Primarily the most attention-grabbing-identified particle within the lepton household is the electron, a key constructing block of topic and central to our concept of electrical energy. But the electron is no longer an handiest child. It has two heavier siblings, the muon and the tau lepton, and collectively they’re identified because the three lepton flavors. Per the Usual Mannequin of particle physics, the handiest incompatibility between the siblings desires to be their mass: the muon is about 200 events heavier than the electron, and the tau-lepton is about 17 events heavier than the muon. It’s a ways a mighty characteristic of the Usual Mannequin that every flavor is equally likely to have interaction with a W boson, which ends from the so-called lepton flavor universality. Lepton flavor universality has been probed in quite a lot of processes and vitality regimes to high precision.
In a brand modern see, described in a paper posted this day on the arXiv and first presented on the LHCP 2020 conference, the ATLAS collaboration items a proper measurement of lepton flavor universality utilizing a label-modern methodology.
ATLAS physicists examined collision events the effect pairs of high quarks decay to pairs of W bosons, and attributable to this fact into leptons. “The LHC is a high-quark factory, and produced 100 million high-quark pairs throughout Plod 2,” says Klaus Moenig, ATLAS Physics Coordinator. “This gave us an infinite goal pattern of W bosons decaying to muons and tau leptons, which used to be critical for this high-precision measurement.”
They then measured the relative likelihood that the lepton resulting from a W-boson decay is a muon or a tau-lepton—a ratio identified as R(τ/μ). Per the Usual Mannequin, R(τ/μ) desires to be unity, because the energy of the interaction with a W boson desires to be the connected for a tau-lepton and a muon. But there has been tension about this ever for the reason that 1990s when experiments on the Fair Electron-Positron (LEP) collider measured R(τ/μ) to be 1.070 ± 0.026, deviating from the Usual Mannequin expectation by 2.7 long-established deviations.
The modern ATLAS measurement offers a value of R(τ/μ) = 0.992 ± 0.013. This is in point of fact the most proper measurement of the ratio to this level, with an uncertainty half of the scale of that from the combination of LEP results. The ATLAS measurement is in agreement with the Usual Mannequin expectation and means that the old LEP discrepancy will more than likely be attributable to a fluctuation.
“The LHC used to be designed as a discovery machine for the Higgs boson and heavy modern physics,” says ATLAS Spokesperson Karl Jakobs. “But this consequence extra demonstrates that the ATLAS experiment is additionally able to measurements on the precision frontier. Our capability for these forms of precision measurements will handiest strengthen as we utilize more knowledge in Plod 3 and beyond.”
Even supposing it has survived this most contemporary test, the precept of lepton flavor universality is doubtlessly no longer entirely out of the woods till the anomalies in B-meson decays recorded by the LHCb experiment have additionally been definitively probed.
Take a look at of the universality of τ and μ lepton couplings in W-boson decays from tt¯ events with the ATLAS detector. arXiv: 2007.14040 [hep-ex]. arxiv.org/abs/2007.14040
Long-standing tension within the Usual Mannequin addressed (2020, July 30)
retrieved 30 July 2020
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