On the definition of temperature in dense granular media

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In this Letter we report the measurement of a pseudo-temperature for compacting granular media on the basis of the Fluctuation-Dissipation relations in the aging dynamics of a model system. From the violation of the Fluctuation-Dissipation Theorem an effective temperature emerges (a dynamical temperature Tdyn) whose ratio with the equilibrium temperature Tdeq depends on the particle density. We compare the results for the Fluctuation-Dissipation Ratio (FDR) Tdyn/Tdeq at several densities with the outcomes of Edwards' approach at the corresponding densities. It turns out that the FDR and the so-called Edwards' ratio coincide at several densities (very different ages of the system), opening in this way the door to experimental checks as well as theoretical constructions.