Speakers
Description
Long-range correlations in azimuthal angle, commonly referred to as the ridge signals, have been observed across different hadronic collision systems and energies, but the underlying origin remains unresolved. High charged-particle multiplicity events have been a central focus in enhancing sensitivity to the possible emergence of collective-like behavior, including in e⁺e⁻ collisions. We present a measurement of two-particle angular correlations using data collected by the ALEPH detector during the LEP2 program in e⁺e⁻ collisions at center-of-mass energies up to √s = 209 GeV, with a focus on enhancing the contribution from two–color-string topologies arising from hadronic W⁺W⁻ decays using a boosted decision tree (BDT). Correlation functions are evaluated across a broad range of pseudorapidity and full azimuth, in bins of charged-particle multiplicity. The correlation functions are further decomposed into a Fourier series, and the resulting harmonic coefficients vn are compared with Monte Carlo (MC) baseline. For multiplicity starting from 30, the observed $v_2$ goes from negative to positive, which marks a sizable deviation from the MC expectations. This result facilitates understanding collective-like behavior emergence in small systems, and may provide new constraints on parton- and hadron-level interactions in particle production mechanisms.
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