New ATLAS outcome weighs in on the W boson
For the previous 40 years, the W boson has been making headlines. Within the Nineteen Eighties, the announcement of its discovery helped verify the speculation of the electroweak interplay – a unified description of electromagnetic and weak forces. At present, measurements of its mass (mW) are testing the consistency of the Normal Mannequin it helped to type.
The W-boson mass is carefully associated to the plenty of nature’s heaviest particles, together with the highest quark and the Higgs boson. Nonetheless, if extra heavy particles exist, the mass would possibly deviate from the Normal Mannequin prediction. By evaluating direct measurements of the W-boson mass to theoretical calculations, physicists are on the lookout for deviations that may very well be an indicator of latest phenomena. To be sufficiently delicate to such deviations, mass measurements must have amazingly small uncertainties, of the order of 0.01%.
In 2017, the ATLAS Collaboration at CERN revealed the LHC’s first measurement of the W-boson mass, giving a price of 80370 MeV with an uncertainty of 19 MeV. On the time, this measurement was essentially the most exact single-experiment outcome, and was in settlement with the Normal Mannequin prediction and all different experimental outcomes. Final yr, the CDF Collaboration at Fermilab revealed an much more exact measurement of the W-boson mass, analysing the complete dataset offered by the Tevatron collider. With a price of 80434 MeV and an uncertainty of 9 MeV it differed considerably from the Normal Mannequin prediction and from the opposite experimental outcomes.
In a new preliminary result launched immediately, the ATLAS Collaboration studies an improved re-analysis of its preliminary W-boson mass measurement. ATLAS finds mW to be 80360 MeV, with an uncertainty of simply 16 MeV. The measured worth is 10 MeV decrease than the earlier ATLAS outcome and is in settlement with the Normal Mannequin.
The ATLAS Collaboration has measured the W boson mass to be 80360 MeV with an uncertainty of 16 MeV – in settlement with the Normal Mannequin.
For this new evaluation, ATLAS physicists revisited its information collected in 2011 at a centre-of-mass power of seven TeV (akin to 4.6 fb-1, additionally utilized in ATLAS’ earlier measurement). Researchers employed improved statistical strategies and refinements within the remedy of the info, enabling them to scale back the uncertainty of their mass measurement by greater than 15%.
Researchers centered on collision occasions the place the W boson decays into an electron or a muon (leptons), and a corresponding neutrino. The W-boson mass was then decided by becoming the kinematic distributions of the decay leptons in simulation to the info. The principle distinction between the 2017 and the brand new measurement is within the technique used to carry out these suits. Whereas the earlier measurement used the accessible information solely to find out the W-boson mass, with systematic uncertainties added after the very fact, the brand new measurement concurrently adjusts the systematic uncertainties along with the W-boson mass. This enchancment diminished a number of systematic uncertainties, significantly these associated to the theoretical modelling of W-boson manufacturing and decay.
The W-boson transverse momentum distribution has as a lot of an affect on the lepton-decay distributions because the W-boson mass itself – and is due to this fact an vital supply of uncertainty. Because the decision of the 2011 information was too poor to confirm the modelling of this distribution intimately, researchers as an alternative used information recorded in 2017 throughout a particular, low-luminosity proton-proton run at a centre-of-mass power of 5 TeV. They discovered the info agreed with predicted distributions, thus validating the mannequin.
Additionally essential to the measurement have been the parton distribution capabilities (PDFs) of the proton, which mannequin the relative momenta of its quark and gluon constituents. PDFs incorporate a large number of information from completely different particle physics experiments. Because the earlier measurement, these units have been refined by together with extra information. The brand new ATLAS measurement evaluated the dependence of the measured W-boson mass on PDFs units contemplating these newer variations.
Future measurements of the W-boson mass are anticipated by different LHC experiments, in addition to additional research by ATLAS utilizing information samples recorded in numerous pile-up circumstances and at completely different centre-of-mass energies. These will present unbiased evaluations of the experimental outcomes obtained thus far.