Measurement of Collins asymmetries in inclusive production of pion pairs at BABAR.

Abstract

Transversity Distribution, which describes the quark transverse polarization inside a transversely polarized nucleon, is the last leading-twist missing piece of the QCD description of the partonic structure of the nucleon. Transversity can be extracted from Semi-Inclusive Deep Inelastic Scattering (SIDIS) data, where, due to its chiralodd nature, it couples to a new, unknown fragmentation function, called Collins function. Independent information on the Collins function can be obtained in e+e- -> qq annihilation processes, where q = u, d, s. In the cross section formula, the Collins effect introduces a term that depends on the polarization of the quark q (antiquark q), which produces an asymmetry in the azimuthal distribution of the fragmenting products. For unpolarized beams we do not know the spin of the quarks arising from the e+e- annihilation process, and hence we cannot measure the spin effects due to the Collins function analyzing the hadron distributions generated in only one jet, which come from the fragmentation of a single quark. The Collins spin effect can instead be studied in inclusive production of two hadrons detected in opposite jets, produced by the fragmentation of a correlated qq pair: e+e- -> qq -> h1h2X. This correlation results in an observable azimuthal asymmetry distribution of the two hadrons. In this thesis, we present the measurement of the Collins asymmetries in the pions system (h1; h2 = π), using a sample of about 468 fb^(-1) of data collected at a center of mass energy of 10.54 and 10.58 GeV, with the BABAR detector at the e+e- collider PEPII at the SLAC National Accelerator Laboratory of Stanford. The Collins asymmetries are calculated in two different frames called thrust reference frame (RF12), and second hadron momentum frame (RF0), and the final results are shown as a function of the fractional energy and, for the first time in e+e- annihilation, as a function of the transverse momenta of the two pions. These results can be used in combination with Belle, HERMES, and COMPASS data in order to improve the extraction of the Transversity function, and Collins fragmentation function.