We consider a supersymmetric hybrid inflation model with two inflaton fields .
The superpotential during inflation is dominated by W = ( \kappa S + \kappa ^ { \prime } S ^ { \prime } ) M ^ { 2 } , where S , S ^ { \prime } are the ( twin ) inflatons carrying the same U ( 1 ) _ { R } charge , \kappa , \kappa ^ { \prime } are dimensionless couplings , and M ( \sim 10 ^ { 15 - 16 } GeV ) is a dimensionful parameter associated with a symmetry breaking scale .
One light mass eigenstate drives inflation , while the other heavier mass eigenstate is stuck to the origin .
The smallness of the lighter inflaton mass for the scalar spectral index n _ { s } \approx 0.963 , which is the center value of WMAP5 , can be controlled by the ratio \kappa ^ { \prime } / \kappa through the supergravity corrections .