Long-term stress in the standard model addressed


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The best-known particle in the lepton family is the electron, a key component of matter and central to our understanding of electricity. But the electron is not an only child. He has two heavier brothers, the muon and the tau lepton, and together they are known as the three flavors of leptons. According to the standard model of particle physics, the only difference between siblings should be their mass: the muon is approximately 200 times heavier than the electron, and the tau-lepton is approximately 17 times heavier than the muon. It is a notable feature of the Standard Model that each flavor has the same probability of interacting with a W boson, which results from the so-called lepton flavor universality. The universality of Lepton’s flavor has been proven in different energy regimes and processes with high precision.


In a new study, described in an article published today in the arXiv and first presented at the LHCP 2020 conference, the ATLAS collaboration presents an accurate measurement of the universality of lepton flavor using a completely new technique.

ATLAS physicists examined collision events in which the pairs of higher quarks break down into pairs of W bosons and, subsequently, into leptons. “The LHC is a top quark factory and produced 100 million top quark pairs during Race 2,” says Klaus Moenig, ATLAS Physics Coordinator. “This gave us a large, unbiased sample of W bosons that break down into tau muons and leptons, which was essential for this high-precision measurement.”

They then measured the relative probability that the lepton resulting from a decay of the W boson is either a muon or a tau-lepton, a relationship known as R (τ / μ). According to the standard model, R (τ / μ) should be unity, since the strength of the interaction with a boson W should be the same for a tau-lepton and a muon. But there has been tension about this since the 1990s, when experiments on the Large Electron Collider (LEP) measured that R (τ / μ) was 1,070 ± 0.026, deviating from the standard model expectation by 2.7 standard deviations.

Researchers from the ATLAS collaboration explain their new measurement of the “universality of leptonic taste,” a unique property of the Standard Model of particle physics. Credit: CERN

The new ATLAS measurement gives a value of R (τ / μ) = 0.992 ± 0.013. This is the most accurate measure of the relationship to date, with an uncertainty half the size of the combined LEP results. The ATLAS measurement is in accordance with the expectations of the Standard Model and suggests that the previous LEP discrepancy may be due to fluctuation.

“The LHC was designed as a discovery machine for the Higgs boson and new heavy physics,” says ATLAS spokesman Karl Jakobs. “But this result shows that the ATLAS experiment is also capable of taking measurements at the precision boundary. Our ability for these types of precision measurements will only improve as we take more data in Race 3 and beyond.”

Although it has survived this last test, the principle of the universality of the taste of lepton will not be completely out of the question until the anomalies in the meson B decays recorded by the LHCb experiment have also been definitively proven.


ATLAS experiment finds evidence of spectacular top four quark production


More information:
Proof of the universality of the lepton couplings τ and μ in the decomposition of the W boson of the tt¯ events with the ATLAS detector. arXiv: 2007.14040 [hep-ex]. arxiv.org/abs/2007.14040

Citation: Long-standing voltage in the addressed Standard Model (2020, July 30) retrieved on July 30, 2020 from https://phys.org/news/2020-07-long-standing-tension-standard.html

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