How Significant are Cosmological Collider Signals in the Planck Data?

Abstract

The search for primordial non-Gaussianities (PNG) is theoretically well motivated but remains observationally challenging. Tight constraints with low significance for the standard non-Gaussian shapes suggest that detection may lie beyond the reach of near-future experiments. However, tests of PNG are highly template-dependent. From a theory perspective, a whole new family of bispectrum shapes arise in the cosmological collider program, with distinct signatures of heavy particles during inflation. In this work, we provide a class of simplified collider templates for these particles that encompasses a broader range of masses, sound speeds, and interactions. We propose that, given the current state of observations, the most effective strategy to search for PNG signals is through orthogonalizing the collider templates, such that they are uncorrelated with the tightly constrained single field predictions. Using the Modal pipeline and Planck CMB data, we perform a systematic parameter scan of the collider templates with the most significant result reaching $2.4σ$ for spin-0, after taking into account the look-elsewhere effect; indicative results for spin-1 and spin-2 peak near 2$σ$. These results indicate that, with refined collider templates and improved data analysis strategies, there are credible prospects with forthcoming observations to detect PNG and also rule out single field inflation.

Figures (2)

• Figure 1: Raw signal-to-noise ratios of the best-fit $f_{\rm NL}$ as a function of the mass parameter $\mu$, for all four templates investigated in this paper: Scalar-I, Scalar-II, Spin-1 and Spin-2, and their orthogonalized counterparts. Before taking into account the look-elsewhere-effect, the most significant signals correspond to Scalar-II ($3.34\sigma$) and Spin-2 ($2.95\sigma$).
• Figure 2: The most favoured primordial shape function from Scalar-II template with $\mu=1.85$ and $c_s=0.012$.