Search for invisible decays of low-mass Higgs bosons at BaBar

We search for a light scalar particle produced in single-photon decays of the $\Upsilon(2S)$ and $\Upsilon(3S)$ resonances through the process $\Upsilon\to \gamma + A^0$, $A^0 \to \hbox{invisible}$. Such an object appears in supersymmetric extensions of the Standard Model, such as NMSSM, where a light $CP$- odd Higgs boson naturally couples strongly to $b$-quarks. If in addition there exist a light stable neutralino, decays of $A^0$ could be preferentially to an invisible final state. We search for events with a single high energy photon and a large missing energy and momentum, consistent with a 2-body decay of $\Upsilon$.