Particle physicists may have finally poked a hole in their understanding of the subatomic realm—which they would relish. A new look at old data suggests an ephemeral particle called the W boson is heavier than predicted by physicists’ “standard model” of particles and forces. The discrepancy could hint at particles not included in the 40-year-old theory, says Doreen Wackeroth, a theorist at the University at Buffalo who was not involved in the work. “I’m very excited about the result!”
But the finding, reported today in Science, also clashes with previous measurements, giving some physicists pause. “All these measurements claim to measure the same quantity,” says Martin Grünewald, an experimental physicist at University College Dublin. “Somebody must be, I will not say wrong, but maybe made a mistake or pushed the error evaluation too aggressively.”
Vexingly successful, the standard model was completed in 2012, when the world’s largest atom smasher, the Large Hadron Collider (LHC) at the European particle physics laboratory CERN, discovered its last missing piece, the long-predicted Higgs boson. The theory accounts for every particle interaction seen so far, but it suffers obvious deficiencies. It includes three forces—electromagnetic, strong, and weak— but leaves out gravity. It also contains no dark matter, the invisible stuff that makes up 85% of the universe’s matter.
Now that all the standard model particles are known, physicists can test the theory’s internal consistency, because each particle’s properties depend on those of others. For example, the mass of the W boson—which conveys the weak nuclear force just as the photon conveys the electromagnetic force—depends on those of the Higgs and a heavy but fleeting subatomic particle called the top quark. So, from those input measurements, physicists can predict the W’s mass and look for a discrepancy with the measured value.
The measurement is tricky. Created in a high energy particle collision, a W quickly decays into either an electron or its heavier cousin, a particle called a muon, and an antineutrino. The antineutrino cannot be detected, so physicists must deduce its presence by summing up the momenta and energies of all the other particles spewing from each collision and looking for events in which something unseen seems to fly out the side of the cylindrical detector. From the energy and momentum of the decay particles, analyzed statistically over many events, they can estimate the W’s mass.
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Now, one team says its reading conflicts with the standard model prediction. The data come from the Collider Detector at Fermi National Accelerator Laboratory (CDF), a particle detector fed by the Tevatron collider, which ran at Fermilab from 1984 until 2011. After a decade of work, Ashutosh Kotwal, a particle physicist at Duke University, and his 397 CDF collaborators find the W boson has a mass of 80,443.5 megaelectron volts—86 times that of a proton. The measurement differs from the predicted mass by seven times the experimental uncertainty.
“What does it mean? That’s the next big question,” Wackeroth says. Physicists have spotted a couple of other small anomalies that suggest the standard model may finally be cracking, she says. For example, she notes that the muon appears to be slightly more magnetic than predicted.
Weighty issue
A new measurement of the mass of a particle called the W boson disagrees strongly with the theoretical prediction—and with previous measurements, including one from the same group.
However, earlier measurements of the W’s mass generally agreed with the standard model (see chart, below). The new result even contradicts the CDF’s previous result, published in 2012, which was based on the first quarter of the current data set, notes Dmitri Denisov, a physicist at Brookhaven National Laboratory who worked on D0, a rival Tevatron detector. “That’s my first concern,” he says.
But CDF researchers made several improvements in the analysis that account for the difference, Kotwal says. “We are confident in the techniques we have used,” he says. “It is a distinct possibility that there is something new in nature that the standard model does not capture.”
Physicists should soon get yet another W boson mass measurement. Scientists with the Compact Muon Solenoid, a detector at the LHC, hope to publish one early next year, says Guillelmo Gomez-Ceballos, a CMS physicist at the Massachusetts Institute of Technology. He is also a CDF member, and although he didn’t work on the new study, he says, “I don’t remember any analysis that has been done with so much care.”
It may take years to reconcile the measurements. But physicists won’t be left rudderless in the meantime. Since 1957, the Particle Data Group (PDG) at Lawrence Berkeley National Laboratory (LBNL) has maintained a compendium of particles and arbitrated disputes over their measured properties. The new W boson mass value comes as the PDG is preparing its latest annual update, says Michael Barnett, a retired LBNL physicist who led the PDG from 1990 to 2015 and still works on it. “We’re going to have to stop the presses, just like we did when the Higgs was discovered,” he says.
For a parameter like the W boson’s mass, the PDG averages the most current and reliable measurements. If they disagree far beyond their uncertainties, the group applies a specific mathematical algorithm that effectively widens the error bars to encompass the discordant individual results, Barnett says. Ironically, even though the CDF has now reported the single most precise measurement of the W mass, the official value will likely become even less certain than before.