Charged-Particle Pseudorapidity Density in Pb-Pb Central Collisions at sNN = 2.76 TeV
R.A. Ricci1, L. Vannucci1 for the ALICE collaboration.
1
INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy.
The distribution was fitted by using the Glauber model in
the region above 150 to avoid the contamination by events
due to electromagnetic processes. The shaded area in fig. 1
is related to the most central collisions and corresponds to
3615 events of the 650000 acquired. In order to minimize
acceptance effects we used, in the analysis, a subset of
2711 events having the reconstructed primary vertex
displaced, along the beam axis, less than 7 cm from the
SPD geometrical center.
INTRODUCTION
In relativistic nucleus-nucleus collisions the dependence
of the charged-particle multiplicity density on energy and
system size reflects the interplay between hard and soft
parton-parton scattering processes. Predictions of models,
that successfully describe particle production at RICH,
vary by a factor of 2 [1,2] at the LHC energies.
ALICE measured the charged-particle pseudorapidity
density in Pb-Pb collisions at sNN = 2.76 TeV to provide
an essential constraint for heavy-ion high-energy models.
EXPERIMENTAL DETAILS
The prompt charged particle density dNch/dη in central
(small impact parameter) Pb-Pb collisions was measured in
the |η| = |-ln(θ/2)| < 0.5 interval (θ is the polar angle).
Our set of prompt particles includes also the decay
products, except the weak decay of strange particles.
In the measurement we used a Pb-Pb four bunches beam,
having a luminosity of about 5.1023 cm-2s-1, the SPD [3],
the VZERO [4] and the ZDC [5] detectors of the ALICE
spectrometer [6].
The trigger signal was provided by the SPD and the
VZERO detectors in coincidence with the LHC bunchcrossing signal. The trigger configuration required at least
two of the following three conditions: (i) two pixel chips
hit in the outer layer of the SPD, (ii) a signal in the
VZERO part upstream to ALICE, (iii) a signal in the
VZERO part downstream to ALICE.
The observed trigger rate was negligible without beam
and about 50 Hz during the physics measurements.
This rate was mainly due (~90%) to electromagnetically
induced processes [7]. These processes have very large
cross-sections at LHC energies but generate final states
with very low multiplicities that can well distinguished
from the high multiplicity events.
Fig. 1 (upper panel) shows the measured correlation
between the energy deposited in the ZDC (proportional to
the number of non-interacting nucleons close the beam
rapidity) and the sum of the amplitude in the VZERO
detector (proportional to the event multiplicity). As the
impact parameter decreases and the event becomes more
and more central the measured multiplicity in the VZERO
detector increases and less energy is deposited in the ZDC.
Therefore the signal amplitude of the ZDC detector is
small in two cases: for peripheral collisions that produce
low multiplicity final states and for central collisions the
produce few particles close the beam rapidity.
The VZERO amplitude histogram is shown in the lower
panel of fig. 1; the inset displays the low amplitude part.
LNL Annual Report
Fig. 1. Upper panel: ZDC and VZERO correlation. Lower panel:
histogram of sum of the VZERO amplitude
Starting from the number of reconstructed tracklets
inside the SPD with |η| < 0.5, we calculated the charged
particle pseudorapidity density according to:
dNch / dη = A x (1-B) x dNtracklet /dη .
In this expression A is the correction factor for the
acceptance and the efficiency and B is the probability to
reconstruct a tracklet from uncorrelated hits.
The hit relative angles, Δφ in the bending plane and Δθ
in the no-bending direction, were used to select the pairs of
hits suitable to form a tracklet. In order to minimize the
value of B we rejected from the reconstruction the pairs of
hits having Δφ2 + Δθ2 exceeding a maximum value. The cut
Δφ corresponds to selecting charged particles with a
minimum transversal momentum of 50 MeV/c; such a
value is so small that the particles mostly are absorbed by
the detector material.
Three different methods are used to take into account the
combinatorial background and estimate the value of B. The
first one is based on simulated events having a similar
number of hits in the SPD as in the real data. In the second
method random background hits are injected in the real
49
Nuclear Physics
events. In the third method the events were modified by
rotating the hits of the inner SPD layer of 180° in φ so that
the hit correlation is completely loss but the global event
features are preserved. From the combination of these three
methods we estimated a combinatorial background of
about 14%.
The correction for the acceptance and the efficiency, A,
is estimated, by means of simulated events, by calculating
the ratio of generated primary charged particles to the reconstructed tracklets after subtraction of the combinatorial
background. In this way, the factor A accounts for geometrical acceptance, detector and reconstruction efficiencies, contamination by weak decay products of strange
particles, conversions, secondary interactions and undetected particles having a transverse momentum below
50 MeV/c. The value of the correction factor A was found
to be ~2 and slightly varying in dependence on the vertex
position.
We estimated also the systematic uncertainties: 2% due
the background subtraction, 1% due to the event particle
composition, 1% due to the weak decays contamination,
2% due to low momentum undetected particles, 3% due to
the centrality definition. The total systematic uncertainty
due to the different components results in ~4.8%.
for heavy-ion than for pp and ͞pp collisions. For a comparison the curves proportional to (sNN)0.15 and (sNN)0.11 are
superimposed to the data.
A significant increase, by a factor 2.2, in the pseudorapidity density is observed at sNN = 2.76 TeV for Pb-Pb
compared to sNN = 0.2 TeV for Au-Au. The average
multiplicity per participant pair, for our centrality
selection, is found to be a factor 1.9 higher than in pp and
͞pp at similar energies.
Our results are significantly larger than those measured
at RHIC indicating an energy dependence stronger than
measured in pp and ͞pp collisions.
RESULTS
In order to compare bulk particle production in different
collision systems and at different energies, and to compare
with model calculations, the charged-particle density was
scaled by the number of participant nucleons determined
by using the Glauber model (fig. 1). The average number
of participants for the 5% most central events was
estimated to be <Npart> ~ 381 with an rms of 18 and a
systematic uncertainty of 1%.
The charged-particle density at midrapidity was found
to be: dNch / dη = 1584 ± 4(stat.) ± 76(syst.), which, normalized per participant pairs, gives a value of:
(dNch / dη) / (<Npart> / 2) = 8.3 ± 0.4(syst.), with negligible
statistical errors.
In fig. 2 the obtained result is compared to the measurements for Au-Au and Pb-Pb, and no single diffractive
pp and ͞pp collisions, over a wide range of interaction
energies. It is evident that the energy dependence is steeper
LNL Annual Report
Fig. 2. Charged particle pseudorapidity density per participant
pairs for central nucleus-nucleus and no single diffractive pp and
͞pp collisions as function of sNN .
[1]
[2]
[3]
[4]
S. Abreu et al., J. Phys., G35 (2008) 054001.
N. Armesto, arXiv:0903.1330.
P. Riedler et al., Nucl. Instr. and Meth., A568 (2006) 284.
G. H. Corral et al., AIP Conf. Proc. of the First ICFA
Workshop on Instrumentation for Particle Physics, Morelia,
Michoacan Mexico, 2002.
[5] Zero Degree Calorimeter ZDC, Technical Design Report,
CERN/LHCC 99-5: ALICE TRD 3.
[6] K. Aamod et al., JINST, 3 (2008) S08002.
[7] G. Baur et al., Phys. Rep., 364 (2002) 359.
50
Nuclear Physics