Speaker
Description
High charged-particle multiplicity event has been a central focus in the study of collective behavior across both large and small collision systems. A previous measurement of two-particle angular correlations in $e^+e^-$ collisions at center-of-mass energies up to $\sqrt{s} = 209,\mathrm{GeV}$, using LEP-II data, revealed intriguing discrepancies with Monte Carlo predictions at high multiplicity, suggesting the possible emergence of long-range near-side correlations even in the simplest collision system. Unlike at lower energies, where quark-antiquark production dominates, $W^+W^-$ processes become increasingly important at multiplicities above 40. Could the observed excess in long-range correlations be explained by the more complex color-string configurations arising from $W^+W^-$ production, or is it simply a consequence of the higher final-state multiplicity enabled by the increased collision energy, independent of the underlying production mechanism?
To address these questions, we present a measurement of two-particle angular correlations in $e^+e^-$ collisions at $\sqrt{s} = 183\text{–}209,\mathrm{GeV}$, with a focus on enhancing the contribution from $W^+W^-$ processes. The analysis uses data collected by the ALEPH detector during the LEP-II program. Correlation functions are evaluated across a broad range of pseudorapidities and full azimuth, in bins of charged-particle multiplicity. The correlation functions are further decomposed into a Fourier series, and the resulting harmonic coefficients $v_n$ are compared with predictions from event generators. These results provide new insights into the emergence of long-range correlations in small systems and offer important context for similar phenomena observed in proton-proton, proton-nucleus, and nucleus-nucleus collisions.
