ATLAS Physics Group

ATLAS is one of the four large detectors at the proton-proton collider LHC (Large Hadron Collider) at the Centre of European Particle Physics CERN (Conseil Européen pour la Recherche Nucléaire). The initial LHC run in the years 2010-2012 brought a variety of new physics results, culminating in the observation of the Higgs boson in 2012. The LHC has begun to touch an energy regime probed never before in an accelerator. Among the main physics goals of ATLAS are the investigation of the Higgs boson, the search for Supersymmetry and other new phenomena beyond the standard model of particle physics (e.g. large extra dimensions), physics of the top quark, of B hadrons and of weak gauge bosons, as well as tests of Quantum Chromodynamics (QCD) at high energies.

Our group is active in many of these areas, analyzing the reactions that occur at the LHC and improving reconstruction and analysis methods to match the rapid increase in size of the available data set. We also share responsibility for operating and monitoring the data quality in particular of the ATLAS Pixel Detector, which operates at a distance of only 5cm to the interaction point, as well as future upgrades of this detector, such as a new innermost layer of the pixel detector at 3.5cm which was installed in 2014.

Research Topics

Higgs Boson

The Higgs boson is an elementary particle postulated by theorist Peter Higgs. This particle is a remnant of the spontaneous symmetry breaking of electroweak interactions. It is this symmetry breaking that gives a mass to elementary particles. Our group has been pursuing the Higgs boson since the early nineties at experiments at LEP, Tevatron and now the Large Hadron Collider at CERN.

The Higgs has been discovered in 2012 culminating in a Nobel prize awarded to Peter Higgs and Francios Englert in 2013.

Our group is part of the ATLAS experiment and we are particularly interested in the fermionic couplings of the Higgs boson. We investigate these via two main search channels: Higgs decaying into a pair of tau leptons and Higgs decaying to b-quark anti-b-quark. We are currently analysing the data from Run 2 of the LHC, which started in 2015.

Top Quark

The top quark is an elementary particle with a very large mass. This large mass is responsible for its properties and unique role in high energy particle physics.  The top decays before forming bound states (hadrons), thus providing a unique opportunity to study a bare quark.

After the measurements of the production cross section, top quark mass and charge asymmetry, the focus of our group has moved to the rare processes ttgamma, ttZ, ttH and decays involving flavor changing neutral decays or lepton flavor violation.

Beyond the Standard Model

Despite its great success, the Standard Model (SM) has a number of shortcomings and is not a complete theory of fundamental physics. Many competing theories have been proposed to address these issues, such as supersymmetry and grand unification. Such theories often exhibit extended gauge and Higgs sectors, predicting the existence of additional gauge and Higgs bosons. Furthermore, while the Higgs boson discovered at the LHC is consistent with the one predicted by the SM, it is also consistent with Higgs bosons from many of these alternate models. Uncovering the real nature of the Higgs sector and of the fundamental interactions which govern our universe are at the forefront of modern particle physics research. 

Our group is actively involved in many searches for physics beyond the SM at ATLAS. We focus on final states involving b-quarks and/or tau-leptons. Such searches are particularly sensitive to additional Higgs bosons and new physics signals which either couple strongly to the third generation or decay themselves to Higgs bosons.

Analytical Techniques

Analysis of particle physics data relies heavily on the accurate reconstruction and identification of particles produced in collisions. Increasing expectations from analysers coupled with the ever harsher experimental conditions at the LHC place continual pressure to develop more sophisticated algorithms. This in turn has lead to growing interaction with the Machine Learning community to integrate recent advancements in Classification and Regression algorithms.

Our group plays a crucial role in the advancement of b-quark tagging and tau-lepton identification at ATLAS. 

Latest Results

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