Physicists from the ATLAS Collaboration at CERN’s Large Hadron Collider (LHC) have observed a process called WWW production: the simultaneous creation of three massive W bosons in high-energy proton-proton collisions.
Display of a candidate WWW→ 3 leptons + neutrinos event; the event is identified by its decay to a muon (red line), two electrons (blue lines) and missing transverse energy (white dashed line). Image credit: CERN.
The W boson, named after the weak force, plays a crucial role in testing the Standard Model of particle physics.
Though discovered nearly four decades ago, it continues to provide physicists with new avenues for exploration.
“Measurements of multiple electroweak bosons production at the LHC constitute an important test of the Standard Model,” said Dr. Jessica Metcalfe, a physicist at the Argonne National Laboratory and a member of the ATLAS Collaboration.
“They are sensitive to the properties of electroweak-boson self-interactions and provide a test of the electroweak theory and of the perturbative quantum chromodynamics.”
As one of the heaviest known elementary particles, the W boson is able to decay in several different ways.
The ATLAS physicists focused their search on the four WWW decay modes that have the best discovery potential due to their reduced number of background events.
In three of these modes, two W bosons decay into charged leptons (electrons or muons), carrying the same positive or negative charge, and neutrinos, while the third W boson decays into a pair of light quarks.
In the fourth decay mode, all three W bosons decay into a charged lepton and a neutrino.
The researchers analyzed the full LHC Run-2 dataset, recorded by the detector between 2015 and 2018.
They analyzed around 20 billion collision events recorded and pre-filtered by the ATLAS experiment in their search for just a few hundred events expected from the WWW process.
They observed the WWW process with a statistical significance of 8.2 standard deviations — well above the 5 standard-deviation threshold needed to declare observation.
“This exciting measurement also allows us to look for hints of new interactions that might exist beyond the current energy reach of the LHC,” they said.
“In particular, we can use the WWW production process to study the quartic gauge boson coupling — where two W bosons scatter off each other — a key property of the Standard Model.”
“New particles could alter the quartic gauge boson coupling through quantum effects, modifying the WWW production cross section.”
“The continued study of WWW and other electroweak processes offers an enticing road ahead.”
Jessica Metcalfe et al. (ATLAS Collaboration). Measurements of multi-boson production at ATLAS. EPS-HEP Conference 2021