Long-range near-side correlation in e+e− collisions at 183-209 GeV with ALEPH archived data

The first measurement of two-particle angular correlations for charged particles with LEP-II data is presented. The study is performed using archived hadronic <math altimg="si1.svg"><msup><mrow><mi>e</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math> data collected by ALEPH at center-of-mass energies up to 209 GeV, above the <math altimg="si2.svg"><msup><mrow><mi>W</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><msup><mrow><mi>W</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math> production threshold, which provide access to unprecedented charged-particle multiplicities and more complex color-string configurations if compared to previous measurements at LEP-I energies. An intriguing long-range near-side excess is observed in the correlation function measured with respect to the thrust axis in the highest multiplicity interval (<math altimg="si3.svg"><msub><mrow><mi mathvariant="normal">N</mi></mrow><mrow><mi mathvariant="normal">trk</mi></mrow></msub><mo>≥</mo><mn mathvariant="normal">50</mn></math>). Such a structure is not predicted by the Monte-Carlo simulation. The harmonic anisotropy coefficients <math altimg="si4.svg"><msub><mrow><mi>v</mi></mrow><mrow><mi>n</mi></mrow></msub></math>, which result from the Fourier expansion of the two-particle correlation functions, were also measured for the first time in <math altimg="si1.svg"><msup><mrow><mi>e</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math> data, and compared to pythia 6 predictions and to the results obtained in proton-proton collisions. The results presented in the Letter provide novel experimental constraints on the formation of collective phenomena in point-like <math altimg="si1.svg"><msup><mrow><mi>e</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math> collisions.