Thesis

Fragmentation, the process by which quarks and gluons turn into hadrons, cannot be computed from first principles in QCD and has to be extracted from data. Transverse-momentum-dependent fragmentation functions (TMDFFs) are particularly poorly known. The cleanest observable for constraining them is the transverse-momentum distribution of nearly back-to-back pi+pi- pairs in e+e- annihilation, but no such measurement exists. The recent public release of the DELPHI data and software through CERN Open Data now makes it possible. The student will first set up access to the archived LEP data (DELPHI or ALEPH) and then extract pion-pair distributions. Pythia8 simulations will be used to identify the observable most sensitive to TMDFFs, and the measurement will come with a full uncertainty budget. The result will enter the MAP Collaboration’s global TMD fits as their first e+e- input, giving the most precise TMDFFs to date. These are key inputs for the EIC and FCC-ee physics programmes. The project is co-supervised by Valerio Bertone (theory) and Francesco Bossù (experiment) at CEA/Irfu. It involves David d’Enterria (CERN) and Ronan McNulty (UCD), with CERN and the MIT group that leads the current LEP data reanalysis directly involved. A highly motivated candidate, who did his M1 with the team and is now doing his M2 in Dublin, has already been identified.