earlier claims. These ups and downs, including the latest measurements from the LHCb experiment, were covered in the reviews to keep readers up to date.

When various data are in substantial conflict, PDG sets error bars that range across the whole span of results, or, in some cases, provides no average at all. Also, about 20 years ago, the PDG instituted a new naming scheme that more logically renamed many particles. All of them stuck except for one – there was an international campaign against that name-change, so the PDG staff deferred in this one instance.

Only a subset of data collected by PDG is available in a downloadable format suitable for further processing. There is a demand for such access from researchers running Monte Carlo programs and others who want to, for example, investigate the statistical properties of the agreement between multiple measurements of the same quantity. “Making all PDG data available in a machine-readable format is a very high priority. We’ve wanted to do this for a long time as there are many uses and a lot of interest from the community. But we can barely keep up with the ongoing updates of the Review, and so the implementation of such new features takes much more time than we would like,” Beringer says.

Rosenfeld, in the conclusion of his 1975 paper assessing the work of PDG, noted challenges even then in supporting the data needs of the scientific community: “As we write this review we wonder if we have not been too modest in our requests for support…we feel that PDG is doing an effective job, but if we could spend, each year, one-fifth of the typical experiment [in those days the typical experiment cost about $3 million], it could provide broader and more timely services.”

The gradual transition from print to primarily online distribution is expected to continue, in line with the overall shift of publishers toward online publication but also, in part, because of the high cost of printing and mailing the books. Nevertheless, as long as there is continuing demand and adequate resources, PDG hopes to continue the printed book. “Producing and updating the Review of Particle Physics in modern formats will remain PDG’s core mission,” says Beringer.

Online access to PDG’s increasingly mobile-friendly web pages, or a PDG smartphone app with the complete contents of the Review in a mobile-friendly format, could in principle replace the PDG booklet. But especially for students, the PDG book and booklet  also carry substantial symbolic value, and the booklets are often distributed in introductory particle-physics classes. “It is a landmark thing for some of the graduate students and postdocs,” Barnett remarks. “When you get your first book, and when you see your data appearing, you feel like you are a particle physicist.”