The mass of the W boson is being disputed. What is it after all?

Different teams have produced different measurements.
Loukia Papadopoulos
Engineers working on the Large Hadron Collider.jpg
Engineers working on the Large Hadron Collider.


Researchers at CERN’s Large Hadron Collider have measured the mass of the W boson and found it to be in line with the Standard Model of particle physics, countering a measurement taken last year by a collaboration of hundreds of scientists at the CDF Collaboration. They found it to be much larger than previously assumed.

Who is right and how big is the discrepancy? The second question can be answered (about ~73 megaelectronvolts between the two measurements), the first not so much.

The CERN team found a boson mass of 80,360 ±16 MeV, 10 MeV lower and 16% more precise than the previous estimate out of ATLAS, while, last year, the CDF Collaboration measured the boson’s mass to be 80,433 ± 9 MeV.

Gizmodo investigated and got some feedback from some leading scientists in the field.

“Since the ATLAS experiment’s document describes a ‘reanalysis’ of the same data that ATLAS already released in 2017, the fact that ATLAS obtains a similar value as before is to be expected,” said Ashutosh Kotwal, a physicist at Duke University and a member of the CDF Collaboration, in an email to Gizmodo

“The reanalysis uses essentially the same technique as the previous publication. It is interesting that a press release was issued to advertise a tweaked analysis of old data.”

“The CDF measurement continues to be the world’s most precise measurement of the W boson mass,” Kotwal added.

David Toback, a physicist at Texas A&M University and a spokesperson for the CDF Collaboration, added that some people might be tempted to conclude that the measurement that is the least different from the Standard Model estimate is the right one. But that is not the case.

“I’m not interested in simple. CDF is not interested in simple. Science does not offer ‘truth’; it offers our best understanding of the moment,” Toback said. “We are looking forward to the ATLAS publication of their conference proceedings with all the gory details, so we can understand them at the same level we understand our own.”

Currently, more measurements of the W boson are being awaited from ATLAS, the Compact Muon Solenoid, and LHCb, all experiments along the Large Hadron Collider. Could they provide the much-needed clarification we all seek? Only time will tell.

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