Graphical small cancellation groups with the Haagerup property

We prove the Haagerup property (= Gromov’s a-T-menability) for finitely generated groups defined by infinite presentations satisfying the graphical C’(lambda)-small cancellation condition with respect to graphs endowed with a compatible wall structure. We deduce that these groups are coarsely embeddable into a Hilbert space and that the strong Baum-Connes conjecture and, hence, the Baum-Connes conjecture with arbitrary coefficients hold for them. As the main step we show that C’(lambda)-complexes satisfy the linear separation property. Our result provides many new examples and a general technique to show the Haagerup property for graphical small cancellation groups.