High-pT Spectra of Charged Hadrons in Au+Au Collisions at sqrt s_(NN)=9.2 GeV in STAR
aa r X i v : . [ nu c l - e x ] A p r HIGH- p T SPECTRA OF CHARGED HADRONSIN Au+Au COLLISIONS AT √ s NN = 9 . GeV IN STAR
M.V. Tokarev for the STAR Collaboration ∗† Joint Institute for Nuclear Research, Dubna, Russia
The production of hadrons in heavy ions collisions at high p T provides an important informationon mechanism of particle formation and constituent energy loss in medium. Such information isneeded for search of a Critical Point and signatures of phase transition. Measurements by the STARCollaboration of charged hadron production in Au+Au collisions at √ s NN =9.2 GeV over a widetransverse momentum p T = 0 . − . GeV/c and at mid-rapidity range are reported. It allows fora first measurement of the spectra for charged hadrons at high p T at this energy. The spectrademonstrate the dependence on centrality which enhances with p T . The constituent energy loss andits dependence on transverse momentum of particle, and centrality of collisions are estimated in the z -scaling approach.PCAS numbers: 25.75.-qKeywords: high energy, heavy ions, charged hadron spectra, energy loss I. INTRODUCTION
Heavy ion collisions at RHIC have provided evidencethat a new state of nuclear matter exists [1]. This newstate is characterized by a suppression of particle produc-tion at high p T [2], a large amount of elliptic flow ( v ),constituent quarks (NCQ) scaling of v at intermediate p T [3] and enhanced correlated yields at large ∆ η and∆ φ ≃ − ) and temperature ( T ≃
170 K) reached incentral Au+Au collisions at RHIC are enough to observesignatures (enhancement of multiplicity, transverse mo-mentum and particle ratios fluctuations, long-rang cor-relations, strange-hadron abundances,...) of a possiblephase transition from hadronic to quark and gluon de-grees of freedom. Nevertheless, a clear indication of sucha transition has yet to be observed. This has been widelydiscussion in the literature [7, 8, 9, 10]. The principalchallenge remains localization of a Critical Point on theQCD phase diagram. Near the QCD Critical Point, sev-eral thermodynamic properties of the system such as theheat capacity, compressibility, correlation length are ex-pected to diverge with a power-law behavior in the vari-able ǫ = ( T − T c ) /T c , where T c is the critical temperature.The rate of the divergence can be described by a set ofcritical exponents. The critical exponents are universalin the sense that they depend only on degrees of free-dom in the theory and their symmetry, but not on other ∗ Speaker † E-mail: [email protected] details of the interactions. This scaling postulate is thecentral concept of the theory of critical phenomena [11].An important step towards understanding the struc-ture of the QCD phase diagram is systematic analysisof particle production as a function of collision energy,centrality and collisions species. Assuming the system isthermalized, temperature T and baryon chemical poten-tial µ B can be determined. A search for the location ofa possible Critical Point on the { T c , µ c } phase diagram,can be done by varying the beam energy. The proposedBeam Energy Scan (BES) has been tasked to carry outthis search [12].A first test run for Au+Au collisions at √ s NN =9 . π ± , K ± , p, ¯ p ) production, azimuthal anisotropy, in-terferometry measurements [13], and on high- p T spectraof charged hadron production, which are reported in thispaper. II. EXPERIMENT AND DATA ANALYSIS
The data presented here are from Au+Au collisions at √ s NN = 9 . ≃ ± . η and the full azimuthal angle. The sensitive volume ofthe TPC contains P10 gas (10% methane, 90% argon)is regulated at 2 mbar above atmospheric pressure. TheTPC data are used to determine particle trajectories, mo- rMultEntries 4037Mean 69.64RMS 64.6Underflow 0Overflow 0Integral 4037 ch N0 50 100 150 200 250 300 N u m b e r o f eve n t s rMultEntries 4037Mean 69.64RMS 64.6Underflow 0Overflow 0Integral 4037 Reference multiplicity
Au+Au & 9.2 GeV
FIG. 1: Uncorrected charged particle multiplicity distribu-tion measured in the TPC within η < √ s NN = 9 . menta, and particle-type through ionization energy loss(dE/dx). STAR’s solenoidal magnetic field used for thislow energy Au+Au test run was 0.5T. In the future, theTime of Flight (ToF) detector [15] (with 2 π azimuthalcoverage and | η | < .
0) will further enhance the PIDcapability. All events were taken with a minimum biastrigger. The trigger detectors used in this data are theBeam-Beam-Counter (BBC) and Vertex Position Detec-tor (VPD)[16]. The BBCs are scintillator annuli mountedaround the beam pipe beyond the east and west pole-tipsof the STAR magnet at about 375 cm from the center ofthe nominal interaction region (IR), and they have a η coverage of 3 . < | η | < . π )coverage. The VPDs are based on the conventional tech-nology of plastic scintillator read-out by photomultipliertubes. They consist of two identical detector setups veryclose to the beam pipe, one on each side at a distance of | V z | = 5 . √ s NN =9.2 GeV is defined using the uncorrected number ofcharged particle tracks reconstructed in the main TPCover the full azimuth, pseudo-rapidity | η | < | V z | <
75 cm. Those primary tracks which originatewithin 3 cm of the primary vertex (distance of the clos-est approach or DCA) and have transverse momentum p T > . π, K, p, ¯ p ) [13] from embed-ding Monte-Carlo (MC) tracks into real events at the rawdata level and subsequently reconstructing these events.The background for identified particles was estimated in[13]. For charged hadrons it was estimated to be about ∼
10% at low p T and decreases up to ∼
2% at higher p T .Figure 1 shows the uncorrected multiplicity distribu- tion for charged tracks from the real data. The centralityclasses 0–10%, 10–30%, 30–60% include 483, 1049, 1391events with the mean value < N ch > of charged tracks199 . ± . , . ± . , . ± .
4, respectively. The re-sults presented in this paper cover the collision centralityrange of 0-60%. The results from more peripheral colli-sions are not presented due to large trigger inefficienciesin this test run, which bias the data in this region [13].
III. RESULTS AND DISCUSSION
1. Spectra
The transverse momentum spectrum of hadrons pro-duced in high energy collisions of heavy ions reflects fea-tures of constituent interactions in the nuclear medium.The medium modification is one of the effects (recombi-nation, coalescence, energy loss, multiple scattering,...)that affects the shape of the spectrum. The properties ofthe created medium are experimentally studied by vari-ation of the event centrality and collision energy.Figure 2 shows the charged hadron yields in Au+Aucollisions at √ s NN = 9 . | η | < . p T . The resultsare shown for the collision centrality classes of 0-10%, 10-30%, 30-60%, and 0-60%. The distributions are measuredin the momentum range 0 . < p T < . GeV/c. Themultiplied factor of 10 is used for visibility. As seen fromFig. 2 spectra fall more than four orders of magnitude.The shape of the spectra indicates the exponential andpower-law behavior at 0 . < p T < . GeV/c and p T > . GeV/c, respectively.The centrality dependence of < p T > is of interest,as for a thermodynamic system this quantity correlateswith the temperature of the system, whereas dN/dη ∝ ln ( √ s NN ) has relevance to its entropy [17]. The meanvalues of the transverse momentum < p T > for thecentrality classes 0–10%, 10–30%, 30–60%, and 0–60%are found to be 553 . ± . . ± . . ± . . ± . < p T > slowly increases with centrality.Similar behavior is observed for pions at √ s NN = 9 . < p T > vs. dN ch /dη could be associated with enhancement of multiparticleinteractions in the medium.
2. The R mult/ (0 − and R CP ratios The ratio of transverse momentum yields for differentcentralities allows us to study features of constituent in-teractions in the medium depending on the scale. Strongsensitivity of the ratio R mult/minbias ( ≃ . dN ch /dη = 1 .
97 and 9.01) at p T ≃ K S , Λ) pro-duction [18]. apt Entries 229013 (GeV/c) T p0 1 2 3 4 - ( G e V / c ) T N / d y dp ) d T p π / N / ( -5 -3 -1 apt Entries 229013
STAR × × × -1 × Preliminary+X ± h → Au+Au = 9.2 GeV NN s |<0.5 η | FIG. 2: Mid-rapidity ( | η | < .
5) transverse momentum spec-tra for charged hadrons produced in Au+Au collisions and en-ergy √ s NN = 9 . Figure 3 shows the R mult/ (0 − ratio of multiplicitybinned p T spectra to multiplicity-integrated (0 − R mult/ (0 − = F scale d N mult / πp T dydp T d N − / πp T dydp T , (1)where the factor F scale is defined as follows F scale = N − evnt < N − ch >N multevnt < N multch > . (2)As seen from Fig. 3 the ratio is sensitive to centrality forhigh p T . It increases from 0.6 to 1.2 at p T ≃ . GeV/cfor low and high centralities, respectively.Figure 4 shows the dependence of the R CP ratio ofyields for the central (C) 0–10% and the peripheral (P)30–60% multiplicity classes on the transverse momentum R CP = d N C / πp T dydp T / < N Cbin >d N P / πp T dydp T / < N Pbin > . (3) apt1
Entries 1443 (GeV/c) T p0 1 2 3 4 m u l t / ( - % ) R apt1 Entries 1443
STARPreliminary +X ± h → Au+Au = 9.2 GeV NN s |<0.5 η |0-10(%) FIG. 3: The R mult/ (0 − ratio of charged hadron yieldsin Au+Au collisions at mid-rapidity ( | η | < .
5) and energy √ s NN = 9 . Errors shown for data are the quadrature sum of sta-tistical uncertainties. The ratio increases with transversemomentum. The similar trend is observed in Au+Aucollisions at √ s NN = 200 GeV for p T < . R CP are different [20]. For p T > . GeV/c the ratio is higherthan unity, while the R CP at 200 GeV never go aboveunity, it decreases for p T > . < p T <
12 GeV/c.
3. Constituent energy loss
The energy loss of particles created in heavy ion col-lisions characterizes properties of the nuclear medium.The nuclear modification factor R AA measured at RHICat √ s NN = 62 . ,
130 and 200 GeV strongly shows a sup-pression of the charged hadron spectra at p T > R AuAu for peripheralcollisions at √ s NN = 200 GeV is close to unity at p T > apt1 Entries 1013 (GeV/c) T p0 1 2 3 C P R apt1 Entries 1013
STAR Preliminary+X ± h → Au+Au = 9.2 GeV NN s |<0.5 η | FIG. 4: The R CP ratio of charged hadron yields in Au+Aucollisions at mid-rapidity ( | η | < .
5) and energy √ s NN =9 . and a Critical Point [24].The measured spectra (Fig.2) allow us to estimateconstituent energy loss in charged hadron productionin Au+Au collisions at √ s NN = 9 . √ s NN = 200 GeV. The estimations are based on amicroscopic scenario of particle production proposed in[24]. The approach relies on a hypothesis about self-similarity of hadron interactions at a constituent level.The assumption of self-similarity transforms to the re-quirement of simultaneous description of transverse mo-mentum spectra corresponding to different collision en-ergies, rapidities, and centralities by the same scalingfunction ψ ( z ) depending on a single variable z . The scal-ing function is expressed in terms of the experimentallymeasured inclusive invariant cross section, the multiplic-ity density, and the total inelastic cross section. It isinterpreted as a probability density to produce an in-clusive particle with the corresponding value of z . Thescaling variable z is expressed via momentum fractions( x , x , y a ), multiplicity density, and three parameters( δ, ǫ, c ). The constituents of the incoming nuclei carryfractions x , x of their momenta. The inclusive particlecarries the momentum fraction y a of the scattered con-stituent. The parameters δ and ǫ describe structure of thecolliding nuclei and fragmentation process, respectively.The parameter c is interpreted as a ”specific heat” ofthe created medium. Simultaneous description of differ-ent spectra with the same ψ ( z ) puts strong constraintson the values of these parameters, and thus allows fortheir determination. It was found that δ and c are con-stant and ǫ depends on multiplicity. For the obtained FIG. 5: The momentum fraction y a for charged hadron pro-duction in Au+Au collisions at mid-rapidity ( | η | < .
5) as afunction of the energy, centrality collision and hadron trans-verse momentum. values of δ, ǫ and c , the momentum fractions are de-termined to minimize the resolution Ω − ( x , x , y a , y b ),which enters in the definition of the variable z . The sys-tem of the equations ∂ Ω /∂x = ∂ Ω /∂x = ∂ Ω /∂y a = ∂ Ω /∂y b = 0 was numerically resolved under the con-straint ( x P + x P − p/y a ) = ( x M + x M + m/y b ) ,which has sense of the momentum conservation law of aconstituent subprocess [24].The scaling behavior of ψ ( z ) for charged hadrons inAu+Au collisions at √ s NN = 200 and 9.2 GeV is con-sistent with δ = 0 . c was found to be 0.11. This value is less than one(0.25) determined from pp data [24]. In this approach theenergy loss of the scattered constituent during its frag-mentation in the inclusive particle is proportional to thevalue (1 − y a ).Figure 5 shows the dependence of the fraction y a on thecentrality of Au+Au collision and transverse momentumat √ s NN = 9 . y a demon-strates a monotonic growth with p T . It means that theenergy loss associated with the production of a high- p T hadron is smaller than for hadron with lower transversemomenta. The decrease of y a with centrality collisionrepresents larger energy loss in the central collisions ascompared with peripheral interactions. The energy dissi-pation grows as the collision energy increases. It is esti-mated to be about 50% at √ s NN = 9 . √ s NN = 200 GeV for p T ≃ z (low p T ) is governed by the single parameter c .The value of c was found to be constant in Au+Au col-lisions at √ s NN = 9 . , . , IV. SUMMARY AND OUTLOOK
In summary, we have presented the first STAR resultsfor charged hadron production in Au+Au collisions at √ s NN = 9 . . < p T < . GeV/c. The centrality dependence of thehadron yields and ratios are studied. We observed thatthe sensitivity of the ratios R mult/ (0 − and R CP tocentrality is enhanced at high p T . Hadron yields can beused to estimate of a constituent energy loss. The en-ergy loss of the secondary constituents passing through the medium created in the Au+Au collisions was esti-mated in the z -scaling approach. It depends on the col-lision energy, transverse momentum, and centrality. Itwas shown that the energy loss increases with the colli-sion energy and centrality, and decreases with p T .These results provide an additional motivation for theBeam Energy Scan program at the RHIC [12]. At theSTAR experiment, the large and uniform acceptanceand extended particle identification (TPC, ToF) is suit-able for a Critical Point search at low energy √ s NN =5 −
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