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In particle physics, knowledge prolonged outlives the detectors that crank out it. A 10 years ago the 4,100-metric-ton Collider Detector at Fermilab (CDF) attained the finish of its everyday living and was shut down, stripped of its components for use in other experiments. Now a fresh investigation of old CDF data has unearthed a spectacular discrepancy in the mass of an elementary particle, the W boson, that could position the way to new, as nevertheless undiscovered particles and interactions.
The W boson is enormous, some 80 moments heavier than a proton. Crucially, the W boson is accountable for selected forms of radioactive decay, enabling neutrons to change into protons. For the reason that its mass is constrained by (and by itself constrains) a lot of other particles and parameters within just the Common Model—particle physicists’ principle of elementary particles and how they behave—the W boson has develop into a target for scientists searching for to recognize wherever and how their best theories fail.
Though physicists have extensive known the W boson’s approximate mass, they continue to do not know it just. Plugging knowledge into the Typical Product framework, nevertheless, predicts that the so-named W mass must be 80,357 mega-electron-volts (MeV), additionally or minus 6 MeV. (One MeV is about twice the mass-energy contained in a solitary electron.) But in a new assessment printed on Thursday in Science, physicists on the CDF collaboration have instead located the W boson mass to be 80,433.5 ± 9.4 MeV. The new measurement, which is far more exact than all earlier measurements mixed, is nearly 77 MeV higher than the Common Model’s prediction. While these numbers differ by only about a single part in 1,000, the uncertainties for each and every are so minuscule that even this small divergence is of massive statistical significance—it is exceedingly not likely to be an illusion manufactured via sheer possibility. The properly-analyzed W boson, it appears to be, still retains a good deal of secrets about the workings of the subatomic world—or at minimum about how we look into it. Taken by surprise, particle physicists are only starting to grapple with the implications.
“Nobody was waiting around for this discrepancy,” states Martijn Mulders, an experimental physicist at CERN in the vicinity of Geneva, who was not associated with the new investigate but co-wrote an accompanying commentary in Science. “It’s quite unanticipated. You virtually feel betrayed because suddenly they’re sawing off just one of the legs that definitely support the full structure of particle physics.”
Questing for Quarks
It was a rough measurement of the W boson mass that permitted physicists in 1990 to forecast the mass of the prime quark with affordable accuracy five many years right before that particle was first noticed. Then, working with the W boson mass and major quark mass, researchers created a identical prediction for the Higgs boson—which bore out spectacularly in 2012. A lot more not too long ago, physicists building these types of measurements have focused less on refining the Standard Model’s main competencies and a lot more on probing its failures—it does not, for occasion, incorporate gravity, darkish issue, neutrino masses or a selection of other troublesome phenomena. Poking at the spots where by the Standard Product breaks or or else deviates from observations, physicists say, is one particular of the very best strategies to search for “new physics,” their catch-all time period for obtaining extra, probably a lot more fundamental making blocks of the universe. Until finally the CDF result, some of the Standard Model’s most promising discrepancies incorporated an anomaly investigated at the Muon g-2 experiment at Fermilab and results from the LHCb (Big Hadron Collider elegance) experiment at CERN.
Compact anomalies are a dime a dozen, and the vast greater part are only statistical fluctuations arising from the definitely enormous quantities of subatomic situations made and recorded by normal particle physics experiments. In such scenarios, individuals anomalies fade absent as even bigger volumes of information are gathered. This newest anomaly appears more promising, while, for the reason that there is presently so substantially preexisting significant-top quality details on the W boson’s mass, and the theoretical prediction of the particle’s mass has quite small uncertainty. And, possibly most importantly, the CDF collaboration has been exceptionally thorough. The experiment was “blinded” to reduce the threat of human bias, that means that physicists examining its knowledge ended up retained in the darkish about its final results right until their do the job was accomplished. When the CDF’s calculated benefit for the W mass was discovered to workforce members in November 2020, “it was a moment of stunned silence,” says the study’s corresponding creator, Ashutosh Kotwal. “The realization of what that unblinded quantity meant—that, of class, is pure gold.”
Considering that then, the results have gone by a number of further more rounds of peer review—but that only assures the physicists have performed their homework, not that they have uncovered new physics.
Mining the Details
To evaluate the mass of a W boson, just one must first build a particle collider. The Tevatron, which ran from 1983 to 2011, was a 3.9-mile (6.3-kilometer) loop in which protons crashed into antiprotons at up to about two tera-electron-volts (TeV)—some 25 periods the mass of a W boson. The CDF experiment, located along the loop, sought signals of W bosons in these collisions from 2002 till the Tevatron shut down.
But one particular simply cannot basically notice a W boson it decays into other particles much much too immediately to sign-up in any detector. Alternatively physicists must infer its presence and properties by researching all those decay products—chiefly electrons and muons. Counting diligently, the CDF crew found about four million activities in the experiment’s info attributable to a W boson decay. By measuring the energy deposited in the CDF detector by all those events’ electrons and muons, the physicists labored backward to figure out how significantly energy—or mass—the W boson originally experienced.
This get the job done took a decade since of the numerous uncertainties in the info, Kotwal states. To access its unprecedented level of precision—twice as specific as the earlier very best single experiment measurement of the W boson mass, which was produced by the ATLAS collaboration——the CDF team quadrupled their dataset and also used new procedures. These involved modeling proton and antiproton collisions and conducting a new, extra extensive examination of the decommissioned detector’s operational quirks—even employing old cosmic-ray info to map its layout down to the micron.
That was ample to elevate the researchers’ anomalous outcome to outstanding heights of statistical importance: virtually 7 sigma, in the parlance of data. 7 sigma indicates that if no new physics influenced the W boson, discrepancies at the very least as large as the a person observed would still crop up from pure opportunity at the time just about every 800 billion instances the experiment was operate. Even in the earth of particle physics, where astronomical figures are the norm, this nearly seems like overkill: the field’s “gold standard” threshold for statistical significance is only 5 sigma, which corresponds to a supplied impact showing up by prospect once each and every 3.5 million runs. Crucially, the seven-sigma value of the CDF team’s new measurement does not necessarily mean that final result has a 99.999999999 p.c possibility of staying new physics. It does not even mean other measurements of the W mass are mistaken. Somewhat a 7-sigma end result means that no matter what the CDF collaboration is seeing is not by chance. It is a simply call to additional inquiry, not a summary.
To figure out the anomaly’s supply, corroboration from other experiments is wanted. “It’s a quite stunning result,” says Guillaume Unal, ATLAS’s physics coordinator, who was not concerned in the new research. “It’s a pretty complex and demanding measurement, and it’s also a very significant a single to definitely probe the Common Model with great accuracy.” ATLAS is at the moment functioning to increase its measurement of the W mass, and Unal claims making use of details from the LHC’s next operate, which concluded in 2018, may perhaps let them to get near to CDF’s precision.
In the meantime, theorists will pounce on this new result to deliver myriad attainable explanations. Despite the fact that the LHC has dominated out numerous permutations of supersymmetry (SUSY)—a set of theories positing that elementary particles have “superparticle” partners—one perpetrator that could be shifting the W boson’s mass ever so slightly is a cohort of reasonably mild supersymmetric particles.
“Of study course, [the LHC constraints] are turning out to be far more and much more stringent,” states Manimala Chakraborti, a theoretical physicist at the Nicolaus Copernicus Astronomical Heart of the Polish Academy of Sciences, who is not aspect of the CDF collaboration. “But even now, you can uncover regions of authorized parameter room for SUSY.”
At a time when new colliders are being proposed, and the LHC is preparing to start an additional campaign of collisions just after a large overhaul, the announcement of a seven-sigma-magnitude anomaly from a lengthy-absent experiment whose detectors have been cannibalized may seem to be bizarre.
But the collaboration continues to meet up with to assess and refine the fruits of the experiment’s operate. “Detective perform alone is what retains us going,” Kotwal suggests. “The clues are all there…. It is like Sherlock Holmes. The person could be absent, but the footprints are however there.”