Addressed long-term stress in the standard model

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The most well-known particle in the lepton family is electron, a major building block of matter and the center of our understanding of electricity. But the electron is not just a child. It has two heavy siblings, Mune and Tau Leptan, and together they are known as the three leptons flavored. According to the Standard Model of Particle Physics, the only difference between siblings should be their mass: the muon is about 200 times heavier than the electron, and the tau-lipton is about 17 times heavier than the muon. It is a notable feature of the standard model that each flavor is equally likely to interact with W. bosone, resulting in so-called leptan flavor universality. Lipton flavor universality has been investigated in various processes and energy regimes for high accuracy.

In a new study, described in a paper posted today on AXAV and first presented at the LHCP 2020 conference, the Atlas collaboration has presented an accurate measurement of Lipton flavor universality using a new technology.

Atlas physicists investigated collision events where pairs of top quarks decay to pairs of W. bosons, and later into leptons. “LHC is a top-quark factory, and 100 million top-quark pairs have been produced during Run 2,” says Atlas Physics Coordinator, Klaus Moeng. “This gave us a large unbiased sample of rotting W bosons to mu bons and tau leptons, which was required for this high precision measurement.”

They then measured the relative probability that the lepton is a muon or tau-lipton, known as R (τ / μ), as a result of W-boson decay. According to the standard model, R (the / μ) must be unity, since the strength of interaction with the W boson should be the same for a tau-leptan and a muon. But this has been tense ever since the 1990s when R (measured / μ) in the Large Electron – Positron (LEP) collider was measured at 1.070 26 0.026, a deviation of 2.7 million deviations from the standard model expectation. was doing.

Researchers at Atlas Collaboration explained their new measurement of “leptan taste universality” – a unique property of the standard of particle physics. Credit: CERN

The new ATLAS measurement gives a value of R (μ / μ) = 0.992 3 0.013. This is the most accurate measurement of the ratio to date, with a half-size of uncertainty associated with combining LEP results. The atlas measurement is in line with the standard model expected and suggests that the previous LEP anomaly may be due to a fluctuation.

“The LHC was designed as a search machine for Higgs bosons and massive new physics,” Atlas spokesman Carl Jacobs says. “But this result further demonstrates that the ATLAS experiment is also capable of performing measurements over an accurate range. Our capability for this type of accurate measurements will only improve when we take more data in Run 3 and beyond.”

While it has survived this latest test, the theory of lepton taste universality will not be completely out of the woods until anomalies in the B-mason anomalies recorded by the LHCB experiment have also been definitively investigated.

Atlas experiment finds evidence of superb four-top quark production

more information:
The universality of τ and μ lepton couplings in the W-boson is tested by tt¯ events with the ATLAS detector. arXiv: 2004.14,060 [hep-ex].

Quotes: The long-term stress (2020, 30 July) addressed in the standard model was retrieved from until 30 July 2020.

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