Recent Results of NA49

Transcription

1 Recent Results of NA49 M. Gaździcki for the NA49 Collaboration Institut für Kernphysik, Univetsität Frankfurt D 6486 Frankfurt, Germany Abstract Results on the energy dependence of pion and strangeness production as well as charge fluctuations in central Pb+Pb collisions at 4, 8 and 158 A GeV are presented and compared with results at lower and higher energies. The measured behaviour is consistent with the hypothesis that a transient state of deconfined matter is created in Pb+Pb collisions for energies larger than about 4 A GeV. 1 Introduction and Experiment The primary purpose of the heavy ion programme at the CERN SPS is the search for evidence of a transient deconfined state of strongly interacting matter during the early stage of nucleus nucleus collisions. When sufficiently high initial energy density is reached a formation of a state of quasi free quarks and gluons, the quark gluon plasma (QGP) is expected. A key problem is the identification of experimental signatures of the QGP creation [1]. Numerous proposals were discussed in the past [2], however the significance of them has come under renewed scrutiny. A possible, promising strategy is a study of the energy dependence of pion and strangeness yields. It was suggested [3, 4] that the transition may lead to anomalies in this dependence: a steepening of the increase of the pion yield and a non monotonic behaviour of the strangeness to pion ratio. First experimental results from Pb+Pb (Au+Au) collisions at top SPS (158 A GeV) and AGS (11 A GeV) energies have suggested [3] that anomalies in pion and strangeness production should take place between these energies. The need for further study of this hypothesis motivated an energy scan at the CERN SPS. Within this ongoing project NA49 has recorded central Pb+Pb collisions at 4 and 8 A GeV during the heavy ion runs in 1999 and 2, respectively. The data at the top SPS energy (158 A GeV) were taken in the previous SPS runs. The energy scan programme is planed to be finished in 22 by taking a data at 2 and 3 A GeV. In this paper we report results on the energy dependence of pion and strangeness production as well as net electric charge fluctuations.

2 The NA49 experimental set up [5] consists of four large volume Time Projection Chambers (TPCs). Two of them, Vertex TPCs, are placed in the magnetic field of two super-conducting dipole magnets and allow a precise measurement of particle momenta p (σ(p)/p 2 (.3 7) 1 4 (GeV/c) 1 ) and electric charge. The other two TPCs, positioned downstream of the magnets were optimised for high precision detection of ionization energy loss de/dx (relative resolution of about 4%) and consequently provide a means of measuring the particle mass. The TPC data yield spectra of identified hadrons above midrapidity. The particle identification capability of the MTPCs is augmented by two Time of Flight (TOF) detector arrays (resolution σ tof 6 ps). The combined TPC and TOF information allow for the measurement of charged kaon spectra at midrapidity. Central collisions (7%, 7% and 5% of all inelastic interactions at 4, 8 and 16 A GeV, respectively) were selected by a trigger using information from a downstream calorimeter, which measured the energy of the projectile spectator nucleons. The geometrical acceptance of the Veto Calorimeter was adjusted for each energy by the proper setting of a collimator. 2 Pion Production dn/dy* π / N w Difference (A+A)-(p+p) /2 F (GeV ) NA49 Bevalac + Dubna /2 y* F (GeV ) Fig. 1. Left: rapidity distribution of π in central Pb+Pb collisons at 4 (squares), 8 (triangles) and 158 (dots) A GeV. Open symbols show values reflected at y = (NA49 preliminary). Right: total pion multiplicity π produced per wounded (participant) nucleon versus the Fermi energy variable F s.25 NN nucleus nucleus collisions (full symbols). AGS RHIC for p+p reactions (open symbols) and central Rapidity distributions of π are plotted in Fig. 1, left. The integrated yields are 312±15, 445±22 and 61±3 at 4, 8 and 158 A GeV respectively. Pions are the dominant produced particle species and thus their number provides a measure of the entropy in a statistical model description of the reaction. The yield of pions (estimated here as π = 1.5 ( π + π + )) divided by p+p, p+ p

3 the number of wounded nucleons (participants) N W is shown versus the Fermi energy variable F ( s 2m N ) 3/4 / s 1/4 NN s 1/4 NN in Fig. 2, right. While p+p data show a linear rise throughout there is a change for A+A collisions (illustrated more clearly in the inset) in the SPS energy range. Below [6] one finds a regime of slight suppression, above [7] a region of enhancement with a steeper linear rise than for p+p reactions [3]. Within the Statistcal Model of the Early Stage (SMES) [4] this steepening has interpreted as indicating the activation of a large number of partonic degrees of freedom at the onset of deconfinement [4]. 3 Strangeness Production + / π + K.25.2 RHIC midrapidity values RQMD URQMD Hadron gas s NN (GeV) Fig. 2. Left: 4π ratio K + / π + versus energy compared to predictions of the RQMD [1] (dashed), UrQMD [11] (dotted) and extended statistical [12] (solid curve) models. Right: the strangeness content measure E s versus the Fermi energy variable F s.25 NN compared to the prediction of the statistical model of the early stage [4] (curves). Measured total kaon yields in central Pb+Pb collisions are 18±1, 29±2, 5±5 for K and 56±3, 79±5, 95±9 for K + at 4, 8, 16 A GeV respectively. Yields of most particles, of course, increase with energy. Changes in the composition of the produced system are better characterised by particle ratios. A special role plays here the full phase space K + to π + ratio whic is closely proportional to the strangeness to entropy ratio. The K + / π + ratio is shown as a function of s in Fig. 2 (left). A steep increase of the ratio in the low (AGS) energy region [6] is followed by a rapid turnover (around 4 A GeV) into a decrease and a successive saturation suggested by preliminary RHIC data [8]. This behaviour is again consistent with the hypothesis of a transition to a QGP occuring close to 4 A GeV. Within SMES [4] the decrease of the

4 ratio K + / π + in the transition region is related to the lower value of the strangeness to entropy ratio in a QGP compared to confined matter. Preliminary results on the K + to π + ratio in central Au+Au collisions at RHIC [9] indicate that it is similar to the ratio measured at top SPS energy. This is again in agreement with the hypothesis of a transition to QGP in the low SPS energy region. Within the model the strangeness to entropy ratio is independent of the collision energy provided that the threshold for deconfiment has been crossed. Also shown in Fig. 2 are predictions of models which do not invoke transition to QGP: RQMD [1], UrQMD [11] and Extended Hadron Gas Model [12]. In Fig. 2 (right) an alternative measure of the strangeness to entropy ratio, E S = ( Λ + K + K )/ π, is plotted as a function of F for A+A collisions and p+p(p) interactions. For A+A collisions the Λ multiplicity was estimated as Λ = ( K + K )/.8, based on strangeness conservation and approximate isospin symmetry of the colliding nuclei. This estimate agrees with the preliminary Λ measurements presented in Ref. [13]. Rich data on Λ and K S production in p+p interaction allow to establish precisely the energy dependence in elementary interactions, much better than it is possible for the K + /π + ratio. Consequently it is possible to conclude that a sharp non monotonic energy dependence might occur as a unique property of heavy ion collisions not observed in elementary interactions. The results on A+A collisons are compared in Fig. 2 (right) with the predictions of a Statistical Model of the Early Stage [4]. As discussed above, within this model, a rapid change of the energy dependence of strangeness to entropy ratio is due to the transition from confined to deconfined matter. 4 Charge Fluctuations The event by event fluctuations of net electric charge are studied as a function of the size of the rapidity window centered around midrapidity. The intensive fluctuation measure Φ q [14] is plotted in Fig. 3 (left) versus an accepted fraction of charged hadrons N ch / N ch tot. No significant energy dependence is observed. The values of Φ q measure are close to expectations for independent particle emission modified by global charge conservation Φ cc q = 1 N ch / N ch tot 1 [15] (line in Fig. 3 left). The difference Φ q Φ cc q is plotted in Fig. 3 right for more clearly illustration. A possible significant reduction of charge fluctuations [16] due smaller charge quanta in the deconfinement phase is not observed. It is argued, how-

5 ever, that in the SPS energy range the fluctuations established at the early stage should be smeared by the rescattering in the hadronic phase [17] and by resonance decay [15]. Φ q.1 - Φ q GeV 8 GeV 16 GeV <N ch > acc / <N ch > tot <N ch > /<N ch > tot Fig. 3. Left: Φ q versus the ratio N ch / N ch tot of the multiplicity in the acceptance window and the total multiplicity in the events; the curve shows the prediction for independent particle emission plus global charge conservation. Right: the difference Φ q Φ cc q (see text). 5 Summary In summary, results on pion and strangeness production as well as net charge fluctuations in central Pb+Pb collisions at 4, 8 and 158 A GeV are presented. The change from a pattern of pion suppression observed at low collision energies to pion enhancement seen at high energies is located at about 4 A GeV. A non monotonic energy dependence of the K + / π + ratio is observed. A maximum is found close to 4 A GeV followed by a nearly constant value at higher energies. Unusal suppression of net charge fluctuations is not seen in the data. The results are consistent with the hypothesis that a transient state of deconfiend matter is created in central Pb+Pb collisions for energies larger than about 4 A GeV. References [1] J. C. Collins and M. J. Perry, Phys. Rev. Lett. 34 (1975) 151. [2] J. Rafelski and B. Müller, Phys. Rev. Lett. 48, 166 (1982), T. Matsui and H. Satz, Phys. Lett. B178 (1986) 416. [3] M. Gaździcki and D. Röhrich, Z. Phys. C65, 215 (1995) and Z. Phys. C71, 55 (1996). [4] M. Gaździcki and M. I. Gorenstein, Acta Phys. Polon. B3, 275 (1999).

6 [5] S. Afanasiev et al., Nucl. Instrum. Meth. A43, 21 (1999). [6] L. Ahle et al. (E82 Collab.), Phys. Rev. C57, 466 (1998), L. Ahle et al. (E82 Collab.), Phys. Rev. C58, 3523 (1998), L. Ahle et al. (E82 Collab), Phys. Rev. C6, 4494 (1999), L. Ahle et al. (E866 Collab. and E917 Collab.), Phys. Lett. B476, 1 (2), L. Ahle et al. (E866 Collab. and E917 Collab.), Phys. Lett. B49, 53 (2), J. Barrette et al. (E877 Collab.), Phys. Rev. C62, 2491 (2) D. Pelte et al. (FOPI Collab.), Z. Phys. A357, 215 (1997). [7] B. B. Back et al. (PHOBOS Collab.), Phys. Rev. Lett. 85, 31 (2). [8] C. Adler et al. (Star Collab.), Phys. Rev. Lett. 86 (21) 4778, B. B. Back et al. (Phobos Collab.), Phys. Rev. Lett. 87 (21) [9] J. Harris et al. (STAR Collab.), Proceedings of 15th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (QM21), Stony Brook, New York, 15-2 Jan 21, to be published in Nucl. Phys. B, B. Jacak et al. (PHENIX Collab.), Proceedings of International Workshop of the Physics of the Quark-Gluon Plasma, Ecole Polytechnique, Palaiseau, France, September 4 7, 21. [1] H. Sorge, H. Stöcker and W. Greiner, Nucl. Phys. A489, 567c (1989) and F. Wang, H. Liu, H. Sorge, N. Xu and J. Yang, Phys. Rev. C61, 6494 (2). [11] S. A. Bass et al., Prog. Part. Nucl. Phys. 41 (1998) 225, H. Weber et al. (UrQMD Collab.), to be published. [12] J. Cleymans and K. Redlich, Phys. Rev. C6, 5498 (1999), P. Braun-Munzinger et al., e-print Archive: hep-ph/1666. [13] A. Mischke et al. (NA49 Collab.), nucl-ex/2112. [14] M. Gaździcki, Eur. Phys. J. C8 (1999) 131, M. Gaździcki and St. Mrówczyński, Z. Phys. C54 (1992) 127. [15] J. Zaranek, preprint hep-ph/ (Nov. 21) [16] M.Asakawa, U.Heinz and B.Müller, Phys. Rev. Lett. 85 (2) 272, S. Jeon and V. Koch, Phys. Rev. Lett. 85 (2) 276. [17] E. V. Shuryak and M. A. Stephanov, Phys. Rev. C63 (21) 6493.