Measurement of heavy-flavour production as a function of multiplicity in pp and p-Pb collisions with ALICE

0.58 MB·English

by  Riccardo Russo

Nuclear
Physics A
NuclearPhysicsA00(2015)1–4
Measurement of heavy-flavour production as a function of
multiplicity in pp and p–Pb collisions with ALICE
RiccardoRusso(fortheALICECollaboration)
5
1 DipartimentodiFisica,Universita’degliStudidiTorinoandINFN,viaPietroGiuria1,10125Torino
0
2

n
a
J

6 Abstract
2
] partiIcnlethmesueltipprloiccieteydiinngpsprceosulllitssioanrespatre√sesnt=ed7fTroemVathnedmp–ePasbucreomlliesniotnosfaotpe√nsNhNea=v5y.0-fl2aTvoeVurrpercoodrudcetdiowniathstahefuAncLtIioCnEodfectehcatrogredin-
x 2010and2013,respectively. D0,D+andD∗+mesonsarereconstructedfromtheirhadronicdecaychannelsinthecentralrapidity
e
region,andtheirproductionyieldsaremeasuredinvariousmultiplicityandp intervals.
- T
l Theper-eventyieldsofDmesonsinthevariousmultiplicityintervals,normalizedtotheirmultiplicity-integratedvalue,and
c
theirevolutionwith p aremeasuredforppandp–PbcollisionstostudythecontributionofMulti-PartonInteractions(MPIs)to
u T
opencharmproductioninthetwosystems. ThenuclearmodificationfactorofDmesonsinp–Pbcollisions,definedastheratio
n
oftheD-mesonyieldsinp–Pbandppcollisionsscaledbytheaveragenumberofbinarycollisions(cid:104)N (cid:105),isdiscussedintermsof
[ coll
itsdependenceontheeventactivity. Severalexperimentalestimatorsoftheeventactivityareusedinordertoassesstheroleof

1 kinematicbiases.
v
5 Keywords: QGP,Heavy-flavour,Multi-PartonInteraction
2
4
1. Introduction
6
0 The measurement of heavy-flavour production as a function of the multiplicity of charged particles produced in
.
1 hadroniccollisionsissensitivetotheinterplaybetweenhardandsoftcontributionstoparticleproductionandcould
0 giveinsightintotheroleofMulti-PartonInteractions(MPIs,i.e.severalhardpartonicinteractionsoccuringinasingle
5 collisionbetweentwonucleons).
1
ParticleproductionattheLHCisexpectedtohaveasubstantialcontributionfromMPIsinpp(p–Pb)collisions,
:
v wherethehighestmultiplicityvaluesobservedaresimilartotheonesofperipheralCu–Cu(Pb–Pb)collisionsatRHIC
Xi (LHC).MeasurementsbytheCMSCollaborationofjetandunderlyingeventpropertieshaveshownbetteragreement
with models including MPIs [1]. Measurements by the ALICE Collaboration of minijets point to an increase of
r
a MPIswithincreasingcharged-particlemultiplicity[2]. Intheheavy-flavoursectorseveralmeasurementshavebeen
performed[3,4].InparticularALICEfoundanapproximatelylinearincreaseofJ/ψyieldasafunctionofmultiplicity
√
inppcollisionsat s=7TeV[5].
Moreoveritisinterestingtocompareheavy-flavourproductioninp–Pbcollisionswithppresultstotestwhether
the yield and the transverse momentum distributions follow a scaling with the number of binary nucleon–nucleon
collisions in the p–Pb collision. This scaling is expected for particles produced in hard (high virtuality) partonic
scattering processes in the absence of nuclear effects in the initial or in the final state of the p–Pb collision. This
isstudiedbymeasuringthenuclearmodificationfactorR , definedastheratioofthe p -differentialcrosssection
pPb T
measuredinp–PbcollisionstothatmeasuredinppcollisionsscaledbythemassnumberAofthePbnucleus. The
D-mesonR inminimumbiasp–Pbcollisionswasfoundconsistentwithunityforp >1GeV/cwithinuncertainties
pPb T
1
Author1etal./NuclearPhysicsA00(2015)1–4 2
ofabout20%[6],showingthatColdNuclearMatter(CNM)effects(nuclearmodificationsofthepartondistribution
functions (PDF), k broadening, energy loss in cold nuclear matter) do not strongly affect charm-quark production
T
in p–Pb collisions [7]. It is interesting to measure the nuclear modification factor in p–Pb collisions in classes of
theeventactivity,becausethelatterisrelatedtothecollisioncentrality(i.e. impactparameteraswellasthenumber
of participating nucleons and binary collisions) of the p–Pb collisions. The measurement of the R requires to
pPb
estimatetheaveragenumberofbinarycollisions(cid:104)N (cid:105)fortheeventactivityintervalsusedintheanalysis. ALICE
coll
hasidentifiedinp–Pbcollisionsseveralsourcesthatcaninduceabiasinthecentralitydeterminationbasedonparticle
multiplicitymeasurement[8,9]. Thisbiashavebeenobservedinthemeasurementofthenuclearmodificationfactor
ofchargedparticlesinp–Pbcollisionsinmultiplicityclasses[9]. Thisworkinvestigateswhethersuchabiasisalso
presentforDmesons.
TheresultsarepresentedinformoftheD-mesonself-normalizedyieldinppandp–Pbcollisions,definedas
d2ND/dydp Ymult/((cid:15)mult)
T = (1)
(cid:104)d2ND/dydp (cid:105) Ytot/((cid:15)tot×(cid:15)trigger)
T
where Ymult is the D-meson per-event yield in multiplicity intervals, Ytot the multiplicity integrated per-event yield,
(cid:15) arethecorrespondingreconstructionandselectionefficienciesand(cid:15)trigger isthetriggerefficiency(onlyrelevantin
pp). Furthermore,thebinaryscalingisstudiedinseveraleventactivityclassesviatheQ ratio,defiledas:
pPb
dNpPb/dp dNpPb/dp
QV0A(p )= mult T Qmult(p )= mult T (2)
pPb T (cid:104)NGlauber(cid:105)dNpp/dp pPb T (cid:104)Nmult(cid:105)dNpp/dp
coll T coll T
for the V0A and ZNA event activity estimators respectively, defined in the next section. These two observables
represent different ways to study the multiplicity dependence of D-meson production in p–Pb collisions: the self-
normalizedyieldsaremorefocusedonthestudyofMPIs,whileQ reflectsthescalingofcharmproductioninp–Pb
pPb
collisionsrelativetoppcollisions.
2. Datasampleandanalysisstrategy
TheALICEdetectorisdescribedin[10]. Thedatasamplesanalyzedarefromthe2010pprun(300·106eventsat
√ √
s=7TeV)andthe2013p–Pbrun(100·106eventsat s =5.02TeV).Detailsonthetriggerandeventselections
NN
canbefoundin[6,11]. Eventsaredividedineventactivityclasses. Threeeventactivityestimatorshavebeenused:
• N : numberoftracksegmentsreconstructedintheSiliconPixelDetector(SPD-twoinnermostlayersof
tracklets
theInnerTrackingSystem-|η|<0.9);
• V0A: signal amplitude of the A-side VZERO scintillator (2.8 < η < 5.1 - Pb going direction for p–Pb colli-
sions);
• ZNA:energyfromnuclearfragmentsintheA-sideZeroDegreeNeutronCalorimeter(112.5mfrominteraction
point-Pbgoingdirectionforp–Pbcollisions).
Theself-normalizedyieldsareobtainedinN intervals,whiletheQ analysisadoptsV0AandZNAmultiplicity
tracklets pPb
classes (0-20%, 20-40%, 40-60%, 60-100%). The analysis is based on the reconstruction of D mesons in their
hadronic decay channels (D0 → K−π+, D+ → K−π+π+, D∗+ → D0π+) in the ALICE central barrel (|η| <0.9),
exploitingtheexcellentvertexresolutionandparticleidentificationcapabilitiesoftheALICEdetectorasdescribedin
[12]. D-mesonefficiencycorrectionsinppandp–PbcollisionsaredeterminedwithMonteCarlosimulationsbased
on PYTHIA 6.4.21 and HIJING event generators. A fraction of the total D-meson yield comes from the decay of
Bmesons. Forthe Q analysisthiscontributionwasestimatedbasedonFONLLpQCDcalculationsasdescribed
pPb
in [12], while for the self-normalized yield analysis no subtraction has been performed, assuming that the fraction
of feed-down D mesons does not depend on multiplicity and cancels in the ratio of Eq. 1. A deviation from this
assumption has been considered to estimate the corresponding systematic uncertainty. The pp and p–Pb corrected
yieldshavebeenusedtocomputethe Q asinEq. 2. Theaveragevaluesof N inthefourV0AandZNAevent
pPb coll
activityclasseshavebeenevaluatedasfollows:
• V0A: the (cid:104)NGlauber(cid:105) values have been obtained for each V0A multiplicity interval with the approach used for
coll
Pb–Pbcollisions,i.e. viaafittotheV0AmultiplicitydistributionbasedontheGlaubermodelforthecollision
geometryandatwo-componentmodelforparticleproduction[13].
2
Author1etal./NuclearPhysicsA00(2015)1–4 3
• ZNA: the (cid:104)Nmult(cid:105) values have been calculated by scaling the (cid:104)N (cid:105) in minimum-bias p–Pb collisions by the
part part
ratiobetweentheaveragemultiplicitydensitymeasuredatmid-rapidityforagivenZNenergyeventclassand
theonemeasuredinminimumbiascollisions. (cid:104)Nmult(cid:105)isobtainedas(cid:104)Nmult(cid:105)=(cid:104)Nmult(cid:105)-1.
coll coll part
p(cid:156)T14 ALICE Preliminary p(cid:156)T14 ALICE Preliminary
D/dyd 12 D0 mppe, sos n=, 7|y Tlaeb|V<0.5 /dydN 12 p-PbA vesrNaNg e= D 50.,0 D2+ ,T De*V+ meson
2Nd) / (cid:157)T180 22p<<-PppbTT,<< 44s GGNNee =VV //5cc.02 TeV 2d) / p(cid:157)T180 JJ|y//(cid:115)(cid:115)lab |(cid:65)(cid:65)<0 .µµ5++,µµ 2--,,< -2p4.T5.0<<<4yy Glalabeb<<V4-/.20c.,5 p, Tp>T0>0
p d
d y
y 6 d 6
d /
D/ N
N 4 2d 4
2d (
( 2 +7%/-3% (3.1%) in pp (p-Pb) 2
normalization unc. not shown
6% (3%) unc. in pp (p-Pb) ±3.1% normalization unc. not shown
on dN/d(cid:100) / < dN/d(cid:100) > not shown ± 3% unc. on dN/d(cid:100) / < dN/d(cid:100) > not shown
n unc. 00..42 B fraction hypothesis in pp and p-Pb: × 1/2 (2) at low (high) multiplicity n unc. 00..42 B fraction hypothesis: × 1/2 (2) at low (high) multiplicity
ow 0 ow 0
d d
d- -0.2 d- -0.2
ee -0.4 ee -0.4
B f 0 1 2 3 4 5 6 B f 0 1 2 3 4 5 6
dN /d(cid:100) / (cid:157)dN /d(cid:100)(cid:156) dN /d(cid:100) / (cid:157)dN /d(cid:100)(cid:156)
ALI−PREL−76733 ch ch ALI−PREL−76649 ch ch
Figure1.Left:self-normalizedD0yieldsfor2<pT<4GeV/casafunctionofmultiplicityforppandp–Pbcollisions.Right:averageD0,D+and
D∗+self-normalizedyieldscomparedtoJ/ψintherapidityintervals2.5<ylab<4.0and−4.0<ylab<−2.5forp–Pbcollisions
3. Results
Theself-normalizedyieldshavebeenmeasuredforpromptD0,D+ andD∗+ mesons. Theyareshownincharged-
particle multiplicity (dN /dη) intervals (Fig.1), since a simulation study has shown that N /(cid:104)N (cid:105) equals
ch tracklets tracklets
dN /dη/(cid:104)dN /dη(cid:105). ForalltheD-mesonspecies,theyieldincreaseswithcharged-particlemultiplicity. No p depen-
ch ch T
denceofthistrendhasbeenobserved.
TheleftpanelofFig.1showsD0self-normalizedyieldsforppandp–Pbcollisions.Bothsystemsshowanincrease
oftheyield withcharged-particlemultiplicity. Thetrendforppcollisions canbeinterpretedasbeing duetostrong
hadronicactivityconnectedwithcharmproductionandtothepresenceofMPIsaffectingthehardmomentumscale
relevantforheavy-quarkproduction. Inthep–Pbcase,itshouldbeconsideredthathigh-multiplicityeventscanalso
originate from a higher number of nucleon-nucleon collisions in the nuclear interaction. The right panel of Fig.1
shows the average values of self-normalized yields for D0, D+ and D∗+ and J/ψ in p–Pb collisions. J/ψ yields have
been measured in 2.5< y <4.0 (p-going direction) and −4.0 < y < −2.5 (Pb-going direction) and they show
lab lab
anincreasewithcharged-particlemultiplicity. HoweveraquantitativecomparisonofJ/ψandD-mesonyieldshasto
takeintoaccounttherapiditydependenceofcoldnuclearmattereffectssuchasgluonshadowing,thatdependsonthe
Bjorkenxvalueofthepartoninvolvedintheprocessproducingcharm,andenergylossincoldnuclearmatter[14].
The average value of Q for prompt D0 and D∗+ mesons is shown in the left panels of Figs. 2 and 3 for the
pPb
V0AandZNAestimators,respectively. AbiascanbeobservedintheV0Ameasurement,wherethelow-multiplicity
Q is below unity in all six p bins, while the ZNA measurement is compatible with unity within systematic and
pPb T
statisticaluncertaintiesatbothlowandhigheventactivity. These Q resultsarecomparedwiththeonesobtained
pPb
for charged particles, shown in the right panels of Figs. 2 and 3. These comparisons demonstrate that the Q of
pPb
high p (>8GeV/c)chargedparticlesfeatureasimilarpatternastheoneofDmesons, confirmingthepresenceof
T
abiasintheV0A-baseddeterminationof(cid:104)NGlauber(cid:105)thatisreducedusingtheZNAestimator. Thisindicatesthatthe
coll
determinationof(cid:104)N (cid:105)dependsontherapidityregioninwhichtheeventactivitymeasurementisperformed. With
coll
theleastbiasedestimator(ZNA)weobserveQ beingcompatiblewithunityforallmultiplicitesand p . Detailson
pPb T
theN biashavebeenpresentedatthisconferenceandtheyaredescribedin[9].
coll
In conclusion, the D-meson self-normalized yields show an increasing trend with increasing charged-particle
multiplicity.Thetrendsobservedforppandp–Pbcollisionsarecompatiblewithinuncertaintes.TheQ resultsforD
pPb
mesonsarequalitativelysimilartotheonesobtainedforhighp chargedparticles.InparticulartheZNAmeasurement
T
shows no multiplicity dependence of the D-meson production in p–Pb collisions relative to binary scaling of pp
productioncrosssectionswhileV0Aresultsshowasimilarbiasasobservedforhigh-p chargedparticles.
3 T
Author1etal./NuclearPhysicsA00(2015)1–4 4
References
[1] V.Khachatryanetal.[TheCMSCollaboration],Eur.Phys.J.C73(2013)2674
[2] A.Abelevetal.[ALICECollaboration],JHEP09(2013)049
[3] MAguilar-Benitez[NA27Collaboration],Z.Phys.C41(1988)191
[4] R.Aaijetal.[LHCbCollaboration],J.HighEnergyPhys.,06(2012)141
[5] A.Abelevetal.[ALICECollaboration],Phys.LettB712(2012)165-175
[6] A.Abelevetal.[ALICECollaboration],arXiv:1405.3452
[7] S.LifortheALICECollaboration,theseproceedings
[8] A.MorschfortheALICECollaboration,arXiv:1309.5525
[9] A.ToiafortheALICECollaboration,theseproceedings
[10] A.Abelevetal.[ALICECollaboration],JINST3(2008)S08002
[11] A.Abelevetal.[ALICECollaboration],JHEP1207(2012)191
[12] R.RussofortheALICECollaboration,arXiv:1305.3435
[13] A.Abelevetal.[ALICECollaboration],Phys.Rev.C88(2013)044909
[14] J.MartinBlancofortheALICECollaboration,theseproceedings
4
Author1etal./NuclearPhysicsA00(2015)1–4 5
pt D 3 ALICE Preliminary V0A Multiplicity V0AQpPb2.5 Vp0-APb AsLNINC E= 5P.R02E LTIeMVINARY 50--150%% 4600--6800%%
V0A promQpPb2.5 Avper-aPgbe, D 0s, NDN* += - 05.9.06 2< TyecmVs <0.04Event Cla06s-02s-e01s%0 (0P%b-side) 2 SSyysstt.. oonn n(cid:157) oTrpmA a(cid:156)liz ation 1200--2400%% 80-100%

2
1.5
1.5
1
1
0.5
0.5
Filled markers : pp rescaled reference
Open markers: pp p-extrapolated reference
T 0 5 10 15 20 25 30
0
0 5 10 15 20 25 30 p (GeV/c)
p (GeV/c)
ALI−PREL−79740 T ALI−PREL−79671 T
Figure2. Left:averagepTdifferentialQpPbofD0andD∗+mesonsinthe0-20%and60-100%V0Aeventactivityclasses. Right: pTdifferential
QpPbofchargedhadronsinsevenV0Aeventactivityclasses.
prompt D2.35 p-APLbI,C Es NPNr e=l i5m.0in2a rTyeV ZN Ener0gy-2 E0v%ent Class multQpPb 111...468 Cph aPrAbgL e IdC EZsp NNaPNr R+t i= EcN lL5emcI.Mso0ul ll2I tN| TAηeR| V<Y 0.3 12 5000 1245000%%%% 846000 1680000%%%
mult QpPb Average D0, D*+ -0.96 < ycms <0.04 60-100% 1.2

2 1
0.8
1.5 0.6
0.4 Syst. on 〈 T 〉
pA
1 0.2 Syst. on normalization
Syst. on dN/dp
T
1.15
0.5 1.1
1.05
Filled markers : pp rescaled reference 1
Open markers: pp pT-extrapolated reference 0.95
0 0.9
0 5 10 15 20 25 30 0.85
p (GeV/c) 0 5 10 15 20 25 30
ALI−PREL−79715 T ALI−PREL−80855 pT (GeV/c)
Figure3. Left: averagepTdifferentialQpPbofD0andD∗+mesonsinthe0-20%and60-100%ZNAeventactivityclasses. Right: pTdifferential
QpPbofchargedhadronsinsevenZNAeventactivityclasses.
5