Hamiltonian Algebroid Symmetries in W-gravity and Poisson sigma-model

Starting from a Lie algebroid ${\cal A}$ over a space V we lift its action to the canonical transformations on the principle affine bundle ${\cal R}$ over the cotangent bundle $T^*V$. Such lifts are classified by the first cohomology $H^1({\cal A})$. The resulting object is the Hamiltonian algebroid ${\cal A}^H$ over ${\cal R}$ with the anchor map from $\G({\cal A}^H)$ to Hamiltonians of canonical transformations. Hamiltonian algebroids generalize the Lie algebras of canonical transformations. We prove that the BRST operator for ${\cal A}^H$ is cubic in the ghost fields as in the Lie algebra case. To illustrate this construction we analyze two topological field theories. First, we define a Lie algebroid over the space $V_3$ of $\SL$-opers on a Riemann curve $\Si_{g,n}$ of genus g with n marked points. The sections of this algebroid are the second order differential operators on $\Si_{g,n}$. The algebroid is lifted to the Hamiltonian algebroid over the phase space of $W_3$-gravity. We describe the BRST operator leading to the moduli space of $W_3$-gravity. In accordance with the general construction the BRST operator is cubic in the ghost fields. We present the Chern-Simons explanation of our results. The second example is the Hamiltonian algebroid structure in the Poisson sigma-model invoked by Cattaneo and Felder to describe the Kontsevich deformation quantization formula. The hamiltonian description of the Poisson sigma-model leads to the Lie algebraic form of the BRST operator.

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