Long-Standing Tension in the Standard Model of Particle Physics Addressed by ATLAS Experiment at CERN – SciTechDaily

A brand fresh ATLAS measurement of a key feature of the Fashioned Model is named lepton flavor universality suggests that a old discrepancy measured by the LEP collider in W boson decays can be because of the a fluctuation.

The particular-known particle within the lepton household is the electron, a key building block of subject and central to our figuring out of electricity. However the electron is now not an fully child. It has two heavier siblings, the muon and the tau lepton, and collectively they’re is named the three lepton flavors. Essentially essentially based on the Fashioned Model of particle physics, the fully distinction between the siblings may perhaps perhaps well furthermore fair mute be their mass: the muon is ready 200 cases heavier than the electron, and the tau-lepton is ready 17 cases heavier than the muon. It’s miles a noteworthy feature of the Fashioned Model that each and every flavor is equally more doubtless to have interaction with a W boson, which outcomes from the so-called lepton flavor universality. Lepton flavor universality has been probed in diverse processes and energy regimes to high precision.

In a brand fresh glimpse, described in a paper posted on July 28, 2020, on the arXiv and first presented at the LHCP 2020 convention, the ATLAS collaboration offers a valid measurement of lepton flavor universality the usage of a trace-fresh methodology.

Researchers from the ATLAS collaboration exhibit their fresh measurement of “lepton flavor universality” – a routine property of the Fashioned Model of particle physics. Credit: CERN

ATLAS physicists examined collision events where pairs of high quarks decay to pairs of W bosons, and therefore into leptons. “The LHC is a high-quark manufacturing facility, and produced 100 million high-quark pairs all the tactic via Flee 2,” says Klaus Moenig, ATLAS Physics Coordinator. “This gave us a natty fair pattern of W bosons decaying to muons and tau leptons, which used to be crucial for this high-precision measurement.”

They then measured the relative probability that the lepton attributable to a W-boson decay is a muon or a tau-lepton – a ratio is named R(τ/μ). Essentially essentially based on the Fashioned Model, R(τ/μ) may perhaps perhaps well furthermore fair mute be solidarity, because the energy of the interplay with a W boson may perhaps perhaps well furthermore fair mute be the the same for a tau-lepton and a muon. But there has been tension about this ever since the 1990s when experiments at the Huge Electron-Positron (LEP) collider measured R(τ/μ) to be 1.070 ± 0.026, deviating from the Fashioned Model expectation by 2.7 usual deviations.

The fresh ATLAS measurement offers a impress of R(τ/μ) = 0.992 ± 0.013. That is mainly the most valid measurement of the ratio up to now, with an uncertainty half the size of that from the aggregate of LEP outcomes. The ATLAS measurement is in settlement with the Fashioned Model expectation and suggests that the old LEP discrepancy can be because of the a fluctuation.

“The LHC used to be designed as a discovery machine for the Higgs boson and heavy fresh physics,” says ATLAS Spokesperson Karl Jakobs. “But this result further demonstrates that the ATLAS experiment is also in a position to measurements at the precision frontier. Our potential for many of these precision measurements will fully give a enhance to as we make a selection extra data in Flee 3 and past.”

Though it has survived this latest take a look at, the precept of lepton flavor universality may perhaps perhaps well furthermore now not be fully out of the woods until the anomalies in B-meson decays recorded by the LHCb experiment private also been definitively probed.

Reference: “Take a look at of the universality of τ and μ lepton couplings in W-boson decays from tt¯ events with the ATLAS detector” by ATLAS Collaboration, 28 July 2020, High Vitality Physics – Experiment.

arXiv: 2007.14040