New physics andCPviolation in singly Cabibbo suppressedDdecays

We analyze various theoretical aspects of $CP$ violation in singly Cabibbo suppressed (SCS) $D$ meson decays, such as $D\to KK,\pi \pi$. In particular, we explore the possibility that $CP$ asymmetries will be measured close to the present level of experimental sensitivity of $\mathcal{O}({10}^{-2})$. Such measurements would signal new physics. We make the following points: (i) The mechanism at work in neutral $D$ decays could be indirect or direct $CP$ violation (or both). (ii) One can experimentally distinguish between these possibilities. (iii) If the dominant $CP$ violation is indirect, then there are clear predictions for other modes. (iv) Tree-level direct $CP$ violation in various known models is constrained to be much smaller than ${10}^{-2}$. (v) SCS decays, unlike Cabibbo favored or doubly Cabibbo suppressed decays, are sensitive to new contributions from QCD penguin operators and especially from chromomagnetic dipole operators. This point is illustrated with supersymmetric gluino-squark loops, which can yield direct $CP$ violating effects of $\mathcal{O}({10}^{-2})$.