With new precision measurements of the Higgs boson, ATLAS has observed its decay to two W bosons.

The discovery in 2012 by the ATLAS and CMS experiments at CERN of a Higgs boson marked an important milestone in the history of particle physics. It confirmed a key prediction of the Standard Model, the theory that describes fundamental particles and their interactions.

The Standard Model makes many different predictions regarding the production and decay properties of the Higgs boson, most of which can be tested at the Large Hadron Collider (LHC). Since the discovery, experimentalists from the ATLAS collaboration have analysed the complete dataset recorded in 2011 and 2012, have improved the calibration of the detector, and have increased substantially the sensitivity of their analyses.

The new measurements constitute the most stringent tests of the Standard Model Higgs sector presented by ATLAS up until now. They feature updated measurements of the decay of the Higgs boson to pairs of photons and Z bosons, and the observation of the decay of the Higgs boson to pairs of W bosons at a level of significance exceeding 6 standard deviations.

The signal strength μ represents the ratio of the observed Higgs production and decay rate to the Standard Model prediction. A μ value consistent with one signifies that the measurement is in agreement with the Standard Model prediction. ATLAS measures signal strengths of μ=1.17 ± 0.27 for the diphoton channel, μ=1.44^+0.40_-0.33 for the ZZ channel, and μ=1.08^+0.22_-0.20 for the WW channel, consistent with Standard Model predictions.

Read more: "New ATLAS Precision Measurements of the Higgs Boson – Observation of WW Decay"