The scalar chemical potential in cosmological collider physics

Abstract

Non-analyticity in co-moving momenta within the non-Gaussian bispectrum is a distinctive sign of on-shell particle production during inflation, presenting a unique opportunity for the “direct detection” of particles with masses as large as the inflationary Hubble scale (H). However, the strength of such non-analyticity ordinarily drops exponentially by a Boltzmann-like factor as masses exceed H. In this paper, we study an exception provided by a dimension-5 derivative coupling of the inflaton to heavy-particle currents, applying it specifically to the case of two real scalars. The operator has a “chemical potential” form, which harnesses the large kinetic energy scale of the inflaton, $$ {\overset{\cdot }{\phi}}_0^{1/2}\approx 60H $$ ϕ ⋅ 0 1 / 2 ≈ 60 H , to act as an efficient source of scalar particle production. Derivative couplings of inflaton ensure radiative stability of the slow-roll potential, which in turn maintains (approximate) scale-invariance of the inflationary correlations. We show that a signal not suffering Boltzmann suppression can be obtained in the bispectrum with strength fNL ∼ $$ \mathcal{O} $$ O (0.01–10) for an extended range of scalar masses $$ \lesssim {\overset{\cdot }{\phi}}_0^{1/2} $$ ≲ ϕ ⋅ 0 1 / 2 , potentially as high as 1015 GeV, within the sensitivity of upcoming LSS and more futuristic 21-cm experiments. The mechanism does not invoke any particular fine-tuning of parameters or breakdown of perturbation-theoretic control. The leading contribution appears at tree-level, which makes the calculation analytically tractable and removes the loop-suppression as compared to earlier chemical potential studies of non-zero spins. The steady particle production allows us to infer the effective mass of the heavy particles and the chemical potential from the variation in bispectrum oscillations as a function of co-moving momenta. Our analysis sets the stage for generalization to heavy bosons with non-zero spin.

TwInflation

“…suppressed by e −πMσ/H , since M σ H. However, the recently discussed "scalar chemical potential" mechanism [63] may eliminate this suppression and be compatible with our twin symmetry structure. We leave an exploration of this to future work.…”

Chemical-potential-assisted particle production in FRW spacetimes

“…This is why (2.4) is only a necessary condition, rather than being sufficient. However, we note that a different opinion is given in [54], where it is argued that a scalar chemical potential can also bring isotropic enhancement to particle production.…”

Disentangling mass spectra of multiple fields in cosmological collider

“…In refs. [38,57,60,61], however, it is pointed out that an introduction of effective chemical potentials via some kinds of dimension-5 derivative operators amplifies the signal, even when their masses are definitely larger than the Hubble scale. Thus, it would be interesting to apply our results to a system with chemical potentials.…”