DOI: 10.17182/hepdata.70016.v1/t18
title: "Table 18" of "Two-particle Bose-Einstein correlations in $pp$ collisions at $\mathbf {\sqrt{s} =}$ 0.9 and 7 TeV measured with the ATLAS detector"

description: The $R_{2}(Q)$ correlation function measured at $7\ TeV$ using unlike-charge particle reference sample for different $k_{T}$ intervals with multiplicity $n_{ch} \ge 2$. The error bars represents only the statistical uncertainties.
description: CERN-LHC. Studies of Bose-Einstein Correlations (BEC) for pairs of like-sign charged particles measured in the kinematic range $p_{T} &gt; 100\ MeV$ and $|\eta| &lt; 2.5$ 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\ \mu$$b^{-1}$ (2009), $190\ \mu$$b^{-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, $Q^{2}$, of the two particles. The studies were performed using the double-ratio correlation function $R_{2}(Q)$. In the double-ratio method, the single-ratio correlation function $C_{2}(Q)$ 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 $\lambda$ (incoherence or chaoticity parameter). To quantitatively characterize the BEC effect, the Gaussian $\Omega^{(G)}$ and the Exponential $\Omega^{(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 $Q \sim 0.7\ GeV$ there is a visible bump which is due to an overestimate of $\rho \to \pi^{+}\pi^{-}$ decays in the Monte Carlo simulation. Therefore the region $0.5 \le Q \le 0.9\ GeV$ is excluded from the fits.

The BEC parameters characterizing the correlation strength $\lambda$ 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 $n_{ch} \ge 2$, $n_{ch} \ge 2$ and $n_{ch} \ge 150$, respectively. The average transverse momentum $k_{T}$ and the multiplicity $n_{ch}$ 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.