Published online by Cambridge University Press: 19 June 2013
For an arbitrary connected reductive group $G$, we consider the motivic integral over the arc space of an arbitrary
$ \mathbb{Q} $-Gorenstein horospherical
$G$-variety
${X}_{\Sigma } $ associated with a colored fan
$\Sigma $ and prove a formula for the stringy
$E$-function of
${X}_{\Sigma } $ which generalizes the one for toric varieties. We remark that, in contrast to toric varieties, the stringy
$E$-function of a Gorenstein horospherical variety
${X}_{\Sigma } $ may be not a polynomial if some cones in
$\Sigma $ have nonempty sets of colors. Using the stringy
$E$-function, we can formulate and prove a new smoothness criterion for locally factorial horospherical varieties. We expect that this smoothness criterion holds for arbitrary spherical varieties.