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11 december 2018

Closed FOM programme

Number 7.
Title Physics at the TeV scale: ATLAS (ATLAS)
Executive organisational unit FOM-Nikhef + BUW
Programme management Prof.dr. ir. P.J. de Jong and Prof.dr. N. de Groot
Duration 1997-2015
Cost estimate M€ 48.1

With the discovery of the Higgs particle in 2012 awarded with the 2013 Physics Nobel prize, one of the objectives of this programme is successfully completed. Next is to determine the properties of the particle, and to search for physics beyond the Standard Model, in particular supersymmetry. The development, construction and future upgrade of the ATLAS detector for those (and other) objectives is crucial in this endeavor.

Background, relevance and implementation
The Large Hadron Collider, situated at CERN, is designed to collide protons at 14 TeV cm energy to serve four experiments. After an initial running period in 2009 at injection energy, the collision energy was increased to 8 TeV in the following years. In 2015 the LHC energy will be increased to its design value. 
Notwithstanding the impressive confirmation of electro-weak theory by e.g. the LEP programme, there are crucial questions left unanswered. The most urgent is the question whether the Higgs mechanism really generates the masses of elementary particles? Are there signs that the different fundamental forces originate from one force? What are the symmetries underlying the patterns of elementary particles? Are there more dimensions of space than the eye can see? What is dark matter, and can we make it in the laboratory?
The ATLAS detector has been designed as a general-purpose experiment to tackle these questions. Nikhef has been responsible for the large outer muon chambers in the barrel, including their alignment and read-out systems, and a large part of the inner tracking detector equipped with silicon sensors. Together with Dutch industry two large superconducting toroidal magnets were constructed. The full detector is installed and operational. Nikhef is taking a leading part in the development of advanced particle reconstruction software, in analyses of top quark production, in the discovery of the Higgs boson, and in searches for new physics in particular supersymmetry. Together with the academic computing centre SARA, Nikhef provides one of the leading (Tier-1) data analysis centres.
The ATLAS experiment has collected 45 pb-1 in the 2010 run, allowing for extensive detector cali­brations and a 're-discovery' of all particles of 50 years of particle physics. The large data set of more than 25 fb-1 collected in 2011 and 2012 has led to a firm conformation of the Standard Model, the discovery of the Higgs particle, and allows for explorations into the unknown.
Using the 2011 and 2012 data, a Higgs-like resonance has been observed with a mass of 125 GeV. Both the decays to two photons, the decay to two Z-bosons to leptons and the decays to two W-bosons confirm the existence of this new resonance. On July 4th 2012 the announcement was made public as the significance of this resonance exceeded 5 sigma. Since then, the significance of the observation has increased and various properties of the Higgs particle, as predicted by the Standard Model, are tested. Dutch scientists play a key role in the analysis of the in Z- and W-boson final states, and the statistical interpretation of the data.
Searches for new phenomena beyond the Standard Model have not yet observed deviations from the Standard Model predictions, and limits on such new phenomena have been set that far surpass limits from previous experiments. Results include limits on new gauge bosons W' and Z' below 2.5 TeV, excited quarks below 3.8 TeV, fourth generation top quark partners below 600 GeV, and extra dimensions and quantum black holes below 1.5 to 4 TeV. Tight limits are also set on supersymmetry; in constrained models squarks and gluinos below 1.5 TeV are excluded. The 2015 LHC run at full design energy will put supersymmetry to further stringent tests.
Dring the period 2000-2012 a subgroup (led by Prof. S.J. de Jong) is participating in the D0 experi­ment at the Tevatron collider at Fermilab near Chicago, USA. The objectives are to study top quark properties and to search for the Higgs boson, in preparation for the LHC as well as to exploit the scientific opportunities of the Tevatron per se. The Tevatron has ceased operation in September 2011.
Partners in this large programme are the UvA and RU. The principal contribution of the FOM group FOM-N-02 is in the electronics, software and graphics connected to the ATLAS muon detector, and the D0 experiment. The entire FOM group has joined the ATLAS collaboration.

The final evaluation of this programme was consisted of a self-evaluation initiated by the programme leader in 2016.

Please find a research highlight that was achieved in 2013 within this FOM programme here.