252 ALICE Collaboration / Physics Letters B 776 (2018) 249–264
for charged jets increased from 40% at p
sub
T
,ch jet
=10 GeV/c to 100%
for p
sub
T
,ch jet
> 40 GeV/c.
Region-to-region
fluctuations of the jet mass and p
T
-scale were
characterized by using the hybrid events and calculating δ p
T
and
δM, defined as the difference between the transverse momentum
or mass of the background-subtracted hybrid jet and the probe jet
[48]. On a jet-by-jet basis a linear correlation between δp
T
and δM
was
observed.
The
jet mass distributions of hybrid jets matched to probe jets
within a certain jet p
T
-interval, showed on average a larger jet
mass with respect to the corresponding spectrum of the probes.
This offset was due to background fluctuations and limited purity
and efficiency within a reconstructed jet p
T
-interval, resulting in
jet migration between p
T
-intervals.
Detector
effects on the jet energy and mass were investigated
by matching detector level jets and particle level jets from a Pythia
simulation and comparing their properties (including Pb–Pb back-
ground).
The jets were matched based on distance, in a way that
guarantees a one-to-one match. The constituents of the detector
Fig. 1. (Color online) Mass response using the area-based background subtraction
method in the 10% most central Pb–Pb collisions for background fluctuations only
(black, shaded histogram), compared to the full response including detector effects
(red, hashed histogram), for anti-k
T
jets with resolution parameter R = 0.4. M
sub
refers to the background-subtracted reconstructed jet mass while M
probe
is the jet
mass of the embedded probe. From top left to bottom right, each panel represent a
p
T,ch jet
region, 40–60, 60–80, 80–100, 100–120 GeVc.
level jets are all assigned the pion mass, as is done for the data
analysis, while the particle mass is used for the particle level
jet reconstruction. A comparison between the jet mass response
due to background fluctuations and the full response, which also
contains detector effects, is shown in Fig. 1. While background
fluctuations induce a positive shift of the reconstructed jet mass,
detector effects, which are dominated by the finite tracking effi-
ciency
and the mass assumption of the jet constituents, reduce the
reconstructed jet mass. This was further characterized by extract-
ing
the mean and the most probable value from the distribution
in Fig. 1, giving a measure of the relative jet mass shift. The rel-
ative
mass shift is shown in Fig. 2 for the area-based subtraction
method (left) and the constituent subtraction method (right). In
the kinematic range of interest, the mass shift does not exhibit
a strong dependence on jet momentum. The performance of the
constituent subtraction method is slightly better than for the area-
based
method since the constituent subtraction corrects partially
for the local background fluctuations while the area-based method
only corrects for the average background.
Since
embedding a full PYTHIA event, including the underly-
ing
event, into the sparse p–Pb event would significantly distort
the p–Pb background estimate, the above procedure, devised with
Pb–Pb collisions in mind, was modified for p–Pb collisions. To min-
imize
the distortion, we instead embedded single tracks, whose
4-vectors correspond to jets reconstructed from a PYTHIA simu-
lation
(tune Perugia2011 [49]) at detector level, into p–Pb events.
After running FastJet on the measured events including embedded
PYTHIA tracks, each resulting jet was matched with the particle
level PYTHIA jet associated to the embedded track.
Fig. 3 sho
ws the jet mass resolution for Pb–Pb and p–Pb colli-
sions
as a function of the jet mass at particle level for probe jets
with 60 < p
T,ch jet
< 80 GeV/c. A strong dependence on M
probe
is
observed. The resolution for jets with a small mass is poor while
for larger jet masses it improves to 25%. Jets with small mass are
very collimated and typically have a small number of constituents.
The influence of the tracking inefficiency and the contamination
of tracks from the background on these jets are large. For large
enough p
T,ch jet
(> 40 GeV/c), jets with a small jet mass are rare
and therefore the poor resolution for very collimated jets with
small number of constituents is not a limiting factor in this analy-
sis,
which was restricted to jets with p
T,ch jet
> 60 GeV/c for Pb–Pb
and
p–Pb collisions. For example, only about 16% of the jets have
a mass smaller than 6 GeV/c
2
within the 60–80 GeV/cp
T,ch jet
in-
terval
in PYTHIA.
The
jet mass scale and resolution in p–Pb collisions are dom-
inated
by tracking inefficiency, the mass assumption for the con-
stituents,
and, less strongly, by track momentum resolution. The jet
Fig. 2. Jet mass scale characterized by the relative mean and most probable value of the response. Jet mass scale is shown as a function of probe jet p
T
for background
fluctuations and the full response including detector effects, using anti-k
T
PYTHIA jets with R = 0.4 embedded into central Pb–Pb collisions. Left: area-based subtraction
method. Right: constituent subtraction method.
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