# Bose-Einstein correlations of same-sign charged pions in the forward region in $pp$ collisions at $\sqrt{s}$ = 7 TeV

[to restricted-access page]

## Abstract

Bose-Einstein correlations of same-sign charged pions, produced in proton-proton collisions at a 7 TeV centre-of-mass energy, are studied using a data sample collected by the LHCb experiment. The signature for Bose-Einstein correlations is observed in the form of an enhancement of pairs of like-sign charged pions with small four-momentum difference squared. The charged-particle multiplicity dependence of the Bose-Einstein 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 charged-particle multiplicity, while the chaoticity parameter is seen to decrease.

## Figures and captions

 Multiplicity of reconstructed VELO tracks assigned to a PV for the 2011 no-bias 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 [147 KiB] Thumbnail [144 KiB] *.C file Results of the fit to the double ratio for like-sign pion pairs with event-mixed 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 [187 KiB] Thumbnail [150 KiB] *.C file Fig2b.pdf [35 KiB] HiDef png [171 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 [96 KiB] Thumbnail [97 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 [92 KiB] Thumbnail [91 KiB] *.C file Animated gif made out of all figures. PAPER-2017-025.gif Thumbnail

## Tables and captions

 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 [73 KiB] Thumbnail [26 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 [39 KiB] Thumbnail [18 KiB] tex code

Created on 11 July 2020.