A theory for jammed granular materials is developed with the aid of a nonequilibrium steady-state distribution function. The approximate nonequilibrium steady-state distribution function is explicitly given in the weak dissipation regime by means of the relaxation time. The theory quantitatively agrees with the results of the molecular dynamics simulation on the critical behavior of the viscosity below the jamming point without introducing any fitting parameter. DOI: 10.1103/PhysRevLett.115.098001 PACS numbers: 45.70.-n, 05.20.Jj, 64.70.ps, 83.50.Ax Introduction.-A description of granular rheology has been a long-term challenge for both science and technology. The problem extends to a vast range, from solidlike creep motion, gaslike, to liquidlike phenomena [1]. Similar to solid-liquid transitions, granular materials acquire rigidity when the density exceeds a critical value [2][3][4][5]. This phenomenon, referred to as the jamming transition, is universely observed in disordered materials such as colloidal suspensions [6], emulsions, and foams [7], as well as granular materials. The jamming transition and its relation to the glass transition have attracted much interest in the last two decades, and various aspects have been revealed [8][9][10][11][12]. In particular, characteristics in the vicinity of the jamming point, including the critical scaling behavior, have been extensively investigated by experiments and numerical simulations [2][3][4][13][14][15][16][17][18][19][20][21][22][23][24][25]. It has been shown that the shear stress, the pressure, and the granular temperature can be expressed by scaling functions with exponents near φ ∼ φ J , where φ J is the jamming transition density. The shear viscosity exhibits a form η ∼ ðφ J − φÞ −λ with λ ≈ 2 for non-Browninan suspensions, foams, and emulsions [26][27][28][29], and a recent careful analysis demonstrated that λ is in the range between 1.67 and 2.55 [30]. It seems that the exponent λ for granular flows takes a larger value than that for suspensions [18,19,25,31], although a value in the range mentioned above has been reported as well [17]. However, these studies are based on numerical simulations or phenomenologies without any foundation of a microscopic theory.Even when we focus only on the flow properties below the jamming point φ J , which can be tracked back to Bagnold's work [32], we have not yet obtained a complete set in describing the rheological properties of dense granular flows. One of the remarkable achievements is the extension of the Boltzmann-Enskog kinetic theory to inelastic hard disks and spheres [33][34][35][36][37][38]. However, it has been recognized that the kinetic theory breaks down at densities with volume fraction φ > φ f ¼ 0.49 [39][40][41][42],