Scalar Cherenkov radiation from high-energy cosmic rays

TL;DR

This paper investigates scalar Cherenkov radiation from high-energy cosmic rays, showing that energy loss depends on the UV cutoff of the effective field theory, which constrains certain scalar fields in cosmology.

Contribution

It provides detailed calculations of scalar wave energy loss and explores how different UV cutoff scales affect the constraints on scalar fields from cosmic ray observations.

Findings

Energy loss scales with the square of the UV cutoff frequency.

Low UV cutoff in dark-energy models makes scalar radiation negligible.

High UV cutoff can lead to significant energy loss, constraining scalar field theories.

Abstract

As first noted by Robert Wagoner in the 1970s, if a scalar field is nonminimally coupled to the Ricci scalar and propagates at subluminal speeds, then there exists the possibility of scalar $\check{\hbox{C}}$erenkov radiation from a moving particle. The mere observation of high-energy cosmic rays could in principle rule out the existence of such scalar fields since any particle moving faster than scalar perturbations would lose energy in the form of scalar waves until it moves slower than those. We compute in detail the energy loss to scalar waves and find that it scales with the square of the ultra-violet (UV) cutoff frequency of the effective field theory (EFT) of gravity. For dark-energy-motivated EFTs, the UV cutoff can be low, in which case that energy loss could always be negligible. In contrast, if viewed as a covariant theory valid at all scales or as an EFT valid at higher energies, perhaps even all the way up to the Planck scale, as may be the case if motivated by quantum-gravity perspectives, then the energy loss to scalar waves may diverge or become dramatically large. In this case, high-energy cosmic rays of extragalactic origin stringently constrain any conformally coupled scalar fields with non-canonical kinetic terms, although a minimum scalar phase velocity is required to trust the EFT.