Study of the dynamics of soft interactions with two-particle angular correlations at ATLAS

Measurements of inclusive two-particle angular correlations in proton-proton collisions at centre-of-mass energies √ s = 900 GeV and 7 TeV are presented. The events were collected with the ATLAS detector at the LHC, using a single- arm minimum-bias trigger, during 2009 and 2010. Correlations are measured for charged particles in the kinematic range defined by a transverse momentum p T > 100 MeV and pseudorapidity | η | < 2 . 5. In total, integrated luminosities of 7 μ b − 1 and 190 μ b − 1 are analysed for 900 GeV and 7 TeV data, respectively. At 900 GeV only events with a charged particle multiplicity n ch ≥ 2 are analysed whereas at 7 TeV, a second phase-space region of n ch ≥ 20, with a suppressed contribution from diffractive events, is also explored. Data are corrected using a novel approach in which the detector effects are applied repeatedly to the observable distribution and then extrapolated to a detector effect of zero. A complex structure in pseudorapidity and azimuth is observed for the cor- relation function at both collision energies. Projections of the two-dimensional correlation distributions are compared to the Monte Carlo generators pythia 8 and herwig ++, as well as the AMBT2B, DW and Perugia 2011 tunes of pythia 6. The strength of the correlations seen in the data is not reproduced by any of the models.