Status of the evidence for the onset of deconfinement and the urgent need for primary Ar beams

This paper summarizes the status of the evidence for the onset of deconfinement in central Pb+Pb collisions at low SPS energies and underlines the urgent need for a timely execution of the NA61/SHINE ion program. Recent results from the RHIC beam energy scan program agree with NA49 measurements in the region of the onset of deconfinement. The new LHC data confirm the interpretation of the structures observed at low SPS energies as due to the onset of deconfinement. Thus the physics justification of the NA61/SHINE ion program, the observation of the onset of deconfinement at the low SPS energies, is strongly confirmed by independent measurements. These recent developments underline the urgency of carrying out the NA61/SHINE ion program as soon as technically possible. In particular, the most urgent data for this program, central Ar+Ca collisions at 13A, 20A, 30A, 40A, 80A and 158A GeV/c should be taken with as little delay as possible. It is also crucial to complete the energy scan with secondary Be beams in 2011 and 2012.