Physicists have nailed a fiendishly tough measurement — the mass of the elemental particle the W boson. The end result, from the CMS experiment on the Giant Hadron Collider (LHC), is according to the predictions of the normal 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’ greatest description of particles and forces.
“The usual mannequin shouldn’t be useless,” mentioned Josh Bendavid, a particle physicist on the Massachusetts Institute of Know-how in Cambridge and member of the CMS collaboration, when he introduced 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. If the discovering had been near the 2022 end result, we’d be declaring the usual mannequin’s loss of life, 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.
On supporting science journalism
For those who’re having fun with this text, think about supporting our award-winning journalism by subscribing. By buying a subscription you’re serving to to make sure the way forward for impactful tales concerning the discoveries and concepts shaping our world immediately.
Reduction for physicists
The 2022 end result, produced by an experiment known as CDF on the Fermi Nationwide Accelerator Laboratory in Batavia, Illinois, used 10-year-old information 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 will probably’t be full as a result of it doesn’t account for mysterious phenomena similar to darkish matter.
The CMS result’s essentially the most exact measurement of the W mass to come back out of the LHC, and its precision is roughly on par with the CDF end result. It’s also according to the 4 measurements that preceded the CDF determine, leaving that worth as an outlier. “They can’t each be proper,” says Ashutosh Kotwal, an experimental particle physicist at Duke College in Durham, North Carolina, who led the CDF examine.
“It might have been most likely higher for the group if we discovered one thing completely completely different from the usual mannequin, as a result of that will have been thrilling for the way forward for our discipline,” says Elisabetta Manca, a particle physicist on the College of California, Los Angeles, who was one of many foremost analysts for the CMS discovering. However by way of confidence within the end result, the worth was a “aid”, she says.
Basic forces
The W boson, together with its sister particle, the Z, are concerned in radioactive decay as a service of the weak nuclear drive, one in all 4 basic forces of nature. Its mass is without doubt one of the few values in the usual mannequin that may be predicted at excessive precision by idea and measured via experiments. This makes it a good way to hunt for cracks within the mannequin. “There usually are not many high-precision observables. That’s what makes it vital and worthwhile,” says Kotwal.
However the W mass is extraordinarily tough 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, similar to 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 principally at muon decays. The workforce reconstructed properties of muons from round 100 million W decays from the LHC with unprecedented precision, says Manca. They then in contrast the information with 4 billion simulated collisions and decays that used completely different values for the W mass — and completely different values for hundreds 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.
The workforce used cutting-edge software program and idea, and calibrated and cross-checked their outcomes with different measurements of the W and towards Z decays to make sure that their strategies had been working as anticipated, says Manca.
That the CMS result’s broadly according to these from different LHC experiments — ATLAS and LHCb — which used completely different detectors and methodologies, provides the workforce confidence that they’ve hit on the best determine, says Manca.
Anomaly evaluation
Nobody can but say why CDF’s end result stands out. One attainable purpose is that the detector used completely different theoretical instruments from CMS to generate its simulations. CDF detected collisions from a proton–antiproton accelerator known as the Tevatron, which closed in 2011, whereas the LHC collides solely protons. “There isn’t any one factor the place we will say ‘that’s the explanation why the result’s so completely different,’” says Manca.
Kotwal says he might want to see the CMS paper, which will likely be printed in coming months, to see the workforce’s methodology. “Individuals have been reviewing how we’ve executed it and we haven’t obtained any clear indication that any flaw has been seen. The identical must be executed for CMS,” he says.
Agreeing on humankind’s greatest 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 depart 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 finally break open the usual mannequin.
This text is reproduced with permission and was first printed on September 17, 2024.
