BoseEinstein correlations of samesign charged pions, produced in protonproton collisions at a 7 TeV centreofmass energy, are studied using a data sample collected by the LHCb experiment. The signature for BoseEinstein correlations is observed in the form of an enhancement of pairs of likesign charged pions with small fourmomentum difference squared. The chargedparticle multiplicity dependence of the BoseEinstein correlation parameters describing the correlation strength and the size of the emitting source is investigated, determining both the correlation radius and the chaoticity parameter. The measured correlation radius is found to increase as a function of increasing chargedparticle multiplicity, while the chaoticity parameter is seen to decrease.
Multiplicity of reconstructed VELO tracks assigned to a PV for the 2011 nobias sample. Different colours indicate three activity classes defined as fractions of the full distribution. The minimum value of the track multiplicity to accept reconstructed PV is five. 
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Results of the fit to the double ratio for likesign pion pairs with eventmixed reference samples and the Coulomb effect subtracted for the three activity classes: (a) low, (b) medium and (c) high activity. The blue solid line denotes the fit result using the parameterisation of Eq. (3). Only statistical uncertainties are shown. 
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Correlation radius $R$ as a function of activity. Error bars indicate the sum in quadrature of the statistical and systematic uncertainties. The points are placed at the centres of the activity bins. 
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Chaoticity parameter $\lambda$ as a function of activity. Error bars indicate the sum in quadrature of the statistical and systematic uncertainties. The points are placed at the centres of the activity bins. 
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Animated gif made out of all figures. 
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Fractional systematic uncertainties on the $R$ and $\lambda$ parameters for the three activity classes, as described in the text. The total uncertainty is the sum in quadrature of the individual contributions. 
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Results of fits to the double ratio $r_{\rm d}(Q)$ for the three different activity classes and corresponding $N_{ch}$ bins, using the parameterisation of Eq. (3). Statistical and systematic uncertainties are given separately. 
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Created on 16 January 2021.