J. Adams 1 , R. Bellwied 2 , M. Heinz 3 , C. Mironov 4 , R. Witt 3 for the STAR Collaboration

1. K0s (V0 vs NLO) z z Strange particle production mechanisms in pp collisions at RHIC J. Adams1, R. Bellwied2, M. Heinz3, C. Mironov4, R. Witt3 for the STAR Collaboration 1University of Birmingham, 2Wayne State University, 3University of Bern, 4Kent State University Das NA49-Experiment untersucht die Produktion von Hadronen in relativistischen Kernreaktionen (p+p, p+A und A+A) am CERN-SPS. Die vier hochauflösenden großvolumigen Spurendriftkammern (TPCs) erlauben den Nachweis neutraler seltsamer Hadronen in einem weiten Phasenraumbereich durch die Rekonstruktion ihrer geladenen Zerfallsprodukte. Die Erhöhung der Produktion seltsamer Teilchen in Kern-Kern- im Vergleich zu Proton-Proton-Stößen wird als mögliche Signatur des Quark-Gluon-Plasmas diskutiert. Die p+p-Daten früherer Experimente weisen jedoch zu hohe statistische und systematische Fehler auf, um signifikante Aussagen zuzulassen. Im Rahmen des Experiments NA49 wurden deshalb die folgenden neutralen seltsamen Teilchen (V0) in inelastischen p+p-Kollisionen bei 158 GeV gemessen: Particle production in elementary proton-proton collisions is widely recognized as a well understood problem in the string fragmentation process. The universality of the fragmentation process between e+e- and pp collisions has been recently confirmed by Kniehl, Kraemer and Poetter (KKP) in a detailed paper based on pion and charged hadron production [1]. Using the factorization theorem with the proper next-to-leading order corrections, the calculation seems to describe the pion and charged hadron production in RHIC pp data well [2,3]. We therefore expected the strange particle production to also be described by the standard fragmentation descriptions. Our measurements are based on two STAR Ph.D.theses [4,5]. Comparison to PYTHIA The most ubiquitous model available for the description of hadron-hadron collisions is PYTHIA. In Fig.1 we have used PYTHIA v6.221 (default settings, with MSEL=1) to compare to measured strange particle pT spectra (solid line). The agreement only improved when the K-factor was raised to three (dashed line). Agreement with the multiplicity dependence of the <pt> could also not be reached with the default setting (solid line in Fig.2), but only when increasing the kT-factor to a value of 4 GeV/c (dashed line in fig.2) Are these large values for kT and K unphysical ? Do they signal new physics in strangeness production ? The kT smearing is expected to increase for intermediate pt fragments and at moderate (RHIC) incident energies. The K-factor which quantifies the importance of next-to-leading order (NLO) effects should also go up at RHIC energies. Eskola et al. showed this in QM02 [6] as seen in Fig.3. Therefore one would expect actual NLO calculations to perform better than the default PYTHIA for strange particle production. References: [1] B.Kniehl et al.,Nucl.Phys.B597 (2001) 337 [6] K.Eskola et al., Nucl.Phys.A713 (2003) [2] C.Adler et al., Phys.Rev.Lett.91 (2003) 241803 [7] D.DeFlorian et al., Phys.Rev.D57 (1998) 5811 [3] J.Adams et al., Phys. Rev.Lett.91 (2003) 172302 [8] S.Albino et al., hep-ph/0502188 [4] M. Heinz, University of Bern (2005) [9] C. Bourrely et al., Phys. Rev.D68 (2003) 014003 [5] J.Adams, University of Birmingham (2005) [10] K.Werner et al., hep-ph/0506232 Comparison to next-to-leading order (NLO) calculations Werner Vogelsang used the KKP parametrization [1] which was successfully applied to the PHENIX p0 and the STAR charged hadron spectra, and used a Lambda fragmentation function by Vogelsang and DeFlorian [7] in order to describe our strange baryon data. Fig.4 shows that the NLO calculations describe neither shape nor magnitude of the strange particle production very well, and that the disagreement is worse for the heavier baryon. The agreement gets significantly better when quark separated contributions to the fragmentation function are used according to Albino, Kniehl and Kraemer (AKK) [8] and Bourelly and Soffer (BS) [9]. Fig. 5 shows the agreement of the STAR K0 data with the AKK parametrization in comparison to UA1 data at sqrt(s) = 630 GeV, Fig.6 shows the quark separated fragmentation function contributions to proton and Lambda spectra in sqrt(s)=91.2 GeV e+e- collisions according to [9]. It is interesting to note that in Fig.5 the quark separation is more important at lower (RHIC) energies than at higher CERN energies, and that in Fig.6 the heavy quark contribution to the light baryon production is not negligible, which is in agreement with the quark separated contributions to the K0 spectrum in Fig.5 according to AKK. Comparison to EPOS Finally we compare to the new EPOS model [10], which takes into account partonic interactions between strings, partons, and partonic remnants of projectile and target (i.e. diquarks in the case of proton-proton interactions). The agreement with the data is remarkable (Fig.7). Summary The good agreement between data and sophisticated NLO calculations, as well as the EPOS calculations, points at the necessity of higher order corrections to the simple string fragmentation picture for the production of strange baryons and mesons. The contributions of each quark flavor to the fragmentation has to be taken into account separately and effects from di-quark remnants or cascading soft non-valence quarks are non-negligible. Therefore strange particles which are produced abundantly at RHIC in pp collisions provide a good test for the quantitative determination of the production mechanism in elementary collisions. Simple string fragmentation is not sufficient to describe hadron production even in pp collisions. Fig.4 Fig.1 Fig.2 Fig.5 Fig.6 Fig.7 Fig.3 STAR