ATLAS is one of two general purpose detectors operating at the LHC at CERN. DESY joined the ATLAS collaboration in 2006 and is engaged in several activities.

The Higgs Boson and searches for new physics

The discovery of the Higgs boson in 2012 at the Large Hadron Collider (LHC) completed the Standard Model (SM) of electroweak and strong interactions and led a year later to the award of the Nobel Prize in physics. However, several experimental results in particle physics and astronomy point to the fact that the SM can only be an effective theory, which needs an appropriate extension for a true description of nature at higher energies. In the next few years, the two most promising means to find this extension are precise measurements of the newly discovered Higgs particle and direct searches for new particles or interactions at the LHC. The DESY ATLAS group contributes to both of these directions, in particular with Higgs to di-photon and b-quark decays. These efforts include Higgs property measurements and searches for new physics with Higgs bosons or Higgs-like signatures in the final state.

This event display shows a Higgs boson candidate decaying to a pair of photons, which deposit their energy in the electromagnetic calorimeter nearly back-to-back in the azimuthal plane. The photon energy deposits are illustrated as yellow cones. This simple event signature is one of the strengths of the di-photon final state. It has an excellent detector resolution and it allows for a simple and efficient online and offline event selection.

Higgs Physics
Higgs physics with photons
Dark Matter with Higgs

The physics of the top quark

The top quark is by far the heaviest of the quarks. Because of this it is expected that measurements involving the top quark should be particularly good for finding as yet undiscovered resonances. The graph shows the results of a search for a possible heavy version of the Z boson decaying into a top-antitop pair. Nothing was found so we can conclude that no such resonance exists unless its mass exceeds 2 TeV. The group is engaged in such searches as well as in studying the properties of the top quark itself (its spin), its state of polarization, and using it to test strong interaction theory by measuring the rate of jet production in events in which a top quark is produced and comparing it to precise QCD predictions.

Top and Top associated Higgs Physics

Standard Model studies

The group is involved in a wide range of studies which include jet structure and W-boson and Z-boson production kinematics. The main interest is in furthering the knowledge of the proton's parton density functions (PDFs) beyond the highly detailed knowledge already provided by HERA. The example shown here is the result of a fit to ATLAS data on W and Z boson production together with the deep inelastic scattering measurements of HERA. The figure shows the density of the quark sea relative to the density of the non-strange sea and suggests that the strange sea is as populated at the non-strange sea. Most of the existing PDFs assume that the strange sea is suppressed since the strange quark is relatively heavy. The predictions of some popular PDFs are indicated as points on the figure.



The Semi-Conducting Tracker (SCT) is a silicon strip detector and is a component of the Inner Tracker. The above photo was taken during the final assembly stage. The DESY group is responsible for the fast calibration loop which is run at the CERN Tier0 center immediately after the data is taken as well as other projects including the commissioning of a SONAR system for measuring gas composition in the cooling system and the implementation of improvements to the detector simulation.

ATLAS SCT Operation

The silicon strip tracker upgrade for the High-Luminosity LHC

End of 2024, the LHC at CERN will be shut down to allow the upgrades to both the machines and the detectors needed for running at high luminosity from 2026 onwards at what will be called the high-luminosity LHC (HL-LHC). ATLAS plans to upgrade several new detector components at this time, and to completely replace the inner tracker. The DESY group is heavily involved in the design and simulation of the new inner detector and will build one of the end-caps for the strip detector. A mechanical drawing of one end-cap is shown.

ATLAS Upgrade : Silicon Strip Tracker Endcap

Monte Carlo generator tuning and interfacing generators with ATLAS software

Monte Carlo generators are used to simulate events produced at the LHC. They are extensively used for estimating the sensitivity of the detector to hypothetical signals and for estimating background processes, particularly from QCD and Electroweak processes. The generators are built around very sophisticated models of the "hard" parts of the interactions but, because of difficulties with extracting predictions from QCD for the so-called "soft" interactions, they are in need of supplementary phenomenological models whose parameters need to be tuned to match the data. The group is involved in producing such tunes. The plot on the left shows a typical tuning variable (in this case the average sum of the transverse momenta (pT) of particles in the event as a function of the pT of the highest-pT particle in the event compared to various generator tunes. In addition to tuning the generators, the group is also responsible for maintaining the interfaces between the generators and the ATLAS analysis software.



The ALFA detector completes the central ATLAS detector in the very forward direction. For this purpose tracking detectors are integrated in 2 Roman Pot stations in a distance of 240m at each side of the ATLAS interaction point. The tracking detectors are placed in so-called Roman Pots and flexible bellows allow to move them in the vertical direction close to the LHC beams. The photo shows two of the Roman Pots during installation, viewed from the beam direction in their closed position, close to the beam. ALFA measures elastic scattering at very small angles to determine the absolute luminosity of ATLAS. Diffractive scattering (together with the central detector) and total cross sections at all LHC energies will also be measured.

ALFA data taking is performed in special LHC runs with high β* optics and low beam intensities. Data taken at β* = 90m in the years 2011 and 2012 allows the extraction of total cross sections at 7 and 8 TeV center of mass energies with uncertainties around 3%. A first run with β* = 1km was performed in the Coulomb-Nuclear interference range. These data are suitable for extracting cross section results which are independent of the luminosity calibration by Van der Meer scans.



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