Measurements of inclusive J/psi production in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV with the ALICE experiment

0.63 MB·English

by  Antonin Maire

y
Measurements of inclusive J/ production in Pb–Pb
collisions at √s = 2.76 TeV with the ALICE
3 NN
1
experiment
0
2

n
a
J

7
1 AntoninMaire ,for the ALICECollaboration
∗

PhysikalischesInstitut-Heidelberg,Germany
]
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E-mail:[email protected]
e
-
p
Charmonium is a prominent probe of the Quark-Gluon Plasma (QGP), expected to be formed
e
h in ultrarelativisticheavy-ion(A–A) collisions. Ithas beenpredictedthat the J/y (cc) particleis
[
dissolvedin the deconfinedmediumcreated in A–A systems. Howeverthis suppressioncan be

1 counterbalancedvia regenerationof the charm/anti-charm boundstate in QGP or via statistical
v productionatthephaseboundary. AtLHCenergies,thelattermechanismsareexpectedtoplay
8
amoreimportantrole,duetoacharmproductioncrosssectionsignificantlylargerthanatlower
5
0 energies.
4 MeasurementsobtainedbytheALICEexperimentforinclusiveJ/y productionareshown,mak-
.
1 inguseofPb–Pbdataat√s =2.76TeV,collectedin2010and2011.Inparticular,thefocusis
NN
0
givenonthenuclearmodificationfactor,R ,derivedforforward(2.5<y<4)andmidrapidities
3 AA
(y <0.9),bothdowntozerotransversemomentum(p ). Thecentrality,yand p dependences
1 | | T T
: ofR arepresentedanddiscussedinthecontextoftheoreticalmodels,togetherwithPHENIX
v AA
i andCMSresults.
X
r
a
Xth QuarkConfinementandtheHadronSpectrum
8–12October2012
TUMCampusGarching,Munich,Germany
Speaker.
∗
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
InclusiveJ/y productioninPb–Pbcollisionsat√s =2.76TeVwithALICE AntoninMaire
NN
1. Introduction : studyingJ/y production inultrarelativisticheavy-ioncollisions
Ultrarelativistic heavy-ion (A–A) collisions are expected to give access to a medium where
quarksandgluonsaredeconfined,theso-calledQuark-GluonPlasma(QGP).Inordertotestexperi-
mentallythedeconfinementofthecreatedmedium,severalsignatureshavebeenproposed. Among
them,themeltingofJ/y withintheQGP[1]. Theideaisthatcolourscreening inthemediumpre-
vents the bound states cc to form. This results in a J/y production suppressed in A–A relative to
thecorrespondingproductioninreferencesystems,likeproton-proton(pp)orproton-nucleus(p–A)
collisions(hadroniccollisionswithoutQGP).TheideadevelopedforJ/y canactuallybeextended
to other charmonium and bottomonium states. The resulting suppression hierarchy is expected to
depend on the binding energy of the considered quarkonium state. This leads to the notion of the
sequential melting [2] : the states that are more weakly bound may dissociate in the deconfined
medium from lower temperatures on ("T [y (2S)].T [c ]<T [J/y ] T [¡ (2S)]<
disso disso cJ disso ≈ disso
T [¡ (1S)]").
disso
However, with experiments moving towards higher A–A collision energies (√s ), possible
NN
scenarios for recombination of c with c¯ into charmonia must be considered [3]. Indeed, the total
production of charm and anti-charm quarks becomes more and more abundant with √s and, if
NN
recombination may appear non-negligible already at RHIC energies, it is expected to be sizeable
at the LHC. There are basically two complementary approaches to describe the (re)generation of
charmonium :
thermal models : charm quarks are thermalised in the QGP and quarkonia are produced at
•
the phase boundary via statistical hadronisation. The implicit hypothesis is that cc is fully
dissolved inthemedium;thepossibility tobindqwithq¯canonlyhappenatthelaststageof
thecollision [4].
transport models : the binding and dissociation of cc take place directly in the QGP, the
•
competition being described by a transport equation. In other words, a partial survival of
charmonium iscomplemented byacontinuous regeneration [5–7].
2. Data analysisand experimental remarks
2.1 Detectorsetupanddatacollection
TheJ/y analysespresentedintheseproceedingsmakeuseofbothdi-leptondecaychannelsof
J/y (see Tab.1). Theanalysisatmidrapidity(|yJ/y |<0.9)isbasedonthee+e− channel. Ithinges
onthecentral-barrel detectorsoftheALICEexperiment[8] : theInnerTrackingSystem(ITS)and
TimeProjectionChamber(TPC),fortrackingandtheTPCandtheTimeOfFlightdetector(TOF),
to identify electrons and partially remove contamination by other particle species. The studies at
forward rapidity (2.5 < y < 4) profit from the m +m channel. They rely on detectors placed
J/y −
beyondanabsorber, i.e.muon-tracking andmuon-trigger chambers.
In both cases, the analysed data samples are Pb–Pb collisions at √s = 2.76 TeV. For mid
NN
rapidity, thisisacombination ofdatacollected in2010(L 2m b 1)and2011(L 13 m b 1,
int − int −
≈ ≈
i.e. about 60 % of Pb–Pb data collected that year). For forward rapidity, this is exclusively data
takenin2011(L 70 m b 1).
int −
≈
2
InclusiveJ/y productioninPb–Pbcollisionsat√s =2.76TeVwithALICE AntoninMaire
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2.2 Remarks: low-p reachandinclusiveJ/y measurement...
T
ItmustbenotedthattheALICEacceptanceenablestheJ/y identificationdowntozerotrans-
verse momentum (p ) in both rapidity ranges, i.e. in the phase space region where the bulk of
T
charmonium isproduced. Thisisaunique featureoftheexperiment forwhatconcerns A–Aatthe
LHC.
Particles mass(GeV/c2) c.t orwidth decaychannel B.R.
J/y e+e 5.94%
J/y (cc) 3.097 92.9keV/c2 → −
→ J/y m +m 5.93%
−
→
pt
m c (cc) 3.415 10.4MeV/c2 c J/y (1S)g 1.17%
pro c c0 (cc) 3.511 0.86MeV/c2 c c0 →J/y (1S)g 34.4 %
← c1 c1 →
c (cc) 3.556 1.98MeV/c2 c J/y (1S)g 19.5 %
c2 c2 →
y (2S)(cc) 3.686 304keV/c2 y (2S) J/y +anything 59.5 %
→
pt B0 (db¯) 5.280 455 m m
m
pro B+ (ub¯) 5.279 492 m m B0,B±,B0s →J/y +anything 1.16%
n- B0 (sb¯) 5.367 449 m m
o s
n
Table 1: Main characteristics of particles contributingto the inclusive J/y signal [9]. The table includes
mass, decaylengthorresonancewidth,aswellasconsidered decaychannelandcorrespondingBranching
Ratio(B.R.).
The measurements presented in the following are for inclusive J/y , i.e. not only for direct
J/y production butalso J/y feddownfrom decays ofparticles withhigher mass. Aspresented in
Tab. 1,twocomponents canbedistinguished :
the prompt component for which the production vertex cannot be separated experimentally
•
from the primary interaction region; it includes J/y issued by prompt decays of c and
cJ
y (2S),togetherwithprimaryJ/y ;
the non-prompt component, originating from beauty mesons, with a decay length which is
•
large enough to allow a possible discrimination between the decay vertex and the primary
one.
Foraninclusive measurement, withaJ/y momentum ranging from p =0toabout 8GeV/c,
T
experimental data1 at TeV scale suggest that the inclusive J/y yield has the following relative
contributions: 35-55%ofdirectproduction, 25-35%from c ,10-15%fromy (2S)and10-15%
cJ
fromBmesons.
3. Experimental results : nuclearmodification factor
Thecomparison ofthe J/y production in A–A and pp is studied via the nuclear modification
factor, R ,definedasfollows:
AA
1InppattheLHCorppattheTevatron;thereisnoequivalentpicturemeasuredinA–A.
3
InclusiveJ/y productioninPb–Pbcollisionsat√s =2.76TeVwithALICE AntoninMaire
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d2NAA(J/y )/dp dy
R (p ,y,centrality classi)= i T (3.1)
AA T hTAAii d2s pp(J/y )/dpTdy
with the J/y yields in Pb–Pb at √s = 2.76 TeV as the basis of the present measurements. The
NN
study can be performed differentially as functions of collision centrality, rapidity and/or momen-
tum, depending on the collected data statistics. The values of the nuclear overlap function TAA
i
h i
arederivedfromaGlauberModel,foreachcentralityinterval,andcorrespondtoanormalisationof
Pb–Pbyieldstotheadaptednumberofbinarynucleon-nucleon collisions. Asfortheppreference,
the inclusive J/y cross sections, s pp(J/y ), are taken from [10], measured also at √s = 2.76 TeV,
forbothmidandforwardrapidity.
1.4
A
A Inclusive J/y , 0<p <8 GeV/c, Pb-Pb s =2.76 TeV
R T NN
1.2 |y|<0.9, L» 15 m b-1
2.5<y<3, L» 70 m b-1
3.5<y<4, L» 70 m b-1
1
0.8
0.6
0.4
0.2
ALICE common global sys. = – 1.9%
0
0 50 100 150 200 250 300 350 400
Æ N æ
part
Figure1: NuclearmodificationfactorR forinclusiveJ/y asafunctionofcollisioncentrality,measured
AA
bytheALICEexperimentinPb–Pbcollisionsat√s =2.76TeV,inthreerapidityintervalsrangingfrom
NN
forward to mid rapidity. Vertical error bars stand for statistical uncertainties; blue empty boxes, for sys-
tematic uncertainties; red brackets and shaded boxes, for partially correlated and uncorrelated systematic
uncertainties,respectively.Thefilledboxessittingatunityindicatetherespectiveppreferenceuncertainties.
The figure 1 shows R of inclusive J/y extracted for three rapidity ranges – from most
AA
forward to mid y – as a function of centrality, embodied here by the average number of nucleons
participating in a Pb–Pb collision, N . One can observe a weak centrality dependence of the
part
h i
suppression, for each rapidity interval, and a suppression which seems less and less pronounced
whilegoingtowardsmidrapidity. Notethat,fortheresults derivedatmidrapidity,theuncertainties
remain rather significant and dominated by systematic uncertainties related to the pp reference as
wellasthesignalextraction inPb–Pb.
ThecentralitydependenceofR inALICEcanbecomparedtoPHENIXresultsobtainedfor
AA
Au–Aucollisionsatlowerenergies(√s =0.2TeV). ThisispresentedinFig.2,wherethecharge
NN
particle density, dN /dh , at mid rapidity is used as centrality observable. The figure exhibits a
ch
centrality dependence which is clearly different between ALICEand PHENIXdata; in particular,
the 0.2 TeV R in most central collisions appears noticeably lower, in each rapidity region, than
AA
theequivalentquantityat2.76TeV. Thispatternisincontrastwiththeexpectationsbasedsolelyon
4
AAALLLIII−−−PPPRRREEELLL−−−333999000111222
InclusiveJ/y productioninPb–Pbcollisionsat√s =2.76TeVwithALICE AntoninMaire
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RAA 11..24 IAPInnHLccIllECuuNssEiiIvv XPee r (JJeP//liyyRm,,C i12n 8..a254r<<y ,(yy 2P<<024b1.,-12 P0),b <p0 pT5T>s4<N098N 1G =G2e )e2,V V.A/7c/u6c - AT egugVl loo,sb bLNaainNllt s=»sy y 07ss..0.=2= m –T–be 1-9V14.2%% RAA 11..24 |AP|yyLH||<<IEC00N..E93I,,5X Pp, ( TpbP>T-P>0R0 bLG G9es8eVN V/N(c2 /,c=0 L 02i n 7.t 7 g)» 6l2 o1 3Tb5e2a mV3l b0s-y11s),. =A –u -1A2u% sNN = 0.2 TeV
1 1
0.8 0.8
0.6 0.6
0.4 0.4
0.2 0.2
0 0
0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400
dNch/dh (cid:231)h =0 dNch/dh (cid:231)h =0
(a) (b)
Figure 2: Centrality dependence of ALICE R (inclusive J/y ) at (a) forward rapidity, compared with
AA
equivalentPHENIXresults[11]and(b)midrapidity,comparedwithcorrespondingPHENIXresults[12].
1.4 1.4
RAA 1.2 ASInLtcaIlCtu. sEHiv aPedr Jero/lyinmi,z i2na.at5iro<yn,y P<M4bo,- dP0eb<l p (TsA<N.8N A =Gn de2rV.o7/n6ci c T e & V ga, lolL.b, » aJ P7l s0Gy m s3b.8=-1 –(2 1041%1) 124081) RAA 1.2 ASALLtaIICCt. EEH ,aP |dyrer|o<lin0mi.z9ina,a tpiroTyn>, 0PMbo-dPebl ( A.s ANnN d=r o2n.7ic6 & T eaVl.,, LJ P»G 1 53 8m b(2-1011) 124081)
Transport Model (X. Zhao & al., NPA 859 (2011) 114) Transport Model (X. Zhao & al., NPA 859 (2011) 114 & priv. comm.)
Transport Model (Y.-P. Liu & al, PLB 678 (2009) 72) Transport Model (Y.-P. Liu & al, PLB 678 (2009) 72)
Shadowing+comovers+recombination (E. Ferreiro, priv.comm.) Shadowing+comovers+recombination (E. Ferreiro, priv.comm.)
1 1
ds /dy=0.4 mb
cc
0.8 ds cc /dy=0.25 mb 0.8
ds /dy=0.3 mb
cc
0.6 0.6
0.4 0.4
ds /dy=0.15 mb
cc
0.2 0.2
0 0
0 50 100 150 200 250 300 350 400 0 50 100 150 200 250 300 350 400
ÆN æ ÆN æ
part part
(a) (b)
Figure 3: ALICE centrality dependence for R (inclusive J/y ) compared with several models (thermal
AA
[13],transport[7,14]andco-movers[15]),at(a)forwardrapidityand(b)midrapidity.
the colour screening model, for which higher energy density (higher dN /dh ) implies a stronger
ch
suppression.
This trend indicating less suppression at the LHC can be explained by several models, as
plotted in Fig. 3 : whether it deals with statistical hadronisation [13], transport [7, 14] or co-
mover[15]models, afairagreement seemstobefound between LHCdataandphenomenological
descriptions. It should be noticed that every model incorporates a significant fraction2 of c and c¯
recombination for most central collisions. Any further interpretation and discrimination between
models is precluded by uncertainties, on data, but also on models. For instance, all of them are
almost equivalently affected by the uncertainty of the total c,c¯ production cross section, i.e. the
absence ofaccuratemeasurementscoveringopencharm(D0,D+,D ,D+ mesonsandL + baryon)
∗ s c
andhidden charm(thewholecharmonium family)overawideregionofphasespace.
2Byconstruction,itis100%inthecaseofstatisticalmodels.
5
AAAAAAAAAAAALLLLLLLLLLLLIIIIIIIIIIII−−−−−−−−−−−−PPPPPPPPPPPPRRRRRRRRRRRREEEEEEEEEEEELLLLLLLLLLLL−−−−−−−−−−−−111333333333666999666777333333666222555777333666111444777999
InclusiveJ/y productioninPb–Pbcollisionsat√s =2.76TeVwithALICE AntoninMaire
NN
1.4 1.4
RAA 1.2 AInLcIlCusEiv Per eJ/liym, inceanryt,r aPlibty-P 0b% -9s0N%N =, 22..57<6y T<e4V , L i n t » g7l0o bmabl- 1sys.= – 7% RAA 1.2 ALICE Preliminary, Pb-Pb sNN = 2.76 TeV, L int » 70 mb-1
CMS (JHEP 1205 (2012) 063), Pb-Pb sNN = 2.76 TeV, L int » 7.3 mb-1 Inclusive J/y , centrality 0%-90%, 2.5<y<4 global sys.= – 7%
Inclusive J/y , centrality 0%-100%, 1.6<|y|<2.4 global sys.= – 8.3%
1 1
Transport Model (X. Zhao & al., NPA 859 (2011) 114)
Total with shadowing
0.8 0.8 Primordial J/y
Regenerated J/y
Total without shadowing
0.6 0.6 Primordial J/y
Regenerated J/y
0.4 0.4
0.2 0.2
0 0
0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8
p (GeV/c) p (GeV/c)
T T
(a) (b)
Figure 4: p dependence of ALICE R (inclusive J/y ) at forward rapidity, integrated over all collision
T AA
centralities(a)comparedwithCMS[16]and(b)comparedtoatransportmodel[7].
With the current data, a last item to complement the description of R (inclusive J/y ) can
AA
be added though. The figure 4 displays the p dependence of the nuclear modification factor,
T
for the forward rapidity region, in a wide centrality interval (0-90%). The resulting differential
measurements spanfrom0to8GeV/candoverlapathigh p withthecorresponding result(under
T
similarcentrality andrapidityselections) bytheCMScollaboration [16](Fig. 4a). Lookingatthe
detailedcomparisonwithonetransportapproach[7](Fig.4b),dataseemtobecorrectlyreproduced
by theory. Based on the given model, the agreement suggests that J/y regeneration phenomena
mayessentiallyvanishfrom5-6GeV/con,whereastheyshouldaccountforasizeablecontribution
( 50%)for p <3GeV/c. Considering recombination asamechanism preferentially workingat
≈ T
lowmomentum,thisemphasizestheimportance ofmeasuring J/y downto p =0totestthiskind
T
ofprediction.
4. Final considerations : complementing the picture
While testing the original idea by Matsui and Satz of J/y suppression in QGP, the study of
J/y inA–Acollisions atLHCenergies maybeexaminedfromtwootherangles.
On the one hand, at high p , J/y appears as the "hidden charm" piece in the puzzle of
T
identified-hadron R . Canweconfirmorinvalidate withproperaccuracyahigh-p picturelike:
AA T
R [h ]<R [D] R [promptJ/y ] R [y (2S)]<R [B J/y ]<R [¡ (1S)]
AA ± AA AA AA AA AA
≈ ≈ →
The question is the flavour dependence and hierarchy of the suppression for particles at high mo-
mentum(p >6GeV/c),rangingfromgluons(R [h ],[17])uptothehiddenbeauty(R [¡ (1S),
T AA ± AA
¡ (2S),¡ (3S)],[18])viaopencharm(R [D],[19])andopenbeauty(R [J/y fromB],[16]).
AA AA
On the other hand, at low p , studies may address the question related to thermalisation of
T
charm quarks. On this aspect, the discussion must be enriched by R measurements of other
AA
charmonium(e.g.y (2S)[20])togetherwithmeasurementofellipticflowofcharmspecies. Inthat
sense, theALICEPreliminary measurement indicating anon-zero elliptic flow(significance close
to3-s in20-40%Pb–Pbcollisionswith p (J/y )>1.5GeV/c[21])iscertainlyaprominentresult
T
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AAAAAALLLLLLIIIIII−−−−−−PPPPPPRRRRRREEEEEELLLLLL−−−−−−111111666666777888777222111333
InclusiveJ/y productioninPb–Pbcollisionsat√s =2.76TeVwithALICE AntoninMaire
NN
thatwillbeclosely followedinthefuture.
Lastbutnotleast,oneshouldconcludethediscussionwiththerelevanceofJ/y measurements
inp–Acollisions. Nothinghasbeenmentionedsofaronthestudiesrelatedtothissystemwhereas
p–AcollisionsdefineacrucialexperimentalmilestoneonthewaygoingfrompptoA–Acollisions.
Therearenon-QGPeffectsforeseen inA–Acollisions : partondistribution functionsinnucleican
be modified (lowering of the J/y production probability due to parton shadowing in the nucleus),
incidentpartoncanlooseenergyinnucleusbeforethehardscattering, ... Allthesephenomenacan
belabelled asColdNuclearMatter(CNM)effects.
InRHICdata,CNMeffectsareexpected toexplainforalargeparttheinclusive J/y suppres-
sion seen in mostcentral Au–Au collisions at √s = 0.2TeV(see forinstance Fig. 87 and 88, p.
NN
126 of [22]). At LHC energies, the same phenomena may account for a less dramatic effect but
this has to be confirmed experimentally. Forthat purpose, results from p–A data taking occurring
at the LHC early 2013 will certainly act as as the keystone of the J/y picture currently emerging
withA–AdataatTeVscale.
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