Studies of dielectrons from open charm and beauty decays

A few millionths of a second after the Big Bang, matter existed in a very different state than today, called Quark-Gluon Plasma (QGP), with quarks and gluons no more confined inside particles. Using heavy-ion collisions, scientists can recreate and study in laboratory this particular state of matter. Studying the production of electron-positron pairs in heavy-ion collisions, with the ALICE experiment at CERN, we can extrapolate several properties of the QGP, like its temperature. One of the main background contributions to these measurements, not yet well understood, is given by the decays of particles containing heavy quarks (charm and beauty). In this project we will try to first quantify better, and then remove, this background. In particular we will use the fact that particles containing heavy quarks show a specific decay pattern, delayed with respect to other particles that produce electron-positron pairs. Our analysis strategy will be complementary to other more traditional approaches. We will use machine learning techniques, the most updated techniques for data analysis, based on developing, testing, and applying predictive algorithms to separate the background from the signal.

The aim of the project was to study the production of electron-positron pairs (also called dielectrons) with the ALICE experiment at the LHC, in order to better understand the properties of the \textit{quark-gluon plasma} (QGP), the state in which matter existed a few microseconds after the Big Bang, with quarks and gluons no more confined inside particles. Dielectrons are connected with important open questions in particle physics, such as the measurement of the temperature of the QGP. The main background contribution to this measurement are dielectrons from decays of particles containing heavy quarks (charm and beauty). It is important to understand this contribution and disentangle it from other dielectron sources, like e.g. dielectrons from thermal radiation. In order to understand this thermal contribution one needs to understand all other contributions first. For this reason I studied dielectron production in small systems, i.e. pp and p--Pb collisions, where we expect that the QGP is not formed. In this project dielectrons were measured at the center-of-mass energy $\sqrt{s_{NN}}$ = 5.02 TeV in both collision systems for the first time. Charm and beauty production cross sections were extracted from the dielectron spectra, using Monte Carlo simulations to get the expected distributions of electrons from charm and beauty hadron decays. The obtained results are useful starting points for future investigations and in particular for the analysis of dielectron in Pb--Pb collisions, which is of course more challenging due to the high background component. The analysis strategy was based on the fact that particles containing charm or beauty quarks show a specific decay pattern, delayed with respect to other particles that produce electron-positron pairs. I applied machine learning techniques, which are the most updated techniques for data analysis, based on developing, testing, and applying predictive algorithms to separate the background from the signal. I also performed the measurement of the charmed baryon $\Lambda_{c}^{+}$ production cross section in pp and in p--Pb collisions at a center-of-mass energy of $\sqrt{s_{NN}}$ = 5.02 TeV. This measurement will be a crucial contribution to the measurement of the total charm production cross section at LHC energies. It was observed that the measured $\Lambda_{c}^{+}$ production cross section is significantly larger than previous measurements obtained in different collisions systems and at different energies, indicating that fragmentation fractions of charm quarks into baryons are different.