arXiv:0711.0547v2 [nucl-ex] 28 Mar 2008
High Transverse Momentum Hadron Spectra at√sNN= 17.3GeV, in Pb+Pb and p+p
C. Alt,9T. Anticic,23B. Baatar,8D. Barna,4J. Bartke,6L. Betev,10H. Bia? lkowska,20C. Blume,9B. Boimska,20
M. Botje,1J. Bracinik,3R. Bramm,9P. Bunˇ ci´ c,10V. Cerny,3P. Christakoglou,2P. Chung,19O. Chvala,14
J.G. Cramer,16P. Csat´ o,4P. Dinkelaker,9V. Eckardt,13D. Flierl,9Z. Fodor,4P. Foka,7V. Friese,7J. G´ al,4
M. Ga´ zdzicki,9,11V. Genchev,18G. Georgopoulos,2E. G? ladysz,6K. Grebieszkow,22S. Hegyi,4C. H¨ ohne,7
K. Kadija,23A. Karev,13D. Kikola,22M. Kliemant,9S. Kniege,9V.I. Kolesnikov,8E. Kornas,6R. Korus,11
M. Kowalski,6I. Kraus,7M. Kreps,3A. Laszlo,4, ∗R. Lacey,19M. van Leeuwen,1P. L´ evai,4L. Litov,17
B. Lungwitz,9M. Makariev,17A.I. Malakhov,8M. Mateev,17G.L. Melkumov,8A. Mischke,1M. Mitrovski,9
J. Moln´ ar,4St. Mr´ owczy´ nski,11V. Nicolic,23G. P´ alla,4A.D. Panagiotou,2D. Panayotov,17A. Petridis,2, †
W. Peryt,22M. Pikna,3J. Pluta,22D. Prindle,16F. P¨ uhlhofer,12R. Renfordt,9C. Roland,5G. Roland,5M.
Rybczy´ nski,11A. Rybicki,6A. Sandoval,7N. Schmitz,13T. Schuster,9P. Seyboth,13F. Sikl´ er,4B. Sitar,3
E. Skrzypczak,21M. Slodkowski,22G. Stefanek,11R. Stock,9C. Strabel,9H. Str¨ obele,9T. Susa,23I. Szentp´ etery,4
J. Sziklai,4M. Szuba,22P. Szymanski,10,20V. Trubnikov,20D. Varga,4,10M. Vassiliou,2G.I. Veres,4,5
G. Vesztergombi,4,4D. Vrani´ c,7A. Wetzler,9Z. W? lodarczyk,11A. Wojtaszek,11I.K. Yoo,15and J. Zim´ anyi4, †
(The NA49 collaboration)
1NIKHEF, Amsterdam, Netherlands.
2Department of Physics, University of Athens, Athens, Greece.
3Comenius University, Bratislava, Slovakia.
4KFKI Research Institute for Particle and Nuclear Physics, Budapest, Hungary.
5MIT, Cambridge, USA.
6Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland.
7Gesellschaft f¨ ur Schwerionenforschung (GSI), Darmstadt, Germany.
8Joint Institute for Nuclear Research, Dubna, Russia.
9Fachbereich Physik der Universit¨ at, Frankfurt, Germany.
10CERN, Geneva, Switzerland.
11Institute of Physics´Swi¸ etokrzyska Academy, Kielce, Poland.
12Fachbereich Physik der Universit¨ at, Marburg, Germany.
13Max-Planck-Institut f¨ ur Physik, Munich, Germany.
14Charles University, Faculty of Mathematics and Physics,
Institute of Particle and Nuclear Physics, Prague, Czech Republic.
15Department of Physics, Pusan National University, Pusan, Republic of Korea.
16Nuclear Physics Laboratory, University of Washington, Seattle, WA, USA.
17Atomic Physics Department, Sofia University St. Kliment Ohridski, Sofia, Bulgaria.
18Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria.
19Department of Chemistry, Stony Brook Univ. (SUNYSB), Stony Brook, USA.
20Institute for Nuclear Studies, Warsaw, Poland.
21Institute for Experimental Physics, University of Warsaw, Warsaw, Poland.
22Faculty of Physics, Warsaw University of Technology, Warsaw, Poland.
23Rudjer Boskovic Institute, Zagreb, Croatia.
Transverse momentum spectra up to 4.5GeV/c around midrapidity of π±, p, ¯ p, K±in Pb+Pb
reactions were measured at√sNN= 17.3GeV by the CERN-NA49 experiment. The nuclear mod-
ification factors RAA for π±and RCP for π±,p, ¯ p,K±were extracted and are compared to RHIC
results at√sNN= 200GeV. The modification factor RAA shows a rapid increase with transverse
momentum in the covered region. This indicates that the Cronin effect is the dominating effect in
our energy range. The modification factor RCP, in which the contribution of the Cronin effect is
reduced, shows a saturation well below unity in the π±channel. The extracted RCP values follow
the 200GeV RHIC results closely in the available transverse momentum range, except for π±above
2.5GeV/c transverse momentum. There the measured suppression is smaller than that observed at
PACS numbers: 25.75.Dw
∗Corresponding author. E-mail address: firstname.lastname@example.org
One of the most interesting features discovered at
RHIC is the suppression of particle production at high
transverse momenta in central nucleus-nucleus reactions
relative to peripheral ones as well as to p+nucleus and to
p+p collisions [1, 2, 3, 4, 5]. This is generally interpreted
as a sign of parton energy loss in hot and dense strongly
The aim of the presented analysis is to investigate the
energy dependence of these effects via a systematic study
of Pb+Pb reactions at top ion-SPS energy, 158AGeV
(√sNN= 17.3GeV), with the CERN-NA49 detector. A
similar study has been published by the CERN-WA98
collaboration for the π0channel .
paper is to extend their results to all charged particle
channels, i.e. π±, p, ¯ p and K±.
The aim of this
Invariant yields were extracted as a function of trans-
verse momentum pTin the range from 0.3 to 4.5GeV/c
in the rapidity interval −0.3 ≤ y ≤ 0.7 (midrapidity), at
different collision centralities. The identification of par-
ticle types is crucial, because the particle composition of
hadron spectra changes rapidly with transverse momen-
tum and differs significantly from that observed at RHIC
energies (at SPS energies p production is comparable to
the π±yields already at moderate values of pT).
The most important results are the nuclear modifica-
tion factors RAA, which were extracted from the iden-
tified charged hadron spectra in Pb+Pb and from the
already published p+p pion spectra . It is important
to note that the use of reference spectra (e.g. p+p), mea-
sured exactly at the same collision energy is necessary,
because of the rapid change of the shape of the parti-
cle spectra around√sNN= 17.3GeV. A baseline, which
is constructed from the existing nearby energy measure-
ments is not sufficient, as discussed in .
Besides the in-medium parton energy loss, the RAA
quantity can also contain other medium effects, such as
the Cronin effect , a term used for the observation of
an increased particle yield at high transverse momentum
in p+nucleus compared to the elementary p+p reactions.
This effect is believed to be a consequence of multiple
scattering of projectile partons (or hadrons) in the nuclei
before the particle production process. The Cronin effect
could cause a large part of the nuclear modification effects
measured by the RAA quantity.
search of the other nuclear effects the RCP (central to
peripheral) modification factors were also extracted from
the data. If the multiple scattering interpretation of the
Cronin effect is correct, its contribution to the RCPratio
is expected to be reduced, as it should already be present
in the peripheral baseline.
For a more sensitive
II. EXPERIMENTAL SETUP
The NA49 detector  is a wide-acceptance hadron
spectrometer for the study of hadron production in col-
lisions of hadrons or heavy ions at the CERN-SPS. The
main components are four large-volume Time Projection
Chambers, TPCs, (Fig. 1) which are capable of detect-
ing 80% of some 1500 charged particles created in a cen-
tral Pb+Pb collision at 158AGeV beam energy. Two
chambers, the Vertex TPCs (VTPC-1 and VTPC-2),
are located in the magnetic field of two superconduct-
ing dipole magnets (1.5 and 1.1T), while the two others
(MTPC-L and MTPC-R) are positioned downstream of
the magnets symmetrically to the beam line. The setup
is supplemented by two Time of Flight (TOF) detector
arrays, which are not used in this analysis, and a set
of calorimeters.The NA49 TPCs allow precise mea-
surements of particle momenta p with a resolution of
σ(p)/p2 ∼= (0.3 − 7) · 10−4(GeV/c)−1.
Pb foils (typically of 224mg/cm2thickness) are used
as target for Pb+Pb collisions, and a liquid hydrogen
cylinder (length 20cm) for p+p interactions. The target
is positioned about 80cm upstream from VTPC-1.
Pb beam particles are identified by means of their
charge as seen by a Helium Gas-ˇCerenkov counter (S2’)
and proton beam particles by a 2mm thick scintillator
(S2).Both detectors are situated in front of the tar-
get. For p beams, interactions in the target are selected
by requiring a valid incoming beam particle and no sig-
nal from a small scintillation counter (S4) placed on the
beam line between the two vertex magnets. For p+p
interactions at 158GeV this counter selects a (trigger)
cross section of 28.5mb out of 31.6mb of the total in-
elastic cross section. For Pb-ion beams an interaction
trigger is provided by a Helium Gas-ˇCerenkov counter
(S3) directly behind the target. The S3 counter is used
to select minimum-bias collisions by requiring a reduc-
tion of theˇCerenkov signal. Since theˇCerenkov signal
is proportional to Z2, this requirement ensures that the
Pb projectile has interacted with a minimal constraint on
the type of interaction. This setup limits the triggers on
non-target interactions to rare beam-gas collisions, the
fraction of which proved to be small after cuts, even in
the case of peripheral Pb+Pb collisions. The resulting
minimum-bias trigger cross section was about 80% of the
total inelastic cross section σInel= 7.15b.
For Pb+Pb reactions, the centrality of a collision is se-
lected (on-line for central Pb+Pb, off-line for minimum-
bias Pb+Pb interactions) by a trigger using information
from a downstream calorimeter (VCAL), which measures
the energy of the projectile spectator nucleons [11, 12].
Table I lists the statistics used in the 158AGeV Pb+Pb
V0 S2 S4
V0 S3 S2’
FIG. 1: (Color online) Setup of the CERN-NA49 experiment showing different beam definitions and target arrangements.
Reaction√sNN[GeV] Centrality Number of events
Pb+Pb 17.3 mid-central
TABLE I: The event statistics, used in the analysis.
A. Event Selection
As the target setup of the experiment is not contained
in a vacuum pipe, non-target background collisions are
also expected to be recorded in the unfiltered event sam-
ple. To reduce this contamination, a cut was applied
to the reconstructed longitudinal position of the colli-
sion point: only those collisions were accepted, which
occurred close to the nominal target position. An exam-
ple plot is shown in Fig. 2 for the case of Pb+Pb reac-
tions, showing the target peak and the small flat non-
target contribution, which was used to estimate the re-
maining background events. The contamination of the
peripheral Pb+Pb spectra remains below 5%. For non-
peripheral spectra this contamination is zero. The pe-
ripheral Pb+Pb spectra are corrected for this effect.
In the case of Pb+Pb reactions, the recorded events
were classified by collision centrality, which is correlated
to the impact parameter – the transverse distance of the
centers of the colliding nuclei at impact. The centrality
is determined via the energy measured by the VCAL: the
non-interacting (spectator) part of the projectile nucleus
travels along the beamline, finally hitting the VCAL’s
surface, and leaving an energy signal in the apparatus,
proportional to the volume of the projectile spectator
matter. The higher this energy is, the more peripheral is
FIG. 2: (Color online) The distribution of the longitudinal co-
ordinate of the reconstructed collision point in minimum-bias
Pb+Pb events, together with the cuts. The other peaks cor-
respond to material in the beamline, while the small constant
background under the target peak corresponds to beam+gas
the given reaction. The event centrality is defined by the
fraction of total inelastic cross section, i.e. by the running
where σInelis the total inelastic cross section, and
is the differential cross section in EVCAL, which is defined
by the EVCALspectrum, normalizing its area to the trig-
ger cross section. The VCAL energy spectrum and event
centrality classification is shown in Fig. 3. A relatively
large centrality interval was used for the selection of pe-
ripheral events in order to gain enough statistics in the
particle spectrum analysis.
-0.1 0.150.40.65 0.9 1.15 1.4
FIG. 3: (Color online) The distribution of the energy de-
posited in the VCAL by the projectile spectators in Pb+Pb
minimum-bias reactions. The centrality intervals selected by
fraction of total inelastic cross section are shown by the ver-
Due to the aging of the photomultiplier tubes of the
calorimeter, the VCAL response was slowly varying in
time in terms of scale offset and amplification. This ef-
fect is corrected by a time-dependence calibration pro-
cedure, which is based on keeping the correlation of the
VCAL energy to the total measured charged multiplicity
time-independent (see ). The mean values of vari-
ous collision parameters (number of wounded nucleons,
number of binary collisions) for the selected centrality
intervals were calculated from the VENUS-4.12 Monte
Carlo model  which was filtered by the detector sim-
ulation. For the non-peripheral region, this was straight-
forward by applying statistical averaging in the centrality
intervals. For the peripheral region, a semi-empiric ap-
proach was used, which also takes the trigger bias into
account: the VCAL energy distribution for the averaging
was taken from the measurement, while the mapping of
VCAL energy into collision parameters was taken from
VENUS + detector simulation.
scribed in .A list of the average values is shown
in Table II together with their systematic errors, which
were estimated by assuming a 5% uncertainty of the to-
tal inelastic cross section, and a 10% uncertainty of the
skin thickness of the nuclear density profile. The stated
values were also confirmed by the GLISSANDO Monte
Carlo model .
The procedure is de-
0 − 5%
5 − 12.5%
12.5 − 23.5% 211 ± 4
23.5 − 33.5% 146 ± 5
33.5 − 43.5% 87 ± 7⋆120 ± 13⋆
43.5 − 80% 40 ± 4⋆
33.5 − 80% 56 ± 7⋆
357 ± 2
288 ± 3
742 ± 18
565 ± 18
379 ± 16
234 ± 14
46 ± 5⋆
70 ± 13⋆
TABLE II: The mean values of the number of wounded nu-
cleons and binary collisions in various centrality intervals in
Pb+Pb collisions, together with their systematic errors.
Averages for the full minimum-bias dataset using the semi-
empiric method, as discussed in the text.
The transverse momentum spectra of particles in
heavy-ion collisions are known to decrease rapidly (ap-
proximately exponentially) toward higher transverse mo-
menta. Therefore, the background of fake tracks becomes
more and more important with increasing pT, especially
for fixed-target experiments, in which the track density is
strongly focused in the forward direction, increasing the
probability for fake track formation. We apply cuts to
minimize the contribution of such artificial tracks. These
background tracks consist of discontinuous track candi-
dates, and track candidates whose trajectories pass close
to the border of the detector volume. The former kind
of tracks mainly arise from erroneous pairing of straight
track pieces in MTPC, outside of the magnetic field, to
some residual points in the VTPCs. The latter kind of
tracks are subject to distortions, as they are likely to
have missing or displaced clusters. The corresponding
trajectories will have wrong fitted curvature, therefore
they should be discarded from the analysis.
The discontinuous track candidates can be easily iden-
tified: they are track candidates that (according to their
reconstructed charge and momentum) would have left
hits in a given TPC (VTPC1, VTPC2, or MTPC), but
did not. These tracks were rejected as one source of fake
tracks. After this cleaning procedure a 3 dimensional
phase space study was performed. Its coordinates are ra-
pidity y, transverse momentum pTand charge-reflected
azimuth φ. Charge-reflected azimuth is defined by the
azimuth angle of the momentum vector in the transverse
plane, whose x-component is multiplied by the charge
sign of the track candidate – the x-direction being per-
pendicular to the magnetic field and to the beamline.
As our detector has an x-reflection symmetry, the use of
charge-reflected azimuth enables us to distinguish tracks
not crossing the plane defined by the beamline and the
magnetic field direction (right-side tracks) from tracks
crossing this plane (wrong-side tracks). An example of
the track quality distribution as a function of φ and pTis
shown in the upper panel of Fig. 4 for a typical rapidity
slice. A domain of high quality right-side tracks reveals
itself: in this region, the fraction of those tracks with
a ratio of the number of measured to number of poten-
tial points below 60% is very low. Selection of this re-
gion by a 3 dimensional momentum space cut provides a
clean track sample. The 3 dimensional momentum space
cut is guided by the isosurfaces of the number of poten-
tial points, and by the requirement of avoiding efficiency
holes, which can be determined from the dropping of the
φ distribution at fixed (y,pT) values (the φ distribution
should be uniform due to the axial symmetry of particle
production). The momentum space cut curve is shown
by the dotted line in the upper panel of Fig. 4, at given
y. The φ distribution is shown in the lower panel of Fig.
4 at given (y,pT), which reveals the efficiency holes to
be avoided, and the high track yield excess at the accep-
tance borders. The dotted arrows indicate the momen-
tum space cut in φ, for this particular (y,pT) slice. The
outlined procedure is discussed in detail in . The re-
sulting phase space, which contains the clean track sam-
ple, covers the rapidity region −0.3 ≤ y < 0.7, the trans-
verse momentum region 0GeV/c ≤ pT< 5GeV/c, and
a (y,pT) dependent φ interval. In the following analysis,
all distributions will be shown as a function of transverse
The particle identification was performed via the spe-
cific energy loss (dE
dx) of the particles.
verse momentum bin thedE
dxspectrum was recorded, and
by using the known shape of the response function (see
[18, 19]) of the given particle species (π±, p, ¯ p, K±, e±),
a fit was performed to thedE
dxhistogram, with the ampli-
tudes of the particle response functions as free parame-
ters. Also other parameters characteristic of the response
function shape, such as the most probable value, were
kept as free parameters. Thus, their statistical errors
are also reflected in the fitted errors of the amplitudes.
As our data was limited by statistics at higher pT, Pois-
son maximum-likelihood fitting was employed. This ap-
proach allows reliable fits even in regions with low statis-
tics (see ). An example can be seen in Fig. 5 which
dxfit procedure for inclusive particle
identification in a typical phase space bin. The full pro-
cedure is described in .
by possible systematic shifts of fit parameters were es-
timated by calculating the statistical regression matrix
from the fitted covariance matrix, and by propagating the
systematic errors of the fit parameters (approximately
estimated to be about 0.5%) to the fitted yields via this
matrix. The resulting systematic errors of yields are es-
In each trans-
Systematic errors, caused
2.5GeV/c ≤ pT< 2.6GeV/c
Clean track sample
0 ≤ yπ± < 0.1
FIG. 4: (Color online) Upper panel: example plot for the mo-
mentum space selection in the rapidity slice 0 ≤ y < 0.1, with
π±mass hypothesis. φ means charge-reflected azimuth. The
size of the boxes indicate the fraction of those track candi-
dates in the given momentum space bin, which have a ratio
of number of measured to number of potential points below
60% (i.e. the fraction of those track candidates, which most
likely do not correspond to real particle trajectories). The
color map indicates the number of potential points as a func-
tion of momentum space. As can be seen, the bad track can-
didates mainly populate the borders of the acceptance. The
dotted line shows the momentum space cut, guided by poten-
tial point isosurfaces. Lower panel: example plot for the φ
distribution at 0 ≤ y < 0.1 and 2.5GeV/c ≤ pT< 2.6GeV/c.
timated to be 4% for K+, 2% for ¯ p, and 1% for π+, π−,
−0.3 ≤ yπ± < 0.7;2.8GeV/c ≤ pT< 3GeV/c
FIG. 5: (Color online) Example plot for the identification of
negative particles by specific energy loss fits in central Pb+Pb
collisions (0-5%). The parameterization of the response func-
tion of each particle species with fixed momentum is known,
but the yields (the amplitudes in the summed
model) are fit parameters.
Monte Carlo methods including a GEANT simulation
of our detector were used to correct the yields of π+,π−
and K+,K−as well as p and ¯ p for tracking inefficien-
cies (below 10%), decay losses (from 20% to 0%), feed-
down from weakly decaying particles (5 − 30%), and fi-
nally for geometric acceptance. The peripheral particle
spectra were also corrected for non-target contamination
(5%) by extracting the non-target contribution from pure
beam+gas events, and subtracting it (for detailed proce-
dure, see ). The yield of fake tracks, after the in-
troduced track cuts, turned out to be negligible. The
momentum space resolution was estimated to be better
than 1% in the whole region. The momentum scale un-
certainty is of the order of 0.1%.
The largest and most sensitive correction is the feed-
down correction. The yield of feed-down particles was
estimated by using a simulation to determine the condi-
tional probabilities of reconstructing a secondary particle
originating from a weakly decaying primary particle as a
primary particle. To calculate the feed-down yields as a
function of momentum, one has to fold these tables of
conditional probabilities with the momentum differenti-
ated yields of the weakly decaying particles. The rele-
vant decay channels are listed in Table III. The yields of
the weakly decaying particles K0
parameterizations of yields, measured previously in our
s, Λ,¯Λ were taken from
experiment [21, 22, 23] (K0
average charged Kaon data). The used Λ,¯Λ yields in-
clude the Σ0→ Λγ and the¯Σ0→¯Λγ contributions, but
are feed-down corrected for Ξ (¯Ξ) decays. Since the Ξ
(¯Ξ) yields are generally below 20% of the Λ (¯Λ) yields,
their contribution is neglected in the correction. How-
ever, it constitutes a source of systematic errors, that
is discussed below. The K0
syield does not suffer from
syield was constructed from
Σ+→ p π0
Σ+→ n π+
Σ−→ n π−
¯Σ−→ ¯ p π0
¯Σ−→ ¯ n π−
¯Σ+→ ¯ n π+
s → π+π−
→ p π−
→ ¯ p π+
≤ 5% to π±
≤ 30% to p
≤ 30% to ¯ p
≤ 6% to p
≤ 6% to ¯ p
TABLE III: The list of relevant feed-down channels.
For π+, only the K0
contribution, while for π−also the Λ → pπ−channel has
to be taken into account. All other contributions to the
π±channels are negligible. For p, the Λ → pπ−gives the
dominant contribution, and for ¯ p, the¯Λ → ¯ pπ+is domi-
nant. The p and ¯ p spectra are also contaminated by the
decays of Σ+and¯Σ−. These are taken into account by
scaling up the Λ,¯Λ yields by 20%. This treatment was
suggested by the VENUS-4.12 model, which predicts that
the relative intensity of the non-Λ contribution to the p
feed-down, or the relative intensity of the non-¯Λ contri-
bution to the ¯ p feed-down is approximately constant at
20%. Assuming a 50% systematic uncertainty of this 20%
scaling, the contribution to the systematic uncertainty of
the p, ¯ p spectra would be 3%. There is an additional un-
certainty of the ¯ p feed-down correction, which is due to
the large errors of the measured¯Λ inverse slope parame-
ters. The systematic error, caused by the poor knowledge
of the¯Λ slopes was estimated by repeating the feed-down
calculation using the Λ temperatures, which should pro-
vide a reasonable upper bound for the¯Λ temperatures.
The systematic error, caused by this variation was esti-
mated to be 5%. The calculated feed-down contribution
to the π±and p, ¯ p yields in Pb+Pb reactions is depicted
in Fig. 6.
The tracking inefficiency was estimated by simulation
and reconstruction of embedded particles in real events,
i.e. by determining the conditional probability of losing
a track of given particle type and momentum, taking
the proper track density environment into account. The
inefficiency estimates are plotted in Fig. 7. These also
include the inefficiency caused by the decay loss for the
weakly decaying particles.
The aim of our analysis is to measure the particle
yields as a function of pTat y = 0, integrated over
−180◦≤ φ < 180◦. Therefore, the particle spectra have
to be corrected for the geometric acceptance. This cor-
s→ π+π−channel gives a sizable
Fraction of feed-down contribution
FIG. 6: (Color online) Ratio of the estimated feed-down con-
tribution to the raw measured particle yields. The top, center
and bottom row of plots correspond, respectively, to Pb+Pb
collisions with centrality ranges of 0-5%, 12.5-23.5% and 33.5-
80% of the inelastic cross section (see Table II).
rection can be performed without simulation, as the ac-
cepted momentum space region is defined explicitly by
our momentum space cut: only the (y,φ) shape of the
particle distributions have to be known in each pTbin.
The φ distribution is uniform in each (y,pT) slice due
to the axial symmetry of particle production. For the
y-dependence we use the fact, that the y spectra around
midrapidity are approximately flat in Pb+Pb reactions
(see ).Therefore, we assume a flat y distribution
in −0.3 ≤ y < 0.7 for the acceptance correction. The
systematic error, caused by this approximation has been
estimated to be below 2%.
When all the discussed corrections are taken into ac-
count, the Pb+Pb particle spectra bear the systematic
errors listed in Table IV.
Points which correspond to a fitted amplitude of less
than 25 particles or with a statistical error above 30%
are not shown. Points below pT= 0.3GeV/c are not
reported at all in order to avoid thedE
(squares), K±(circles), p and ¯ p (triangles). The top, cen-
ter and bottom row of plots correspond to Pb+Pb collisions
with centrality ranges of 0-5%, 12.5-23.5% and 33.5-80% of
the inelastic cross section (see Table II). The decay loss for
π±and K±is included in the inefficiencies.
(Color online) Detection inefficiencies for π±
dxacceptance feed-down feed-down quadratic
TABLE IV: The list of estimated systematic errors and their
sources on the
A.Charged Hadron Spectra
The resulting hadron spectra are shown in Fig. 8.
Preliminary versions, without the discussed corrections
have been shown in [25, 26, 27]. The new spectra show
very good agreement with earlier low pTPb+Pb results
[24, 28]. Also shown in Fig. 8 (lowest panel) are our
π+,π−spectra measured in p+p interactions .
FIG. 8: (Color online) Invariant yields per inelastic collision
of π±(squares), K±(circles) and p, ¯ p (triangles) versus trans-
verse momentum pTat midrapidity and√sNN= 17.3GeV
energy. Results are shown for Pb+Pb collisions with central-
ity ranges of 0-5% (top row), 12.5-23.5% (second row) and
33.5-80% (third row) of the inelastic cross section (see Table
II). Previously published results for central collisions in the
lower pTrange [24, 28] are indicated by the bands in the top
row. The bottom row shows published NA49 results  for π+
(left) and π−(right) production in inelastic p+p collisions.
B.Nuclear Modification Factors
The nuclear modification effects are measured by the
nuclear modification factors, which are particle yield ra-
tios of two reactions, with appropriate scaling factors.
The nuclear modification factors of a reaction A1+ A2
relative to A3+ A4are most generally defined as:
?NBC(A1+ A2)?·yield(A1+ A2)
?NW(A1+ A2)?·yield(A1+ A2)
The ratio RA+A/p+pis in the following abbreviated by
the commonly used RAA, while for Rp+A/p+pthe nota-
tion RpA will be used. The scaling factors ?NBC? and
?NW? are the average numbers of binary collisions and
of wounded nucleons, respectively, which are calculated
according to the Glauber model by Monte Carlo methods
which simulate the geometrical configurations. The two
different kinds of scaling assumptions are motivated by
extreme scenarios of particle production. A pQCD-like
particle production scheme would imply a binary collision
scaling (particles are produced in binary parton-parton
collisions). However, a scenario of a superposition of soft
collisions (e.g. particle production via projectile excita-
tion and decay) would rather suggest the scaling of par-
ticle production with the number of wounded nucleons,
as proposed in e.g.  (each collision increases the en-
ergy content of the excited nucleon). As the particle pro-
duction scheme has not been completely determined at
SPS energies, especially below 2GeV/c transverse mo-
mentum, both extreme scaling assumptions are exam-
To verify the presence or absence of nuclear effects, we
calculate the modification factors RAAand RCP (A+A
central/peripheral) for hadrons using our particle spectra
at√sNN= 17.3GeV. We compare our results to those
obtained at√sNN= 200GeV, measured by the PHENIX
experiment at RHIC [2, 4]. As the production of π+and
π−is already very similar at our energy, we only show
the average of π±for charged pions. The heavier parti-
cle yields (p, ¯ p, K+,K−) proved not yet to be symmetric
with respect to charge conjugation at our energy and we
show them separately. The antiparticle/particle asym-
metry of our spectra can be seen in Fig. 9.
is a maximally isospin-asymmetric system, the π−/π+
ratio is slightly below one. Pb+Pb is less asymmetric
in isospin, thus its π−/π+production ratio is closer to
unity. The isospin argument does not hold for the case
of K−/K+and ¯ p/p ratios, which are strongly influenced
by the net-baryon density.
The nuclear modification factor RAAis shown in Fig.
10 for the π±channel at top SPS and RHIC collision
energies. The RHIC data are compared to RdA at the
same center-of-mass energy, while for the SPS data RpA,
¯ p/p × 6
FIG. 9: (Color online) Ratios between antiparticle and par-
ticle yields versus transverse momentum pTat midrapidity
and√sNN= 17.3GeV energy. Results are shown for π−/π+
(squares), K−/K+(circles) and ¯ p/p (scaled up by a factor
6, triangles). Plots correspond to Pb+Pb collisions with cen-
trality ranges of 0-5% (top left), 12.5-23.5% (top right) and
33.5-80% (bottom left) of the inelastic cross section (see Table
II). Ratios from published NA49 results on π−and π+yields
in p+p collisions  are plotted in the bottom right panel.
measured at√sNN= 19.4GeV , is shown in addition.
Since the RpA factors exhibit only a weak energy de-
pendence for 0 ≤ pT< 4.5GeV/c in the energy range
19.4 ≤√sNN≤ 27.4GeV , the comparison to the
√sNN= 17.3GeV data is to a certain extent justified.
However, the shrinking of the available momentum space
region with decreasing collision energy makes this extrap-
olation in energy somewhat uncertain, as the modifica-
tion factors are expected to behave singularly close to the
momentum space kinematic limit.
The main observations with the binary collision scaling
assumption are the following. Both the RAA and the
RpA data show a similar low pTincrease at√sNN=
17.3GeV and√sNN= 200GeV, up to pT≤ 1GeV/c.
The RpAratios rise above 1 at both collision energies with
similar slopes, showing an excess of particle yield, which
is often called Cronin effect. The 17.3GeV RAA data
keeps rising approximately linearly up to 2GeV/c, where
the statistics of the p+p reference spectrum runs out. In
the case of 200GeV RAAdata, the ratio starts to decrease
in this region, showing a large suppression both relative
to the constant 1 line and to the RpAratio (the “Cronin
baseline”). The evolution of the 17.3GeV data at pT>
2GeV/c is not clear, because the available experiments
√sNN= 17.3 GeV (NA49)
√sNN= 200 GeV (PHENIX)
midrapidity in central nucleus+nucleus and inelastic nu-
cleon+nucleon collisions scaled by the number of binary col-
lisions (RBC, top row) and number of wounded nucleons
(RW, bottom row).The left column shows results from
NA49 at the SPS (√sNN= 17.3GeV) for the ratio of cen-
tral (0-5%) Pb+Pb collisions to inelastic p+p reactions (filled
squares) compared to the ratio of p+W to p+p collisions at
√sNN= 19.4GeV  (the dotted line is shown to guide the
eye).The right column shows measurements from RHIC
(√sNN= 200GeV) [2, 4] for the ratio of central (0-5%)
Au+Au collisions (open squares) and d+Au reactions (open
triangles). (The error bars attached to the constant 1 line
indicate normalization uncertainty.)
(Color online) Ratios of charged pion yields at
on p+p have no statistics beyond pT= 2GeV/c. In the
covered pTregion, the RAA ratio stays below the RpA
Cronin baseline also at 17.3GeV.
When assuming wounded nucleon scaling, there are
some differences compared to the above picture. Namely,
the modification factors start from one at both energies,
and show a rapid increase with similar slopes at the be-
ginning. The RpA ratios stay below the RAA result as
opposed to the binary collision case. The 200GeV RAA
data show a tendency of returning to the constant 1 line
at pT≈ 3GeV/c.
A widely believed interpretation of the Cronin effect
is momentum transfer from the longitudinal degrees of
freedom to the transverse degrees of freedom by multi-
ple scattering of partons or hadrons, depending on the
picture. However, then this effect must also be present
in A+A collisions, and may mask other nuclear effects.
On the other hand the nuclear modification factor RCP
would not contain most of the contribution of the Cronin
effect. The nuclear modification factor RCP is shown in
Fig. 11 for different particle types at top SPS and RHIC
collision energies. When assuming binary collision scal-
ing, the π±ratios show an amazing similarity at the two
energies at pT≤ 2GeV/c, above which the 17.3GeV ra-
tio seems to stay constant (below unity), and the 200GeV
ratio begins to show a large suppression. The p ratios
seem to saturate at pT≥ 2GeV/c at both energies and
are almost identical over the whole pTrange. The ¯ p ra-
tios differ at the two energies. The K+,K−results are
very similar at the two energies in the common pTrange.
In fact, the NA49 K±results cover a slightly larger pT
interval than the published PHENIX measurements.
When using wounded nucleon scaling, the modification
factors start from one, and show a rapid increase. The
π±,K±modification factors show differences between the
top SPS and RHIC collision energies. The difference of ¯ p
results is even larger. The p modification factors at top
SPS and RHIC energies, however, are quite similar.
In Fig. 12, the available RCP measurements for π±
and π0at SPS and RHIC are compared [2, 6, 30]. It is
seen that the agreement between the result for π±from
NA49 and π0from WA98 for pT> 0.8GeV/c is quite
good within errors. The difference between the PHENIX
π±and π0measurements is larger and outside errors.
In conclusion, RCP ratios for identified charged parti-
cles scaled by the number of binary collisions are remark-
ably similar at SPS and RHIC energies. Our present
results at the SPS only reach the medium pTregion,
where soft production processes and the quark coales-
cence mechanism  may still dominate. It remains to
be tested by future experiments, having access to an ex-
tended pTrange, whether energy loss of hard scattered
partons in dense matter is also a significant process at
Invariant yields of π±, p, ¯ p, K±particles were mea-
sured in Pb+Pb reactions at√sNN= 17.3GeV collision
energy around midrapidity as a function of transverse
momentum up to 4.5GeV/c, with overall systematic er-
rors below ≈ 5%. Using these spectra and the previously
published π±yields in p+p collisions, the nuclear mod-
ification factor RAA for π±and RCP for π±, p, ¯ p, K±
The extracted RAA ratio, scaled with the number of
binary collisions, shows a rapid rise with transverse mo-
mentum in the covered pTregion and does not show the
strong suppression observed at RHIC. Interestingly, the
RCP ratios for π±,K±, and p in the available pTregion
stay rather close to the√sNN= 200GeV RHIC results.
In fact, RCP ratios for p are almost identical. However,
although the π±ratio stays below one also at the SPS, it
shows a much larger suppression at RHIC above 2GeV/c
transverse momentum. When scaled with the number
of wounded nucleons, the modification factors RAAand
0 0.51 1.522.53
0 0.51 1.52 2.53 3.5
FIG. 11: (Color online) Ratio of yields in central to peripheral
Pb+Pb and Au+Au collisions at√sNN= 17.3GeV (squares)
and 200GeV  (triangles). Left column shows ratio RBC
using binary collision scaling, right column shows ratio RW
using wounded nucleon scaling. Centrality intervals are spec-
ified in the legend of the top left panel. (The error bars at-
tached to the constant 1 line indicate normalization uncer-
Pb+Pb(33−80%)π± √sNN= 17.3GeV (NA49)
Pb+Pb(43−88%)π0 √sNN= 17.3GeV (WA98)
Au+Au(30−92%)π± √sNN= 200 GeV (PHENIX)
Au+Au(30−92%)π0 √sNN= 200 GeV (PHENIX)
FIG. 12: (Color online) Comparison of nuclear modification
factors for π±and π0using binary collision scaling (RBC
top) and wounded nucleon scaling (RW
at the SPS (√sNN= 17.3GeV) in Pb+Pb collisions from
NA49 on π±are shown by squares, from WA98 on π0
by circles. Measurements from RHIC (√sNN= 200GeV) in
Au+Au collisions for π± are plotted as triangles, for π0
 as diamonds. (The error bars attached to the constant 1
line indicate normalization uncertainty.)
CP, bottom). Results
RCP start from one and show a rapid rise with pT, fur-
thermore the RpAratio stays below RAAin the covered
pTregion for both SPS and RHIC collision energies.
The pTrange of our modification factor measurements
is limited by the statistics of the p+p and peripheral
Pb+Pb collision reference spectra. A recently started
second generation experiment, CERN-NA61 [32, 33, 34],
will make it possible to extract reference spectra up to
higher pTof 4GeV/c in p+p and p+Pb collisions match-
ing the pTrange of the currently available central Pb+Pb
data. These future measurements will allow a more sen-
sitive test for the presence of high pTparticle suppression
at SPS energy.
This work was supported by the US Department of En-
ergy Grant DE-FG03-97ER41020/A000, the Bundesmin-
isterium fur Bildung und Forschung, Germany, the Vir-
tual Institute VI-146 of Helmholtz Gemeinschaft, Ger-
many, the Polish State Committee for Scientific Research
(1 P03B 006 30, 1 P03B 097 29, 1 PO3B 121 29, 1 P03B
127 30), the Hungarian Scientific Research Fund (OTKA
68506), the Polish-German Foundation, the Korea Sci-
ence & Engineering Foundation (R01-2005-000-10334-0),
the Bulgarian National Science Fund (Ph-09/05) and
the Croatian Ministry of Science, Education and Sport
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