The behavior of constrained caloric curves as ultimate signature of a phase transition for hot nuclei, B. Borderie, CNRS-IN2P3/Institut de Physique Nucléaire/Université Paris-Sud 11, France − Experimental data obtained from multifragmenting quasi-fused nuclei produced in central 129Xe+natSn collisions and related simulations  have been used to deduce event by event freeze-out properties on the thermal excitation energy range 4-12 AMeV. From such properties and by using, on protons emitted at freeze-out, a new thermometer based on quantum momentum fluctuations , constrained caloric curves have been derived. At constant average volumes caloric curves exhibit a monotonous behavior whereas for constrained pressures backbending are observed. Those results definitively sign the existence of a first order phase transition for hot nuclei.
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