Investigation of the p–Σ
0
interaction via femtoscopy ALICE Collaboration
hypothesis for a proton and the experimental measurement, normalized by the detector resolution σ . For
candidates with p < 0.75 GeV/c, PID is performed with the TPC only, requiring |n
σ
|< 3. For larger mo-
menta, the PID information of TPC and TOF are combined. Secondary particles stemming from weak
decays or the interaction of primary particles with the detector material contaminate the signal. The cor-
responding fraction of primary and secondary protons are extracted using Monte Carlo (MC) template
fits to the measured distribution of the Distance of Closest Approach (DCA) of the track to the primary
vertex [27]. The MC templates are generated using PYTHIA 8.2 [37] and filtered through the ALICE
detector [38] and reconstruction algorithm [30]. The resulting purity of protons is found to be 99%, with
a primary fraction of 82%.
The Σ
0
is reconstructed via the decay channel Σ
0
→ Λγ with a branching ratio of almost 100% [6]. The
decay is characterized by a short life time rendering the decay products indistinguishable from primary
particles produced in the initial collision. Due to the small mass difference between the Λ and the Σ
0
of about 77 MeV/c
2
, the γ has typically energies of only few hundreds of MeV. Therefore, it is recon-
structed relying on conversions to e
+
e
−
pairs in the detector material of the central barrel exploiting the
unique capability of the ALICE detector to identify electrons down to transverse momenta of 0.05 GeV/c.
For transverse radii R < 180 cm and |η| < 0.9 the material budget corresponds to (11.4 ±0.5) % of a
radiation length X
0
, and accordingly to a conversion probability of (8.6 ±0.4) % [39]. Details of the
photon conversion analysis and the corresponding selection criteria are described in [39, 40]. The re-
construction relies on the identification of secondary vertices by forming so-called V
0
decay candidates
from two oppositely-charged tracks using a procedure described in detail in [41]. The products of the po-
tential γ conversion are reconstructed with the TPC and the ITS in the kinematic range p
T
> 0.05 GeV/c
and |η| < 0.9. The candidates for the e
+
e
−
pair are identified by a broad PID selection in the TPC
−6 < n
σ
< 7. The resulting γ candidate is obtained as the combination of the daughter tracks. Only
candidates with p
T
> 0.02 GeV/c and within |η| < 0.9 are accepted. Combinatorial background from
primary e
+
e
−
pairs, or Dalitz decays of the π
0
and η mesons is removed by requiring that the radial dis-
tance of the conversion point, with respect to the detector centre, ranges from 5 cm to 180 cm. Residual
contaminations from K
0
S
and Λ are removed by a selection in the Armenteros-Podolandski space [40, 42].
Random combinations of electrons and positrons are further suppressed by a two-dimensional selection
on the angle between the plane defined by the e
+
e
−
pair, and the magnetic field [43] in combination with
the reduced χ
2
of a refit of the reconstructed V
0
assuming that the particle originates from the primary
vertex and has M
V
0
= 0 [40]. The Cosine of the Pointing Angle (CPA) between the γ momentum and
the vector pointing from the PV to the decay vertex is required to be CPA > 0.999. In addition to the
tight CPA selection, the contribution of particles stemming from out-of-bunch pile-up is suppressed by
restricting the DCA of the photon to be along the beam direction (DCA
z
< 0.5 cm). After application of
the selection criteria, about 946 ×10
6
γ candidates with a purity of about 95.4% are available for further
processing.
The Λ particle candidates are reconstructed via the subsequent decay Λ → pπ
−
with a branching ratio
of 63.9% [6], following the procedures described in [27, 28]. For the Λ the charge conjugate decay is
exploited, and the same selection criteria are applied. The decay products are reconstructed with the
TPC and the ITS within |η| < 0.9. The daughter candidates are identified by a broad PID selection in
the TPC |n
σ
| < 5. The resulting Λ candidate is obtained as the combination of the daughter tracks. The
contribution of fake candidates is reduced by requesting a minimum p
T
> 0.3 GeV/c. The coarse PID
selection of the daughter tracks introduces a residual K
0
S
contamination in the sample of the Λ candidates.
This contamination is removed by a 1.5σ rejection on the invariant mass assuming a decay into π
+
π
−
,
where σ corresponds to the width of a Gaussian fitted to the K
0
S
signal. Topological selections further
enhance the purity of the Λ sample. The radial distance of the decay vertex with respect to the detector
centre ranges from 0.2 cm to 100 cm and CPA > 0.999. In addition to the tight CPA selection, particles
stemming from out-of-bunch pile-up are rejected using the timing information of the SPD and SSD, and
the TOF detector. One of the two daughter tracks is required to have a hit in one of these detectors. After
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