Particle physics may very well be rewritten after shock W boson measurement
The usual mannequin of particle physics has stood the take a look at of time for many years, however now a brand new measurement of a particle known as the W boson might point out a chink in its armour
The Tevatron particle accelerator at Fermilab, Illinois, pictured in 1992 GRANGER/Alamy
A brand new measurement of a elementary particle known as the W boson seems to defy the usual mannequin of particle physics, our present understanding of how the fundamental constructing blocks of the universe work together. The consequence, which was a decade within the making, will likely be closely scrutinised, but when it holds true, it might result in solely new theories of physics.
“It might be the largest discovery since, nicely, for the reason that begin of the usual mannequin 60 years in the past,” says Martijn Mulders on the CERN particle physics laboratory close to Geneva, Switzerland, who has written a commentary on the consequence for the journal Science.
The usual mannequin describes three distinct forces: electromagnetism, the robust pressure and the weak pressure. Particles known as bosons function mediators for these forces between particles of matter. The weak pressure, which is chargeable for radioactive decay, makes use of the W boson as certainly one of its messengers.
The W boson is so central to the usual mannequin that physicists have tried to measure its mass with ever higher precision because it was first noticed in 1983. These measurements have all broadly agreed with one another, an obvious affirmation of the usual mannequin’s validity.
However we all know that the usual mannequin is flawed. It has no clarification for gravity, darkish matter and the absence of antimatter in our universe, so physicists are always looking out for deviant measurements that would result in new theories.
Now, Ashutosh Kotwal at Duke College in North Carolina and his colleagues have introduced a brand new measurement for the W boson’s mass utilizing knowledge from the Tevatron collider in Illinois, placing it at 80.4335 gigaelectronvolts.
The usually accepted mass for the W boson is 80.379 gigaelectronvolts, and whereas the discrepancy could appear small, the brand new worth is probably the most exact to this point, equal to measuring your physique weight to inside beneath 10 grams.
Extra importantly, its distinction from the widely accepted worth of the W boson mass has a statistical significance of round 5 sigma, akin to a chance of about 1 in 3.5 million that a sample of knowledge like this may present up as a statistical fluke.
Physicists usually use 5 sigma as the extent of significance to depend one thing as a “discovery”, however the distinction between the brand new mass measurement and that predicted by the usual mannequin is even increased, at 7 sigma. This corresponds to round a 1 in 780 billion chance of seeing a consequence like this by probability.
Kotwal and his group are conscious that they're making a rare declare that would overturn physics as we all know it, however he says they've completed all of the assessments they'll consider to verify it. A small quantity of systematic uncertainty – primarily potential errors throughout the experimental set-up – stays, however now it's time for others to weigh in on the consequence, he says. “We predict the reply is holding as much as our personal scrutiny,” he says.
Measuring W boson’s mass
The group measured the boson’s mass by smashing beams of protons and antiprotons collectively and analysing the particles produced within the collision. The evaluation was so advanced that the consequence took greater than a decade to supply, after the Tevatron shut down in 2011, however its potential implications are big.
“If the W boson mass is deviating that a lot from the usual mannequin expectation, and if we perceive all of the [systematic] uncertainties, then it’s an enormous deal,” says Ulrik Egede at Monash College in Australia.
The “if” is the essential level for a lot of physicists who, whereas excited on the consequence, are cautious about its divergence from earlier measurements. “We want first to grasp the discrepancy between [this result] and all different experiments earlier than we take into consideration explanations from physics past the usual mannequin,” says Matthias Schott at CERN, who labored on a earlier W boson measurement utilizing knowledge from the ATLAS experiment gathered on the Giant Hadron Collider (LHC) as much as its shutdown in 2018.
Determining the supply of the discrepancy is not any straightforward process. W bosons rapidly decay into different particles, both an electron and an electron neutrino, or a heavier muon and muon neutrino. Neutrinos are onerous to detect, so Kotwal and his group needed to infer the place they had been from massive quantities of knowledge. “[W boson masses] are recognised to be among the experimentally most tough measurements to make,” says Egede.
The 2018 ATLAS measurement for the W boson mass is the latest to this point, however it could additionally not be a lot assist in fixing the riddle. ATLAS used two beams of protons, fairly than a second certainly one of antiprotons, making the outcomes more durable to check, says Kotwal.
If physicists can’t discover an issue with Kotwal and his group’s work, then the following step will likely be producing one other measurement, which might come from three experiments on the LHC. “It’s the one collider with a excessive sufficient power to create W bosons,” says Harry Cliff on the College of Cambridge. The LHC is gearing up for a brand new run this yr after being offline since 2018, however Mulders says knowledge collected for the CMS experiment throughout the earlier run might yield a brand new W boson measurement by subsequent yr.
If the result's borne out, it might be a part of different unexplained anomalies like these from the Muon g-2 experiment and discrepancies picked up on the LHC referring to subatomic particles known as backside quarks, which could require new theories of physics to clarify. Whereas there are not any clear contenders for such a principle at current, Kotwal says that some variants of supersymmetry, which requires the existence of an entire new set of particles, would possibly accommodate the upper W boson mass.
Regardless of the consequence taking 10 years to supply, Kotwal says that is just the start for understanding its significance as physicists all over the world get their fingers on the information. “The science will likely be investigated and we are going to proceed to consider it,” he says.
Journal reference: Science, DOI: 10.1126/science.abk1781
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