ATLAS experiment finds evidence of three massive vector boson productions

Data compared to expectation for the distribution of the invariant mass of two jets. The signal (VVV, in yellow) is scaled to the measured value.
Credit: ATLAS Collaboration/CERN

The ATLAS Experiment at CERN has just released evidence for the simultaneous production of three W or Z bosons in proton–proton collisions at the Large Hadron Collider (LHC). The W and Z bosons are the mediator particles of the weak force – one of the four known fundamental forces – which is responsible for the phenomenon of radioactivity as well as an essential ingredient to our Sun’s thermonuclear process.

A new window for exploration

The new ATLAS result is based on data collected by ATLAS during 2015–2017 at a collision energy of 13 TeV. It provides evidence of “tri-boson” events with a significance of 4 standard deviations. This indication is but the latest chapter in a decades-long history of measurements with weak bosons. The W and the Z bosons were discovered in 1983 at CERN’s proton-antiproton collider. In 1996, at CERN’s Large Electron-Positron (LEP) collider, events with two W bosons were first observed, and shortly thereafter ZZ events were found. A decade after that, WW, WZ and ZZ events were observed at Fermilab’s Tevatron collider. Large rates of diboson events are now produced at the LHC, allowing for precise measurements.

Rare tri-boson production processes are predicted by the Standard Model of particle physics. Their production involves self-interaction among the weak bosons, so-called triple and quartic gauge boson couplings, which are sensitive to possible contributions from yet unknown particles or forces.

Since weak bosons are unstable, they are reconstructed in the detector via their decays to pairs of leptons (including invisible neutrinos) or quarks – the latter forming sprays of particles, called “jets.” ATLAS physicists combined searches for different decay modes and different types of tri-boson production, including events with three W bosons (“WWW”), and events with one W boson, one Z boson and a third boson of either variety. The latter are known as “WVZ” events, where the “V” is a shorthand for “W or Z.”

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