The best known particle in the lepton family is the electron, a major building block of matter and central to our knowledge of electricity. But electrons are not just a child. It has two heavier siblings, the muon and the leu tau, and together they are known as the three flavors of lepton. According to the Standard Model of particle physics, the only difference between the siblings should be their mass: the muon is about 200 times heavier than the electron, and the tau-lepton is about 17 times once heavier than the muon. It is a notable feature of the Standard Model that each flavor is likely to interact with a W boson, resulting from the so-called universality of the lepton flavor. The universality of lepton flavor has been embedded in different processes and energy regimes with high precision.
In a new study, described in a paper posted today on arXiv and presented for the first time at the LHCP 2020 conference, the ATLAS collaboration presents an accurate measurement of the universality of leptone flavor using a brand new technique.
ATLAS physicists have examined collision events where pairs of upper quarks decay to pairs of W bosons, and subsequently into leptons. “The LHC is a top quark factory, and produced 100 million top-quark pairs during Leadership 2,” says Klaus Moenig, ATLAS Physics Coordinator. “This gave us a large unbiased sample of W bosons that had already connected muons and tau leptons, which was essential for this high-precision measurement.”
They then measured the relative probability that the lepton resulting from W-boson decay is muon or tau-lepton – a ratio known as R (τ / μ). According to the Standard Model, R (τ / μ) should be the bond, whereas the strength of the interaction with boson W should be the same for tau-lepton and muon. But there has been tension about this since the 1990s when experiments in the large Electron-Positron (LEP) collector measured R (τ / μ) to be 1,070 ± 0.026, deviating from the Expectation of the Standard Model by 2.7 deviations standard.
The new ATLAS measurement gives a value of R (τ / μ) = 0.992 ± 0.013. This is the most accurate measurement of the ratio to date, with uncertainty half the size of that from the combination of LEP results. The ATLAS measurement is in line with the expectation of the Standard Model and suggests that the discrepancy of the previous LEP may be due to shifts.
“The LHC was designed as a detection machine for the Higgs boson and for the new heavy physics,” says ATLAS Spokesman Karl Jakobs. “But this result further shows that the ATLAS experiment is also able to measure at the frontier of accuracy. Our ability for these types of precision measurements will only improve by taking more data into Ra 3 and beyond. “
Although it has survived from this last test, the principle of universality of lepton flavor will not be completely out of the woods until the anomalies in the deletion of meson B recorded by the LHCb experiment are also have definitely been examined.
The ATLAS experiment finds evidence of a spectacular production of four top quarks
Test of the universality of the coupling of lepton τ and μ in a narrow boson W from tt¯ events with the ATLAS detector. arXiv: 2007.14040 [hep-ex]. arxiv.org/abs/2007.14040
Citation: Long-term stress in the addressed Standard Model (2020, 30 July) obtained on 30 July 2020 from https://phys.org/news/2020-07-long-stand-tension-standard.html
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