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 Björken- 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 production. 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. 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Forward Pion Production in Hadron Collisions at STAR

Similar Forward Pion Production in Hadron Collisions at STAR