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. 
Fig1.pdf [14 KiB] HiDef png [167 KiB] Thumbnail [151 KiB] *.C file 

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. 
Fig2a.pdf [35 KiB] HiDef png [186 KiB] Thumbnail [150 KiB] *.C file 

Fig2b.pdf [35 KiB] HiDef png [170 KiB] Thumbnail [146 KiB] *.C file 

Fig2c.pdf [35 KiB] HiDef png [164 KiB] Thumbnail [141 KiB] *.C file 

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. 
Fig3.pdf [13 KiB] HiDef png [95 KiB] Thumbnail [98 KiB] *.C file 

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. 
Fig4.pdf [13 KiB] HiDef png [91 KiB] Thumbnail [92 KiB] *.C file 

Animated gif made out of all figures. 
PAPER2017025.gif Thumbnail 
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. 
Table_1.pdf [28 KiB] HiDef png [88 KiB] Thumbnail [34 KiB] tex code 

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. 
Table_2.pdf [53 KiB] HiDef png [45 KiB] Thumbnail [23 KiB] tex code 
Created on 14 September 2019.