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Physics Letters B 772 (2017) 567–577

Contents lists available at ScienceDirect

Physics Letters B

www.elsevier.com/locate/physletb

Centrality dependence of the pseudorapidity density distribution for

charged particles in Pb–Pb collisions at

√

= 5.02 TeV

.ALICE Collaboration



a r t i c l e i n f o a b s t r a c t

Article history:

Received

9 January 2017

Received

in revised form 19 June 2017

Accepted

9 July 2017

Available

online 14 July 2017

Editor:

L. Rolandi

We present the charged-particle pseudorapidity density in Pb–Pb collisions at

√

= 5.02 TeV in

centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from −3.5

5, which is suﬃcient for reliable estimates of the total number of charged particles produced in the

collisions. For the most central (0–5%) collisions we ﬁnd 21 400 ± 1 300, while for the most peripheral

(80–90%) we ﬁnd 230 ± 38. This corresponds to an increase of (27 ± 4)%over the results at

√

2.76 TeV previously reported by ALICE. The energy dependence of the total number of charged particles

produced in heavy-ion collisions is found to obey a modiﬁed power-law like behaviour. The charged-

particle

pseudorapidity density of the most central collisions is compared to model calculations — none

of which fully describes the measured distribution. We also present an estimate of the rapidity density

of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to

the beam rapidity, independent of the collision energy from the top SPS to LHC energies.

(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP

1. Introduction

In ultra-relativistic heavy-ion collisions a dense and hot phase

of nuclear matter is created [1–4]. This phase of QCD matter is

considered to be a plasma of strongly interacting quarks and glu-

ons

and is therefore labelled the sQGP [5]. The multiplicity of

primary, charged particles produced in heavy-ion collisions is a

key observable to characterise the properties of the matter created

in these collisions [6]. The study of the primary charged-particle

pseudorapidity density (dN

/dη) over a wide pseudorapidity (η)

range and its dependence on colliding system, centre-of-mass en-

ergy,

and collision geometry is important to understand the rela-

tive

contributions to particle production from hard scatterings and

soft processes, and may provide insight into the partonic structure

of the interacting nuclei.

We havepreviously reported measurementson primary charged-

particle

pseudorapidity densities over a wide pseudorapidity range

in Pb–Pb collisions at the centre-of-mass energy per nucleon pair

√

= 2.76 TeV [7]. In this Letter, we study these distributions in

the pseudorapidity interval from −3.5to 5at a collision energy of

√

= 5.02 TeV as a function of the centrality. Pseudorapidity is

deﬁned as η ≡− log(tan(ϑ/2)), where ϑ is the angle between the

charged-particle trajectory and the beam axis (z-axis). Nuclei are

extended objects, and their collisions can be characterised by cen-

trality

— the experimental proxy for the un-measurable distance



E-mail address: alice-publications@cern.ch.

between the centres of the colliding nuclei (impact parameter).

Aprimary particle is a particle with a mean proper lifetime τ

larger than 1cm/c, which is either a) produced directly in the

interaction, or b) from decays of particles with τ smaller than

1cm/c, restricted to decay chains leading to the interaction [8]. In

this Letter, all quantities reported are for primary charged parti-

cles,

though we will omit “primary” for brevity.

With the large pseudorapidity coverage available in ALICE, we

can reliably estimate, for all centrality classes, the total number

of charged particles produced in the collisions. We therefore also

present the ﬁrst measurement of the total charged-particle multi-

plicity

in Pb–Pb collisions at

√

= 5.02 TeV as a function of the

number of nucleons participating in the collisions (N

part

Finally, we transform the measured dN

/dη distribution for

the 5% most central collisions into charged-particle rapidity density

(dN

/dy), and we examine the centre-of-mass energy dependence

of the width of that distribution. The rapidity (y) of a particle

with energy E and momentum component p

along the beam axis

is deﬁned as y ≡

log

(

[

E + p

]/[E − p

]

)

. The comparison of the

width of the dN

/dy at different collision energies provides an

insight into the constraints on the overall production mechanism

of charged particles.

2. Experimental setup

A detailed description of ALICE and its performance can be

found elsewhere [9,10]. In the following, we brieﬂy describe the

detectors relevant to this analysis.

http://dx.doi.org/10.1016/j.physletb.2017.07.017

0370-2693/

SCOAP

568 ALICE Collaboration / Physics Letters B 772 (2017) 567–577

The Silicon Pixel Detector (SPD), the innermost part of the Inner

Tracking System (ITS), consists of two cylindrical layers of hybrid

silicon pixel assemblies covering |η| < 2 and |η| < 1.4for the inner

and outer layers, respectively. Combinations of hits on each of the

two layers consistent with tracks originating from the interaction

point form tracklets.

The

Forward Multiplicity Detector (FMD) is a silicon strip detec-

tor

which, records the energy deposited by particles traversing the

it. The detector covers the pseudorapidity regions −3.5 < η < −1.8

and

1.8 < η < 5, and has almost full coverage in azimuth (ϕ), and

high granularity in the radial (η) direction.

The

third detector system used in this analysis is the V0. It

consists of two sub-detectors: V0-A and V0-C covering the pseudo-

rapidity

regions 2.8 < η < 5.1 and −3.7 < η < −1.7, respectively,

each made up of scintillator tiles with a timing resolution < 1ns.

The fast signals from either of V0-A or V0-C are combined in a

programmable logic to form a trigger signal and to reject back-

ground

events. Furthermore, the combined pulse height signal of

both sub-detectors forms the basis for the classiﬁcation of events

into different centrality classes [11].

The

Zero-Degree Calorimeter (ZDC) measures the energy of

spectator (non-interacting) nucleons with two components: one

measures protons and the other measures neutrons. The ZDC is

located at about 112.5 m from the interaction point on both sides

of the experiment [9]. The ZDC also provides timing information

used to select collisions in the off-line data processing.

3. Data sample and analysis method

The results presented here are based on data collected by AL-

ICE

in 2015 during the Pb–Pb collision run of the LHC at

√

5.02 TeV. About 100 000 events with a minimum bias trigger re-

quirement

[12] were analysed in the centrality range from 0% to

90%. The minimum bias trigger for Pb–Pb collisions in ALICE, which

deﬁnes the so-called visible cross-section, is deﬁned as a coinci-

dence

between the A (z > 0) and C (z < 0) sides of the V0 detector.

The standard ALICE event selection [13] and centrality estima-

tor

based on the V0–amplitude [11] are used in this analysis. The

event selection consists of: exclusion of background events using

the timing information from the ZDC and V0 detectors; veriﬁca-

tion

of the trigger conditions; and a reconstructed position of the

collision. As discussed elsewhere [11], the 90–100% centrality class

has substantial contributions from QED processes and is therefore

not included in the results presented here.

The

measurement of the charged-particle pseudorapidity den-

sity

at mid-rapidity (|η| < 2) is obtained from a tracklet analysis

using the two layers of the SPD. The analysis method used is iden-

tical

to what has previously been presented [12,14,15]. Note that

no attempt is made to correct for known deﬁciencies, such as devi-

ations

in the number of strange particles or transverse momentum

) distributions compared to experimental measurements [11,16,

17],

in the event generators used to obtain the corrections from

simulations (e.g., HIJING). It is found, through simulation studies,

that tracklet reconstruction ﬁrst and foremost depends on the local

hit density and only weakly on particle mix and transverse mo-

mentum.

For example, the deﬁcit of strange particles in the event

generator effects the result by less than 2%. Since the event genera-

tors

generally, after detector simulation, produce a local hit density

that is consistent with what is observed in data, we observe a cor-

respondence

between the tracklet samples of both simulations and

data. On the other hand, changing the number of tracklets corre-

sponding

to strange particles a postiori to match the measured rel-

ative

yields dramatically biases the simulated tracklet sample away

from the measured, thus entailing systematic uncertainties that are

beyond the effect of the known event generator deﬁciencies, and

Fig. 1. [Colour online.] Charged-particle pseudorapidity density for ten centrality

classes over a broad η range in Pb–Pb collisions at

√

= 5.02 TeV. Boxes around

the points reﬂect the total uncorrelated systematic uncertainties, while the ﬁlled

squares on the right reﬂect the correlated systematic uncertainty (evaluated at

η = 0). Statistical errors are generally insigniﬁcant and smaller than the markers.

Also shown is the reﬂection of the 3.5 < η < 5values around η = 0(open circles).

The line corresponds to ﬁts of the difference between two Gaussians centred at

η = 0(f

) [7] to the data.

as such do not improve the accuracy of the measurements. Instead,

variations on the event generators are used to estimate the system-

atic

uncertainties as detailed elsewhere [12,14,15].

the forward regions (−3.5 < η < −1.8 and 1.8 < η < 5), the

measurement is provided by the analysis of the deposited energy

signal in the FMD. The analysis method used is identical to what

has previously been presented [7,14]: astatistical approach to cal-

culate

the inclusive number of charged particles; and a data-driven

correction — derived from previous satellite-main collisions — to

remove the large background from secondary particles.

4. Systematic uncertainties

For the measurements at mid-rapidity the sources and de-

pendencies

of the systematic uncertainties are detailed elsewhere

[7,12,15]. The magnitude of the systematic uncertainties is un-

changed

with respect to previous results, and amounts to 2.6% at

η = 0 and 2.9% at η = 2, most of which is correlated over |η| < 2,

and largely independent of centrality.

The

systematic uncertainty on the forward analysis is evaluated

using the same technique as for previous results [7]. We ﬁnd that

the uncertainty is uncorrelated across η an that it amounts to 6.9%

for

η > 3.5and 6.4% elsewhere within the forward regions.

The

systematic uncertainty on dN

/dη due to the centrality

class deﬁnition is estimated as 0.6% for the most central and 9.5%

for

the most peripheral class [15]. The uncertainty is estimated

by using alternative centrality deﬁnitions based on SPD hit mul-

tiplicities

and by varying the fraction of the visible hadronic cross-

section.

The 80–90% centrality class has some residual contam-

ination

from electromagnetic processes detailed elsewhere [11],

which gives rise to a 4% additional systematic uncertainty on the

measurements.

summary, the total systematic uncertainty varies from 2.6%

mid-rapidity in the most central collisions to 12.4% at the very

forward rapidities for the most peripheral collisions.

5. Results

Fig. 1 presents the charged-particle pseudorapidity density as

a function of pseudorapidity for ten centrality classes. The mea-

surements

from the SPD and FMD are combined in regions of

overlap (1.8 < |η| < 2) between the two detectors by taking the

weighted average using the non-shared uncertainties as weights.

Finally, based on the symmetry of the collision system, the result

is symmetrised around η = 0, and extended into the non-measured

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