aa r X i v : . [ nu c l - e x ] J un Proc. 21st Winter Workshop onNuclear Dynamics (2005) 000–000 ¯ on Nuclear Dynamics ¯¯ San Diego, California,USAM¯ arch 12–18, 2006
Resonance Production in RHIC Collisions
C. Markert for the STAR collaboration Physics Department, Kent State University, Kent, OH 44242, USA
Abstract.
Results of resonance particle production measured at RHIC in √ s NN = 200 GeV Au+Au collisions are compared to measurements in p+pand d+Au collisions in order to verify the existence of an extended hardron-ically interacting medium. Yield and momentum distributions of resonancesmaybe modified during the fireball lifetime due to resonance decay and thesubsequent rescattering of their decay daughters as well as the regeneration ofresonances from their decay products. Modified momentum spectra in heavyion collisions may change the nuclear modification factor R AA . The influenceon the elliptic flow v due to late regeneration of resonances is discussed. Keywords: resonance, lifetime, strange, freeze-out, medium
PACS:
1. Introduction
If the life time of the hadronic medium created in a heavy ion reaction is longenough (on the order of a few fm/c) resonances decay inside the medium and theirdecay product can therefore interact with the medium as well. In a dense hadronicmedium resonances can also be regenerated from their decay products. This isa dynamical process of creating and decaying resonances (detailed balance). Themeasured resonances are therefore a composition of early and late produced anddecayed resonances. The largest fraction comes from the late decay since the prob-ability of another interaction of the decay particles from early decays is larger thanfrom late decays. Therefore by using the yield of resonances and knowing their de-cay, re-scattering and regeneration cross section we are able to estimate the lifetimeof the hadronic phase.A resonance signal loss in the low momentum region is caused by a largerre-scattering than regeneration cross section. This will also influence the low mo-mentum region of the nuclear modification factor R AA spectrum, which is the ratioof the heavy ion transverse momentum spectrum divided by the p+p spectrumscaled by the number of binary collisions. C. MarkertMore than 60% of the stable particles originate from weak and resonance de-cays. This raises the question how the elliptic flow v of stable particles is affectedif regeneration of resonances after chemical freeze-out is present. Furthermore re-combination models predict an enhancement of v for resonances due to the largerv contribution from regenerated resonances. The yield would scale according toconstituent quark scaling of the recombining hadrons, e.g. K + π → K(892) (2+2=4)and Λ + π → Σ(1385) (3+2=5). The observed enhancement of v would depend onthe fraction of regenerated resonances [ 1].
2. Rescattering and Regeneration Cross Sections
Figure 1 shows the resonance to stable particle ratios for p+p and Au+Au collisionssystems [ 2, 3]. The ratios are normalized to unity for p + p collisions. The deviationof the ratio from unity in Au+Au collisions indicate a late hadronically interactingmedium where decay of resonances and the rescattering of the decay particles islarger than the regeneration of resonances. A ranking of the overall regenerationover rescattering cross section can be deduced as follows: R Λ(1520) < R K (892) Resonance to stable particle ratios of Σ(1385) / Λ, φ/K − , K(892) / K − andΛ(1520) / Λ for p + p and Au + Au collisions at √ s NN = 200 GeV. The ratiosare normalized to unity in p + p collisions. The quadratic sum of statistical andsystematic uncertainties are included in the error bars. Thermal and UrQMD modelpredictions are presented as well [ 4, 5, 6].Sascha Vogel showed at this workshop a microscopic model calculation (UrQMD)of the regeneration cross sections of resonances and confirms the ranking as derivedfrom the data [ 7]. This model predicts a signal loss in the low momentum region dueesonance Production in RHIC Collisions 3to rescattering. A comparison of the transverse momentum spectrum of p+p andAu+Au collisions using the nuclear modification factor R AA shows for the K(892)a larger suppression in the momentum region from p T = 0-2 GeV compared to thelong lived φ and the K (Figure 2). These data support the concept of a hadronicinteracting medium after chemical and before kinetic freeze-out which changes themeasured resonance yield and the momentum spectra. Thermal model predictionsby W. Florkowski et. al show a shift to higher values compared to the measuredK(892) transverse momentum distribution in the low momentum region [ 8]. There-fore nuclear suppression factors (R AA ) of resonances are not directly comparableto stable particles as long as the momentum dependent signal loss of resonances inthe hadronic phase is not taken into account. T p R _ AA )/2 + +h - (h f S0 KK*(892) participants scaling binary scalingSTAR Preliminary Fig. 2. The ratio of the transverse momentum of Au+Au divided by p+p collisionsnormalized to the number of binary collisions.The low mean transverse momentum for heavy multi strange particles (Ξ and Ω)in Au+Au collisions was interpreted as a sign of early decoupling from the hadronicsystem at high temperature and low transverse velocity. In Figure 3 the Σ(1385)shows the same trend as Ξ and Ω. Since the regeneration cross section of the Σ(1385)from π +Λ is very large [ 7] we expect to measure more late produced Σ(1385), i.e.resonances produced close to the kinetic freeze-out. This would suggest a latedecoupling of the Σ(1385) from the hadronic phase, and thus possibly contradictthe simple connection between and the decoupling parameters. Based on the C. Markertprimordial Λ spectra, a Λ from a Σ(1385) decay is not expected to exhibit muchearly decoupling. Thus it is important to measure the Ξ(1530) resonance in heavyion collisions, verify its regeneration cross section and estimate the contribution ofΞ’s from the Ξ(1530) decays. [ G e V / c ] æ(cid:13) T p Æ(cid:13) Mass [GeV/c ] p s0 K - K r * K p f f L - X * S * L - W Central Au+AuMinBias p+pBlast-wave Parametrization STAR Preliminary 0.05 =0.59 æ(cid:13) T b Æ(cid:13) 10 MeV, =89 fo T =170 MeV, fo T =0 T b –(cid:13) –(cid:13) Fig. 3. The h p T i vs particle mass measured in min-bias p + p and most central Au + Au collisions at √ s NN = 200 GeV. Statistical and systematic uncertaintiesare shown [ 2, 3]. 3. d+Au Collisions Since no extended hadronic medium for d+Au collisions is created, the momentumdistribution of p+p and d+Au collisions for resonances are expected to be similarcompared to the stable particles. Figures 4 shows the preliminary ratio of the trans-verse momentum spectra measured in d+Au divided by p+p spectra normalized tothe number of binary collisions. The R dAu of Σ(1385) seams to follow the protonR dAu (right), while the K(892) deviates from the K and π in the low momentumregion. This result is unexpected, if we assume no rescattering in d+Au. The datahave to be further investigated and the yields have to be checked for consistency. 4. Elliptic Flow v Anisotropic flow results are often used as a strong evidence for the formation ofthe QGP in Au+Au Collisions at RHIC. The magnitude and centrality dependenceof the elliptic flow v is used as a proof of early thermalization. The so-called”mass splitting”, the characteristic dependence of v (p T ) on the particle mass, iswell described when using a QGP Equation of State. In addition the constituentesonance Production in RHIC Collisions 5 [GeV/c] T p [GeV/c] T p d A u R participant scaling binary scaling STAR PreliminarydAu minbias + +K - K + p + - p TOF )/2 + +h - (h (892) K* (892) – K* [GeV/c] T p [GeV/c] T p d A u R participant scaling binary scaling STAR PreliminarydAu minbias pTOF p+ )/2 + +h - (h * S * + S Fig. 4. The ratio of the transverse momentum of d+Au divided by p+p collisionsnormalized to the number of binary collisions for meson (left) and baryons (right).quark scaling in the intermediate transverse momentum region is often cited asa proof of deconfinement and partonic (pre-hadronic) collectivity. One thereforewould like to study the elliptic flow of resonances in order to test the probability offormation at the early stage of the collision and the effect the expansion dynamics ofthe source including rescattering and regeneration of resonances will have on theirv . The elliptic flow of the long lived φ meson resonance shows the expected massdependence in the low momentum region and a constituent quark scaling similar tothe K S meson in the intermediate transverse momentum region. In the near futurewe will analyze the elliptic flow of the short lived K(892) and ∆(1232) resonances. 5. Conclusions Resonances are a unique tool to probe the hadronic medium in heavy ion collisions.It allows us to estimate the lifetime of the hadronically interacting medium and de-rive the rescattering over regeneration cross section ranking for different resonances.Using a microscopic models actual regeneration cross sections can be determinedbased on the data. Further more we gain in a better understanding of the hadronicinteraction probabilities which helps us to distinguish between early and late de-coupling of particle species from the hadronic medium. Acknowledgments I would like to thank Sascha Vogel and Marcus Bleicher for their detailed UrQMDanalysis to get a better understanding of hadronic cross sections in terms of reso-nance regeneration. And I also would like to thank the STAR collaboration for thesupport in presenting this data. C. Markert (GeV/c) T p0 1 2 3 4 5 v S0 K L + Lf STAR Preliminary Fig. 5. The v of φ meson compared to K S meson and Λ baryon as a function ofp T . Statistical and systematical errors are included [ 10]. References 1. C. Nonaka et. al., Phys.Rev. C69 (2004) 031902, nucl-th/0312081.2. J. Adams, et al. , (STAR collboration), Subm. to Phys. Rev. Lett,nucl-ex/0604019.3. Sevil Salur, PhD Thesis Yale University 2006.4. F. Becattini, Nucl. Phys. A702 , 336 (2002).5. M. Bleicher et al. , Phys. Lett. B530 , 81 (2002).6. M. Bleicher and H. St¨ocker, J. Phys. G30 (2004) 111.7. S. Vogel, These proceedings.8. W. Florkowski, W. Broniowski and P. Bozek, J.Phys.