Measurements of charged pion and kaon production in central PbϩPb collisions at 40, 80, and 158 A GeV are presented. These are compared with data at lower and higher energies as well as with results from pϩp interactions. The mean pion multiplicity per wounded nucleon increases approximately linearly with s NN 1/4 with a change of slope starting in the region 15-40 A GeV. The change from pion suppression with respect to p ϩp interactions, as observed at low collision energies, to pion enhancement at high energies occurs at about 40A GeV. A nonmonotonic energy dependence of the ratio of K ϩ to ϩ yields is observed, with a maximum close to 40A GeV and an indication of a nearly constant value at higher energies. The measured dependences may be related to an increase of the entropy production and a decrease of the strangeness to entropy ratio in central PbϩPb collisions in the low SPS energy range, which is consistent with the hypothesis that a transient state of deconfined matter is created above these energies. Other interpretations of the data are also discussed.

Results on charged pion and kaon production in central Pb+Pb collisions at 20A and 30A GeV are presented and compared to data at lower and higher energies. Around 30A GeV a rapid change of the energy dependence for the yields of pions and kaons as well as for the shape of the transverse mass spectra is observed. The change is compatible with the prediction that the threshold for production of a state of deconfined matter at the early stage of the collisions is located at low CERN Super Proton Synchroton energies.

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NA61/SHINE (SPS Heavy Ion and Neutrino Experiment) is a multi-purpose experimental facility to study hadron production in hadron-proton, hadron-nucleus and nucleus-nucleus collisions at the CERN Super Proton Synchrotron. It recorded the first physics data with hadron beams in 2009 and with ion beams (secondary 7 Be beams) in 2011.NA61/SHINE has greatly profited from the long development of the CERN proton and ion sources and the accelerator chain as well as the H2 beamline of the CERN North Area. The latter has recently been modified to also serve as a fragment separator as needed to produce the Be beams for NA61/SHINE. Numerous components of the NA61/SHINE set-up were inherited from its predecessors, in particular, the last one, the NA49 experiment. Important new detectors and upgrades of the legacy equipment were introduced by the NA61/SHINE Collaboration.This paper describes the state of the NA61/SHINE facility -the beams and the detector system -before the CERN Long Shutdown I, which started in March 2013.

We look for fluctuations expected for the QCD critical point using an intermittency analysis in the transverse momentum phase space of protons produced around midrapidity in the 12.5% most central C+C, Si+Si and Pb+Pb collisions at the maximum SPS energy of 158A GeV. We find evidence of power-law fluctuations for the Si+Si data. The fitted power-law exponent φ2 = 0.96 +0.38 −0.25 (stat.) ±0.16 (syst.) is consistent with the value expected for critical fluctuations. Power-law fluctuations had previously also been observed in low-mass π + π − pairs in the same Si+Si collisions.