Forward Pion Production in Hadron Collisions at STAR

0.17 MB·English

by  G. Rakness

Forward Pion Production in Hadron Collisions at STAR
5
0 G. Raknessa, for the STAR Collaboration
0
2 aPenn State University/BrookhavenNational Laboratory,
n Physics 510-A, Upton, NY 11973
a
J Measurementsarereportedoftheproductionofhighenergyπ0mesonsatlargepseudorapidity,coincidentwith

6 chargedhadronsatmid-rapidity,forproton+protonanddeuteron+goldcollisionsat√sNN =200GeV.Thep+p
2 cross section for inclusive π0 production follows expectations from next-to-leading order perturbative QCD. A

suppression of the back-to-back azimuthal correlations was observed in d+Au, qualitatively consistent with the
1
gluon saturation picture. Experimental uncertainties regarding theinclusive measurement are discussed.
v
6
2
0
1 1. INTRODUCTION tions. Inparticular,strongazimuthalcorrelations
0
of hadron pairs are expected when particle pro-
5 In contrast to the case for the nucleon, lit-
duction arises from 2 2 parton scattering.
0
tle is known about the distribution of gluons →
/ This paper reports cross sections for for-
x in nuclei, especially in the region of Bj¨orken- ward inclusive π0 production for p+p colli-
e x<0.01, where x is the fraction of the nucleon’s
- sions at √s=200 GeV. The azimuthal correla-
cl momentum carried by the parton. Studies of tions between a forward π0 ( ηπ =4.0) and mid-
deuteron(proton)+nucleuscollisionsatlargecen- h i
u rapidity charged hadrons were studied. In addi-
n terofmassenergy(√sNN=200GeV)canprovide tion, exploratorystudies with d+Au collisions at
: constraints on the gluon density in heavy nuclei.
v √sNN=200GeVarereported,andazimuthalcor-
In the perturbative QCD explanation of parti-
i relations of hadron pairs are compared to those
X
cle production at large pseudorapidity, a large- for p+p collisions. The inclusive yields of π0
r x parton scatters from a low-x parton and then
a mesonsinp+pandd+Aucollisionswillbe forth-
fragments into the observed particle(s). Forward
coming, and details pertinent to this analysis are
charged particle production is found to be sup- presented here. Forward π0 production in d+Au
pressedind+Aucollisions[1],consistentwiththe collisions refers to observation of the π0 in the
expectationofgluonsaturation[2,3], possibly in-
direction of the incident deuteron.
dicating a different mechanism for particle pro-
duction. Explanations of the suppression based
2. EXPERIMENT
onleading-twistpQCDcalculationsusingamodel
of gluon shadowing have also been suggested [4].
The Solenoidal Tracker At RHIC (STAR) is
Further tests of the possible role played by gluon
a multipurpose detector at Brookhaven National
saturation at RHIC energies are provided by the
Laboratory. One of its principal components
study of particle correlations [5,6].
is a time projection chamber used for tracking
At √s=200 GeV and larger collision energies,
charged particles produced at η <1.2. A for-
there is quantitative agreement between NLO | |
wardπ0detector(FPD)comprising7 7matrices
pQCD calculations and measured p+p cross sec- ×
of 3.8 3.8 45 cm3 Pb-glass detectors (towers)
tions at mid-rapidity [7]. This agreement has × ×
was installed 800cm from the interaction re-
been found to extend to π0 production at η = ≈
h i gion near the beam pipe to detect high energy
3.8 [8]. Further tests of the underlying dynamics
π0 mesons with 3.3 < η < 4.1. Data were col-
responsible for forward particle production can
lected over two years of RHIC operations. In the
be obtained from the study of particle correla-
2002run,p+pcollisionswerestudied withapro-
1
2
2) (cid:214) s = 200 GeV p + p fi p 0 + X
V
e10 Æhæ =3.8 - PRL 92 (2004) 171801
G Æhæ =3.3 - hep-ex/0403012
/
3
c NLO pQCD calc.

b
KKP F.F.
m ( 1 Kretzer F.F.
3
p
d
/
3sd -1
10
E
Æhæ =3.8
-2
10 Normalization
Uncertainty = 17% Æhæ =3.3
Æp æ = 1.5 1.7 1.9 2.1 2.2 GeV/c --
-3 T
10 Æp æ = 2.3 2.8 3.0 3.4 GeV/c --
T
25 30 35 40 45 50 55 60 65
E (GeV)
p
Figure 1. Inclusive π0 production cross section Figure 2. Diphoton invariant mass spectra as
versus π0 energy (E ) at average pseudorapidi- a function of energy deposited in the calorime-
π
ties ( η ) 3.3 and 3.8. The inner error bars are ter for d+Au data and HIJING+GEANT simu-
h i
statistical, and are smaller than the symbols for lations. The histograms are reconstructed simu-
mostpoints. Theoutererrorscombinethesewith lationevents. Thepointsaredatawithstatistical
E -dependent systematic errors. The curves are errors, normalized to equal area in each bin.
π
NLOpQCDcalculationsevaluatedatη =3.3and
3.8 using different fragmentation functions.
siveyieldwasalsofoundtoagreewithpredictions
of the PYTHIA event generator [11], which uses
totype FPD [8]. In the 2003 run, p+p collisions initial- and final-state parton showers to model
werestudiedandexploratorymeasurementswere effects beyond leading order.
performed with d+Au collisions. Nucleareffectsonparticleproductionarequan-
tified by R , the ratio of the inclusive yield of
dAu
3. DIFFERENTIAL CROSS SECTION particlesproducedind+Aucollisionstop+pcol-
lisions. The value ofR for forwardπ0 mesons
dAu
The differential cross section for inclusive π0 is expected to be smaller than was seen for nega-
production for 30 < E < 55 GeV at η = 3.8 tive hadrons [1] due to isospin suppression of the
π
was previously reported [8]. The evehntirecon- p + p h− + X process [12]. We proceed to
→
struction and normalization methods were ex- discussconsiderationsrelevanttoanaccuratede-
tended to allow measurement of the differential termination of R for forward π0 production.
dAu
cross section at η = 3.3 [9]. The results are The data in Fig. 1 show that the cross section
h i
showninFig. 1incomparisontoNLOpQCDcal- falls rapidly with E . Accurate yield determi-
π
culations evaluated at η=3.3 and 3.8. The NLO nations require accurate energy calibrations and
pQCD calculations are consistent with the data, good energy resolution. The energy calibration
incontrasttoπ0dataatlower√s[10]. Theinclu- of the FPD is performedby reconstructionof the
3
p 0 + h– correlations, simulation
(cid:214) s = 200 GeV, |Æh æ| = 4.0, |h | < 0.75
p h
p + p d + Au
S = 11.8 – 0.2 % S = 8.4 – 0.5 %
1) B = 40.9 – 0.2 % B = 75.5 – 0.5 %
-ad.0.2 s S = 0.77 – 0.01 0.2 s S = 0.82 – 0.04 ÆpT,pæ=1.1 GeV/c
r
b. ( ÆpT,LCPæ=1.1 GeV/c
o
Pr0.1 0.1 ÆxFæ=0.28
n.
oi 25<E<35 GeV
C
0 0
0 2 4 6 0 2 4 6
Df = f - f
p LCP
Figure 3. Simulation study of the fractional p 0 + h– correlations, preliminary STAR data
difference of reconstructed and simulated energy (cid:214) s = 200 GeV, |Æh æ| = 4.0, |h | < 0.75
p h
versus simulated π0 energy. The energy calibra- p + p d + Au
tion is determined to an accuracy of 1%. Sys- S = 11 – 1 % S = 2 – 1.1 %
≈ 1) B = 50.4 – 1 % B = 88.1 – 1.6 %
tematics of reconstructing the di-photon opening -ad.0.2 s S = 1.13 – 0.07 0.2 s S = 0.54 – 0.27 ÆpT,pæ=1.1 GeV/c
angle are seen beginning around 50GeV. r
b. ( ÆpT,LCPæ=1.4 GeV/c
o
Pr0.1 0.1 ÆxFæ=0.28
n.
invariant mass of the π0 meson, Mγγ. An analy- Coi 25<E<35 GeV
0 0
sisofthetopologyoftheenergydeposition,based 0 2 4 6 0 2 4 6
Df = f - f
onmeasuredshowershapes[13],isusedtorecon- p LCP
structthe relative energyof the two photons and
their separation(d ). The π0 energy is takento
γγ
be the total energy in the calorimeter. The gain Figure 4. Coincidence probability as function
of each tower is determined iteratively by associ- of azimuthal angle difference between a forward
atingMγγ withthetowercontainingthemosten- π0 andamidrapidity leadingchargedparticle for
ergy in the event. After convergence, an energy- p+p (left) and d+Au (right) collisions. The up-
dependent correction is applied to account for perplotsaresimulationusingPYTHIA6.222and
material between the interaction region and the HIJING 1.381 described in the text, while the
detector, which causes decay photons to shower lower plots are data.
priortoreachingtheFPD.Italsocorrectsforthe
non-linearity implied by using digitizers with a
finite number of bits. The energy can be accu-
rately associated with M up to a point set by well for M as well as several other kinematic
γγ γγ
thesystematicsofreconstructingd . At50GeV, variables. The simulations are used to determine
γγ
d 1.1 the linear dimension of a tower. The the efficiencyofπ0 detection. As wasobservedin
γγ
≈ ×
M distribution can be seen in Fig. 2, with res- [8], the detection efficiency is dominated by the
γγ
olution 20MeV/c2 from 30 55GeV. geometrical acceptance of the calorimeter.
≤ −
A simulation is used to model the events us- Usingthesimulation,acomparisonofthekine-
ingthePYTHIA(HIJING)[14]generatorforp+p matics of events that contain π0 mesons with re-
(d+Au) collisions, together with a GEANT sim- constructedvariablesgivesan estimate ofthe ac-
ulationto modelthe detectorresponse,including curacy of the analysis techniques used. The frac-
intervening material. After calibrating the simu- tional difference in energy betweenthe generated
lation with the same technique as the data, the and reconstructed π0 meson in bins of generated
overlay of data and simulation for M can be π0 energy can be seen in Fig. 3. The uncertainty
γγ
seen in Fig. 2. The simulationdescribes the data on the energy calibration is 1%, implying an
≈
4
uncertainty on the yield of 10%. 5. SUMMARY
≈
The data in Fig. 1 show that the cross section
In summary, cross sections for the inclusive
increases rapidlywith η. Accurate yielddetermi-
production of π0 mesons in p+p collisions at
nations requireaccuratepositiondeterminations.
√s=200 GeV, at η =3.3 and3.8 are consistent
A Beam Position Monitor (BPM) is located in h πi
with NLO pQCD calculations. The azimuthal
the vicinity of the FPD, and has been accurately
correlationbetweenpairsofhadronsseparatedby
surveyed. Relative to the BPM, the absolute po-
large∆ηisdescribedbyPYTHIA[11],whichuses
sition of the FPD is presently measured with an
leadingorderpQCDwithpartonshowers. Agree-
accuracy of5mm, giving an absolute accuracyof
mentofthesecalculationswiththeinclusivecross
δη = 0.02 at η = 4.00. This implies an uncer-
section and di-hadron correlations suggest that
tainty on the yield of 11%.
forward π0 production arises from partonic scat-
tering at this collision energy. Exploratory stud-
4. CORRELATIONS
ies of forward π0 production in d+Au collisions
suggest that the azimuthally correlated compo-
Correlations between a π0 produced at large
nentofhadronpairsseparatedbylarge∆ηissup-
rapidity with large Feynman x and charged par-
pressed relative to p+p collisions. More data for
ticles produced at midrapidity were studied with
forward particle production and di-hadron corre-
p+p collisions and d+Au collisions. Details of
lations in d+Aucollisionsare requiredto reacha
the analysis, including detailed comparisons to
definitive conclusion aboutthe possible existence
simulations, may be found in Ref. [5]. The
of gluon saturation in the Au nucleus.
leading charged particle (LCP) analysis selects
the mid-rapidity track with the highest p >0.5
T
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