The bispectrum of primordial curvature perturbations in the squeezed configuration , in which one wavenumber , k _ { 3 } , is much smaller than the other two , k _ { 3 } \ll k _ { 1 } \approx k _ { 2 } , plays a special role in constraining the physics of inflation .
In this paper we study a new phenomenological signature in the squeezed-limit bispectrum : namely , the amplitude of the squeezed-limit bispectrum depends on an angle between { \bf k } _ { 1 } and { \bf k } _ { 3 } such that B _ { \zeta } ( k _ { 1 } ,k _ { 2 } ,k _ { 3 } ) \to 2 \sum _ { L } c _ { L } P _ { L } ( \hat { \bf k } _ { 1 } \cdot \hat { % \bf k } _ { 3 } ) P _ { \zeta } ( k _ { 1 } ) P _ { \zeta } ( k _ { 3 } ) , where P _ { L } are the Legendre polynomials .
While c _ { 0 } is related to the usual local-form f _ { NL } parameter as c _ { 0 } = 6 f _ { NL } / 5 , the higher-multipole coefficients , c _ { 1 } , c _ { 2 } , etc. , have not been constrained by the data .
Primordial curvature perturbations sourced by large-scale magnetic fields generate non-vanishing c _ { 0 } , c _ { 1 } , and c _ { 2 } .
Inflation models whose action contains a term like I ( \phi ) ^ { 2 } F ^ { 2 } generate c _ { 2 } = c _ { 0 } / 2 .
A recently proposed “ solid inflation ” model generates c _ { 2 } \gg c _ { 0 } .
A cosmic-variance-limited experiment measuring temperature anisotropy of the cosmic microwave background up to \ell _ { max } = 2000 is able to measure these coefficients down to \delta c _ { 0 } = 4.4 , \delta c _ { 1 } = 61 , and \delta c _ { 2 } = 13 ( 68 % CL ) .
We also find that c _ { 0 } and c _ { 1 } , and c _ { 0 } and c _ { 2 } , are nearly uncorrelated .
Measurements of these coefficients will open up a new window into the physics of inflation such as the existence of vector fields during inflation or non-trivial symmetry structure of inflaton fields .
Finally , we show that the original form of the Suyama-Yamaguchi inequality does not apply to the case involving higher-spin fields , but a generalized form does .