Pion and kaon production in A+A collisions at low SPS energies C. Alt 1 , C. Blume 1 , P. Dinkelaker 1 , D. Flierl 1 , V. Friese 2 , M. Ga´ zdzicki 1 , C. Hoehne 2 , S. Kniege 1 , I. Kraus 2 , M. Kliemant 1 , B. Lungwitz 1 , C. Meurer 2 , M. Mitrovski 1 , R. Renfordt 1 , A. Richard 1 , A. Sandoval 2 , T. Schuster 1 , R. Stock 1 , C. Strabel 1 , H. Str¨ obele 1 , and the NA49 Collaboration 1 Fachbereich Physik der Universit¨ at Frankfurt 2 GSI Darmstadt The results from the NA49 energy scan programme at the SPS have revealed an anomalous behavior of the en- ergy dependence of the 〈K + 〉/〈π + 〉 ratio in central Pb+Pb collisions [1] which is absent in p+p interactions. Here we report the results on charged kaon and pion production in central collisions at 20 and 30 A·GeV as well as their sys- tem size dependence at 40 A·GeV. The former data extend the energy range of the NA49 data to a region well below the observed structure. The latter studies the onset of the anomalous behavior by comparing particle ratios obtained in p+p, C+C, Si+Si, and minimum bias Pb+Pb collisions. In order to identify kaons, an analysis of the energy loss dE/dx in the NA49 time projection chambers is per- formed. Raw K + and K - yields were extracted from fits of the distributions of dE/dx and tof (where available) in narrow bins of momentum and transverse momentum. The yields at mid-rapidity are obtained using the com- bined dE/dx and tof information (tof + dE/dx analysis). To obtain the π - yields, all negatively charged particles were binned in rapidity (calculated assuming the π–mass) and p T . The contamination from K - , p and e - from the interaction vertex as well as non–vertex hadrons originat- ing from strange particle decays and secondary interac- tions was subtracted. The resulting K ± and π - yields were corrected for geo- metrical acceptance, losses due to in–flight decays, ineffi- ciencies of the tracking algorithms and quality cuts. The details on the correction procedure can be found in [1]. Spectra of transverse mass m T =  p 2 T + m 2 (m is the rest mass of the particle) for K + , K - (tof + dE/dx analy- sis) and π - mesons produced near mid-rapidity in central Pb+Pb collisions (0-7.2%) at 20 and 30 A·GeV (Fig. 1 left) and in C+C, Si+Si, and Pb+Pb with different centrality selections (Fig. 1 right) are fitted by Eq. 1 in the range 0.2 GeV <m T − m< 0.7 GeV: dn m T dm T dy = C · exp  − m T T  . (1) The five centrality selected event samples were obtained from minimum bias Pb+Pb collisions on the basis of the energy deposited by the projectile spectator nucleons in the forward calorimeter [2]. The resulting fractions of the total inelastic cross–section are 0-5%, 5-12.5%, 12.5-23.5%, 23.5-33.5%, and 33.5-43.5%. The rapidity distributions dn/dy plotted in Fig. 2 were obtained by summing the measured m T spectra and using the fitted exponential function (Eq. 1) to extrapolate to full m T . For most bins the necessary correction is small ( ∼ = 5%). The rapidity spectra were parameterized by the sum of two Gaussian distributions placed symmetrically with respect to mid-rapidity. The results of the fits are in- -m (GeV) T m 0 0.5 1 1.5 ) -2 ) (GeV T n/(dydm 2 ) d T (1/m 1 10 3 20 AGeV - π + K - K (GeV) 0 -m t m 0 0.2 0.4 0.6 0.8 1 t dn/dm t 1/m 1 10 10 2 10 3 - π Pb+Pb Si+Si C+C -m (GeV) T m 0 0.5 1 1.5 ) -2 ) (GeV T n/(dydm 2 ) d T (1/m 1 10 3 30 AGeV - π + K - K (GeV) 0 -m t m 0 0.2 0.4 0.6 0.8 1 t dn/dm t 1/m 10 -1 1 10 10 2 + K Pb+Pb Si+Si C+C (GeV) 0 -m t m 0 0.2 0.4 0.6 0.8 1 t dn/dm t 1/m 10 -1 1 10 10 2 - K Pb+Pb Si+Si C+C Figure 1: left: Transverse mass spectra of π - , K + and K - mesons produced at mid-rapidity (|y| < 0.1 for kaons in the tof + dE/dx analysis, and 0 <y< 0.2 for pions) for central Pb+Pb collisions at 20 and 30 A·GeV. right: 40 A·GeV transverse mass spectra of π - , K + , and K - at midrapidity (−0.5 <y< 0.5) for C+C, Si+Si, and minimum bias Pb+Pb collisions. dicated by the full lines in Fig. 2. The mean multiplicities were obtained by integration of the fitted curves. Figure 3 shows the system size dependence of the 〈π - 〉/〈N W 〉, 〈K + 〉/〈N W 〉, 〈K - 〉/〈N W 〉, 〈K + 〉/〈K - 〉, 〈K + 〉/〈π + 〉, and 〈K - 〉/〈π - 〉 ratios. 〈N W 〉 is the mean number of wounded nucleons calculated within a Glauber model. For p+p interactions no measurements of these ra- tios exist at this energy, therefore an interpolation of mea- surements at other energies is used [3]. To ease the com- parison to the isospin symmetric light nuclei, the π yield in isospin averaged nucleon-nucleon interactions (N+N) was estimated as 〈π - 〉 (NN) =0.5 · (〈π + 〉 (pp) + 〈π - 〉 (pp) ) and the ratios which use this yield are also shown in the figures. The 〈π - 〉/〈N W 〉 ratio shows a fast rise for the smaller