Cosmological Collider Physics

TL;DR

The paper develops the framework of cosmological collider physics, showing how particles with masses around the Hubble scale during inflation imprint characteristic non-Gaussian signatures in primordial fluctuations, especially in the squeezed limit. By exploiting de Sitter symmetry and its relation to 3D conformal invariance, it derives general forms for two-, three-, and four-point functions with massive scalars and higher-spin exchanges, including detailed OPE-like limits and the mass/spin dependence encoded in oscillatory phases and angular textures. It also extends the analysis to inflationary backgrounds, examining how slow-roll breaking modifies correlators and how loop corrections and couplings to general operators affect the signatures, with expressions that reveal the prospect of reading off new particle spectra from cosmological data. Overall, the work emphasizes that the squeezed-limit non-Gaussianities carry direct information about the spectrum of fields present during inflation, offering a principled route to probe high-energy physics with cosmological observations, albeit with challenging detectability unless couplings are sizable or novel observational channels are exploited.

Abstract

We study the imprint of new particles on the primordial cosmological fluctuations. New particles with masses comparable to the Hubble scale produce a distinctive signature on the non-gaussianities. This feature arises in the squeezed limit of the correlation functions of primordial fluctuations. It consists of particular power law, or oscillatory, behavior that contains information about the masses of new particles. There is an angular dependence that gives information about the spin. We also have a relative phase that crucially depends on the quantum mechanical nature of the fluctuations and can be viewed as arising from the interference between two processes. While some of these features were noted before in the context of specific inflationary scenarios, here we give a general description emphasizing the role of symmetries in determining the final result.

Figures (13)

• Figure 1: (a) The three momenta of the three point function form a closed triangle. In the squeezed limit, one of the sides, $k_{\rm long}$, is much smaller than the other two, $k_{\rm short}$. (b) In this limit we can also define the angle $\gamma$ between the short and long mode momentum vectors. (c) In position space we have two insertions at a short distance from each other, associated to $k_{\rm short}$ and one at a longer distance, $k_{\rm long}$. We are interested in considering the effects of massive fields, $\sigma$, that can decay into pairs of inflatons. In an inflationary background we can replace an inflaton fluctuation by the classical $\dot \phi_0$ background, so that we get a contribution to the three point function.
• Figure 2: (a) Two point function of massive fields (b) Three point functions of two massless fields and a massive field. (c) Four point function of massless fields with an exchange of a massive field. We can view this diagram as the amplitude to create a pair of massive fields which then decay into inflatons. (d) Contributions from Higgs field exchange, or the exchange of any other gauge non-invariant field.
• Figure 3: The singularity at $k_t\to 0$ arises when the whole diagram occurs at very early times. This translates into very short distances where we can make the flat space and high energy approximation, obtaining (\ref{['FlatLi']}).
• Figure 4: (a) We see a pair of created particles that approach a given comoving points at late time in de-Sitter. There is a subgroup of the conformal group that preserves this two points. These include transformations which act as rotations around each of the two points as indicated. (b) If two points are separated along the direction $3$, then the spins of the physical particles in directions, say $J_{23}$ are the same. So the two particles would be spinning as shown. The spins in the direction $J_{12}$ are opposite.
• Figure 5: (a) Generic configuration of momenta for the three point function. (b) Nearly "collapsed triangle" configuration with $k_3 \sim k_1 + k_2$.