Identified hadron production at high transverse momenta in p+p collisions at sqrt(NN) = 200 GeV in STAR
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Identified hadron production at high transverse momenta in p+pcollisions at √ s N N = 200 GeV in STAR
Yichun Xu (for the STAR collaboration) a Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USAReceived: date / Revised version: date
Abstract.
We report the transverse momentum ( p T ) distributions for identified charged pions, protonsand anti-protons using events triggered by high deposit energy in the Barrel Electro-Magnetic Calorimeter(BEMC) from p + p collisions at √ s NN = 200 GeV. The spectra are measured around mid-rapidity ( | y | < < p T <
15 GeV/ c with particle identification (PID) by the relativistic ionizationenergy loss ( rdE/dx ) in the Time Projection Chamber (TPC) in the Solenoidal Tracker at RHIC (STAR).The charged pion, proton and anti-proton spectra at high p T are compared with published results fromminimum bias triggered events and the Next-Leading-Order perturbative quantum chromodynamic (NLOpQCD) calculations (DSS, KKP and AKK 2008). In addition, we present the particle ratios of π − /π + , p/p , p/π + and p/π − in p + p collisions. PACS.
The study of identified hadron ( π ± , K ± , p ( p )) spectra at high p T in p+p collisions provides a good test of perturbativequantum chromodynamics (pQCD) [1]. In different NLO pQCD calculations, the inclusive production of single hadronis described by the convolution of parton distribution functions (PDFs), parton interaction cross-sections, and frag-mentation functions (FFs) which are parameterized by measured hadron spectra. From the minimum-bias triggered p + p collisions in the year 2003, the p ( p ) and charged pion spectra were measured at p T ≤ c and p T ≤ c with significant systematic errors due to the uncertainties in mean dE/dx position for protons and kaons [2].In order to understand mechanism of hadron production, it is necessary to make a strict constraint on the quark andgluon FFs by comparing theory with experimental data. In addition, it’s also a good baseline for studying color chargeeffect of parton energy loss in heavy ion collisions, in which hadron spectra were measured up to 12 GeV/ c [3].In this article, we present the p T spectra for identified charge pions, protons and anti-protons in p + p collisionsat √ s NN = 200 GeV by the STAR experiment at RHIC. The results will be compared with NLO pQCD calculations.The particle ratios of π − /π + , p/p , p/π + and p/π − in p + p collisions will be presented and and compared with resultsin d + Au collisions [2,4]. The STAR main tracking detector, the TPC, covering full azimuthal angle (2 π ) and | η | < dE/dx information of charged particles. Inaddition, the BEMC covering 2 π azimuthal angle and 0 < η < rdE/dx [5], which will be discussed later.Due to empirical parameters in theory, gas multiplication gains and noise of the TPC electronics, and pileup inhigh luminosity environment, measured rdE/dx values are deviated from theoretical predictions. Dominant pion yieldsshadow kaons and protons in the rdE/dx distribution, although the rdE/dx separations among π ± , K ± , and p( p )are about 1-3 σ . This results in large systematic errors due to the uncertainty of rdE/dx positions. Knowledge of the a Present address: [email protected] Please give a shorter version with: \authorrunning and \titlerunning prior to \maketitle /mass T p t heo r y ) x ps -( n f i t ) x ps ( n -0.8-0.6-0.4-0.2-00.20.4 L proton from pion from Kspion with EMCelectron with EMC/ndf = 95/47 c (GeV/c) T p pps - n x ps n -4-3-2-10123456 Bichsel pp s -n p p s n Bichsel pp s -n p k s n Bichsel pp s -n p e s n corrected pp s -n p p s n corrected pp s -n p k s n corrected pp s -n p e s n Fig. 1.
On the left panel, the dE/dx deviation in σ as functions of transverse momentum divided by mass ( p T /mass). Theblue crosses are proton decayed from Λ , the red filled dots are pion from K S , and blue open circles and red stars are pion andelectron from electron enhancement sample respectively. On the right panel, comparison of the relative dE/dx peak position of nσ Kπ , nσ pπ , nσ eπ . All solid dots depict theoretical values, and open ones are re-calibrated results. precise rdE/dx position for those hadrons is important to understand the efficiencies of particle identification (PID)selection and to reduce the systematic uncertainty in identified hadron yields. In order to improve the PID at high p T ,the re-calibration method [5] is used to locate the rdE/dx positions for different charged particles with good precision.To separate pion and electron clearly and get rdE/dx of electron precisely, information of the BEMC is used to getelectron enhancement data sample. Meanwhile, Strange hadrons Λ → p + π − ( Λ → p + π + ) and K S → π + + π − are reconstructed by their decay topology to identify their decay daughters in the TPC. This provides rdE/dx peakposition from samples of pure protons and charged pions [5].With these precise rdE/dx information, the left panel of Figure 1 shows the deviation of rdE/dx normalized bypion rdE/dx between data and theoretical values. This distribution is described by a function, f ( x ) = A + BC + x .Meanwhile, the separation between identified hadrons and pions are derived from this fit function, as shown on theright panel of Figure 1.In order to extract charged pion and proton (anti-proton) yields, a total of 5.6 million BEMC triggered events inyear 2005 (with transverse energy E T > rdE/dx distribution using 8-Gaussian with four fixed re-calibrated parameters for peak positions. The left panel ofFigure 2 shows the rdE/dx distribution at 5.0 < p T < | η | < rdE/dx . The other method is fitting dE/dx distribution with fixed proton peak position. The final yields we used in this analysis are averaged from these twomethods. Half of the difference ( < p T by dividing BEMC triggered spectra byminimum bias triggered spectra. With the trigger enhancement factors and tracking efficiencies ( ∼
88% at 3 < p T < c ), the charged pion spectra are corrected and shown on the left panel of Figure 3. The corrected spectra of pionsare consistent with minimum bias results at the overlapped p T range, and the NLO pQCD calculations. With p/π + and p/π − ratios, the invariant yields of proton (anti-proton) are calculated by pion spectra times p/π + ( p/π − )ratios. Thespectra of protons and anti-protons are shown on the right panel in Figure 3. Almost all these NLO pQCD predictionscan not describe our proton and anti-proton spectra well at high p T , especially AKK 2008 for anti-proton, which isgoing down to zero at 9 GeV/ c (from S. Albino private communication). The AKK 2008 [7] includes BRAHMS datapoints at high rapidity (2.95 < y < lease give a shorter version with: \authorrunning and \titlerunning prior to \maketitle p s n -6 -3 0 3 6 -6 -3 0 3 6 C oun t <5.50 T ht trg pi+- distribution in 5.00
Fig. 2.
On the left panel, nσ hπ distribution at 3.75 < p T < c for positive (right part) and negative (left part) particles.The black line is the fit curve by 8-Gaussian function, including pion (red dashed line), kaon (blue dot-dashed line), proton(green dotted line), and electron (green solid line). On the right panel, pion spectra from minimum bias events, BEMC triggeredevents, and simulated minimum bias and BEMC triggered events. (GeV/c) T p h d T dp T p p N / d e v en t / N -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 EMC trigger + p EMC trigger - p minimum bias + p minimum bias - p CTEQ6M5+DSS NLO + p CTEQ6M5+DSS NLO - p KKP NLO p AKK 2008 + p AKK 2008 - p STAR Preliminary (GeV/c) T p h d T dp T p p N / d e v en t / N -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 p EMC trigger EMC triggerpp minimum bias minimum biaspp CTEQ6M5+DSS NLO CTEQ6M5+DSS NLOpp AKK 2008 AKK 2008p Fig. 3.
Spectra for pion (on the left panel) and proton (on the right panel) with NLO pQCD calculations including DSS, AKK2008.
The total systematic uncertainties associated with pion are estimated to be less than 15%. The systematic uncer-tainty consists of uncertainty of peak position ( < < < < ∼
13% contribution.The particle ratios at mid-rapidity as a function of p T are shown on Figure 4, and compared with published resultsfrom minimum bias p + p and d + Au collisions. The predictions from different model, such as PYTHIA, DSS, and AKK2008 are also shown. All the boxes and bars represent systematical and statistical errors. Our results are consistentwith those from minimum bias p + p collisions. For the models, only PYTHIA can describe these ratios well, DSSover-predicts anti-proton, AKK 2008 over-predicts proton, and under-predicts anti-proton. The π − /π + ratio fromBEMC triggered events decreases with increasing p T , which indicates significant valence quark contribution to chargepion production. The decrease of p /p ratio also indicates a significant quark contribution to baryon production. The p/π + and p/π − ratios decrease at intermediate p T range (2 < p T < c ) and approach constant at high p T , whichare consistent with PYTHIA simulation. The p/π + ratio in p + p collision is lower than that in d + Au collisions inthe intermediate p T range. We have presented transverse momentum spectra for identified charged pions, protons and anti-protons from p + p collisions triggered by the BEMC at √ s NN = 200 GeV. With PYTHIA and GEANT simulation, the transversemomentum spectra around mid-rapidity ( | η | < p T >
15 GeV/ c with PID by the rdE/dx in the TPC. Comparison of spectra to NLO pQCD predictions, DSS and AKK 2008 can describe pion spectra well, Please give a shorter version with: \authorrunning and \titlerunning prior to \maketitle (GeV/c) T p R a t i o + p / - p pp jp2 triggered datapp minbiasdAu minbiasPYTHIA minbiasCTEQ6M5+DSSAKK 2008 (GeV/c) T p / p R a t i op pp jp2 triggered datapp minbiasdAu minbiasPYTHIA minbiasCTEQ6M5+DSSAKK 2008 STAR Preliminary (GeV/c) T p R a t i o + p p / pp jp2 triggered datapp minbiasdAu minbiasPYTHIA minbiasCTEQ6M5+DSSAKK 2008 (GeV/c) T p R a t i o - p / p pp jp2 triggered datapp minbiasdAu minbiasPYTHIA minbiasCTEQ6M5+DSSAKK 2008 Fig. 4.