An elementary particle collision never before observed

In the standard model of particle physics, the W⁺, W⁻, and Z are so-called vector bosons that transmit the weak force responsible for nuclear decay. But those bosons can also scatter off each other with a cross section that is sensitive to the many details of the theory. Vector-boson scattering is rare; for example, at the Large Hadron Collider the process happens less often than Higgs boson production. Thus, it had never been used to probe particle theory’s accepted paradigm. But ATLAS team members at the LHC have now spotted 34 candidate WW scattering events. The W bosons are neither the particles crashed together by the LHC (those are protons) nor the outgoing particles detected by ATLAS. Rather, as illustrated in the Feynman diagram, during the collision a W (here a W⁺) is radiated by a quark in each of the protons. After scattering, each W⁺ decays into a positron or antimuon—both members of a larger particle class called leptons—and an undetected neutrino. The outgoing quarks, which cannot exist in isolation, are manifest as particle jets. The experimental signature of the W⁺W⁺ scattering is thus two positive leptons, two jets, and missing neutrino energy. With sophisticated numerical calculations and background corrections, the ATLAS researchers conclude that their observations are consistent with the standard model. As the LHC collision energy rises and precision improves, vector-boson scattering may test the Higgs mechanism of the model in detail and even point to new, nonstandard physics. (G. Aad et al., ATLAS collaboration, Phys. Rev. Lett., in press.)