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The dependence of the anomalous J/ψ suppression on the number of participant nucleons
M C. Abreu, B. Alessandro, C. Alexa, R. Arnaldi, M. Atayan, C. Baglin, A.
Baldit, M. Bedjidian, S. Beole, V. Boldea, et al.
To cite this version:
M C. Abreu, B. Alessandro, C. Alexa, R. Arnaldi, M. Atayan, et al.. The dependence of the anomalous J/ψsuppression on the number of participant nucleons. Physics Letters B, Elsevier, 2001, 521, pp.195- 203. �in2p3-00010537�
CERN{EP/2001{069
October11,2001
The dependence of the anomalous J/ suppression
on the number of participant nucleons
M.C.Abreu 6;a)
, B.Alessandro 10)
,C. Alexa 3)
,R.Arnaldi 10)
,M. Atayan 12)
,C. Baglin 1)
,
A. Baldit 2)
,M. Bedjidian 11)
,S.Beole 10)
,V.Boldea 3)
,P.Bordalo 6;b)
,G. Borges 6)
A. Bussiere 1)
,L.Capelli 11)
,J.Castor 2)
,C. Castanier 2)
,B. Chaurand 9)
,I. Chevrot 2)
,
B.Cheynis 11)
,E. Chiavassa 10)
,C. Cicalo 4)
,T.Claudino 6)
,M.P.Comets 8)
,N.Constans 9)
,
S. Constantinescu 3)
,P.Cortese 10;c)
,N. DeMarco 10)
,A. DeFalco 4)
, G.Dellacasa 10;c)
,
A.Devaux 2)
,S.Dita 3)
,O. Drapier 9)
,L.Ducroux 11)
,B. Espagnon 2)
,J. Fargeix 2)
,
P.Force 2)
,M. Gallio 10)
,Y.K.Gavrilov 7)
,C. Gerschel 8)
,P.Giubellino 10)
,M.B.Golubeva 7)
,
M. Gonin 9)
,A.A. Grigorian 12)
, S.Grigorian 12)
,J.Y. Grossiord 11)
,F.F. Guber 7)
,
A.Guichard 11)
,H.Gulkanyan 12)
,R.Hakobyan 12)
,R.Haroutunian 11)
,M. Idzik 10;d)
,
D. Jouan 8)
,T.L. Karavitcheva 7)
,L.Kluberg 9)
,A.B.Kurepin 7)
,Y. Le Bornec 8)
,
C. Lourenco 5)
,P.Macciotta 4)
,M. MacCormick 8)
,A. Marzari-Chiesa 10)
,M. Masera 10)
,
A.Masoni 4)
, M. Monteno 10)
,A.Musso 10)
,P.Petiau 9)
,A. Piccotti 10)
,J.R.Pizzi 11)
,
F.Prino 10)
,G. Puddu 4)
,C. Quintans 6)
,S.Ramos 6;b)
,L.Ramello 10;c)
,P.Rato Mendes 6)
,
L.Riccati 10)
,A.Romana 9)
,H.Santos 6)
,P. Saturnini 2)
,E.Scalas 10;c)
,E. Scomparin 10)
,
S.Serci 4)
,R.Shahoyan 6;e)
,F. Sigaudo 10)
,S.Silva 6)
,M. Sitta 10;c)
,P.Sonderegger 5;b)
,
X.Tarrago 8)
,N.S. Topilskaya 7)
,G.L. Usai 4)
,E.Vercellin 10)
,L.Villatte 8)
,N.Willis 8)
.
NA50 Collaboration
Abstract
The observation of an anomalous J/ suppression in Pb-Pb collisions by the NA50
collaboration can beconsideredas themoststrikingindication forthedeconnement
ofquarksandgluonsatSPSenergies. Inthisletter,we determinetheJ/ suppression
patternasafunctionoftheforwardhadronicenergyE
ZDC
measured ina ZeroDegree
Calorimeter (ZDC). The direct connection between E
ZDC
and the geometry of the
collisionallowsustocalculate,withinaGlauberapproach,thepreciserelationbetween
thenumberofparticipantnucleonsN
part andE
ZDC
. Then,wecheckiftheexperimental
data can be better explained by a sudden or a smooth onset of the anomalous J/
suppressionasa functionof thenumberofparticipants.
Accepted by Phys. Lett. B
LAPP,CNRS-IN2P3, Annecy-le-Vieux,France.
2)
LPC,Univ. BlaisePascaland CNRS-IN2P3,Aubiere, France.
3)
IFA, Bucharest, Romania.
4)
Universita diCagliari/INFN,Cagliari, Italy.
5)
CERN,Geneva,Switzerland.
6)
LIP,Lisbon,Portugal.
7)
INR,Moscow, Russia.
8)
IPN,Univ. de Paris-Sudand CNRS-IN2P3,Orsay, France.
9)
LPNHE,Ecole Polytechniqueand CNRS-IN2P3,Palaiseau, France.
10)
Universitadi Torino/INFN, Torino, Italy.
11)
IPN,Univ. Claude BernardLyon-Iand CNRS-IN2P3,Villeurbanne,France.
12)
YerPhI, Yerevan,Armenia.
a)
also at FCT, Universidadede Algarve, Faro, Portugal.
b)
also at IST,UniversidadeTecnicade Lisboa,Lisbon,Portugal.
c)
Universitadel PiemonteOrientale,Alessandriaand INFN-Torino,Italy.
d)
also at Facultyof Physicsand Nuclear Techniques,Academy ofMiningand Metallurgy,
Cracow, Poland.
e)
onleave ofabsenceof YerPhI, Yerevan, Armenia.
J/ suppression has been proposed along time ago as aclear and unambiguoussig-
natureforthe deconnementofquarksandgluonsinultrarelativisticnucleus-nucleus
collisions[1]. It became however clearvery soonthat alsoabsorption mechanisms at
thehadroniclevelcouldleadtoasizeableJ/ suppression[2,3]. Consequently,inthe
followingyears, the eorts onthe experimentalside were aimedatthe establishment
of a solid systematics on charmonia production with various projectile and target
combinations. High statistics data have been collected by the NA38 collaboration
both for p-A collisions [4], where no QGP-induced suppression is expected, and for
interactions induced by lightions (oxygen,sulphur) [5,6].
The study of Pb-Pb collisions at 158 GeV/c incident momentum has marked a
turningpointin the evolutionof this eld. In fact,experimental datafromNA50 on
J/ suppression as a function of the centrality estimator E
T
, the neutral transverse
energy released in the pseudorapidity window1.1<< 2.3, have shown that:
there is an additional J/ suppression mechanism in Pb-Pb collisions, not
present inS-U interactions (the so-calledanomalous suppression) [7];
theonset of theanomalous J/ suppression occurs inanarrowE
T
range,indi-
catingthe presence of a threshold eect [8];
theJ/ suppression patternshows, fromperipheraltocentralcollisions,atwo-
step behaviour, possibly linked with the successive melting in a deconned
medium of two charmonium resonances: the
c
, which, through its radiative
decay, isan important J/ source, and the more strongly bound J/ itself[9].
Even if there is still debate on the interpretation of the E
T
structure of J/
suppression[10, 11], theseresults can not be satisfactorilyreproducedby theoretical
approaches based oncharmonia dissociationby comovinghadrons [12].
Inthis letterwegoastep furtherinthe analysisof theNA50 Pb-Pbresults,with
atwo-foldpurpose. Ononesideweinvestigateifthesametwo-stepstructure,seen as
a function of E
T
, isstill present when the J/ suppression pattern is studiedversus
another centrality estimator, namely E
ZDC
, the energy released in the NA50 Zero
Degree Calorimeter. On the other side, we determine the correlation of E
ZDC with
the number of projectile spectator nucleons, and consequently with the number of
participantsN
part
,takingintoaccountthesmearingontheN
part
measurementdueto
the experimental resolution on E
ZDC
. We nally investigatethe nature of the onset
of the suppression asa function of the numberof participantnucleons.
2 Experimental set-up and data reduction
AdetaileddescriptionoftheNA50experimentalapparatuscanbefoundin[13,14,15]
and we only recall its basic features hereafter. The main component of the set-up
centrality of the interaction is estimated with three detectors: an electromagnetic
calorimeter, which measures the neutral transverse energy E
T
released in the re-
gion 1.1< <2.3, a silicon microstrip detector (MD), which allows to estimate the
chargedmultiplicityintherange1.5< <3.9, anda zerodegreecalorimeter(ZDC),
measuringtheforwardenergyE
ZDC
,mainlycarriedbyprojectilenucleonswhichhave
not taken part in the collision. This calorimeter is placed on the beam line, inside
the hadronabsorber, and covers the pseudorapidity region 6:3.
The data analyzed in this paper have been collected with the NA50 set-ups of
1996 and 1998. The dierence between the two set-ups concerns only the target
region. In1996, thetargetassemblywasmadeof7sub-targetswith atotalthickness
correspondingto30%of
I
,whilein1998asinglethinnertarget(7%of
I
)has been
employed. The average incident beam intensity was 510 7
Pb ions/burst, with a5 s
spill. Data have been collected with two dierent kinds of trigger. The main one,
called \dimuon trigger", selects events where the spectrometer detects two muons
produced inthe target region. The second one, called \minimum bias trigger",res
whenever a small amount of energy is released in the ZDC. More details on data
taking conditions can be found in [8,9].
The event selectionprocedure is very similar to the one described in [8, 9]. The
onlynotabledierenceisthatthe present analysisdoesnot require theidentication
oftheindividualsub-targetwheretheinteractionhastakenplace,aselectioncriterion
used in previous analyses and based on specic detectors located near each one of
the sub-targets. In fact, it has been realized [16] that the use of this cut, in a
ZDC-basedanalysis, may induce large systematicerrors onthe measurement of J/
suppression for peripheralevents. The possible contribution from Pb-air events has
been removed bymeans of special \empty target"runs, recorded periodicallyduring
the standard datataking by movingthe targetaway fromthe beam line. Thesize of
thiscontamination,negligibleforcentralcollisions,isoftheorderof9%forminimum
bias events with E
ZDC
>20 TeV. Since the dimuon trigger requires the two muons
topoint tothe target, and this requirementis reinforcedby oine cuts atthe single
muon level, the target-out contribution to the dimuon event sample is obviously
smallerand amountsto only2.5% in the same E
ZDC
range.
Apart from Pb-air interactions, the minimum bias sample also contains events
corresponding to non-interacting Pb ions. To eliminate such events, a very low E
T
cut has been imposed on the data. The cut has been tuned in order to retain in
the event sampleused forthe analysis anumberof events correspondingtothe total
hadronic Pb-Pb cross section, calculated inthe frame of the Glauber model.
Finally,for dimuon triggerswe have imposed the cuts 2:92 y
l ab
3:92 (corre-
spondingto0 y
cm
1)and jcos
CS
j< 0:5, where
CS
isthe polardecay angleof
one muon in the Collins-Soper reference frame, in order to remove events produced
atthe edgesof the acceptanceof the spectrometer.
ZDC
The geometry of a nucleus-nucleus collision is usually characterized by the value of
the impactparameter b,which ofcourse isnot directly accessible tothe experiment.
Nevertheless, this quantity can be deduced from the measured E
ZDC
value. In fact,
being placed on the beam line, the ZDC intercepts all the projectile spectator nu-
cleons. These particles are not aected by the collisionand reach the detector with
their initial energy. Any loss of spectators is prevented by choosing for the ZDC
an angular acceptance larger than the transverse spread induced by Fermi motion.
Hence a smallamount of zero degree energy corresponds to a smallnumber of pro-
jectile spectator nucleons, and therefore to a central collision; on the contrary, in a
peripheralcollision only fewnucleonsundergo aninteraction, the number of specta-
tors islarge and alarge amount ofenergy isreleased inthe ZDC.In amore detailed
approach,oneshouldtakeintoaccountthatalsosomeparticipantnucleonsplussome
secondary particlescan beemittedwithin the angularacceptanceof the calorimeter,
contributingto E
ZDC
. However, it willbe shown that this contribution is negligible
for most events and becomes sizeableonly for very centralcollisions.
From the previous considerations, we can express, for a generic Pb-Pb collision
with an impact parameter b, the average E
ZDC
as a sum of two terms, a dominant
one, E spec
ZDC
(b), proportionaltothe number of projectilespectators N
spec
, plus asmall
contributionE part
ZDC
(b) proportionaltothe number of participants N
part :
hE
ZDC
(b)i = E spec
ZDC
(b)+E part
ZDC
(b)=158N
spec
(b)+N
part (b)
= 158 208 N
part (b)
2
!
+N
part
(b) (1)
where 158 GeV is the energy per spectator nucleon and N
part
is the energy
released inthe ZDCby participantsand secondary particles. The linkbetween N
part
(or N
spec
) and the impact parameter b has been obtained with a calculation based
onaGlaubermodelofnucleus-nucleuscollisions,usingWoods-Saxonnucleardensity
proles,with the parameters tabulated in[17]. The same N
part
versus b dependence
hasbeenreproducedusingtheVENUS4.12[18]andRQMD2.3[19]eventgenerators.
Eq. 1 gives the average value of E
ZDC
for a given b. In order to describe the
measuredE
ZDC
spectrawemusttakeintoaccountthatforagiven impactparameter
b,becauseoftheexperimentalresolutionofthedetectorandofuctuationsonN
part at
xed b,the valuesof E
ZDC
are gaussiandistributed. Thewidth
E
ZDC
(b)is expressed
asthe quadraticsum:
E
ZDC
(b)=( q
E
ZDC
(b)+E
ZDC
(b))Æ
Npart
(b) (2)
The and parameters arerelatedtothe resolutionof thedetector and theirvalues
have been xed with measurements done with low intensity proton and ion beams
[14]; the Æ term takes into account the smearing of the signal due to the pedestal
width and to calibration uncertainties. The quantity
N
part
(b) represents the size
ZDC
betweenbandN
part
. Ithasbeenestimatedthroughasimulation,usingeitherVENUS
or RQMD as inputs, with fully compatible results. The values of the remaining
parameters have been xed by means of a t to the measured minimum bias E
ZDC
distribution. We get, for the 1996 data sample, the values =5.67 GeV, =3.39
GeV 1=2
, =0.062, Æ =1227 GeV. In Fig. 1 we plot the E
ZDC
spectrum, for the
1996 data sample,together with our result based onthe Glaubermodel, shown as a
continuous line. The fair agreement between the data and our calculation allows us
to conclude that this approach reproduces reasonably well the connection between
E
ZDC
and the geometry of the collision.
As discussed before, the parameter, which parametrizes the size of the partic-
ipantcontribution to E
ZDC
, is directly tted on the measured data. However, event
generatorsgivequantitativepredictions forthis contribution. Therefore wehavealso
run a Monte-Carlo simulation to directly determine an E
ZDC
spectrum where the
energy carried by participants is included at the generation level, through VENUS
and RQMD. Afterintroducingthe smearingdue tothe detector'sresolution we have
comparedthe simulatedspectrumwith our experimentaldata and with the resultof
the Glaubercalculation (see Fig. 1). The qualitative agreement between the various
approaches conrms that the participant contributionis properly taken intoaccount
in our model. Quantitatively, it is almost negligible for most centralities, since it
accountsfor less than 10% ofE
ZDC for E
ZDC
values larger than12TeV.On the con-
trary, it is sizeable for very central collisions, reaching 50% of the measured E
ZDC
when E
ZDC
3 TeV.
HavingsuccessfullydescribedthemeasuredE
ZDC
spectrum,wecandeterminethe
distribution of the various centrality variables as a function of the measured E
ZDC .
Weplot inFig.2the distributionofN
part
versus E
ZDC
, whichisparticularlyrelevant
for the J/ suppression analysis described hereafter.
4 Study of the J/ suppression
Asinthecase oftheanalysisasafunctionof theneutraltransverse energy,twocom-
plementarytechniques, the so-called\standard analysis" and \minimum-biasanaly-
sis" have been used to study the J/ suppression pattern versus E
ZDC
. They have
been explained indetail in previouslypublished papers [8,9].
Very shortly, in the standard analysis the high-mass Drell-Yan (DY) events are
directly used as a reference for the J/ suppression study. The ratio
J=
=
DY is
obtained by means of a t to the measured invariant mass spectrum. The shapes
ofthe invariantmass distributionsfor thevariousphysicalprocesses are obtained by
meansof a Monte-Carlosimulation. In the minimumbias analysis we use instead as
a reference the much larger sample of minimum bias (MB) triggers. To be able to
comparetheresultsofthetwoanalyses,a\calculatedDY"(DY
)isobtainedstarting
from the measured minimum bias reference. In particular, we calculate, using the
MB yields as a function of E
ZDC
. Multiplying this quantity, for each E
ZDC
bin, by
the measured number of MB events, we get the DY
distribution. In this second
approachthenumberofJ/ eventsforeachE
ZDC
binisobtained bysimplycounting
the events in the mass range 2.9 M 3.3 GeV/c 2
, after subtraction of the small
dimuon continuum contribution.
The result of the standard analysis for the 1996 and 1998 data sets is shown in
Fig.3. Forthe1998sample,becauseoftheverythinsingletarget,the contamination
due to Pb-air events is important for peripheralevents [9]. Forthis reason, we have
limitedtheanalysisofthe1998datatotheregionE
ZDC
<22TeV.Thecontinuousline
inFig.3representsthevalueof
J=
=
DY
expectedincaseofpurenuclearabsorption.
Ithas been obtainedassumingfor theinteraction cross sectionofthe pre-resonantcc
pair that will formthe J/ [20] the value
abs
=6.4mb[6]. This value results from
aGlauberanalysis ofJ/ productioninp-A andS-Ucollisions,wherenoanomalous
suppression ispresent [6].
The results of the standard analysis, even if almost free from systematic errors,
are aected by large error bars, due to the lowDrell-Yanstatistics. This problem is
solved introducing the minimum bias analysis. First, as anintermediate step in the
presentationoftheresults,weplotinFig.4theJ/ distributionversusE
ZDC
directly
dividedbythe measuredminimumbiassample,forthe1996 and1998datasets. The
comparisonofthedatawiththecontinuouslinerepresentingordinarynuclearabsorp-
tion reveals two clear features: a departure fromthe absorption curve atE
ZDC 26
TeV, and a change of slope in the region around E
ZDC
=10 TeV. Furthermore, the
two sets of data show a discrepancy for very central events. This problem, already
discussed in [9]for the E
T
analysis, isconnected with the use, inthe 1996 set-up,of
arelatively thick target. In this situation,itis known that possiblereinteractions of
projectile fragments inthe target could bias the centrality measurement, leading to
asystematic error for centralevents. On the contrarythese eects are negligiblefor
the thin target used inthe 1998 set-up.
InFig.5weshowtheresultoftheminimumbiasanalysisversusE
ZDC
. Forclarity,
in this plot the 1996 data set is used only down to E
ZDC
=9 TeV, i.e. in the region
wherethebiasinducedby reinteractions isnegligible. Theabsolutenormalizationfor
the ratio
J=
=
DY
has been calculated using the results of the standard analysis,
in the range 9 E
ZDC
16 TeV. In Fig. 6 we plot the same ratio, divided by the
normal absorption curve. The results presented in Fig. 5 and 6 exhibit the same
features already visiblein the (J/ )/MB ratio (Fig.4), namely two clear changes of
slope. TherstonecorrespondstotheE
ZDC
regionaround26TeV,whereweobserve
a departure from the trend of the normal absorption curve, while the second one is
visiblein the zone correspondingto the most centralcollisions.
A suppression pattern with the same characteristics has already been obtained
from the analysis of the J/ yield as a function of E
T
[9]. The result presented
hereas afunction ofE
ZDC
thereby conrms the two-step structure ofthe anomalous
J/ suppression in Pb-Pb collisions. A quantitativecomparison of the results of the
