W boson width measured for first time at Large Hadron Collider

Physicists have made a measurement at the Large Hadron Collider (LHC) which could expand our understanding beyond the Standard Model of Particle Physics.

Since the discovery of the Higgs boson in 2012, questions have remained open in fundamental physics about what lies beyond the Standard Model’s framework which describes all of the particles and forces in the universe.

One such parameter which has, until now, remained a mystery is the width of the W boson particle.

The W boson along with the Z boson is responsible for communicating the weak nuclear force – the force responsible for radioactive decay.

Width in particle physics doesn’t mean the physical size of the particle. Width, instead, is related to the lifetime of the particle – in other words, how quickly does the particle decay into other particles. A measurement of the W boson width which is unexpected or doesn’t match up with theory could indicate that there is physics beyond what’s in the Standard Model that is yet to be discovered.

A new study published on the arXiv preprint server includes the first ever measurement of the W boson width at the LHC. It was taken by the ATLAS collaboration during Run 1 of the LHC which took place 2009–2013.

The W boson width had previously been measured at CERN’s Large Electron-Positron collider and Fermilab’s Tevatron collider. The average of those results placed the width of the W boson at 2,085 million electronvolts (MeV) with an error of 42 MeV. This sits within the range predicted by the Standard Model of 2,088 MeV.

But the new measurement at the more powerful LHC suggests a different result – 2,202 MeV, with an error of 47 MeV. However, this result is still within acceptable statistical variations of the theoretical result.

It is the most accurate measurement of the W boson width to date.

To take the measurements, physicists needed to understand how W bosons are produced in proton-proton collisions and the inner structure of protons.

The W boson width was also measured alongside the particle’s mass. It has now been updated to 80,367 ± 16 MeV, improving on previous measurements using the same dataset.

Further tests at LHC and developing theoretical models will help physicists continue to test whether there are any gaps in the Standard Model which would suggest new physics.

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