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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| ATLAS experiment | 2/5 | https://en.wikipedia.org/wiki/ATLAS_experiment | reference | science, encyclopedia | 2026-05-05T13:02:57.471723+00:00 | kb-cron |
masses; channels of production, decay and mean lifetimes; interaction mechanisms and coupling constants for electroweak and strong interactions. For example, the data collected by ATLAS made it possible in 2018 to measure the mass [(80,370±19) MeV] of the W boson, one of the two mediators of the weak interaction, with a measurement uncertainty of ±2.4‰.
==== Higgs boson ====
One of the most important goals of ATLAS was to investigate a missing piece of the Standard Model, the Higgs boson. The Higgs mechanism, which includes the Higgs boson, gives mass to elementary particles, leading to differences between the weak force and electromagnetism by giving the W and Z bosons mass while leaving the photon massless. On July 4, 2012, ATLAS — together with CMS, its sister experiment at the LHC — reported evidence for the existence of a particle consistent with the Higgs boson at a confidence level of 5 sigma, with a mass around 125 GeV, or 133 times the proton mass. This new "Higgs-like" particle was detected by its decay into two photons (
H
→
γ
γ
{\displaystyle H\rightarrow \gamma \gamma }
) and its decay to four leptons (
H
→
Z
Z
∗
→
4
l
{\displaystyle H\rightarrow ZZ^{*}\rightarrow 4l}
and
H
→
W
W
∗
→
e
ν
μ
ν
{\displaystyle H\rightarrow WW^{*}\rightarrow e\nu \mu \nu }
). In March 2013, following the updated results from ATLAS and CMS, CERN announced that the newly discovered particle was indeed a Higgs boson. The experiments were also able to show that the properties of the particle as well as the ways it interacts with other particles were well-matched with those of a Higgs boson, which is expected to have spin 0 and positive parity. Analysis of more properties of the particle and data collected in 2015 and 2016 confirmed this further. In October 2013, two of the theoretical physicists who predicted the existence of the Standard Model Higgs boson, Peter Higgs and François Englert, were awarded the Nobel Prize in Physics.
==== Top quark properties ==== The properties of the top quark, discovered at Fermilab in 1995, had been measured approximately. With much greater energy and greater collision rates, the LHC produces a tremendous number of top quarks, allowing ATLAS to make much more precise measurements of its mass and interactions with other particles. These measurements provide indirect information on the details of the Standard Model, with the possibility of revealing inconsistencies that point to new physics.
=== Beyond the Standard Model === While the Standard Model predicts that quarks, leptons and neutrinos should exist, it does not explain why the masses of these particles are so different (they differ by orders of magnitude). Furthermore, the mass of the neutrinos should be, according to the Standard Model, exactly zero as that of the photon. Instead, neutrinos have mass. In 1998 research results at detector Super-Kamiokande determined that neutrinos can oscillate from one flavor to another, which dictates that they have a mass other than zero. For these and other reasons, many particle physicists believe it is possible that the Standard Model will break down at energies at the teraelectronvolt (TeV) scale or higher. Most alternative theories, the Grand Unified Theories (GUTs) including Supersymmetry (SUSY), predicts the existence of new particles with masses greater than those of Standard Model.
==== Supersymmetry ==== Most of the currently proposed theories predict new higher-mass particles, some of which may be light enough to be observed by ATLAS. Models of supersymmetry involve new, highly massive particles. In many cases these decay into high-energy quarks and stable heavy particles that are very unlikely to interact with ordinary matter. The stable particles would escape the detector, leaving as a signal one or more high-energy quark jets and a large amount of "missing" momentum. Other hypothetical massive particles, like those in the Kaluza–Klein theory, might leave a similar signature. The data collected up to the end of LHC Run II do not show evidence of supersymmetric or unexpected particles, the research of which will continue in the data that will be collected from Run III onwards.
==== CP violation ==== The asymmetry between the behavior of matter and antimatter, known as CP violation, is also being investigated. Recent experiments dedicated to measurements of CP violation, such as BaBar and Belle, have not detected sufficient CP violation in the Standard Model to explain the lack of detectable antimatter in the universe. It is possible that new models of physics will introduce additional CP violation, shedding light on this problem. Evidence supporting these models might either be detected directly by the production of new particles, or indirectly by measurements of the properties of B- and D-mesons. LHCb, an LHC experiment dedicated to B-mesons, is likely to be better suited to the latter.
==== Microscopic black holes ==== Some hypotheses, based on the ADD model, involve large extra dimensions and predict that micro black holes could be formed by the LHC. These would decay immediately by means of Hawking radiation, producing all particles in the Standard Model in equal numbers and leaving an unequivocal signature in the ATLAS detector.
== ATLAS detector == The ATLAS detector is 46 metres long, 25 metres in diameter, and weighs about 7,000 tonnes; it contains some 3,000 km of cable. At 27 km in circumference, the Large Hadron Collider (LHC) at CERN collides two beams of protons together, with each proton carrying up to 6.8 TeV of energy – enough to produce particles with masses significantly greater than any particles currently known, if these particles exist. When the proton beams produced by the Large Hadron Collider interact in the center of the detector, a variety of different particles with a broad range of energies are produced.