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Search for new physics at the LHC and preparations for the CMS detector upgrade

Final Report Summary - LHC-PHYSICS (Search for new physics at the LHC and preparations for the CMS detector upgrade)

The field of high-energy elementary particle physics focuses on the study of the fundamental building blocks of nature, the elementary particles, and the interactions between them. A major part of the present-day knowledge in the field was obtained through large, complex particle accelerators, where particles are accelerated to very high energies and brought to collision. The Large Hadron Collider (LHC) at CERN, Geneva, due to its unprecedented collision energy, provides the unique laboratory to perform studies of elementary particles at energy scales that were never reached before. The LHC has already operated colliding proton beams at an energy of 7-8 TeV from 2010 to 2012. The data obtained in this period led to the discovery of long sought Higgs boson. In Summer 2015, the LHC is set to restart operations at a nearly doubled energy (from 8 TeV to 13 TeV) and at significantly higher collision rates (so-called luminosity). One of the two objectives of the proposed project is the preparation for the analysis of the upcoming LHC data with the CMS detector and eventually analyze the data (the second year of the fellowship) for a particular research topic. Despite the great success of the current theoretical model of particles and their interactions, called Standard Model (SM), in explaining the experimental data so far, there exist a number of phenomena in nature that cannot be accommodated in this theory. This fact hints towards the existence of a more fundamental theory. The aim of the planned data analysis is to search for experimental signatures of hypothetical particles (additional to those that are already observed experimentally) that are predicted by theories beyond the SM.
As stated above, for what concerns the analysis of the LHC with CMS detector, the main objective of the proposal during 2014-2015 was the preparation for the analysis. These preparations included the development of a particular online event selection algorithm (called Trigger), the optimization of the particle (i.e. electrons and muons) identification techniques as well as the development of a method to quantify the expected SM background from the W and Z boson production. For the latter subject, fellow Dobur supervised a student of Prof. Clerbaux to develop the technique using Monte Carlo (MC) simulations. The results of this study are such that the idea put forward by the fellow and studied by the student proves the concept of principle and applicable on the actual data analysis. Dr. Dobur's work on the development of the triggers has been completed and the algorithm developed by the fellow is currently a part of the integral Trigger menu that will be used in CMS detector once the collisions start in Summer 2015.

The Fellow Dobur worked very closely with Prof. Clerbaux on other fronts of the LHC data analysis, in particular for the searches for new physics on a front where one looks for high mass resonances that decay into electron-positron pairs. Dobur, joined the weekly meetings with Prof. Clerbaux and her team where the cross-pollination of ideas on the subject was a key element.

Additional to the work summarized above on physics analysis front, the fellow worked on finalizing some of the work initiated by her before the start of the fellowship. This was related to the analysis of the LHC data taken in 2012 with the goal of searching for new physics using CMS detector. This work led to two journal publications. Furthermore, Dr. Dobur worked on a journal publication which documented the results on the re-interpretation of the LHC data published by the CMS collaboration. This work involved experimental physicists and theorist from IIHE (ULB-VUB) (Inter-university Institute for High Energies) and University of Strasbourg, France. The fellow Dobur acted as an expert bridging between the theorists and the experimental data from CMS on which she has had worked before. With this successful work, the objective on the interdisciplinary aspect of the proposed project is accomplished. The results of this work has been submitted to Physics Letters B journal for publication.

Before the end of this decade (around 2018), the LHC will enter a second long shutdown to prepare to operate above its original design collision rate. Another two-year shutdown in the period 2022-2023 will aim to bring the machine’s luminosity to over five times the design value. The resulting high collision rate will necessitate a substantial upgrade of the sub-systems of the CMS detector, to cope with the much more demanding environment of 100 or more simultaneous collisions. A completely new inner tracking detector (tracker) will be needed, along with the complete replacement of various electronics systems. The second objective of the proposed project was to join development of track trigger algorithms using simulation, and test their performance in electronics. To initiate this work, the fellow joined weekly meetings at the lab, where software activities for the tracker upgrade were being initiated among a group of newcomers. She analyzed and summarized the state-of-the-art in the trigger simulation developments, and identified an important problem with possible high impact: although the efficiency of the fast identification of the muons were acceptably high, for the low energy electrons, due to their often non-helical tracks, this efficiency was significantly low and needed further investigations. To tackle the problem most accurately, she needed particular training on the usage of the software tools. Therefore she joined a two-day hands-on tutorial at CERN, Geneva in July 2014. In parallel to the R&D work for the tracker upgrade, the fellow joined the ULB team that is active on GEM detector developments where she worked together with a MSc. student to test the performance of a prototype of the GEM detector using cosmic rays. The obtained results confirmed rather high detection efficiency between 90 and 95%.

During the fellowship, Dobur has gone through a successful application on an assistant professor position in Ghent university in Belgium. Needless to say, the prestige Prof. Dobur added in her curriculum with the IEF Marie Curie grant and the fruitful work she had at ULB during the fellowship helped on this very important step up on her academic career.