Abstract
Bose-Einstein correlations in pairs of identical charged pions produced in a sample of 4.3 million Z $^0$ hadronic decays are studied as a function of the three components of the momentum difference, transverse (“out” and “side”) and longitudinal with respect to the thrust direction of the event. A significant difference between the transverse, r $_{t_{side}}$ , and longitudinal, r $_l$ , dimensions is observed, indicating that the emitting source of identical pions, as observed in the Longitudinally CoMoving System, has an elongated shape. This is observed with a variety of selection techniques. Specifically, the values of the parameters obtained by fitting the extended Goldhaber parametrisation to the correlation function ${\mathrm C'} ={\mathrm C^{DATA}}/{\mathrm C^{MC}}$ for two-jet events, selected with the Durham algorithm and resolution parameter y $_{cut}$ = 0.04, are r $_{t_{side}}$ = (0.809 $\pm$ 0.009 (stat) $^{+0.019}_{-0.032}$ (syst)) fm, r $_l$ = (0.989 $\pm$ 0.011 (stat) $^{+0.030}_{-0.015}$ ({\it syst})) fm and r $_l$ /r $_{t_{side}}$ = 1.222 $\pm$ 0.027 (stat) $^{+0.075}_{-0.012}$ (syst). The results are discussed in the context of a recent model of Bose-Einstein correlations based on string fragmentation. The results of a unidimensional analysis are also presented.
Highlights
Bose-Einstein correlations (BECs) [1] in pairs of identical bosons, mainly π±π±, have been widely studied at various energies for hadronic final states produced by different initial states: e+e− [2, 3], ep [4], pp [5], πp, K±p [6] and heavy ion collisions [7]
The correlation function obtained with this reference sample has to be normalized and suffers, at large four-momentum differences, from long-range correlations due to energy, momentum and charge conservation
In order to compare the experimental results with the predictions of [13], which are relevant to two-jet events, and to study the dependence of the results on the “jettyness” of the event the same analysis is done on samples of events defined as “two-jet events” by the Durham jet-finding algorithm [18], with various values of the resolution parameter ycut
Summary
Bose-Einstein correlations (BECs) [1] in pairs of identical bosons, mainly π±π±, have been widely studied at various energies for hadronic final states produced by different initial states: e+e− [2, 3], ep [4], pp [5], πp, K±p [6] and heavy ion collisions [7]. BECs are manifested as enhancements in the production of identical bosons which are close to one another in phase space They can be analysed in terms of the correlation function. We use a large sample of Monte Carlo events without the simulation of Bose-Einstein effects in order to obtain a correlation function which is self normalized and which has a reduced contamination from correlated unlike-charge pairs. This correlation function, called C′(p1, p2), is used in the paper to obtain the reference results. We present the results of a unidimensional analysis of BECs
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