The collaborations working on the world’s leading particle-collider experiments have joined forces, combined their data and produced the first joint result from Fermilab’s Tevatron collider and CERN’s Large Hadron Collider. Scientists from the four experiments involved – ATLAS, CDF, CMS and D0 – announced their joint findings on the mass of the top quark at the 2014 Rencontres de Moriond international physics conference on 19 March. The four collaborations pooled their data-analysis power to arrive at a world’s best value for the mass of the top quark of 173.34±0.76 GeV/c2.

Experiments at the LHC and the Tevatron collider are the only ones that have observed the top quark – the heaviest-known elementary particle. Its large mass makes it one of the most important tools in the quest to understand the nature of the universe.

The CDF and D0 experiments discovered the top quark in 1995, and the Tevatron produced some 300,000 top-quark events during its 25-year lifetime, before it finally shut down in 2011 (CERN Courier December 2011 p43). Now the LHC is the world’s leading top-quark factory, having produced close to 18-million events with top quarks since it started collider physics operations in 2009.

Each of the four collaborations had previously released their individual measurements of the top-quark mass. Combining them together required close collaboration between the four large groups of researchers, and a detailed understanding of each other’s techniques and uncertainties. Each experiment measured the mass of the top quark using several different methods. The analyses involved a variety of top-quark decay channels, employing sophisticated techniques that have been developed and improved over more than 20 years of top-quark research, beginning at the Tevatron and continuing at the LHC.

More than 6000 researchers from more than 50 countries participated in the four experimental collaborations.

While this article was in preparation, the CMS Collaboration released the world’s most precise single measurement of the top-quark mass in the semileptonic decay channel, using the experiment’s full sample of data at 8 TeV. Combined with the previous CMS results, this gives a mass of 172.22±0.73 GeV/c2. More details will appear in the next edition of CERN Courier.