ATLAS Experiment

ATLAS is the largest general-purpose particle detector experiment at the Large Hadron Collider (LHC), a particle accelerator at CERN (the European Organization for Nuclear Research) in Switzerland. The experiment is designed to take advantage of the unprecedented energy available at the LHC and observe phenomena that involve highly massive particles which were not observable using earlier lower- energy accelerators. ATLAS was one of the two LHC experiments involved in the discovery of the Higgs boson in July 2012. It was also designed to search for evidence of theories of particle physics beyond the Standard Model.

The experiment is a collaboration involving 6,003 members, out of which 3,822 are physicists (last update: June 26, 2022) from 257 institutions in 42 countries.

History

Particle accelerator growth

The first cyclotron, an early type of particle accelerator, was built by Ernest O. Lawrence in 1931, with a radius of just a few centimetres and a particle energy of 1 megaelectronvolt (MeV). Since then, accelerators have grown enormously in the quest to produce new particles of greater and greater mass. As accelerators have grown, so too has the list of known particles that they might be used to investigate.

ATLAS Collaboration

The ATLAS Collaboration, the international group of physicists belonging to different universities and research centres who built and run the detector, was formed in 1992 when the proposed EAGLE (Experiment for Accurate Gamma, Lepton and Energy Measurements) and ASCOT (Apparatus with Super Conducting Toroids) collaborations merged their efforts to build a single, general-purpose particle detector for a new particle accelerator, the Large Hadron Collider. At present, the ATLAS Collaboration involves 5,767 members, out of which 2,646 are physicists (last census: September 9, 2021) from 180 institutions in 40 countries.

Detector design and construction

The design was a combination of two previous projects for LHC, EAGLE and ASCOT, and also benefitted from the detector research and development that had been done for the Superconducting Super Collider, a US project interrupted in 1993. The ATLAS experiment was proposed in its current form in 1994, and officially funded by the CERN member countries in 1995. Additional countries, universities, and laboratories have joined in subsequent years. Construction work began at individual institutions, with detector components then being shipped to CERN and assembled in the ATLAS experiment pit starting in 2003.

Detector operation

Construction was completed in 2008 and the experiment detected its first single proton beam events on 10 September of that year.

Data-taking was then interrupted for over a year due to an LHC magnet quench incident. On 23 November 2009, the first proton–proton collisions occurred at the LHC and were recorded by ATLAS, at a relatively low injection energy of 900 GeV in the center of mass of the collision. Since then, the LHC energy has been increasing: 1.8 TeV at the end of 2009, 7 TeV for the whole of 2010 and 2011, then 8 TeV in 2012.
The first data-taking period performed between 2010 and 2012 is referred to as Run I. After a long shutdown (LS1) in 2013 and 2014, in 2015 ATLAS saw 13 TeV collisions.
The second data-taking period, Run II, was completed, always at 13 TeV energy, at the end of 2018 with a recorded integrated luminosity of nearly 140 fb⁻¹ (inverse femto barn).
A second long shutdown (LS2) in 2019-21 has followed, while ATLAS is being upgraded for Run III, starting in May 2022.

Leadership

The ATLAS Collaboration is currently led by Spokesperson Andreas Hoecker and Deputy Spokespersons Marumi Kado and Manuella Vincter. Former Spokespersons have been:

Experimental program

In the field of particle physics, ATLAS studies different types of processes detected or detectable in energetic collisions at the Large Hadron Collider (LHC). For the processes already known, it is a matter of measuring more and more accurately the properties of known particles or finding quantitative confirmations of the Standard model. Processes not observed so far would allow, if detected, to discover new particles or to have confirmation of physical theories that go beyond the Standard model.

Standard Model

The Standard model of particle physics is the theory describing three of the four known fundamental forces (the electromagnetic, weak, and strong interactions, while omitting gravity) in the universe, as well as classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists around the world, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, confirmation of the top quark (1995), the tau neutrino (2000), and the Higgs boson (2012) have added further credence to the Standard model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.

Although the Standard model is believed to be theoretically self-consistent and has demonstrated huge successes in providing experimental predictions, it leaves some phenomena unexplained and falls short of being a complete theory of fundamental interactions. It does not fully explain baryon asymmetry, incorporate the full theory of gravitationSean Carroll, PhD, Caltech, 2007, The Teaching Company, ''Dark Matter, Dark Energy: The Dark Side of the Universe'', Guidebook Part 2 page 59, Accessed 7 Oct. 2013, "...Standard Model of Particle Physics: The modern theory of elementary particles and their interactions ... It does not, strictly speaking, include gravity, although it's often convenient to include gravitons among the known particles of nature..." as described by general relativity, or account for the accelerating expansion of the universe as possibly described by dark energy. The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not incorporate neutrino oscillations and their non-zero masses.

Precision measurements

With the important exception of the Higgs boson, detected by the ATLAS and the CMS experiments in 2012, all of the particles predicted by the Standard Model had been observed by previous experiments. In this field, in addition to the discovery of the Higgs boson, the experimental work of ATLAS has focused on precision measurements, aimed at determining with ever greater accuracy the many physical parameters of theory.
In particular for
# the Higgs boson;
# W and Z bosons;
# the top and bottom quarks
ATLAS measures:
# 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 MeV">Electronvolt.html" ;"title="8037±19) Electronvolt">MeVof the W boson">Electronvolt">MeV<_a>.html" ;"title="Electronvolt.html" ;"title="8037±19) Electronvolt">MeV">Electronvolt.html" ;"title="8037±19) MeVof the weak interaction, with a measurement uncertainty">Electronvolt">MeVof the W boson, one of the two mediators of the electroweak interaction">weak interaction, with a measurement uncertainty of ±2.4Per mille">‰.

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 ($H\rightarrow\gamma\gamma$) and its decay to four leptons ($H\rightarrow ZZ^*\rightarrow 4l$ and $H\rightarrow WW^*\rightarrow e\nu\mu\nu$).

In March 2013, in the light of the updated ATLAS and CMS results, CERN announced that the new 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

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