A team of 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, the overarching theory that describes the four fundamental forces and the characteristics of the smallest units of matter.
The team’s finding counters a precise measurement taken last year by a collaboration of hundreds of scientists at the CDF Collaboration, who were happily surprised to find that the W boson, an elementary particle responsible for the weak nuclear force, was much more massive than previously believed — a finding that breached the Standard Model’s expectations.
The ~73 megaelectronvolt discrepancy between the two measurements is the difference between the boson’s true mass being nearly in agreement with the Standard Model and significantly at odds with it.
The CERN team announced their result during the Rencontres de Moriond conference last month. Their figure comes from a reanalysis of some 14 million W boson candidates produced in collisions between protons in the Large Hadron Collider as part of the ATLAS experiment back in 2011.
They found a boson mass of 80,360 ±16 MeV, 10 MeV lower and 16% more precise than the previous estimate out of ATLAS, according to a CERN release.
“This updated result from ATLAS provides a stringent test, and confirms the consistency of our theoretical understanding of electroweak interactions,” Andreas Hoecker, a spokesperson for the ATLAS experiment, said in the release.
But things aren’t so straightforward. Last year, the CDF Collaboration measured the boson’s mass to be 80,433 ± 9 MeV, based on collisions done at the Fermilab’s Tevatron accelerator in Illinois, also in 2011. (Tevatron shut down shortly after the 2011 experimental run.)
The difference between the two measurements taken at CERN and Fermilab seems small, but it’s massive on a subatomic scale and has significant implications for the Standard Model. For perspective, the CDF Collaboration’s estimated boson mass is about 80 times the mass of a proton.
“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.
These separate experiments, which together involve thousands of scientists, came up with very different numbers for the mass of this fundamental particle. Neither team has identified anything amiss with their approaches.
“We’ve spent the last year showing the results all over the world, and there have been no substantive concerns found about the methods or the cross checks,” said David Toback, a physicist at Texas A&M University and a spokesperson for the CDF Collaboration, in an email to Gizmodo. “A combination of the collaborations from around the world has been convened to see if they could understand the differences, and they have also not come up with an explanation for the differences.”
So what’s going on? The Standard Model has been the guiding framework for our understanding of particle physics since the early 1970s. It’s not perfect, a fact physicists are well aware of: The model doesn’t account for dark matter, the catch-all term for the enigmatic something we cannot directly observe, dark energy, which makes up 68% of the universe and is apparently responsible for the universe’s accelerating expansion, or gravity, a grounding force in all our lives but one that doesn’t appear to exist on subatomic scales.
Getting exact measurements for fundamental particles like the W boson helps physicists understand the limits of the Standard Model; once those limits are identified, scientists are better set up to discover new things.
In other words, known unknowns are rife in the subatomic world, and sometimes getting really wacky results is a good thing. Figuring out what’s a genuine discovery and what’s a fluke in data is the challenge.
Toback said that some people may be tempted to conclude that the measurement with the smallest error bars or the one least different from the Standard Model estimate — that is to say, the ATLAS figure — is the correct one.
“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.”
Besides the work of CDF and the recent ATLAS analysis, more measurements of the W boson are expected from ATLAS, the Compact Muon Solenoid, and LHCb, all experiments along the Large Hadron Collider.
It’s possible that the Large Hadron Collider is simply too low energy to produce particles that could help clarify what the Standard Model overlooks. If that’s the case, we’d have to wait for an upgraded LHC or a much more massive collider to induce new reactions between particles.