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Title: Classification of nonthermal fixed points and their scaling exponents
Plain: Experiments in one-, two- and three-dimensional atomic gases find self-similar stretching of correlations, with exponents that differ between systems. Which distinct universal classes exist, and what selects the class, is not known.
Precise: In an isolated Bose gas after a quench, the momentum occupation obeys $f(k,t)=t^{\alpha} f_s(t^{\beta} k)$ in a scaling window. Determine the set of fixed points ($\alpha, \beta$ and the scaling function $f_s$) and the properties that select them: dimension d, number of field components and symmetry (scalar or spinor), conservation laws, and topological defects. Experimental inputs: 1D (Erne et al., Nature 563, 225, 2018), spinor (Prufer et al., Nature 563, 217, 2018), 3D homogeneous (Glidden et al., Nature Physics 17, 457, 2021). Answer: a classification with computed exponents matching the measured ones.
Settled by: A renormalization-group or kinetic theory predicting $\alpha$ and $\beta$ for each experimental system, confirmed by measurements in which $d$ or the symmetry is varied.
Kind: well-posed
Answer type: classification
References: (empty)
Literature status: partially-resolved
Status note: Wave-kinetic and defect-dominated fixed points are identified in theory; not every measured exponent set is matched to a predicted class (moderate confidence).
Posed since: (empty)