Long-running tension in the Standard Model of particle physics addressed by the ATLAS experiment at CERN

A new ATLAS measurement of a key Standard Model feature known as lepton flavor universality suggests that an earlier discrepancy measured by the LEP collider in W boson decays may be due to fluctuation.

The best-known particle of the lepton family is the electron, a key element of matter and central to our understanding of electricity. But the electron is not an only child. It has two heavier siblings, the muon and the tau lepton, and together they are known as the three flavors of lepton. 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-lepton is about 17 times heavier than the muon. . It is a remarkable feature of the Standard Model that each flavor is equally likely to interact with a W boson, resulting in what is known as lepton flavor universality. The universality of lepton flavor has been probed in different energy processes and regimes with great precision.

In a new study, described in a paper published July 28, 2020 on arXiv and first presented at the LHCP 2020 conference, the ATLAS collaboration presents a precise measurement of lepton flavor universality using of a whole new technique.


Researchers from the ATLAS collaboration explain their new measure of “lepton flavor universality” – a property unique to the Standard Model of particle physics. Credit: CERN

ATLAS physicists have examined collision events where pairs of top quarks decay into pairs of W bosons and then into leptons. “The LHC is a top quark factory and produced 100 million top quark pairs during Run 2,” explains Klaus Moenig, ATLAS physics coordinator. “This gave us a large, unbiased sample of W bosons decaying into muons and tau 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 would be a muon or a tau-lepton – a ratio known as R(τ/μ). According to the standard model, R(τ/μ) should be equal to unity, because the strength of the interaction with a W boson should be the same for a tau-lepton and a muon. But there have been tensions over this since the 1990s, when experiments at the Large Electron-Positron Collider (LEP) measured R(τ/μ) at 1.070 ± 0.026, deviating from the Standard Model expectation. of 2.7 standard deviations.

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

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

Although it survived this last test, the principle of lepton flavor universality will not be fully out of the woods until the B meson decay anomalies recorded by the LHCb experiment have been resolved. definitely surveyed.

Reference: “Testing the Universality of τ and μ Lepton Couplings in W Boson Decays from tt¯ Events with the ATLAS Detector” by ATLAS Collaboration, July 28, 2020, High Energy Physics – Experiment.
arXiv:2007.14040

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