JHEP01(2018)045
Source
Uncertainty [%]
5.02 TeV 8.16 TeV
Data and
simulation
Detector misalignment 0.2 – 1.0 0.2 – 1.0
Pixel hit reconstruction inefficiency 1.0 1.0
Pixel cluster splitting 0.3 – 0.8 0.3 – 0.6
MC
corrections
Background modeling 1.3 – 3.2 1.5 – 2.5
Signal and sideband region selection 0.5 – 1.5 0.6 – 1.5
Choice of parametrization variable 1.6 – 2.5 1.5 – 3.5
NSD selection 1.2 1.2
Total uncertainty 3.0 – 4.3 3.7 – 4.6
Table 1. Summary of the systematic uncertainties from various sources, for pPb collisions at 5.02
and 8.16 TeV. The range of values indicates the minimum and maximum uncertainties across the
η
lab
range.
layer 2 for tracklet type 2+3). The maximum deviation in each η
lab
interval, 1.5–2.5%, is
quoted as an uncertainty. An uncertainty is assigned for the selection of NSD events. The
fraction of the single-diffractive events removed by the event selection, as determined from
the Epos generator, is 16% when the tracklet multiplicity in the event is less than 10, and
falls quickly to 0% with increasing tracklet multiplicity. This fraction is varied from 0% to
twice the nominal value, and the maximum deviation from the final results, 1.2%, is quoted
as the uncertainty. A summary of the systematic uncertainties for the measurements at
5.02 and 8.16 TeV is shown in table 1.
5 Results
Pseudorapidity density distributions of charged hadrons in the region |η
lab
| < 2.4 for NSD
pPb collisions are shown in figure 2. The distributions shown are the average of the
measured distributions from the three types of tracklets (1+2, 1+3, and 2+3), which are
consistent with each other within 3%. A clear difference in the particle densities between
the lead ion (η
lab
< 0) and the proton (η
lab
> 0) beam directions is observed. The
measured dN
ch
/dη
lab
distribution at 5.02 TeV agrees with the measurement by the ALICE
Collaboration [30]. The multiplicities at 8.16 TeV are significantly higher than those at
5.02 TeV.
Figure 3 shows a comparison between the measurement at 8.16 TeV and theoretical
calculations from the Hijing (versions 1.3 and 2.1), Epos LHC (v3400), and Dpmjet-III
MC generators, and the KLN model. The Hijing and Epos generators were tuned to data
from RHIC and the LHC, respectively. Calculations from Hijing 2.1, a two-component
model that combines perturbative QCD descriptions of hard parton scatterings with a
string excitation model for soft interactions, agree with the experimental data in the re-
gion −0.5 < η
lab
< 1.5 when the nuclear modification of the initial parton distributions
(shadowing) is included in the calculation. The Hijing 1.3 calculation overpredicts the par-
ticle density because it has an older implementation of the gluon shadowing effects. The
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