Sunday, 2 August 2015

First Run 2 ATLAS results with strong Wits contribution

The first results from LHC Run 2 was shown at the European Physical Society Meeting in Vienna last week. While we are excited about the next discovery with the new data, the first task with a new collision energy data is always to make sure we understand our detector performance and verify that the Standard Model predictions are correct.

The former was very important because the ATLAS detector went through a fair bit of improvement during the long shutdown. A new tracking detector (the insertable B-layer, or the IBL) was added next to the beam pipe, which dramatically improved the accuracy of the track reconstruction and the identification of jets originating from bottom quarks, which is important for many searches. Shown below is the event display of a proton-proton collision event recorded by ATLAS on 3 June 2015, with the first LHC stable beams at a collision energy of 13 TeV.

The Monte Carlo event generators are extensively used in measurements and searches. While the perturbative part is modelled based on very accurate theoretical prediction, the non-perturbative part is constrained by measurements. So it was crucial to test the phenomenological energy extrapolation models used in these generators by looking at the 13TeV data. The usual "minimum-bias" and "underlying event" measurements were performed, where different distributions with charged particles were measured, and the models seemed to be in reasonable agreement with data. This gave us confidence that we are ready to use these models for our exciting new physics searches. Dr. Deepak Kar, who joined the Wits group recently, played a major role in getting this results public in a such a short time as the ATLAS soft-QCD subgroup convener and an analyser.

The figure below shows the the activity in an event which is mostly not coming from the hard collision, i.e our process of interest. This is termed underlying event, and represents the soft background which needs to be modelled well by the generators. The 13 TeV data is compared to several such models, and the ones to be used by ATLAS describe the data reasonably well.

More details can be found at the following ATLAS briefing.

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