Monday, 3 August 2015

Prof. Jean Cleymans, head of the SA-CERN consortium, visits Wits

Professor Jean Cleymans, head of the SA-CERN consortium, visits the School of Physics of the University of the Witwatersrand. Wits is actively involved in CERN activities contributing to the four   directions in the consortium: ALICE, ATLAS, Isolde and Theory.

Prof. Cleymans visited the various facilities used at Wits for CERN-related activities. This includes synergistic activities with Nuclear and Radiation physics, Material Sciences, Electrical and Information engineering. Overall, over 50 people are involved in CERN activities, ranging from undergraduate, graduate students, post-doctoral fellows, to technical and academic staff. Most of people involved in the program are students.

After visiting the facilities we had a meeting with people involved with the CERN programme at Wits. Below is a picture that includes Professor Joao Rodrigues, the Deputy Dean of the Faculty of Sciences and former Head of the School. Dr Xifeng Ruan, Dr Luis March and Kehinde Tomiwa were connected remotely. Missing in the picture are Prof. Zeblon Vilakazi, two post-doctoral fellows at ALICE, a PhD student from Isolde, Vincent Govender, Jonathan Padavatan,  and ATLAS students Shell-may Liao, Guillermo Hamity, Titus Masike, Marc Sacks and Kamela Sekonya, among others:

Prof. Jean Cleymans and Prof. Bruce Mellado addressing the audience. In the background, Dr. Xifeng Ruan, Dr. Luis March, and Kehinde Tomima, based abroad.

Some of the Wits academic staff involved at SA-CERN with Prof. Jean Cleymans. From left to right, Prof. Elias Sideras-Haddad, Dr. Deepak Kar, Prof. Deena Naidoo, Prof. Bruce Mellado, Prof. Alan Cornell.

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.