A fundamental quantity in (1+1)-dimensional quantum field theories is Zamolodchikov's c-function. A function of a renormalization group distance parameter r, which interpolates between ultraviolet and infrared fixed points, its value is usually interpreted as a measure of the number of degrees of freedom of a model at a particular energy scale. The c-theorem establishes that c(r) is a monotonically decreasing function of r and that its derivative may only vanish at quantum critical points (r = 0 and r = ∞). At those points, c(r) becomes the central charge of the conformal field theory which describes the critical point. In this communication, we argue that a different function proposed by Calabrese and Cardy, defined in terms of the two-point function , which involves the branch-point twist field and the trace of the stress–energy tensor Θ, has exactly the same qualitative features as c(r).
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