Science & Technology

ATLAS Collaboration Detects Process Even Rarer Than Higgs Particle

Candidate occasion for WW → WW scattering within the ATLAS detector on the Giant Hadron Collider.

A brand new revealed research from the ATLAS Collaboration stories the primary proof of a course of that can be utilized to check the mechanism by which the just lately found Higgs particle imparts mass to different elementary particles.

Scientists working the ATLAS experiment on the Giant Hadron Collider (LHC), the world’s largest and strongest “atom smasher,” report the primary proof of a course of that can be utilized to check the mechanism by which the just lately found Higgs particle imparts mass to different elementary particles. Extra uncommon than the manufacturing of the Higgs itself, this course of—a scattering of two same-charged particles referred to as W bosons off each other—additionally supplies a brand new stringent check of the Commonplace Mannequin of particle physics. The findings, which up to now are in settlement with predictions of the Commonplace Mannequin, are reported in a paper simply accepted by Bodily Evaluate Letters.

“Solely about one in 100 trillion proton-proton collisions would produce one in every of these occasions,” mentioned Marc-André Pleier, a physicist on the U.S. Division of Power’s Brookhaven Nationwide Laboratory who performed a management position within the evaluation of this end result for the ATLAS collaboration. Complicating issues additional, discovering one such uncommon occasion is just not sufficient. “You have to observe many to see if the manufacturing fee is above or on par with predictions,” Pleier mentioned. “We regarded by means of billions of proton-proton collisions produced on the LHC for a signature of those occasions—decay merchandise that enable us to deduce like Sherlock Holmes what occurred within the occasion.”

The evaluation efforts began two years in the past and have been carried out particularly by teams from Brookhaven, Lawrence Berkeley Nationwide Laboratory, College of Michigan, and Technische Universität Dresden, Germany. Preliminary outcomes have been introduced by Pleier on the “Rencontres de Moriond – QCD and Excessive Power Interactions” convention in March 2014. Now finalized based mostly on a complete of 34 noticed occasions, the measured interplay fee is in good settlement with that predicted by the Commonplace Mannequin, a concept describing all recognized elementary particles and their interactions.

“The Commonplace Mannequin has up to now survived all assessments, however we all know that it’s incomplete as a result of there are observations of darkish matter, darkish power, and the antimatter/matter asymmetry within the universe that may’t be defined by the Commonplace Mannequin,” Pleier mentioned. So physicists are at all times on the lookout for new methods to check the idea, to search out the place and the way it may break down.

“This technique of W boson interactions is one we might by no means check,” Pleier mentioned, “as a result of we didn’t have sufficient power or massive sufficient knowledge units wanted to see this very uncommon course of—till we constructed the LHC.”

Now with the LHC knowledge in hand, the measured fee agrees with the prevailing concept’s predictions and establishes a sign at a significance degree of three.6 sigma—sturdy proof, in line with Pleier. “The chance for this measurement to be a mere background fluctuation could be very small—about one in 6000,” he mentioned. However the physicists wish to be extra sure by gathering extra knowledge to scale back uncertainties and improve the extent of significance.

There’s one more reason for persevering with the hunt: “By measuring this course of we will test whether or not the Higgs particle we found does its job the way in which we anticipate it to,” Pleier mentioned. “A wealth of fashions along with the Higgs mechanism exists to attempt to clarify how elementary particles get their mass. Measurements of such scattering processes can thus be each a elementary check of the Commonplace Mannequin and a window to new physics.”

To check the Higgs mechanism, the scientists examine distributions of decay merchandise of the W scattering course of—how typically they observe specific merchandise at a selected power and geometrical configuration.

“It’s like a fingerprint,” Pleier mentioned.“We have now a predicted fingerprint and we have now the fingerprint we measure. If the fingerprints match, we all know that the Higgs does its job of mass era the way in which it ought to. But when it deviates, we all know that another physics mechanism helps out as a result of the Higgs alone is just not doing what we anticipate.”

Once more, up to now, the info point out that the Higgs is working as anticipated.

“For the primary time, we will rule out sure fashions or predictions that we couldn’t earlier than,” Pleier mentioned. “To finish the job, we want extra knowledge, at greater power, so we will see the fingerprint extra clearly.”

The LHC will resume knowledge taking at elevated collision energies—13 tera-electronvolts (TeV) as a substitute of 8 TeV—in spring of 2015. The datasets collected will likely be as much as 150 occasions the dimensions of the at present accessible knowledge and can enable for an in depth behind-the-scenes have a look at the Higgs at work.

The ATLAS experiment at LHC is supported by DOE’s Workplace of Science and the Nationwide Science Basis.

Brookhaven Nationwide Laboratory serves because the U.S. host laboratory for the ATLAS experiment on the LHC, and performs a number of roles on this worldwide collaboration, from building and challenge administration to knowledge storage, distribution, and evaluation, funded by the DOE Workplace of Science (HEP).

DOE’s Workplace of Science is the one largest supporter of primary analysis within the bodily sciences in america, and is working to deal with a number of the most urgent challenges of our time.

Publication: G. Aad et al. (ATLAS Collaboration), “Seek for darkish matter in occasions with a Z boson and lacking transverse momentum in pp collisions at s√=8 TeV with the ATLAS detector,” Phys. Rev. D 90, 012004 – Revealed 10 July 2014; doi:10.1103/PhysRevD.90.012004

PDF Copy of the Examine: Evidence for Electroweak Production of W±W±jj in pp Collisions at s√=8 TeV with the ATLAS Detector

Picture: Brookhaven Nationwide Laboratory

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