First High-Precision Measurement Of The W Boson's Mass Obtained At CERN

Display of a candidate event for a W boson decaying into one muon and one neutrino from proton-proton collisions recorded by ATLAS. CERN

Researchers using the Large Hadron Collider (LHC), the flagship experiment of CERN, have obtained the first high-precision mass measurement of one of the heaviest fundamental particles in the universe, the W boson. The boson is one of the carriers of weak nuclear force, which is crucial to phenomena like radioactive decay and nuclear fission.

The W boson has a mass of 80370±19 MeV, which is about 86 times the mass of a single proton. The measurement, reported in the European Physical Journal C, is based on about 14 million W bosons observed in the LHC during its first run back in 2011.

The collider is now working at twice the energy it had during the first run and researchers are excited about the possibility of refining such a measurement in the near future. The value currently obtained is not a paradigm shift and is consistent with what has been previously measured at the LEP, the predecessor to the LHC, and in the US at the Fermilab’s Tevatron particle accelerator.

“Achieving such a precise measurement despite the demanding conditions present in a hadron collider such as the LHC is a great challenge,” said the physics coordinator of the ATLAS Collaboration, Tancredi Carli, in a statement. “Reaching similar precision, as previously obtained at other colliders, with only one year of Run 1 data is remarkable. It is an extremely promising indication of our ability to improve our knowledge of the Standard Model and look for signs of new physics through highly accurate measurements.”

The Standard Model of particle physics is the theoretical scaffolding that connects all the fundamental particles to their interactions. It is one of the finest scientific achievements of the human race but it remains limited, as, for example, it doesn’t include gravity. Physicists hope to move past the model and are currently looking for measurements that don’t fit the theory well.

The mass of the W boson could be one of these measurements. The Standard Model gives us a more precise value than what we can currently measure with our experiments, so it could potentially deviate. The mass of the W boson is also linked to the masses of the Higgs boson and the top quark by quantum physics relations, so by improving on one measurement, researchers can better understand the whole model.

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