Physicists have nailed a fiendishly troublesome measurement — the mass of the elemental particle the W boson. The end result, from the CMS experiment on the Massive Hadron Collider (LHC), is according to the predictions of the commonplace mannequin, and pours chilly water on an anomaly within the W boson mass that surfaced in 2022. That measurement had hinted on the existence of phenomena past the usual mannequin, physicists’ finest description of particles and forces.
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“The usual mannequin isn’t useless,” mentioned Josh Bendavid, a particle physicist on the Massachusetts Institute of Know-how in Cambridge and member of the CMS collaboration, when he offered the end result on 17 September. Rapturous applause met the announcement, made at a seminar at CERN, Europe’s particle-physics laboratory close to Geneva, Switzerland, which hosts the LHC. CMS’s end result was was 10 years within the making, and produced a mass of 80,360.2 million electronvolts for the W boson, which is concerned in carrying the weak nuclear drive (see ‘The W boson puzzle’). If the discovering had been near the 2022 end result, we’d be declaring the usual mannequin’s demise, mentioned Bendavid.
“The group will likely be excited by the truth that we will attain this precision and have this understanding of the usual mannequin at this degree,” says Florencia Canelli, an experimental particle physicist on the College of Zurich in Switzerland, who works on the CMS experiment however was not concerned within the end result.
Supply: CERN/compiled by Nature
Aid for physicists
The 2022 end result1, produced by an experiment referred to as CDF on the Fermi Nationwide Accelerator Laboratory in Batavia, Illinois, used 10-year-old knowledge to calculate that the W boson was heavier than predicted, opening the potential for a crack in the usual mannequin that excited physicists. Though the mannequin is extremely profitable, physicists know it may well’t be full as a result of it doesn’t account for mysterious phenomena resembling darkish matter.
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The CMS result’s essentially the most exact measurement of the W mass to return out of the LHC, and its precision is roughly on par with the CDF end result. Additionally it is according to the 4 measurements that preceded the CDF determine, leaving that worth as an outlier. “They can not each be proper,” says Ashutosh Kotwal, an experimental particle physicist at Duke College in Durham, North Carolina, who led the CDF research.
“It might have been in all probability higher for the group if we discovered one thing completely totally different from the usual mannequin, as a result of that may have been thrilling for the way forward for our subject,” says Elisabetta Manca, a particle physicist on the College of California, Los Angeles, who was one of many most important analysts for the CMS discovering. However by way of confidence within the end result, the worth was a “reduction”, she says.
Elementary forces
W bosons, together with its sister particle, the Z, are concerned in radioactive decay as a service of the weak nuclear drive, one among 4 elementary forces of nature. It is likely one of the few values in the usual mannequin that may be predicted at excessive precision by principle and in addition measured by experiments. This makes it an effective way to hunt for cracks within the mannequin. “There should not many high-precision observables. That’s what makes it essential and worthwhile,” says Kotwal.
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However the W mass is extraordinarily troublesome to measure. The LHC makes the bosons by colliding protons at excessive energies. These shortly decay into different particles that the experiments detect. However for the W mass, half of the decay is lacking, as a result of the boson transforms into just one detectable particle — a lepton, resembling an electron or its heavier cousin the muon. The opposite particle, a neutrino, zips straight out of the detector leaving no hint.
Muon decays
The CMS evaluation regarded largely at muon decays. The group reconstructed properties of muons from round 100 million W decays from the LHC with unprecedented precision, says Manca. They then in contrast the info with 4 billion simulated collisions and decays that used totally different values for the W mass — and totally different values for 1000’s of different parameters that might bias the outcomes — and regarded for the very best match. “The one which matches is the one we extract,” says Canelli.
Illustration of a CMS collision occasion by which a W boson decays right into a muon (purple line).Credit score: CMS/CERN
The group used cutting-edge software program and principle, and calibrated and cross-checked their outcomes with different measurements of the W and in opposition to Z decays to make sure that their strategies have been working as anticipated, says Manca.
That the CMS result’s broadly according to these from different LHC experiments — ATLAS and LHCb — which used totally different detectors and methodologies, offers the group confidence that they’ve hit on the proper determine, says Manca.
Anomaly evaluation
Nobody can but say why CDF’s end result stands out. One doable purpose is that the detector used totally different theoretical instruments from CMS to generate its simulations. CDF detected collisions from a proton–antiproton accelerator referred to as the Tevatron, which closed in 2011, whereas the LHC collides solely protons. “There isn’t a one factor the place we will say ‘that’s the rationale why the result’s so totally different,’” says Manca.
Kotwal says he might want to see the CMS paper, which will likely be revealed in coming months, to see the group’s methodology. “Folks have been reviewing how we’ve carried out it and we haven’t obtained any clear indication that any flaw has been observed. The identical must be carried out for CMS,” he says.
Agreeing on humankind’s finest guess of the W’s mass will imply bringing collectively specialists from every of the experiments, in addition to theorists, to attempt to perceive the differing outcomes. “We shouldn’t go away the CDF end result as an outlier, we have to perceive why or how it’s there,” says Canelli.
Though CMS didn’t discover an anomaly, the 10-year course of created instruments that permit physicists to make different precision measurements. Such high-precision comparisons are what Manca thinks will in the end break open the usual mannequin.