ATLASTTH · Observation of the Higgs production mode in the ttH channel at ATLAS

Observation of the Higgs production mode in the ttH channel at ATLAS

The project I worked on was carried out within the ATLAS collaboration which built and is exploiting one of the four major experiments at the Large Hadron Collider (LHC) at CERN. My main scientific contribution focussed on improving the understanding of the recently discovered Higgs boson by measuring one of its properties, the coupling with the Top quark. There is a great interest in this measurement because the Top quark has a very large mass; hence the strength of the coupling is very high, much higher than any other coupling. Furthermore this coupling has a special role in many theories that extend the Standard Model (SM) of particle physics, the best theory we have to describe the fundamental Laws of Nature. In order to discriminate between the value predicted from the SM and the one predicted by the other theories, a precise measurement is required. I worked with a large team of particle physicists, and in particular with the PhD students at Royal Holloway, to analyse the data collected by ATLAS during the first period of data-taking. The result of the analysis was published in 2015; unfortunately the data collected was not enough to perform the measurement, hence an upper limit to the value of the coupling was set. To date this is still the most precise measurement of the coupling between Top quark and Higgs. After a three year period in which LHC and the experiments underwent major improvements, last year ATLAS started it second period of data-taking which is still ongoing. I am currently involved in improving the analysis developed in the past years aiming at a publication later this year when the data collected by ATLAS will allow us to publish an improved results with respect to the one published last year. When I joined ATLAS I also contributed to the experiment itself by developing software required for the operation during the second period of data-taking. Using my experience as software developer I improved an application required for operating ATLAS while collecting data. Specifically the application is responsible for the handling of the online trigger database. In addition to adding all the required new functionalities, I also significantly improved the performance of the application and the code structure in order to increase the maintainability of the application. Since the Royal Holloway ATLAS group is also heavily involved in other Top quark analyses with PhD students and other post-docs, I also become involved in those activities. My major contribution was the publication of the measurement of the Top pair differential cross section using the data collected in the first data-taking period. Having performed two analysed using the Top group software framework and because of my experience in the trigger group, I was invited by the Top conveners to join the Top group management with the role of trigger contact person. In such role I liaised between the two groups proposing new trigger required by Top analyses and reporting to the Top group any news from the trigger group. Because of this role I become an expert in the triggers used in the very first few weeks of data-taking in 2015 and played a crucial role in the publication of the first results by the Top group using the collected data. In summer 2015 I also sponsored a summer studentship to work on a study in the framework of the International Linear Collider (ILC), my previous areas of research. I selected and trained a Master student from Italy which worked under my supervision on the analysis of the Higgs boson decaying to two muons. The work was used by the student for her major undergraduate project receiving high praises and later presented by me at the major ILC conference, the LCWS16 hold in Whistler. The proceeding for the presentation is in preparation. The Fellowship allowed me to resume my scientific career and to join the high energy physics community in an exciting period. The Higgs discovery opened a new era in my field of research and I was able to contribute to increase our understanding of this new particle by studying its associated production with a pair of top quarks. Given the fundamental implication of the discovery, the media have continued to cover anything related to the Higgs boson. Therefore the research I carried out can appeal the interest of the non-specialised public and therefore can create a positive feedback loop, especially with the young public that may be more inclined in following a STEM career. During the fellowship I directly helped in training several PhD students in using advanced statistical tool such as Neural Networks and Boosted Decision Trees which are tools I used in my analyses. Such tools are at the centre of what is commonly referred to as Big Data as they allow maximising the amount of information extracted from any data sample. One of the students was even offered a job at one company developing one of such tools. I tried to contribute to the ongoing effort in the physics community to achieve a gender balance. Specifically I selected a female summer student which proved to be the best summer student at Royal Holloway. I also encouraged her to pursue the scientific career and the ATLAS group would be happy to have her as a PhD student after she completes her Master. In the last few months of the Fellowship I worked with one of the professors in the ATLAS group to expand the research activities of the group by applying to an ITN. The proposed projects are heavily based on the work I carried out and, if approved, will be integrated with the activities of other four European Universities, several companies and research institutes from various disciplines.

Data: CORDIS, © European Union