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Abstract

We present the first two-particle angular correlations measurement of charged particles produced in e+e- collisions up to √s=209 GeV, utilizing LEP-II hadronic e+e- data collected by the ALEPH detector at LEP. The angular correlation functions have been measured across a wide range of pseudorapidities and the full azimuth in bins of charged particle multiplicity.

The previous measurement with LEP-I data at √s=91 GeV exhibits no significant long-range correlations in either lab coordinate or thrust coordinate analyses, with associated yield distributions in agreement with PYTHIA v6.1 generator predictions. However, this analysis with LEP-II data at higher collision energies shows intriguing findings. The higher collision energy not only allows events with higher charged multiplicity, but also introduces additional production channels other than the dominant e+e- -> gamma/Z -> qq process, with W^+W^- processes become increasingly important at multiplicities above 40. The measurement shows a tantalizing disagreement with MC at highest multiplicity bin (Ntrk ≥ 50), hinting at the potential of collective phenomena in small systems.

In this presentation, the two-particle correlation functions were decomposed using a Fourier series. In events with high multiplicity featuring more than 50 particles, the data shows an intriguing trend compared to the simulation, and the magnitudes of v2 and v3 are larger than those in the Monte Carlo reference. The observed trend suggests that if subsequent experiments observe a significant collective phenomena, the final state interaction effect may be attributed to such signals.

Additionally, ongoing studies with LEP2 archived data explore QCD phenomenology through observables such as jet substructure and energy-energy correlation (EEC) measurements. These analyses provide further insight into QCD dynamics in a clean, vacuum-like environment and could serve as precision tests of perturbative QCD. Together, these efforts enhance our understanding of small-system references in contrast to the more complex hadronic environments.