Identified particle transverse momentum spectra in p+p and d+Au collisions at sqrt{s_NN} = 200 GeV

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by  Pawan Kumar Netrakanti

Identified particle transverse momentum spectra in p+p and
d+Au collisions at √s = 200 GeV
NN
Pawan Kumar Netrakanti (for STAR COLLABORATION)
VariableEnergyCyclotronCentre,Kolkata
6 Abstract. Thetransversemomentum(pT)spectraforidentifiedchargedpions,protonsandanti-protonsfromp+pandd+Au
0 collisionsaremeasured around midrapidity (y <0.5) over therange of 0.3< pT <10GeV/cat√sNN =200 GeV. The
| |
0 chargedpionandproton+anti-protonspectraathigh pTinp+pcollisionshavebeencomparedwiththenext-to-leadingorder
2 perturbativequantumchromodynamic(NLOpQCD)calculationswithaspecificfragmentationscheme.The p/p +and p¯/p −
hasbeenstudiedathigh p .Thenuclearmodificationfactor(R )showsthattheidentifiedparticleCronineffectsaround
T dAu
n midrapidityaresignificantlynon-zero forchargedpionsandtobeeven largerforprotonsatintermediate pT (2< pT <5
a GeV/c).
J
Keywords: Particleproduction,perturbativequantumchromodynamics,fragmentationfunction
8
PACS: 25.75.-q,25.75.Dw,24.85.+p
1

1 INTRODUCTION
v
2 Thestudyofidentifiedhadronspectraatlargetransverse 102 p+p STAR (p + + p -)/2
2
0 momentum(pT)in p+pcollisionscanbeusedtotestthe 3) STAR (p + p)/2 X 0.1
c
1 predictions from perturbative quantum chromodynam- -2V 1 AKKNLO
0 ics (pQCD) [1]. Comparisons between experimentally e
6 measured p spectra and theory can help to constrain G STAR Preliminary
0 thequarkanTdgluonfragmentationfunctions.Withinthe mb 10-2
x/ framework of pQCD, the expected initial-state nuclear 3 (p
e effectsind+Aucollisionsaremultiplescattering(Cronin d 10-4
l- effect[2])andshadowingofthepartondistributionfunc- 3s / m =pT/2
uc tion. The studyof the nuclearmodificationfactor(RdAu) Ed 10-6
willhelpusinunderstandingthenucleareffectsinvolved
v:n isntradi+nsApuarctoicllliesipornosd.uTchtieonpamrtoidcleelsraatnidosalastohgigivhespuTnciqoune- 10-80 1 2 3 4 5 6 7m =28pT 9 10
i
X dataonFFratios,althoughextractionofthisinformation
ismodel-dependent.Forexample, p/p +reflectstherel- Transverse Momentum (p ) (GeV/c)
r T
a ative probability of a parton to fragment into proton or
pion at high pT [3]. The above aspects have been dis- FIGURE1. Midrapidityinvariantyieldsfor(p ++p )/2and
−
cussedinthismanuscript. (p+p¯)/2athighp forminimumbiasp+pcollisionscompared
T
toresultsfromNLOpQCDcalculationsusingAKK[12](PDF:
CTEQ6M) set of fragmentation functions. The calculations
EXPERIMENT AND ANALYSIS from AKK are for three different factorization scales: m =
pT/2,m =pT,andm =2pT.
The detectors used in the present analysis are the Time
Projection Chamber (TPC), the Time-Of-Flight (TOF)
has been described in Ref. [7].For p >2.5 GeV/c, we
detector, a set of trigger detectors used for obtaining T
usedatafromtheTPC.Particleidentificationatp inthe
the minimum bias data, and the Forward Time Projec- T
TPCcomesfromtherelativisticriseoftheionizationen-
tion Chamber (FTPC) for the collision centrality deter-
ergyloss(rdE/dx).Detailsofthemethodaredescribed
mination in d+Au collisions in STAR experiment. The
inRef.[8].
detailsof the designandothercharacteristicsofthe de-
tectors can be found in Ref. [4]. The data from TOF is
usedtoobtaintheidentifiedhadronspectrafor p <2.5
T
GeV/c.Theprocedureforparticleidentificationin TOF
3.0 duction.Itisdefinedasaratiooftheinvariantyieldsof
p M.B. theproducedparticlesind+Aucollisionstothoseinp+p
2.5 p+p M.B. collisions scaled by the underlyingnumber of nucleon-
) h M.B. |h |<0.5 nucleonbinarycollisions.
5 2.0
.
<0 d2NdAu/dydpT
y| 1.5 RdAu(pT) = N /s ineld2s /dydp ,
(| h bini pp pp T

u 1.0
A where N is the average number of binary nucleon-
Rd nucleonh(NbinNi)collisionsperevent,and N /s inelisthe
0.5 STAR Preliminary h bini pp
nuclearoverlapfunctionT (b)[5,6].Thes inel istaken
A pp
0.0 tobe42mb.
0 1 2 3 4 5 6 7 8 9 In Fig. 2 shows the RdAu for charged pions
((p ++p )/2) and combined proton and anti-proton
Transverse Momentum p (GeV/c) −
T (p+p¯)inminimum-biascollisionsat y <0.5.TheRdAu
| |
> 1 indicates a slight enhancementof high p charged
T
pions yields in d+Au collisions compared to binary
FIGURE 2. Nuclear modification factor R for charged
dAu collision scaled charged pion yields in p+p collisions
pions(p ++p )/2andp+p¯at y <0.5inminimumbiasd+Au
collisions.For−comparisonresu|lt|soninclusivechargedhadrons within the measured (y, pT) range. The RdAu for p+p¯
(STAR)fromRef.[5]at h <0.5areshown.Theshadedband is again greater than unity for pT > 1.0 GeV/c and is
isthenormalizationunce|rta|intyfromtriggerandluminosityin larger than that of the charged pions. The RdAu results
p+pandd+Aucollisions. foridentifiedparticleshasalsobeencomparedtothein-
clusivechargedhadrons.Theuncertaintyindetermining
the numberofbinarycollisionsin d+Au minimum-bias
COMPARISONTO NLO PQCDAND collisionsis 5.3%.
∼
MODEL CALCULATIONS
In Fig. 1 we compare (p + + p )/2 and (p+p¯)/2 yields PARTICLERATIO
−
in minimumbias p+pcollisionsatmidrapidityforhigh
pT to those from NLO pQCD calculations. The NLO The p/p + and p¯/p − atmidrapidityasa functionof pT
pQCD resultsare based oncalculationsperformedwith forp+pandd+Auminimumbiascollisionsareshownin
Albino-Kniehl-Kramer(AKK)setoffragmentationfunc- Fig.3.AtRHIC,thep/p +andp¯/p −ratiosincreasewith
tions [12]. We observe that our charged pion data for pT up to 2 GeV/c and then start to decrease for higher
pT > 2 GeV/c in p+p collisions are reasonably well- pT in both p+p and d+Au collisions. The p¯/p − ratio
explained by the NLO pQCD calculations using the rapidlyapproachesavalueof0.2,whichisalsoobserved
AKK set of FFs. The calculations for the factorization in e++e− collisions for both quark and gluon jets [9].
scales of m = p /2, m = p , and m = 2p have been The p/p + ratios in p+p collisions compare well with
T T T
shown. The combined proton and anti-proton yield in results from lower energy ISR and FNAL fixed target
p+p is lower comparedto NLO pQCD calculations us- experiments[10,11],while p¯/p −ratiosathigh pT have
ing AKK FFs for the factorization scale m = p . The astrongenergydependencewithlargervaluesathigher
T
(p+p¯)/2 yield in p+p collisions, however,is reasonably beamenergies.
well-explainedbyAKKsetofFFsfor m =2p .Forthe
T
firsttimeinp+pcollisionsweobserveareasonablygood
SUMMARY
agreementbetween the NLO pQCD calculations(using
AKK FFs) and data at high p . This reflects the im-
T
portanceoftheflavor-separatedmeasurementsine++e We have measured the transverse momentum spectra
−
collisions in determining the FFs to baryons as used in for identified charged pions, protons and anti-protons
AKKFFscalculation. from p+p and d+Au collisions at √sNN = 200 GeV
around midrapidity (y < 0.5) over the range of 0.3 <
| |
p < 10 GeV/c. For particle identification we use the
T
NUCLEAR MODIFICATIONFACTOR ionizationenergylossanditsrelativisticriseintheTime
Projection Chamber and the Time-of-Flight in STAR.
The nuclear modification factor (R ) can be used to The charged pions, combined proton and anti-proton
dAu
study the effectsof cold nuclearmatter on particle pro- spectra in p+p and collisions have been compared to
calculations with the next-to-leading order perturbative
1.0
p/p + p+p :STAR 200 GeV p/p -
d+Au:STAR 200 GeV
0.8 p+p :ISR 53 GeV
STAR Preliminary
p+W :FNAL 38.8 GeV
0.6
0.4
0.2
STAR Preliminary
0.0
0 2 4 6 8 10 0 2 4 6 8 10
Transverse Momentum p (GeV/c)
T
FIGURE3. Ratioof p/p +and p¯/p −atmidrapidity(y <0.5)asafunctionof pTin p+pandd+Auminimumbiascollisions.
ForcomparisontheresultsfromlowerenergiesatISRa|nd|FNALarealsoshownfor p/p + and p¯/p .Theerrorsrepresentedby
−
boxesarethepoint–to–pointsystematic.
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ACKNOWLEDGMENTS
We would like to thank Simon Albino for providingus
theNLOpQCDresults.WealsothankWernerVogelsang
andStefanKretzerforusefuldiscussions.