The most famous particle of the lepton family is the electron, the key building block of matter and central in our understanding of electricity. But the electron is not the only child. It has two heavier counterparts, muon and lepta tau, and together they are known as the three aromas of lepton. According to the Standard Model of Particle Physics, the only difference between their counterparts should be their mass: a muon is about 200 times heavier than an electron, and a tau lepton is about 17 times heavier than a muon. A notable feature of the Standard Model is that each fragrance interacts equally with the W-boson, which is the result of the so-called universality of the taste of lepton. The versatility of Lepton fragrance has been studied in various processes and energy modes with high precision.
In a new study described in a report posted today at arXiv and first presented at the LHCP 2020 conference, the ATLAS collaboration presents an accurate measurement of the universality of lepton taste using a new methodology.
ATLAS physicists have studied collision events in which pairs of upper quarks break up into pairs of W bosons and later into leptons. “The LHC is the factory with the highest quarks and produced 100 million pairs of top quarks in run 2,” said Klaus Monig, ATLAS Physics Coordinator. “This gave us a large objective sample of W bosons decaying into muons and tau leptons, which was important for this high-precision measurement.”
They then measured the relative probability that the lepton resulting from the decay of the W-boson is a muon or tau-lepton, a ratio known as R (τ / μm). According to the standard model, R (τ / μ) should be a unity, because the force of interaction with the W boson should be the same for tau-lepton and muon. But there has been tension about this since the 1990s, when experiments on a large electron-positron (LEP) collider measured an R (τ / μm) of 1,070 ± 0.026, deviating from the standard model expectation by 2.7 standard deviations.
The new ATLAS measurement gives the value R (τ / μ) = 0.992 ± 0.013. This is the most accurate measurement of the relationship to date, with an uncertainty half that of the combination of LEP results. The ATLAS measurement is consistent with the expectation of the Standard Model and suggests that the previous LEP mismatch may be due to fluctuations.
“The LHC was designed as a discovery machine for the Higgs boson and new heavy physics,” said ATLAS spokesman Carl Jacobs. “But this result further demonstrates that the ATLAS experiment is also capable of measuring at the limit of accuracy. Our ability for these types of accuracy measurements will only improve as we get more data on Run 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 woods until the anomalies of B-meson disorders recorded by the LHCb experiment are also finally investigated.
The ATLAS experiment reveals evidence of impressive quark production at four vertices
Checking the universality of τ and μ lepton couplings in the W-boson decays from tt¯ events with the ATLAS detector. arXiv: 2007.14040 [hep-ex]. arxiv.org/abs/2007.14040
Citation: Prolonged tension in the standard model under consideration (2020, July 30), obtained on July 30, 2020 from https://phys.org/news/2020-07-long-standing-tension-standard.html
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