Narrow Results

The combined measurements of Higgs boson production and decay are performed based on up to 80 fb${}^{-1}$ of proton–proton collision data collected at $\sqrt{s}$ = 13 TeV by the ATLAS detector. The overall signal strength, defined as measured signal yield divided by the Standard Model prediction, is $1.11_{-0.08}^{+0.09}$. The inclusive and differential cross-sections of gluon–gluon fusion, vector-boson fusion, associated production with vector boson, and associated production with top quarks processes are also measured. The measurements are interpreted with leading-order motivated coupling strength modifiers. No significant deviations from the Standard Model predictions are observed.

The central tracker of the ATLAS experiment is built using both silicon and gaseous detectors immersed in a 2 T solenoidal magnetic field. To better match the topology of the tracks emerging from the proton–proton collisions, the tracker is separated into a central barrel part (measuring below pseudorapidity |η|≈1.2) and two end-caps (measuring from ≈1.2 to 2.5). Different technologies are used at different radii to optimize the cost–performance ratio. The innermost part, immediately surrounding the beam pipe and up to ≈15 cm, is made up of silicon pixels for best pattern recognition and maximal radiation resistance. The intermediate region (radii from 30 to 60 cm) uses microstrip detectors and provides excellent space resolution over a large area. The outer layer (radii from 60 to 95 cm) is made up of a large number of small diameter drift tubes (straws) which provide good space resolution in the track bending plane and greatly contribute to pattern recognition with multiple measurements. The transition radiation detection capability of this gaseous detector also helps in electron identification.

In this paper it is shown how a sampling electromagnetic calorimeter based on the liquid argon technique satisfies the very demanding requirements of an experiment at the LHC. Section 2 discusses, using a simplified model, the performance that can be achieved in terms of response time, energy resolution and transverse granularity. Section 3 describes how the calorimeters are realized in ATLAS, their segmentation and how from the readout pulses the energy deposited in the calorimeter is computed. The motivations of a presampler detector in front of the calorimeter are also discussed. Section 4 describes how the energy, position and direction of an electron and a photon are computed. Finally, Sec. 5 briefly illustrates the rejection power of the calorimeter against the hadrons and mentions how a Higgs boson signal in the γγ channel can already be detected with a luminosity of 10 fb^-1.

The readout electronics development of ATLAS has been driven by the large number of channels of the different subdetectors and by particular environment constraints. This paper gives an overview of the developments made for the front-end electronics, the front-end links, the back-end electronics and the power distribution. It also shows how common issues across the experiment have been handled.

This paper describes the concepts behind the TileCal design, as well as the requirements and the constraints that have finally produced the ATLAS central hadronic calorimeter.

The journey in search for the Higgs boson with the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN started more than two decades ago. But the first discussions motivating the LHC project dream date back even further into the 1980s. This article will recall some of these early historical considerations, mention some of the LHC machine milestones and achievements, focus as an example of a technological challenge on the unique ATLAS superconducting magnet system, and then give an account of the physics results so far, leading to, and featuring particularly, the Higgs boson results, and sketching finally prospects for the future. With its emphasis on the ATLAS experiment it is complementary to the preceding article by Tejinder S. Virdee which focused on the CMS experiment.

Since 2010 there has been a rich harvest of results on standard model physics by the ATLAS and CMS experiments operating on the Large Hadron Collider. In the summer of 2012, a spectacular discovery was made by these experiments of a new, heavy particle. All the subsequently analysed data point strongly to the properties of this particle as those expected for the Higgs boson associated with the Brout–Englert–Higgs mechanism postulated to explain the spontaneous symmetry breaking in the electroweak sector, thereby explaining how elementary particles acquire mass. This article focuses on the CMS experiment, the technological challenges encountered in its construction, describing some of the physics results obtained so far, including the discovery of the Higgs boson, and searches for the widely anticipated new physics beyond the standard model, and peer into the future involving the high-luminosity phase of the LHC. This article is complementary to the one by Peter Jenni⁴ that focuses on the ATLAS experiment.

The Standard Model predicts several rare Higgs boson decays, which have not yet been observed, but that could be enhanced in theories beyond the Standard Model. Among these are decays to light leptons, e.g. $H\to \mu \mu$. In addition, theories beyond the Standard Model may predict lepton-flavor violating decays of the Higgs boson. Results for these searches based on LHC $pp$ collision data recorded at 13 TeV by the ATLAS experiment will be presented.

The upcoming luminosity upgrade of the LHC will impose new requirements for the detector installations. To perform under these conditions the Micromegas (MM) technology was selected to be adopted in the New Small Wheel (NSW) upgrade, dedicated to precision tracking. A large surface of the forward regions of the Muon Spectrometer will be equipped with 8 layers of MM modules forming a total active area of 1200 m². The NSW is planned to be installed in the forward region of $1.3<\left|\eta \right|<2.7$ of ATLAS. This new system will have to operate in a high background radiation region, while reconstructing muon tracks as well as furnishing information for the Level-1 trigger. The project requires fully efficient MM chambers with spatial resolution down to 100 $\mu$m, a rate capability up to about 15 kHz/cm² and operation in a moderate (highly inhomogeneous) magnetic field up to $B=0.3$T. The required tracking is linked to the intrinsic spatial resolution in combination with the demanding mechanical accuracy. An overview of the design, construction and QA/QC procedures followed at the Aristotle University of Thessaloniki for the Micromegas LM2 Drift panels production will be presented.

Since 2010 there has been a rich harvest of results on standard model physics by the ATLAS and CMS experiments operating on the Large Hadron Collider. In the summer of 2012, a spectacular discovery was made by these experiments of a new, heavy particle. All the subsequently analysed data point strongly to the properties of this particle as those expected for the Higgs boson associated with the Brout–Englert–Higgs mechanism postulated to explain the spontaneous symmetry breaking in the electroweak sector, thereby explaining how elementary particles acquire mass. This article focuses on the CMS experiment, the technological challenges encountered in its construction, describing some of the physics results obtained so far, including the discovery of the Higgs boson, and searches for the widely anticipated new physics beyond the standard model, and peer into the future involving the high-luminosity phase of the LHC. This article is complementary to the one by Peter Jenni that focuses on the ATLAS experiment.

The journey in search for the Higgs boson with the ATLAS and CMS experiments at the Large Hadron Collider (LHC) at CERN started more than two decades ago. But the first discussions motivating the LHC project dream date back even further into the 1980s. This article will recall some of these early historical considerations, mention some of the LHC machine milestones and achievements, focus as an example of a technological challenge on the unique ATLAS superconducting magnet system, and then give an account of the physics results so far, leading to, and featuring particularly, the Higgs boson results, and sketching finally prospects for the future. With its emphasis on the ATLAS experiment it is complementary to the preceding article by Tejinder S. Virdee which focused on the CMS experiment.

One of the main goal of the Large Hadron Collider (LHC) experiments at CERN in Switzerland is to aim to solve the “origin of the mass” by discovering the Higgs boson and understanding the interaction of the Higgs boson with the elementary particles. The ATLAS, which is one of the LHC experiments has taken about 5 fb⁻¹ of physics quality data and published several results with regard to the “origin of the mass” since March 2010. This presentation focuses on the latest results of the heaviest elementary particle, namely, top quark physics and the Higgs boson searches from ATLAS.

The ATLAS detector was designed and built to study proton-proton collisions produced at the LHC at centre-of-mass energies up to 14 TeV and instantaneous luminosities up to 10³⁴cm⁻²s⁻¹. Liquid argon (LAr) sampling calorimeters are employed for all electromagnetic calorimetry in the pseudorapidity region |η| <3.2, and for hadronic calorimetry in the region from |η| =1.5 to |η| =4.9. The ATLAS LAr calorimeters produce a total of 182,486 signals which are digitized and processed by the front-end and back-end electronics at every triggered event. In addition, the front-end electronics sums analog signals to provide coarsely grained energy sums to the Level-1 trigger system, which is optimized for nominal LHC luminosities. In 2018, an instantaneous luminosity of 2-3 ×10³⁴cm⁻²s⁻¹ is expected, far beyond the nominal one for which the detector was designed. In order to cope with this increased trigger rate, an improved spatial granularity of the trigger primitives is proposed to improve the identification performance for trigger signatures, like electrons, photons, tau leptons, jets, total and missing energy, at high background rejection rates. For these purposes, a new LAr Trigger Digitizer Board (LTDB) is being designed to receive higher granularity signals, digitize them on detector and send them via fast optical links to a new LAr digital processing system (LDPS). The LDPS applies a digital filtering and identifies significant energy depositions in each trigger channel. The refined trigger primitives are then transmitted to the Level-1 trigger system to extract improved trigger signatures.

For the past three years, experiments at the Large Hadron Collider (LHC) have started exploring physics at the high-energy frontier. Thanks to the superb turn-on of the LHC already a rich harvest of initial physics results has been obtained by the general-purpose experiment ATLAS, as well as its sister-experiment CMS. The initial data has allowed a test, at the highest collision energies ever reached in a laboratory, of the Standard Model (SM) of elementary particles, and to make many searches Beyond the Standard Model (BSM). Important results have already been obtained in the search for the Higgs Boson, which would establish the postulated electro-weak symmetry breaking mechanism in the SM, as well as for BSM physics like Supersymmetry (SUSY), heavy new particles, quark compositeness, and others. The important, and successful, SM physics measurements are giving confidence that the experiment is in good shape for their journey into the uncharted territory of new physics anticipated at the LHC.