Two-particle Bose–Einstein correlations in pp collisions at √s = 0.9 and 7 TeV measured with the ATLAS detector


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CERN-LHC. Studies of Bose-Einstein Correlations (BEC) for pairs of like-sign charged particles measured in the kinematic range pT>100 MeV and |η|<2.5 in proton-proton collisions at centre-of-mass energies of 0.9 and 7 TeV with the ATLAS detector at the CERN Large Hadron Collider are presented. In addition to minimum-bias data, high-multiplicity data recorded at 7 TeV using a dedicated trigger are investigated. The integrated luminosities are approximately 7 μb−1 (2009), 190 μb−1 (2010) and 12.4 nb−1 (2010) for 0.9 TeV, 7 TeV minimum-bias and 7 TeV high-multiplicity (HM) data samples, respectively.

Bose-Einstein correlations are measured in terms of a two-particle correlation function. The density function is parameterized in terms of the Lorentz-invariant four-momentum difference squared, Q2, of the two particles. The studies were performed using the double-ratio correlation function R2(Q). In the double-ratio method, the single-ratio correlation function C2(Q) obtained from the data is divided by a similar single-ratio calculated using Monte Carlo events, which do not have BEC effects. The reference sample for each of the two single-ratios is constructed from unlike-sign charged-particle pairs.

A clear signal of Bose-Einstein correlations is observed in the region of small four-momentum difference. The BEC effect is usually described by a function with two parameters: the effective radius R and the strength parameter λ (incoherence or chaoticity parameter). To quantitatively characterize the BEC effect, the Gaussian Ω(G) and the Exponential Ω(E)parametrizations are fit to the measured correlation functions. As the Gaussian parametrization provides a poor description of the BEC-enhanced region and hence the Exponential parametrization is used for the final results. The fits are performed in the Q range 0.02 GeV to 2 GeV and with a bin width of 0.02 GeV. Around Q∼0.7 GeV there is a visible bump which is due to an overestimate of ρ→π+π− decays in the Monte Carlo simulation. Therefore the region 0.5≤Q≤0.9 GeV is excluded from the fits.

The BEC parameters characterizing the correlation strength λ and the correlation source size R are measured for 0.9 TeV, 7 TeV minimum-bias charged-particle events and 7 TeV high-multiplicity charged-particle events with multiplicities nch≥2, nch≥2 and nch≥150, respectively. The average transverse momentum kT and the multiplicity nch dependences of the BEC parameters are investigated for charged-particle multiplicities of up to 240. A saturation effect in the multiplicity dependence of the correlation source size is observed using the high-multiplicity 7 TeV data samples.

See Eur. Phys. J. C75 (2015) 466 for more details.
Date made available4 Jan 2015

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