aa r X i v : . [ nu c l - e x ] J un Proc. 23rd Winter Workshop onNuclear Dynamics (2007) 000–000 ¯ on Nuclear Dynamics ¯¯ Big Sky, Montana, USAF¯ebruary 11–18, 2007
Resonance Production in Jets
Christina Markert for the STAR Collaboration University of Texas, Austin, Texas 78712, USA
Abstract.
Hadronic resonances with short life times and strong coupling tothe dense medium may exhibit mass shifts and width broadening as signaturesof chiral symmetry restoration at the phase transition between hadronic andpartonic matter. Resonances with different lifetimes are also used to extract in-formation about the time evolution and temperature of the expanding hadronicmedium. In order to collect information about the early stage (at the phasetransition) of a heavy-ion collision, resonances and decay particles which areunaffected by the hadronic medium have to be used. We explore a possiblenew technique to extract signals from the early stage through the selection ofresonances from jets. A first attempt of this analysis, using the reconstructed φ (1020) from 200 GeV Au+Au collisions in STAR, is presented. Keywords: resonance, jets, lifetime, strange, freeze-out, medium
PACS:
1. Introduction
Yields of resonances measured via their hadronic decays are sensitive to the hadroniclifetime of the nuclear medium in a heavy-ion reaction. Together with the pion HBTlifetime measurement (∆ τ = 5 −
12 fm/c) [ 1], which determines the time fromthe beginning of the collision to kinetic freeze-out, we, extract a partonic lifetime,under the assumption that the chemical freeze-out occurs at hadronization [ 2].The extended hadronic medium which exists between chemical and kinetic freeze-out in heavy-ion reactions may change the yield and spectra of resonances dueto the re-scattering of the decay particles and possible regeneration of resonances.The resonance over non-resonance ratios from STAR [ 2] can be described with amicroscopic transport model (UrQMD) which assumes a lifetime between chemicaland kinetic freeze-out of ∆ τ = 10 ± τ > T = 0-2 GeV/c) of the resonance spectra. Therefore high momentum resonances andtheir decay particles are less likely to be affected by the hadronic medium. Inorder to test the impact of the partonic medium on resonance properties we needto reconstruct high momentum resonances which are produced early. This mightbe possible through the selection of resonances from the away-side jet of a triggereddi-jet event.
2. Jet Resonance Correlations
A leading trigger-particle correlation analysis requires a high momentum triggerparticle to identify the jet axis and the jet side which is less affected by the medium(same-side). Therefore, the away-side (∆ φ = π ) correlations will measure themedium modified jet. High momentum resonances from the away-side jet are iden-tified via the angle with respect to the jet axis or leading particle (see Figure 1).A high transverse momentum resonance in the away-side jet cone is likely to beproduced early, which, depending on its formation time, can interact with the earlypartonic medium, but leaves the medium fast enough to not exhibit any interactionin the late hadronic phase. The formation time of a resonance in the string frag-mentation process depends on the momentum fraction z carried by the resonance.In addition there is a parton and resonance mass dependence which leads to shorterformation times for heavy resonances. Two approaches were proposed recently:firstly, a study based on the string fragmentation implementation in PYTHIA [ 11];secondly, a quantum mechanical treatment of heavy meson formation in heavy-ioncollisions [ 12]. Both cases demonstrate that the probability of high momentumheavy hadron (or resonance) formation in the partonic medium is finite.Quantitative studies of resonance properties such as yield, mass, width, and branch-ing ratio as a function of resonance momentum, emission angle, jet energy, and jettag, will directly address the question of chiral symmetry restoration. The lowmomentum resonances produced at an angle of ∆ φ = 1 / π or 3 / π with respectto the jet axis or leading particle are identified as late produced thermal reso-nances from the bulk matter of the collision. They predominantly interact in thelate hadronic medium. Therefore their masses and widths are expected to be inagreement with vacuum conditions unless phase space effects from late regener-ation change the shape of the invariant mass signal. The high p T same-side jetresonances are also expected to exhibit the vacuum width and mass because theinitial quark is expected to fragment outside of the medium (surface bias effect).Therefore a differential measurement of high momentum resonances as a function ofesonance Production in Jets 3the angle to the jet axis might have a built-in reference system and may not requirecomparison to measurements in proton+proton collisions, which are very statisticslimited for the rare hadronic resonance channels. Fig. 1.
Sketch of jet fragmentation into resonances (Λ*, φ (1020),...) in the mediumcreated in a heavy-ion collision. Same-side correlations of resonances are not affectedby the medium, whereas the away-side high p T resonance might be affected by theearly (chiral restored) medium. Thermal resonances, which are affected by the latehadronic medium are at π /2 with respect to the trigger particle.
3. Resonance Correlations in STAR
We recently initiated a first attempt to study the high momentum charged hadron- φ (1020) resonance correlation using the STAR detector. We are using the φ (1020)as the associated particle since its reconstructed mass spectrum has the largest sig-nificance of all short lived resonances in STAR. However the lifetime of the φ (1020)is about 45 fm/c, which means that the majority will decay outside of the medium.The number of Au+Au events analyzed (4.5M 0-20% most central) is not sufficientto place an effective high momentum cut on the φ (1020) spectrum and therefore weare not sensitive to mass shifts or width broadenings. The momentum of the kaoncandidates for the φ (1020) reconstruction is restricted to p T [0.2-1.0] GeV/c in orderto achieve clean pid in the TPC. The mean transverse momentum of the φ (1020)is h p T i ∼ . φ (1020) resonances arefrom the thermal medium rather than from an early fragmenting jet. In the futurewe need to select the higher momentum φ (1020)s with better significance by usingthe additional Time of Flight (TOF) detector [ 8] which allows us to identify kaonsup to p = 1.5-2 GeV/c. We will show in these proceedings the analysis techniquesto study resonances from jets and discuss the potential results in general terms. Christina Markert et al.
4. Angle dependent Invariant Mass Distribution of φ (1020) Hadrons with p T > trigT /event = 0.12)and correlated with φ (1020)s ( h p T i ∼ . φ (1020) invariant mass distributions before and after mixed-eventbackground subtraction for 4.5 Million 0-20% most central Au+Au collisions con-taining at least one charged hadron with p T > φ (1020) signal peak is about 230,000 ( φ (1020)/event = 0.05). The derived φ (1020) mass is m = 1.0188 ± and the width is Γ = 4.0 ± from a Gaussian combined with a linear fit, which are in agreement withthe PDG value, folded with the detector momentum resolution and the energy lossin the detectors. ] ) [GeV/c - +K + (K inv m0.98 1 1.02 1.04 1.06 1.08 1.1 - E n t r i e s x -100102030405060 ] ) [GeV/c - +K + (K inv m0.98 1 1.021.041.061.08 1.1 - E n t r i e s x = 200 GeV NN s0-20% central Au+Au (1020) f STAR Preliminary
Fig. 2.
Invariant mass distributions before and after mixed-event background sub-traction for φ (1020) from 4.5 M 0-20% most central 200 GeV Au+Au events in-cluding at least one hadron with p T > φ (1020) angular orien-tation with respect to the leading (jet) particle. Two quadrants in the jet plane:1. same-side ∆ φ = [ − / π, +1 / π ]: 2. away-side ∆ φ = [+3 / π, +5 / π ] and twoquadrants out of the jet plane: 3. with ∆ φ = [+1 / π, +3 / π ] and 4. ∆ φ =+[5 / π, +7 / π ]. Figure 3 shows the φ (1020) invariant mass distributions aftermixed-event background subtraction for the same-side (left) and away-side (right)angular correlations with respect to the trigger hadron. Figure 4 shows the φ (1020)invariant mass signal for the out of jet plane angular correlations with respect tothe trigger hadron. The masses and widths of the φ (1020) signals for the differentangular selections are in agreement with the PDG value, folded with the detectormomentum resolution. The φ (1020) yields in the mass region of 1019.5 ± for the four angle ranges are shown in table 1 with the statistical errors. The sys-esonance Production in Jets 5tematical error due to the normalization of the background, the invariant range andthe linear fit is on the order of 10%. The yield of the signal on the away-side is26% ±
19% higher than on the same-side, which means that there is a trend of alarger resonance production in the away-side ∆ φ correlation compared to the same-side, which might be due to energy conservation (trigger bias). This would meanthat it is less likely to produce a massive resonance if the high momentum particlein a jet takes a large fraction of the energy. From Figure 5 we derive that thesplitting of ∆ φ into 4 equal parts might not contain the full jet on the away-side.The size of the same- and away-side jet in ∆ φ has to be studied in more detail. ] ) [GeV/c - +K + (K inv m0.98 1 1.02 1.04 1.06 1.08 1.1 - E n t r i e s x -505101520 Y0 9289.8 – – -44881.7 C0 9.226 – – – = 200 GeV NN s0-20% central Au+Au (1020) f STAR Preliminaryin jet-plane same-side p – = 0 f D ] ) [GeV/c - +K + (K inv m0.98 1 1.02 1.04 1.06 1.08 1.1 - E n t r i e s x -505101520 Y0 9488.1 – – -25252.6 C0 9.675 – – – = 200 GeV NN s0-20% central Au+Au (1020) f STAR Preliminaryin jet-plane away-side p – p = f D Fig. 3. φ (1020) invariant mass distributions after mixed-event background subtrac-tion for their same-side (left) and away-side (right) angular correlation with respectto the trigger hadron of p T > ] ) [GeV/c - +K + (K inv m0.98 1 1.02 1.04 1.06 1.08 1.1 - E n t r i e s x -505101520 Y0 9416.6 – – -39502.8 C0 9.030 – – – = 200 GeV NN s0-20% central Au+Au (1020) f STAR Preliminaryout of jet-plane side1 p – p = 1/2 f D ] ) [GeV/c - +K + (K inv m0.98 1 1.02 1.04 1.06 1.08 1.1 - E n t r i e s x -505101520 Y0 9372.3 – – -21180.8 C0 9.874 – – – = 200 GeV NN s0-20% central Au+Au (1020) f STAR Preliminaryout of jet-plane side2 p – p = 3/2 f D Fig. 4. φ (1020) invariant mass distributions after mixed-event background sub-traction for the out of jet plane angular correlation ∆ φ = [1 / π, +3 / π ] (left) and∆ φ = [5 / π, +7 / π ] (right) with respect to the trigger hadron of p T > , π ] 231085 ± ± − / π, +1 / π ] 51385 ± ± / π, +5 / π ] 64498 ± ± / π, +3 / π ] 61043 ± ± / π, +7 / π ] 54893 ± ± Table 1. φ (1020) yields in the mass region of 1019.5 ± for the four angleranges.
5. Charged hadron- φ (1020) Correlations in STAR In order to plot the ∆ φ distribution between the hadron trigger particle (p T > φ (1020) meson ( h p T i ∼ . φ (1020) mesons iden-tified via an invariant mass cut (1019.4 ± ) on the decay kaon pair. The φ (1020) signal/background is only 2.2%. In order to subtract the background, thesame angular ∆ φ correlations are generated from a mixed event sample and thetwo histograms are subtracted and normalized using the zero yield at minimum(ZYAM) [ 10]. fD -1 0 1 2 3 4 ) fD d ( d N · t r i g N STAR Preliminary mixed event (1020) f h- · (1020) - C f h- > 4 GeV/c trigT p > ~ 0.9 GeV/c assT
Fig. 5.
STAR, angular correlation of hadron- φ (1020) resonance. Hadron triggerp T > φ (1020) h p T i ∼ . φ (1020) correlation. Thispreliminary result is not corrected for elliptic flow (v ) contribution and has nosystematical error estimation. However the trend of a larger resonance productionin the away-side of the ∆ φ correlation compared to the same side is present as itis in the angle dependent mass distribution in the previous chapter, which mightbe due to energy conservation (trigger bias). Similar results were shown previouslyesonance Production in Jets 7 Fig. 6.
STAR, angular correlation of hadron-hadron for 0-12% cental Au+Au andd+Au collisions. Hadron trigger 4 GeV/c < p T < < p T <
6. Conclusions
Hadronic decays of resonances with different lifetimes are used to extract informa-tion about the time evolution and temperature of the expanding hadronic medium.To derive any additional information about the early partonic stage of a heavy-ion collision, resonances and decay particles need to be unaffected by the hadronicmedium. These proceedings describe a first attempt to select resonances from jets,through angular correlation of a trigger hadron and an associated resonance particle.Due to the low momenta of the identified decay particles only thermally producedresonances are presently reconstructed. Further studies on selection criteria will bedone to select higher momentum resonances and the possibility of a full jet recon-struction with the STAR detectors will be explored. Further theoretical studies offormation time are needed to extract the momentum range of resonances which areformed in, and modified by, the early partonic medium.
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