To change the place to live, habits, home is always as exciting as traumatic. This is why today I want to tell you about my first days in Louvain-la-neuve, Belgium, the city where I will do most of my work during the years of my PhD career.
Before talking about my work (I guess I will have many other opportunities to do that in the next months!) let me just tell you about my first impression on the city and the university.
During the 1960s, some linguistic quarrels between the French-speaking part of Belgium and the Dutch-speaking one took place. Thus, the Catholic University of Leuven was split into two different Institutions, the Katholieke Universiteit Leuven, which remained in Leuven (Louvain in French), and the Université Catholique Louvain, founded in Louvain-la-neuve, at 30 km from Brussels.
Before the construction of the University in the 1970s, the city of Louvain-la-neuve didn’t exist at all. It totally developed around the campus in the following years, and that’s why the inhabitants of this city are mostly students. It could appear kind of strange at first sight: there is no history here, no ancient cathedrals or spectacular buildings, it’s just a village on a human scale with lots of shops and supermarkets. But for me it has the wonderful atmosphere of a new adventure to come, of an unmissable opportunity to grow more and more.
My first impression of the Physics Department has also been great. I haven’t worked on the implementation of codes and stuff like that yet. Everyone knows that the first week is mostly dedicated to all the administrative things, which are very annoying, especially if you come from abroad. However, I’m already familiar with the concept behind my PhD thesis, since my supervisor and the guys from the group introduced me to the general idea. And that’s what I’ll tell you in a few words.
One of the main tasks of LHC Physics is the discrimination of different processes that provide similar final state signatures at the detector level. The Matrix Element Method (MEM) is an efficient technique to do that since it allows to discriminate between two theoretical assumptions using the maximum amount of information.
To make this concept clearer, let’s think about a method to retrieve the mass of a resonance. One way could be to do it with a model-independent method, e.g. one can choose specific variables that are mostly sensitive to the mass of the resonance. Another way is to use a model-dependent method that exploits all the theoretical information provided by different scenarios and discriminates them according to how good (or bad) their predictive power on the measured sample is. The latter is the basic concept behind the Matrix Element Method.
In other words, given a certain theoretical assumption, a, for each experimental event (characterized by a set of momenta p) one can associate a weight P, that is the probability to create and observe the event within the theoretical frame a.
The weight P is proportional to the integral of the squared matrix element for the given process, of the transfer function (that takes into account the detector effects) and of the parton-level phase-space measure (that includes the parton distribution functions).
The Matrix Element Method combines these weights into a likelihood function L. The best estimation of a is the one that maximizes L. And so one can extract the information they need…
Practically, every integral can be numerically estimated by a dedicated Monte Carlo tool called MadWeight. It is a validated solution to estimate these integrals, but it can still be improved. Indeed, MadWeight is written in FORTRAN (a programming language not widely used anymore) and its input and output are text files, difficult to handle. The goal of my PhD program is to rewrite this code in a modular way by using C++ classes in order to get a code which is easier to handle and more general and intuitive. For now, my research will be focused on one specific channel, that is the production of double Higgs decaying into two leptons, two b-jets and MET.
This will be my task for the next months. Do not hesitate to ask if you have questions and… wish me good work!