The best known particle in the lepton family is the electron, a key element in matter and fundamental to our understanding of electricity. But the electron is not an only child. It has two heavier siblings, muon and lepton tau, 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 died. It is a notable feature of the standard model that each flavor is equally likely to interact with a W boson, a result of the so-called universality of the lepton flavor. The universality of Lepton flavor has been tested in different processes and high-precision energy regimes.
In a new study, described in an article published today in arXiv and presented for the first time at the LHCP 2020 conference, the ATLAS collaboration presents an accurate measure of the universality of lepton flavor using a new technique.
ATLAS physicists examined collision events in which pairs of upper quarks decay pairs of W bosons and subsequently into leptons. “The LHC is a top quark factory, and it produced 100 million pairs of top quarks during Run 2,” says Klaus Moenig, ATLAS physics coordinator. “This gave us a large unbiased sample of decaying W bosons in tau muons and leptons, which was essential for this high-precision measurement.”
They then measured the relative probability that the leptone resulting from a decay of the W boson is a muon or a tau-lepton, a ratio known as R (τ / μ). According to the standard model, R (τ / μ) should be the unit, since the force of the interaction with a W boson should be the same for a tau-lepton and a muon. But there have been tensions in this regard since the 1990s when experiments with the large electron-positron collider (LEP) measured R (τ / μ) to be 1,070 ± 0.026, deviating from the model’s expectation. standard by 2.7 standard deviations.
The new ATLAS measure gives a value of R (τ / μ) = 0.992 ± 0.013. This is the most accurate measure of the ratio to date, with an uncertainty of half that of the combination of LEP results. The ATLAS measure is in line with the expectations of the standard model and suggests that the discrepancy in the previous LEP may be due to fluctuation.
“The LHC was designed as a Higgs boson discovery machine and a new physical heavyweight,” says ATLAS spokesman Karl Jakobs. “But this result further demonstrates that the ATLAS experiment is also able to measure at the precision frontier. Our ability for these types of precision measurements will only improve as we take more data in run 3 and beyond. . “
Although it has survived the latter test, the principle of universality of the lepton flavor will not be completely out of the forest until the anomalies of meson B decay recorded by the LHCb experiment have been definitively tested.
The ATLAS experiment finds evidence of a spectacular production of four-pointed quarks
Proof of the universality of lepton couplings τ and μ in W boson decays from tt · events with the ATLAS detector. arXiv: 2007.14040 [hep-ex]. arxiv.org/abs/2007.14040
Citation: Long-term voltage in the standard model addressed (2020, July 30) retrieved July 30, 2020 at https://phys.org/news/2020-07-long-stand-tension-standard.html
This document is subject to copyright. Apart from any fair dealings for the purpose of private studies or research, no part may be reproduced without written permission. Content is provided for information only.