The most famous particle in the lepton family is the electron, the key building block of matter, and the basis of our understanding of electricity. However, the electron is not just a child. It has two heavier siblings, muon and tau lepton, and together they are known as three lepton flavors. According to the standard model of particle physics, the only difference between siblings 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. It is a remarkable feature of the standard model that each aroma probably interacts with the W boson, which is the result of the so-called Lepton’s universal aroma. The versatility of lepton flavors has been tested in various processes and energy modes with high accuracy.
In a new study, described in a document published today on arXiv and first presented at the LHCP 2020 conference, ATLAS collaboration represents an accurate measurement of the universality of lepton flavors using a completely new technique.
ATLAS physicists investigated collision events in which pairs of top quarks decayed into pairs of W bosons and subsequently into leptons. “The LHC is a top quark factory and has produced 100 million pairs of top quarks during run 2,” says Klaus Moenig, ATLAS physics coordinator. “This provided us with a large unbiased sample of W bosons decaying into muons and tau leptons, which was necessary for this highly accurate measurement.”
They then measured the relative probability that the lepton resulting from the decay of the W-boson was a muon or tau-lepton – a ratio known as R (τ / μ). According to the standard model, R (τ / μ) should be unified, because the strength of the interaction with the boson W should be the same for tau-lepton and muon. However, this has been the voltage since the 1990s, when experiments with a large electron positron (LEP) collector measured R (τ / μ) at 1.070 ± 0.026, deviating from the standard model 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 ratio to date, with uncertainty half the magnitude of the uncertainty resulting from the combination of LEP results. The ATLAS measurement is in line with the expectations of the standard model and suggests that a previous LEP discrepancy may be due to fluctuations.
“The LHC was designed as a discovery machine for the Higgs boson and new physics,” said ATLAS spokesman Karl Jakobs. “However, this result further proves that the ATLAS experiment is able to measure at the limit of accuracy. Our capacity for these types of accuracy measurements will only improve if we take more data in run 3 and beyond. ”
Although he survived this last test, the principle of the universality of lepton flavor will not be completely out of forests until the anomalies in B-meson decays recorded by the LHCb experiment have been definitively investigated.
The ATLAS experiment will find evidence of spectacular production of quarks of the highest quality
The universality test of τ and μ lepton bonds in W-bosons decayed from tt¯ events using an ATLAS detector. archive: 2.007.14040 [hep-ex], arxiv.org/abs/2007.14040
Citations: Long-term stress in the standard model (2020, July 30), obtained on July 30, 2020 from https://phys.org/news/2020-07-long-standing-tension-standard.html
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