of the analysed observables. In the low-Q region, the Coulomb repulsion between two
identically charged hadrons alters the correlation function C
2
(Q) by decreasing the BEC
effect. This effect is corrected for with the Gamov penetration factor [44, 45], G
2
(Q), by
applying a weight per particle pair 1/G
2
(Q), where G
2
(Q) =
2πζ
e
2πζ
−1
, ζ = ±
αm
Q
, and m and
α denote the particle rest mass and the fine-structure constant, respectively. The sign of ζ
is positive for same-charge and negative for opposite-charge pairs of hadrons.
The Coulomb interactions are not present in the simulated samples used in the analysis.
This effect therefore has to be corrected for in the data.
3 Detector and dataset
The LHCb detector [46] is a single-arm forward spectrometer designed for the study of
particles containing b or c quarks. The detector includes a high-precision tracking system
consisting of a silicon-strip vertex detector (VELO) [47] surrounding the pp interaction
region and covering the pseudorapidity range 2 < η < 5, a large-area silicon-strip de-
tector located upstream of a dipole magnet with a bending power of about 4 Tm, and
three stations of silicon-strip detectors and straw drift tubes [48] placed downstream of the
magnet. The tracking system provides a measurement of momentum, p, with a relative
uncertainty that varies from 0.5% at low momentum to 1% at 200 GeV/c. The minimum
distance of a track to a primary vertex (PV), the impact parameter (IP), is measured with
a resolution of (15 + 29/p
T
) µm, where p
T
is the component of p transverse to the beam,
in GeV/c. Different types of charged hadrons are distinguished using information from two
ring-imaging Cherenkov detectors [49]. Photon, electron and hadron candidates are iden-
tified by a calorimeter system consisting of scintillating-pad and preshower detectors, an
electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system
composed of alternating layers of iron and multiwire proportional chambers [50]. The trig-
ger [51] consists of a hardware stage, based on information from the calorimeter and muon
systems, followed by a software stage, which applies a full event reconstruction.
In the present analysis, a dataset of no-bias and minimum-bias triggered events col-
lected in 2011 at a centre-of-mass energy of
√
s = 7 TeV is used. The no-bias trigger selects
events randomly, while the minimum-bias trigger requires at least one reconstructed VELO
track. The data were collected with an average number of visible interactions per bunch
crossing
1
(pile-up) of 1.4 [52]. In order to eliminate biases related to the trigger require-
ments, a sample of “independent pp interactions” is constructed as described in section 4.
In the simulation, pp collisions are generated using Pythia 8 [53] with a specific LHCb
configuration [54] and without including the BEC effect. Decays of hadronic particles are
described by EvtGen [55], in which final-state radiation is generated using Photos [56].
The interaction of the generated particles with the detector and its response are imple-
mented using the Geant4 toolkit [57, 58], as described in ref. [59]. To study systematic
effects, an additional sample is simulated using Pythia 6.4 [60] with the Perugia0 [61] tune.
1
A visible interaction corresponds to the PV reconstructed with at least five VELO tracks.
– 4 –