In particle physics, information lengthy outlives the detectors that generate it. A decade in the past the 4,100-metric-ton Collider Detector at Fermilab (CDF) reached the tip of its life and was shut down, stripped of its components to be used in different experiments. Now a recent evaluation of outdated CDF information has unearthed a surprising discrepancy within the mass of an elementary particle, the W boson, that might level the best way to new, as but undiscovered particles and interactions.
The W boson is very large, some 80 instances heavier than a proton. Crucially, the W boson is chargeable for sure types of radioactive decay, permitting neutrons to transform into protons. As a result of its mass is constrained by (and itself constrains) many different particles and parameters inside the Normal Mannequin—particle physicists’ principle of basic particles and the way they behave—the W boson has change into a goal for researchers searching for to know the place and the way their greatest theories fail.
Though physicists have lengthy identified the W boson’s approximate mass, they nonetheless have no idea it precisely. Plugging information into the Normal Mannequin framework, nevertheless, predicts that the so-called W mass must be 80,357 mega-electron-volts (MeV), plus or minus 6 MeV. (One MeV is about twice the mass-energy contained inside a single electron.) However in a brand new evaluation printed on Thursday in Science, physicists on the CDF collaboration have as an alternative discovered the W boson mass to be 80,433.5 ± 9.Four MeV. The brand new measurement, which is extra exact than all earlier measurements mixed, is sort of 77 MeV larger than the Normal Mannequin’s prediction. Though these numbers differ by solely about one half in 1,000, the uncertainties for every are so minuscule that even this small divergence is of monumental statistical significance—it’s exceedingly unlikely to be an phantasm produced by sheer likelihood. The well-studied W boson, it appears, nonetheless holds loads of secrets and techniques in regards to the workings of the subatomic world—or no less than about how we examine it. Taken without warning, particle physicists are solely starting to grapple with the implications.
“No one was ready for this discrepancy,” says Martijn Mulders, an experimental physicist at CERN close to Geneva, who was not concerned with the brand new analysis however co-wrote an accompanying commentary in Science. “It’s very sudden. You nearly really feel betrayed as a result of abruptly they’re sawing off one of many legs that actually help the entire construction of particle physics.”
Questing for Quarks
It was a tough measurement of the W boson mass that allowed physicists in 1990 to predict the mass of the highest quark with affordable accuracy 5 years earlier than that particle was first noticed. Then, utilizing the W boson mass and high quark mass, researchers made a comparable prediction for the Higgs boson—which bore out spectacularly in 2012. Extra just lately, physicists making such measurements have centered much less on refining the Normal Mannequin’s core competencies and extra on probing its failures—it doesn’t, as an illustration, incorporate gravity, darkish matter, neutrino lots or a variety of different troublesome phenomena. Poking on the locations the place the Normal Mannequin breaks or in any other case deviates from observations, physicists say, is likely one of the greatest methods to seek for “new physics,” their catch-all time period for locating further, probably extra basic constructing blocks of the universe. Till the CDF end result, among the Normal Mannequin’s most promising discrepancies included an anomaly investigated on the Muon g-2 experiment at Fermilab and outcomes from the LHCb (Giant Hadron Collider magnificence) experiment at CERN.
Small anomalies are a dime a dozen, and the overwhelming majority are merely statistical fluctuations arising from the really monumental numbers of subatomic occasions produced and recorded by typical particle physics experiments. In such instances, these anomalies fade away as even higher volumes of information are gathered. This newest anomaly seems extra promising, although, as a result of there may be already a lot preexisting high-quality info on the W boson’s mass, and the theoretical prediction of the particle’s mass has very low uncertainty. And, maybe most significantly, the CDF collaboration has been extraordinarily cautious. The experiment was “blinded” to reduce the chance of human bias, that means that physicists analyzing its information have been saved at the hours of darkness about its outcomes till their work was accomplished. When the CDF’s measured worth for the W mass was revealed to crew members in November 2020, “it was a second of surprised silence,” says the examine’s corresponding writer, Ashutosh Kotwal. “The conclusion of what that unblinded quantity meant—that, after all, is pure gold.”
Since then, the outcomes have gone by a number of additional rounds of peer assessment—however that solely ensures the physicists have finished their homework, not that they’ve discovered new physics.
Mining the Knowledge
To measure the mass of a W boson, one should first construct a particle collider. The Tevatron, which ran from 1983 to 2011, was a 3.9-mile (6.3-kilometer) loop the place protons crashed into antiprotons at as much as about two tera-electron-volts (TeV)—some 25 instances the mass of a W boson. The CDF experiment, positioned alongside the loop, sought indicators of W bosons in these collisions from 2002 till the Tevatron shut down.
However one can not merely observe a W boson; it decays into different particles far too rapidly to register in any detector. As an alternative physicists should infer its presence and properties by finding out these decay merchandise—mainly electrons and muons. Counting rigorously, the CDF crew discovered about 4 million occasions within the experiment’s information attributable to a W boson decay. By measuring the power deposited within the CDF detector by these occasions’ electrons and muons, the physicists labored backward to determine how a lot power—or mass—the W boson initially had.
This work took a decade due to the quite a few uncertainties within the information, Kotwal says. To succeed in its unprecedented degree of precision—twice as exact because the earlier greatest single experiment measurement of the W boson mass, which was made by the ATLAS collaboration——the CDF crew quadrupled their dataset and in addition used new strategies. These included modeling proton and antiproton collisions and conducting a brand new, extra thorough examination of the decommissioned detector’s operational quirks—even utilizing outdated cosmic-ray information to map its structure all the way down to the micron.
That was sufficient to raise the researchers’ anomalous end result to exceptional heights of statistical significance: almost seven sigma, within the parlance of statistics. Seven sigma implies that if no new physics affected the W boson, discrepancies no less than as massive because the one noticed would nonetheless come up from pure likelihood as soon as each 800 billion instances the experiment was run. Even on this planet of particle physics, the place astronomical numbers are the norm, this nearly looks like overkill: the sphere’s “gold normal” threshold for statistical significance is just 5 sigma, which corresponds to a given impact showing by likelihood as soon as each 3.5 million runs. Crucially, the seven-sigma worth of the CDF crew’s new measurement does not imply that end result has a 99.999999999 p.c likelihood of being new physics. It doesn’t even imply different measurements of the W mass are incorrect. Somewhat a seven-sigma end result implies that regardless of the CDF collaboration is seeing shouldn’t be by likelihood. It’s a name to additional inquiry, not a conclusion.
To find out the anomaly’s supply, corroboration from different experiments is required. “It’s a really spectacular end result,” says Guillaume Unal, ATLAS’s physics coordinator, who was not concerned within the new examine. “It’s a really complicated and difficult measurement, and it’s additionally a vital one to essentially probe the Normal Mannequin with good accuracy.” ATLAS is presently working to enhance its measurement of the W mass, and Unal says utilizing information from the LHC’s second run, which concluded in 2018, might enable them to get near CDF’s precision.
Within the meantime, theorists will pounce on this new end result to provide myriad attainable explanations. Though the LHC has dominated out many permutations of supersymmetry (SUSY)—a set of theories positing that elementary particles have “superparticle” companions—one perpetrator that might be shifting the W boson’s mass ever so barely is a cohort of comparatively gentle supersymmetric particles.
“After all, [the LHC constraints] have gotten increasingly stringent,” says Manimala Chakraborti, a theoretical physicist on the Nicolaus Copernicus Astronomical Middle of the Polish Academy of Sciences, who shouldn’t be a part of the CDF collaboration. “However nonetheless, you will discover areas of allowed parameter house for SUSY.”
At a time when new colliders are being proposed, and the LHC is making ready to launch one other marketing campaign of collisions after a large overhaul, the announcement of a seven-sigma-magnitude anomaly from a long-gone experiment whose detectors have been cannibalized could appear unusual.
However the collaboration continues to fulfill to evaluate and refine the fruits of the experiment’s run. “Detective work itself is what retains us going,” Kotwal says. “The clues are all there…. It’s like Sherlock Holmes. The individual could also be gone, however the footprints are nonetheless there.”