Particle production at high baryon density in central Au+Au reactions at 11.6<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>GeV/<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>c</mml:mi></mml:math>

Semi-inclusive proton and pion distributions from central Au+Au reactions at 11.6$A$ GeV/$c$ have been measured. The proton rapidity distribution shows significantly increased stopping compared to lighter systems, providing strong evidence for the formation of a state of matter with baryon density substantially greater than normal nuclear matter. Unlike reactions at this energy induced by lighter heavy ions, at low ${m}_{t}\ensuremath{-}{m}_{0}$ the proton invariant spectra deviate from a single exponential shape and become flatter, while ${\ensuremath{\pi}}^{\ensuremath{-}}$ spectra are found to rise faster than the ${\ensuremath{\pi}}^{+}$ spectra.