CERN experiment confirms Standard Model got mass of W boson right

ATLAS reanalysed its 2011 sample of W bosons, improving the precision of its previous measurement.

The eight toroidal magnets surrounding the calorimeter of the ATLAS experiment at CERN. This calorimeter measures the energies of particles produced when protons collide in the centre of the detector.  | Photo Credit: CERN

Scientists at CERN claimed that a new precision measurement of the mass of the W boson by the ATLAS experiment was in conformity with the prediction by the Standard Model (SM) of particle physics. This preliminary result is based on a reanalysis of a sample of 14 million W boson candidates produced in proton-proton (p-p) collisions at the Large Hadron Collider (LHC), the particle accelerator at CERN.

The W boson and the Z boson, its electrically neutral counterpart, are the carriers of the weak nuclear force, the fundamental force that causes radioactivity and initiates the nuclear fusion reaction that powers the sun.

The new ATLAS measurement is is more precise than all previous W boson mass measurements except one: the latest measurement from the CDF experiment at the Tevatron, an accelerator at Fermilab that was shut down in 2011 (Frontline, May 6, 2022).

In the SM, the W boson mass is constrained to weigh 80,354 million electronvolts (MeV), with an uncertainty of 7 MeV. Any departure from this prediction would indicate new physics, such as new particles or interactions. In 2017, ATLAS released its first measurement of the W boson mass, which was determined using a sample of W bosons that it recorded in 2011, when the LHC was running at a collision energy of 7 trillion eV. The mass was 80,370 MeV, with an uncertainty of 19 MeV, which agreed with the SM prediction to a good degree. This was the most precise measurement of the mass until then.

Last year, the CDF collaboration announced an even more precise measurement based on an analysis of its full data set collected at the Tevatron. The result, 80,434 MeV with an uncertainty of 9 MeV, differed significantly from the SM prediction and from the other experimental results.

In its new study, ATLAS reanalysed its 2011 sample of W bosons, improving the precision of its previous measurement. The new W boson mass, 80,360 MeV with an uncertainty of 16 MeV, is 10 MeV lower than the previous ATLAS result and 16 per cent more precise, in apparent agreement with the SM.

“Since the ATLAS document describes a ‘reanalysis’ of the same 2017 data, obtaining a similar value as before is to be expected!” said Ashutosh Kotwal, the main author of the CDF result in an email to Frontline. “Only the data-fitting [a technique to determine mass] aspect of the analysis is slightly updated…,” he added.