The measurement of event-by-event fluctuations and charge-dependent particle
correlations are used to study properties of nuclear matter at high
temperatures as produced in ultrarelativistic heavy-ion collisions. We present
results for event-by-event net-charge fluctuations and charge balance functions
in \Delta\eta\ and \Delta\phi\ in Pb-Pb collisions at sqrt(s_NN)= 2.76 TeV.Comment: 4 pages, 4 figures, Proceedings for Quark Matter 2012, Washington
  D.C., August 13-18, 2012 (accepted version by QM reviewers, additional figure
  for balance functions

Weber, Michael

English

arXiv

The measurement of event-by-event fluctuations and charge-dependent particle correlations are used to study properties of nuclear matter at high temperatures as produced in ultrarelativistic heavy-ion collisions. We present results for event-by-event net-charge fluctuations and charge balance functions in $\Delta\eta$ and $\Delta\phi$ in Pb-Pb collisions at $\sqrt{s_{NN}}$= 2.76$ TeV.The measurement of event-by-event fluctuations and charge-dependent particle correlations are used to study properties of nuclear matter at high temperatures as produced in ultrarelativistic heavy-ion collisions. We present results for event-by-event net-charge fluctuations and charge balance functions in @D@h and @D@f in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76TeV.The measurement of event–by–event fluctuations and charge–dependent particle correlations are used to study properties of nuclear matter at high temperatures as produced in ultrarelativistic heavy–ion collisions. We present results for event–by–event net–charge fluctuations and charge balance functions in Δ η and Δ φ in Pb–Pb collisions at sNN=2.76TeV .The measurement of event-by-event fluctuations and charge-dependent particle correlations are used to study properties of nuclear matter at high temperatures as produced in ultrarelativistic heavy-ion collisions. We present results for event-by-event net-charge fluctuations and charge balance functions in \Delta\eta\ and \Delta\phi\ in Pb-Pb collisions at sqrt(s_NN)= 2.76 TeV

CERN Document Server

Net-charge fluctuations and balance functions at the LHC

M. Weber

Crossref

Nuclear Physics A

AbstractThe measurement of event–by–event fluctuations and charge–dependent particle correlations are used to study properties of nuclear matter at high temperatures as produced in ultrarelativistic heavy–ion collisions. We present results for event–by–event net–charge fluctuations and charge balance functions in Δη and Δφ in Pb–Pb collisions at sNN=2.76TeV

Weber, M.

Elsevier - Publisher Connector 

The measurement of event–by–event fluctuations and charge–dependent particle correlations are
used to study properties of nuclear matter at high temperatures as produced in ultrarelativistic
heavy–ion collisions. We present results for event–by–event net–charge fluctuations and charge
balance functions in Δη and Δϕ in Pb–Pb collisions at √sNN = 2.76 TeV.
1. Motivation
sNN = 2.76 TeV and make use of the
excellent tracking capabilities of the ALICE Time Projection Chamber (TPC).
The study of strongly interacting matter at extreme energy densities as produced in ultrarel-
ativistic heavy–ion collisions is the aim of the ALICE detector [1] at the Large Hadron Collider
(LHC) at CERN. Quantum chromodynamics (QCD) [2] predicts that at suﬃciently high energy
density, of the order of 0.5 GeV/fm3 [3], a deconfined state of quarks and gluons is produced, the
so-called quark gluon plasma (QGP). Many diﬀerent observables are used to find experimental
evidence and subsequently study the properties of the QGP. In this contribution we will focus
on the measurement of event–by–event net–charge fluctuations and charge balance functions.
The analyses use the data taken in Pb–Pb collisions at √
Email address: m.weber@cern.ch (M. Weber, for the ALICE Collaboration)
1A list of members of the ALICE Collaboration and acknowledgements can be found at the end of this issue.
〈Nch〉
≈ 〈Nch〉ν+−,dyn + 4 (1)
with 〈δQ2〉 the variance of the net–charge Q = N+ − N− and 〈Nch〉 = 〈N+ + N−〉 the average
number of all measured charged particles per event. The variable D is close to 1 for the case of a
QGP, and is predicted to be close to 3 for a hadron (resonance) gas [5].
Insight into the possible existence of a QGP can be gained by determining the relevant de-
grees of freedom for the electric charge of the system produced in a heavy–ion collision [4]. In
a hadron (resonance) gas these are mesons and baryons, while in the QGP phase these would
be quarks in the simplest case (neglecting more complex bound states). A significant diﬀerence
between the two phases is expected in measuring event–by–event net–charge fluctuations per
charged degree of freedom:
D = 4
〈δQ2〉
2. Net–charge fluctuations
Abstract
University of Houston, Physics Department, Houston, TX 77204
Net-charge fluctuations and balance functions at the LHC
M. Weber, for the ALICE Collaboration1
Available online at www.sciencedirect.com
Nuclear Physics A 904–905 (2013) 467c–470c
0375-9474/ © 2013 CERN Published by Elsevier B.V. All rights reserved.
www.elsevier.com/locate/nuclphysa
http://dx.doi.org/10.1016/j.nuclphysa.2013.02.050
© CERN for the benefit of the ALICE Collaboration.
In the experiment, the net–charge fluctuations are best studied by calculating the quantity
ν+−,dyn, which is strongly connected to the variable D via the relationship shown in Eq. 1 and is
defined as:
ν+−,dyn =
〈N+(N+ − 1)〉
〈N+〉2
+
〈N−(N− − 1)〉
〈N−〉2
− 2 〈N−N+〉〈N−〉〈N+〉
(2)
Taking into account global charge conservation and finite acceptance, one obtains the corrected
value of ν+−,dyn:
νcorr
+−,dyn = ν+−,dyn +
4
〈Ntotal〉
(3)
where 〈Ntotal〉 is the average total number of charged particles produced over the full phase space.
Figure 1: (color online) Left: 〈Nch〉νcorr+−,dyn (left axis) and D (right axis) as a function of the number of participants for
Δη = 1 and Δη = 1.6 in Pb–Pb collisions at √sNN = 2.76 TeV. Also shown are the results from the HIJING event
generator for both the Δη windows and in the shaded bands the expectations for a hadron resonance gas and the QGP [5].
Right: 〈Nch〉νcorr+−,dyn (left axis) and D (right axis) as a function of Δη for diﬀerent centrality percentiles. The data points
are fitted with the functional form, er f ( Δη√
8σf
). The dashed lines correspond to the extrapolation of the fitted curves. Both
statistical (error bar) and systematic (box) errors are shown. Figures from [4].
In the left panel of Fig. 1 the values of 〈Nch〉νcorr+−,dyn are shown for pp and Pb–Pb collisions as
a function of the mean number of participating nucleons 〈Npart〉 in two pseudorapidity windows
Δη = 1 and Δη = 1.6. The shaded bands indicate the predictions for a hadron resonance gas (HG)
and the QGP [5]. The data points show a monotonic decreasing dependence with 〈Npart〉 and lie
clearly below the HG, but above the QGP expectation. On the other hand, the results from the
HIJING event generator for both pseudorapidity windows do not show any dependence on Npart
and are in the vicinity of the HG band. The observed pseudorapidity dependence, as shown in
the right panel of Fig. 1, hints to a dilution of the primordial fluctuations during the evolution of
the system from hadronization to kinetic freeze–out because of the diﬀusion of charged hadrons
in rapidity [6]. It can be parametrized with
er f (
Δη√
8σf
) (4)
with σf being a measure for the diﬀusion.
 QGP
 Hadron Gas
 = 1.6ηΔ
 = 1.0ηΔ
PbPb    pp   HIJING
1
1.5
2
2.5
3
3.5
4
D
-3
-2.5
-2
-1.5
-1
-0.5
0
(+
-,
dy
n)
co
rr
ν〉
ch
N〈
0 50 100 150 200 250 300 350 400
〉
part
N〈
Extrapolation
Function Erf
ηΔ
fσ8
50-60%
30-40%
0-5%
1.5
2
2.5
3
3.5
4
D
-2.5
-2
-1.5
-1
-0.5
0
(+
-,
dy
n)
co
rr
ν〉
ch
N〈
0 0.5 1 1.5 2 2.5 3 3.5 4
ηΔ
M. Weber / Nuclear Physics A 904–905 (2013) 467c–470c468c
3. Balance functions
Figure 2: (color online) The balance function distributions for the 5% most central (a,b) and peripheral (c,d) Pb–Pb
collisions at √sNN = 2.76 TeV [9] as a function of the relative pseudorapidity Δη (a,c) and the relative azimuthal angle
Δϕ (b,d). The data points are compared to results from HIJING, from AMPT (string melting) and from a thermal blast
wave. The ratios to the blast wave fit are shown in the lower panels. The models are normalized to the same integral as
the ALICE data.
It was proposed to measure the creation time of particles via the correlations between positive
and negatively charged pairs as a function of rapidity [7]. Assuming these pairs being created at
the same space–time point and correlated in momentum space due to a strong collective expan-
sion, their separation in pseudorapidity Δη depends not only on the initial momentum diﬀerence
but also on the length of the rescattering phase. Furthermore, it was shown that the balance
function for the relative azimuthal angle of the charge–anticharge pair can probe the collective
motion of the produced system and in particular its radial flow [8].
The definition of the balance function (e.g. for the pseudorapidity diﬀerence Δη) reads:
B(Δη) =
1
2
[N+−(Δη) − N−−(Δη)
N−
+
N−+(Δη) − N++(Δη)
N+
]
(5)
M. Weber / Nuclear Physics A 904–905 (2013) 467c–470c 469c
with N−−(++,+−)(Δη) the number of − − (++,+−) particle pairs and N+(−) the number of positive
(negative) particles in the analyzed phase–space.
In Fig. 2 the balance functions are shown for the 5%most central and formore peripheral (60–
70%) Pb–Pb collisions as a function of Δη and Δϕ [9]. Furthermore, a comparison to results from
diﬀerent MC event generators is shown. HIJING [10] produces charges early in the collision
history, mainly via string fragmentation, and includes no collective motion, e.g. radial flow. It
is not able to match the data points for B(Δη) and B(Δϕ) for central collisions. Only in the
most peripheral events the HIJING results for B(Δη) are consistent with the ALICE data, which
shows a strong centrality dependence, that is not observed in HIJING. AMPT (string melting)
[11], on the other hand, with parameters tuned to reproduce the measured elliptic flow values of
non–identified particles at the LHC, shows an agreement in B(Δϕ), not only in the most central
bin but for the whole centrality dependence. This can be understood as collective flow being the
determining source of balancing charge correlation in Δϕ. For B(Δη) the balance functions are
similar to HIJING.
4. Summary and outlook
In summary, dynamical net–charge fluctuations were presented as a function of centrality and
pseudorapidity and have a value below the expectation for a hadron resonance gas and above that
of the QGP. The balance functions for charged particles show a strong narrowing with increasing
centrality, which can be reproduced in Δϕwith a model including collective eﬀects, e.g. flow, but
not in Δη. The analysis of net–charge fluctuations will be extended to higher moments and bal-
ance functions will be studied for identified particles and regarding their transverse momentum
and event–plane dependence.
References
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M. Weber / Nuclear Physics A 904–905 (2013) 467c–470c470c


Net-charge fluctuations and balance functions at the LHC

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