Disputation: From Strange to Charm: Meson production in electron-positron collisions

  • Date:
  • Location: Å80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala
  • Doctoral student: Pettersson, Joachim
  • About the dissertation
  • Organiser: Kärnfysik
  • Contact person: Pettersson, Joachim
  • Disputation

Disputation

Decays of pseudoscalar mesons into a lepton-antilepton pair are very rare within the Standard Model since they cannot proceed via a single photon process. The C-even neutral mesons can be produced in electron-positron collisions by a two-photon intermediate state. Due to the rarity the of Pl+l- decays, measurements of their branching fractions are sensitive to physics beyond the Standard Model. This thesis report the first search for the process e+e-→ηusing the K±K0Sπ±K+0 and K+K-η final states to determine the branching fraction of the time reverse process ηc→e+e-. This study is an extension of the previous measurements of π0→e+e- and η→μ+μ-, and the upper limits set for η→e+e- and η'→μ+μ-. The analysis presented in this thesis uses data collected by the BESIII detector situated at the e+e- collider BEPCII in Beijing, China. The data set consist of 22 center-of-mass energy points between 2.000 to 3.080 GeV where a data point at the energy 2.981 GeV was recorded specifically for this study. Monte Carlo simulations are used to develop selection criteria and to estimate selection efficiency and corrections due to initial state radiation.

No signal from e+e-→ηc was observed. Therefore, an upper limit is set for the branching fraction of the process ηce+e- using a profile likelihood method. The result for the 90% C.L. limit is BF(ηce+e-)<8.1×10-7 where systematic uncertainties are included. 

In addition to the search for ηc→e+e-, the cross sections of the processes e+e-→K±K0π±e+e-→K+K-π0 and e+e-→K+K-η are measured at all 22 energies. These cross section measurements can be used to search for exotic resonances and to achieve an understanding of exotic states such as Y(2175) and X(2370).