The W boson may not be heavier than anticipated in spite of everything

The battle over the heft of a hard-to-detect particle is heating up. What’s at stake? Only the main idea describing all identified matter within the universe.

A recalculation of the mass of an elementary particle, the W boson, has elevated the stress between measurements from competing particle collider experiments. The final consequence may bolster the usual mannequin of particle physics, which describes the elemental forces and quantum bits that make up all the pieces we see within the cosmos. Or it may reveal indicators of the usual mannequin’s breakdown, relying on which lab’s reply prevails.

A reanalysis of outdated knowledge from the Large Hadron Collider’s ATLAS experiment yields a W boson mass of about 80,360 million electron volts, or MeV. Researchers with the experiment, at CERN in Geneva, reported the measurement March 23 on the Rencontres de Moriond convention in La Thuile, Italy. The revised worth is carefully aligned with predictions from the usual mannequin.

It additionally boasts diminished uncertainty from the researchers’ earlier evaluation of the info, which they reported in 2018, growing their confidence that they acquired the mass proper.

But the up to date mass is at odds with that of one other group. In 2022, scientists from the Collider Detector at Fermilab, or CDF, experiment shocked the physics group with a measurement of 80,434 MeV — about 100 MeV heavier than anticipated (SN: 4/7/22). If the CDF report is right, it implies that one thing is off with the usual mannequin that has persevered within the face of each experimental problem thrown at it over the past 50 years.

The W boson is accountable for the weak power, considered one of three elementary forces in the usual mannequin (SN: 2/5/83). And “it’s the one mass of a particle in the usual mannequin that may be calculated,” says theoretical physicist Sven Heinemeyer of the Karlsruhe Institute of Technology in Germany. That is, the usual mannequin idea yields a particular mass for the W boson, whereas the lots of different particles akin to electrons and quarks are inputs and could be — so far as the speculation is anxious — any worth. Finding a W boson mass that’s completely different from customary mannequin predictions would present the present idea is improper.

The ATLAS reanalysis affords a stronger counterpoint to the CDF declare than the sooner ATLAS evaluation of the identical knowledge. “The new evaluation is a vital affirmation of our earlier consequence,” says Andreas Hoecker, a physicist at CERN.

The newest ATLAS worth widens the chasm that separates CDF’s mass measurement from the herd of different research. But it shouldn’t be seen as erasing CDF’s customary mannequin problem, says Duke University physicist Ashutosh Kotwal, a member of the CDF collaboration.

“The perspective on the CDF [announcement of a heavy W boson in 2022] doesn’t change due to the ATLAS reanalysis,” Kotwal says. Because the reanalysis relies on knowledge that ATLAS already launched in 2017, he says, “the truth that ATLAS obtains the same worth as earlier than is to be anticipated.”

Heinemeyer, who just isn’t affiliated with ATLAS or CDF, sees a shift within the W boson mass panorama, however no signal of a decision of the discrepancy.

“Having one new measurement just isn’t sufficient,” Heinemeyer says. “If increasingly measurements had been to come back out now from ATLAS and [other experiments], and they might all be in the identical ballpark, sooner or later the group would resolve CDF did one thing improper.”

The subsequent phrase on the W boson mass will in all probability include pending research from ATLAS and different experiments at CERN. The CDF experiment shut down in 2011, so it is not going to contribute additional to the talk.

In the meantime, researchers hope to scrutinize one another’s analyses to seek for clues which may assist clarify discrepancies in W boson mass measurements. “The CDF April 2022 paper gives quite a few cross-checks of the CDF methodology and is clear,” Kotwal says. “I stay up for detailed discussions of the ATLAS methodology.”

In the tip, the battle may reveal a brand new crack in the usual mannequin. Or it may grow to be one other instance of one of the vital profitable theories in historical past standing robust.