Inter-string Bose-Einstein correlations in hadronic Z decays using the L3 detector at LEP

Bose-Einstein Correlations (BEC) of identical bosons can be used for the femtoscopy of the pro- duction properties of bosons in high energy particle collisions. This quantum mechanical BEC effect is a direct consequence of the symmetrization of the wave function of a boson system and is frequently used on photons in Astophysics to measure the angular size and other properties of distant stars. In particle collisions, the effect can be observed experimentally as an enhancement of the production of identical bosons with small four-momentum difference Q relative to a production that would occur in a world without Bose-Einstein statistics. In this thesis, BEC are studied between identical pions produced in electron-positron collisions at a center-of-mass energy of 91 GeV in the LEP e+ e− Collider of CERN, near Geneva. The final-state particles of these collisions are detected in the detector of the L3 experiment, which is positioned at one of the four intersections of LEP. According to the present picture of boson production in electron-positron collisions, these two ̄ colliding particles annihilate to first produce a pair of a quark, q, and an antiquark, q, having the ̄ center-of-mass energy of the original electron-positron pair. The q and q try to separate from each other but are held back by the strong color force acting between them. In the simplest case, ̄ this string-like field breaks into a number of further q q pairs which finally combine into particles. These particles are observed in the detector in the form of two jets of particles moving in opposite directions. ̄ However, in the quantum chromodynamic field spanned between a q and a q, field quanta, so-called gluons, g, can be radiated. Contrary to photons in the case of quantum electrodynamics, these gluons themselves carry color charge and therefore act themselves as sources of the color field. In the next to simplest case, therefore, two string-like fields are spanned, one between the radiated gluon and the quark and one between that gluon and the antiquark. This leads to the production of three particle jets flying apart in different directions, but with the gluon jet containing two color string ends lying close to each other, both in momentum and configuration space. The motivation for our investigation is the question whether bosons originating from different but overlapping strings or string pieces show BEC, i.e., whether so-called inter-string BEC exist between ovelapping strings or string pieces. Statistics tells us that in the case of an increasing number of overlapping sources, the correlation strength λ will decrease if these sources are independent. On the other hand, the source radius R is expected to increase when the sources are dependent, since the average distance between two bosons is larger in this case. In particular, one would expect 110 1. If there are no inter-string BEC but the two strings overlap: λ2-string < λ1-string R2-string ≈ R1-string 2. If there are inter-string BEC and overlap between the two strings: λ2-string ≈ λ1-string R2-string R1-string 3. If there are no BEC between two non-overlapping sources: λ2 ≈ λ 1 R2 ≈ R 1 4. If there are BEC between two non-overlapping sources: λ2 ≈ λ 1 R2 > R 1 . In this thesis, we have searched for differences in BEC between event configurations involving two color strings such as gluon jets and those involving single strings such as quark jets. Com- parisons have been made of two-jet and three-jet events, of quark and gluon jets, and of pairs of identical pions originating from the same and from different sides of a gluon jet. The strength parameter λ and the size parameter R of the Bose-Einstein correlation function R2 are determined and compared for various configurations. The comparison of two-jet and three- jet events is performed for various values of jet resolution and in different multiplicity intervals. Quark and gluon jets are compared for various energy ranges and in various windows of particle momentum. No significant difference is observed between two-jet and three-jet events, or between quark and gluon jets. Furthermore, R2 is compared for pions originating from the same side and from different sides of the gluon jet. Studies are performed in the gluon jet of various three-jet topologies (so-called Yq events, Mercedes events and Yg events), and in different particle momentum windows, both for light-quark and heavy-quark events. Again, no obvious difference is found between the same-side and the different-side pairs. Referring back to our original motivation quoted above, at the given level of statistical signifi- cance, our results support our options (2) of the presence of BEC in pairs of pions originating from two different sources when these two sources are overlapping, or (3) of the lack of any overlap at all. Further analysis will be needed to distinguish between these two remaining alternatives.