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用ALICE检测器在质心能量s NN = 2.76的Pb-Pb碰撞中,用ALICE检测器测量了瞬态迷人介子D 0,D +和D ∗ +及其反粒子的核修饰因子R AA。 $$ \ sqrt {s _ {\ mathrm {N} \; \\ mathrm {N}}} = 2.76 $$ TeV在两个横向动量间隔中,5 <p T <8 GeV / c和8 <p T <16 GeV / c和六个碰撞中心性类。 在最中央的10%碰撞中,AA的最大抑制系数是5-6。 抑制作用及其对中心点的依赖性在不确定性方面与带电介子是兼容的。 通过CMS协作,与B介子衰变的非提示J /ψ的R AA进行比较,表明在最中心的碰撞中D介子的抑制更大。
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JHEP11(2015)205
Published for SISSA by Springer
Received: August 7, 2015
Accepted: October 28, 2015
Published: November 30, 2015
Centrality dependence of high-p
T
D meson
suppression in Pb-Pb collisions at
√
s
NN
= 2.76 TeV
The ALICE collaboration
E-mail: ALICE-publications@cern.ch
Abstract: The nuclear modification factor, R
AA
, of the prompt charmed mesons D
0
, D
+
and D
∗+
, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions
at a centre-of-mass energy
√
s
NN
= 2.76 TeV in two transverse momentum intervals, 5 <
p
T
< 8 GeV/c and 8 < p
T
< 16 GeV/c, and in six collision centrality classes. The R
AA
shows a maximum suppression of a factor of 5–6 in the 10% most central collisions. The
suppression and its centrality dependence are compatible within uncertainties with those
of charged pions. A comparison with the R
AA
of non-prompt J/ψ from B meson decays,
measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most
central collisions.
Keywords: Charm physics, Heavy Ions, Heavy-ion collision
ArXiv ePrint: 1506.06604
Open Access, Copyright CERN,
for the benefit of the ALICE Collaboration.
Article funded by SCOAP
3
.
doi:10.1007/JHEP11(2015)205
JHEP11(2015)205
Contents
1 Introduction 1
2 Experimental apparatus and data sample 2
3 Data analysis 3
4 Results and discussion 7
5 Summary 11
The ALICE collaboration 17
1 Introduction
When heavy nuclei collide at high energy, a state of strongly-interacting matter with high
energy density is expected to form. According to Quantum Chromodynamics (QCD) cal-
culations on the lattice, this state of matter, the so-called Quark-Gluon Plasma (QGP) is
characterised by the deconfinement of the colour charge (see e.g. [1–4]). High-momentum
partons, produced at the early stage of the nuclear collision, lose energy as they interact
with the QGP constituents. This energy loss is expected to proceed via both inelastic
(gluon radiation) [5, 6] and elastic (collisional) processes [7–9].
The nuclear modification factor R
AA
is used to characterise parton energy loss by com-
paring particle production yields in nucleus-nucleus collisions to a scaled proton-proton (pp)
reference, that corresponds to a superposition of independent nucleon-nucleon collisions.
R
AA
is defined as
R
AA
=
1
hT
AA
i
·
dN
AA
/dp
T
dσ
pp
/dp
T
, (1.1)
where dσ
pp
/dp
T
and dN
AA
/dp
T
are the transverse momentum (p
T
) differential cross section
and yield in proton-proton and nucleus-nucleus (AA) collisions, respectively. hT
AA
i is
the average nuclear overlap function, estimated within the Glauber model of the nucleus-
nucleus collision geometry, and proportional to the average number of nucleon-nucleon
(binary) collisions [10, 11]. Energy loss shifts the momentum of quarks and gluons, and
thus hadrons, towards lower values, leading to a suppression of hadron yields with respect
to binary scaling at p
T
larger than few GeV/c (R
AA
< 1).
Energy loss is expected to be smaller for quarks than for gluons because the colour
charge factor of quarks is smaller than that of gluons [5, 6]. In the energy regime of the
Large Hadron Collider (LHC), light-flavour hadrons with p
T
ranging from 5 to 20 GeV/c
originate predominantly from gluon fragmentation (see e.g. [12]). At variance, charmed
mesons provide an experimental tag for a quark parent. Because of their large mass m
c,b
– 1 –
JHEP11(2015)205
(m
c
≈ 1.3 GeV/c
2
, m
b
≈ 4.5 GeV/c
2
[13]), heavy quarks are produced at the initial stage
of heavy-ion collisions in hard scattering processes that are characterised by a timescale
∆t < 1/(2 m
c,b
) ∼ 0.1 (0.01) fm/c for c (b) quarks. This time is shorter than the formation
time of the QGP medium (a recent estimate for the LHC energy is about 0.3 fm/c [14]).
As discussed in ref. [15], this should be the case also for charm and beauty quarks produced
in gluon splitting processes, if their transverse momentum is lower than about 50 GeV/c.
Therefore, the comparison of the heavy-flavour hadron R
AA
with that of pions allows the
colour-charge dependence of parton energy loss to be tested. The softer fragmentation of
gluons than that of charm quarks, and the observed increase of the charged hadron R
AA
towards high p
T
[16], tend to counterbalance the effect of the larger energy loss of gluons on
the R
AA
. The model predictions range from a rather moderate effect R
π
AA
< R
D
AA
[17–20]
to an overall compensation R
π
AA
≈ R
D
AA
(as recently shown in [12]) in the p
T
interval from
5 to about 15 GeV/c.
Several mass-dependent effects are expected to influence the energy loss for quarks
(see [15] for a recent review). The dead-cone effect should reduce small-angle gluon radi-
ation for quarks that have moderate energy-over-mass values, i.e. for c and b quarks with
momenta up to about 10 and 30 GeV/c, respectively [18, 21–24]. Likewise, collisional
energy loss is expected to be reduced for heavier quarks, because the spatial diffusion coef-
ficient that regulates the momentum exchange with the medium is expected to scale as the
inverse of the quark mass [25]. In the p
T
interval up to about 20 GeV/c, where the masses
of heavy quarks are not negligible with respect to their momenta, essentially all models
predict R
D
AA
< R
B
AA
[17–20, 26–35], which stems directly from the mass dependence of the
quark-medium interaction and is only moderately affected by the different production and
fragmentation kinematics of c and b quarks (see e.g. [36]).
A first comparison of light-flavour, charm and beauty hadron nuclear modification
factors based on measurements by the ALICE and CMS Collaborations [16, 37, 38] from
the 2010 LHC Pb-Pb data at a centre-of-mass energy
√
s
NN
= 2.76 TeV was presented
in [37]. In this paper we present the centrality dependence of the D meson R
AA
in Pb-Pb
collisions at the same energy, measured with the ALICE detector [39] using data from both
2010 and 2011 periods (integrated luminosities of about 2.2 and 21 µb
−1
, respectively). The
focus here is on the study of the parton energy loss; therefore, the data are presented for
the high-p
T
interval 5–16 GeV/c, where the largest suppression relative to binary scaling
was observed [37]. The results are compared with charged pions, measured by the ALICE
Collaboration [40], with non-prompt J/ψ mesons, measured by the CMS Collaboration [38],
and with model predictions.
2 Experimental apparatus and data sample
The Pb-Pb collisions were recorded using a minimum-bias interaction trigger, based on
the information of the signal coincidence of the V0 scintillator detectors that cover the full
azimuth in the pseudo-rapidity intervals −3.7 < η < −1.7 and 2.8 < η < 5.1 [41]. The
measurement of the summed signal amplitudes from the V0 detectors was used to sort
the events in classes of collision centrality, defined in terms of percentiles of the Pb-Pb
– 2 –
JHEP11(2015)205
hadronic cross section [42]. The trigger efficiency is 100% for the events considered in this
analysis, which correspond to the most central 80% of the Pb-Pb hadronic cross section.
An online selection based on the information of the V0 detectors was applied to increase
the statistics of central collisions for the 2011 data sample. An offline selection using the
V0 and the neutron Zero-Degree Calorimeters (ZDC) was applied to remove background
from interactions of the beams with residual atoms in the vacuum tube. Events with a
reconstructed primary vertex outside the interval ±10 cm from the interaction point along
the beam direction (z coordinate) were removed. The event sample used in the analysis
corresponds to an integrated luminosity L
int
= (21.3 ± 0.7) µb
−1
in the 0–10% centrality
class (16.4 × 10
6
events) and (5.8 ± 0.2) µb
−1
in each of the 10–20%, 20–30%, 30–40%,
40–50% classes (4.5 × 10
6
events per class). In the 50–80% class, where 2010 data were
used, the analyzed event sample corresponds to (2.2 ± 0.1) µb
−1
(5.1 × 10
6
events).
The decays D
0
→ K
−
π
+
, D
+
→ K
−
π
+
π
+
and D
∗+
→ D
0
π
+
, and their charge conju-
gates, were reconstructed as described in [37] using the central barrel detectors, which are
located in a solenoid that generates a 0.5 T magnetic field parallel to the beam direction.
Charged particle tracks were reconstructed with the Time Projection Chamber (TPC) [43]
and the Inner Tracking System (ITS), which consists of six cylindrical layers of silicon de-
tectors [44]. Both detectors provide full azimuthal coverage in the interval |η| < 0.9. D
0
and
D
+
candidates were formed from pairs and triplets of tracks with |η| < 0.8, p
T
> 0.4 GeV/c,
at least 70 associated space points in the TPC, and at least two hits in the ITS, out of
which one had to be in either of the two innermost layers. D
∗+
candidates were formed by
combining D
0
candidates with tracks with |η| < 0.8, p
T
> 0.1 GeV/c, and at least three
associated hits in the ITS for the 10% most central collisions (two in the other centrality
classes). The decay tracks of the candidate D mesons were identified on the basis of their
specific ionization energy deposition dE/dx in the TPC and of their flight times to the
Time Of Flight (TOF) detector, which has the same η acceptance as the TPC. Particles
were identified as pions (kaons) by requiring the measured signal to be within three times
the resolution (±3 σ) around the expected mean values of dE/dx and time-of-flight for
pions (kaons). Only D meson candidates with rapidity |y| < 0.8 were considered, because
the acceptance decreases rapidly outside this interval.
3 Data analysis
The selection of the D meson decay topology is mainly based on the displacement of
the decay tracks from the primary vertex, and on the pointing of the reconstructed D
meson momentum to the primary vertex [37]. The raw yields were determined in each
centrality and p
T
interval using fits to the distributions of invariant mass M(K
−
π
+
) and
M(K
−
π
+
π
+
), in the case of D
0
and D
+
mesons, and of the difference M(K
−
π
+
π
+
) −
M(K
−
π
+
) for D
∗+
mesons. The fit function is the sum of a Gaussian, for the signal, and
either an exponential function (D
0
and D
+
) or a power-law multiplied with an exponential
function (D
∗+
) to describe the background distribution [37].
For D
0
mesons, an additional term was included in the fit function to account for the so-
called ‘reflections’, i.e. signal candidates that are present in the invariant mass distribution
– 3 –
JHEP11(2015)205
5 < p
T
< 8 GeV/c 8 < p
T
< 16 GeV/c
D
0
D
+
D
∗+
D
0
D
+
D
∗+
Pb-Pb yields:
Yield Extraction 6 8 6 7 8 7
Tracking efficiency 10 15 15 10 15 15
PID identification 5 5 5 5 5 5
Cut efficiency 5 10 5 5 10 5
D p
T
distribution in sim. 2 2 2 2 2 2
Feed-down subtraction
+12
−13
+10
−10
+6
−8
+12
−12
+10
−10
+ 7
−10
hT
AA
i [42] 4 4
pp reference 16 20 17 16 19 17
Reference scaling in
√
s
+ 6
−12
+5
−6
Centrality limits < 0.1
Table 1. Systematic uncertainties (%) on R
AA
of prompt D mesons with 5 < p
T
< 8 GeV/c and
8 < p
T
< 16 GeV/c in the 0–10% centrality class.
also when the (K, π) mass hypothesis for the decay tracks is swapped. A large fraction
(about 70%) of these reflections is rejected by the particle identification selection. The
residual contribution was studied with Monte Carlo simulations (described later in this
section). It was found that the reflections have a broad invariant mass distribution, which
is well described by a sum of two Gaussians, and its integral amounts to about 30% of
the yield of the signal in the p
T
interval used in the analysis presented in this article. In
order to account for the contribution of reflections in the data, a template consisting of
two Gaussians was included in the fit. The centroids and widths, as well as the ratios of
the integrals of these Gaussians to the signal integral, were fixed to the values obtained in
the simulation (see [45] for more details).
In the most central centrality class (0–10%), the statistical significance of the invariant
mass signal peaks varies from 8 to 18 depending on the D meson species and p
T
, while the
signal-over-background ratio ranges from 0.1 to 0.4. In the most peripheral centrality class
(50–80%), the statistical significance varies from 4 to 11, while the signal-over-background
ranges from 0.4 to 1.5. In figure 1 the invariant mass distributions of the three meson
species are shown in the 0–10% centrality class and in the transverse momentum intervals
5 < p
T
< 8 GeV/c and 8 < p
T
< 16 GeV/c.
The correction for acceptance and efficiency was determined using Monte Carlo simula-
tions. Pb-Pb events were simulated using the HIJING generator [46] and D meson signals
were added with the PYTHIA 6 generator [47]. The p
T
distribution of the D mesons
was weighted in order to match the shape measured for D
0
mesons in central Pb-Pb colli-
sions [37]. A detailed description of the detector response, based on the GEANT3 transport
package [48], was included. The contribution of feed-down from B → D+X to the inclusive
D meson raw yield depends on p
T
and on the geometrical selection criteria, because the
secondary vertices of D mesons from B-hadron decays are typically more displaced from
– 4 –
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