Abstract:We investigate the soft behaviour of scalar effective field theories (EFTs) when there is a number of distinct derivative power counting parameters, ρ 1 < ρ 2 < . . . < ρ Q . We clarify the notion of an enhanced soft limit and use these to extend the scope of onshell recursion techniques for scalar EFTs. As an example, we perform a detailed study of theories with two power counting parameters, ρ 1 = 1 and ρ 2 = 2, that include the shift symmetric generalised galileons. We demonstrate that the minimally enhanced soft limit uniquely picks out the Dirac-Born-Infeld (DBI) symmetry, including DBI galileons. For the exceptional soft limit we uniquely pick out the special galileon within the class of theories under investigation. We study the DBI galileon amplitudes more closely, verifying the validity of the recursion techniques in generating the six point amplitude, and explicitly demonstrating the invariance of all amplitudes under DBI galileon duality.

The Vainshtein screening mechanism relies on nonlinear interaction terms becoming dominant close to a compact source. However, theories displaying this mechanism are generally understood to be low-energy theories: it is unclear that operators emerging from UV completion do not interfere with terms inducing Vainshtein screening. In this work, we find a set of interacting massive Galileon theories that exhibit Vainshtein screening; examining potential UV completions of these theories, we determine that the screening does not survive the extension. We find that neglecting operators when integrating out a heavy field is non-trivial, and either care must be taken to ensure that omitted terms are small for the whole domain, or one is forced to work solely with the UV theory. We also comment on massive deformations of the familiar Wess-Zumino Galileons.

Recently, it was argued that the conformal coupling of the chameleon to matter fields created an issue for early universe cosmology. As standard model degrees of freedom become non-relativistic in the early universe, the chameleon is attracted towards a "surfing" solution, so that it arrives at the potential minimum with too large a velocity. This leads to rapid variations in the chameleon's mass and excitation of high energy modes, casting doubts on the classical treatment at Big Bang Nucleosynthesis. Here we present the DBI chameleon, a consistent high energy modification of the chameleon theory that dynamically renders it weakly coupled to matter during the early universe thereby eliminating the adverse effects of the 'kicks'. This is done without any fine tuning of the coupling between the chameleon and matter fields, and retains its screening ability in the solar system. We demonstrate this explicitly with a combination of analytic and numerical results. arXiv:1511.05761v3 [hep-th]

Given the two-port description of a hair cell from Weiss [Hear. Res. 7, 353–360 (1982)], with the Davis model for the angle-dependent conductance representing the transduction channels [Leong and Weiss, Hear. Res. 20, 175–195 (1985)], and the data of Crawford and Fettiplace [J. Physiol. 364, 359–379 (1985)], Howard and Hudspeth [Neuron 1, 189–199 (1988)], and Holton and Hudspeth [J. Physiol. 375, 195–227 (1986)], analysis shows that (1) the feedback inherent in the two-port must be positive, and (2) degenerative (outward-flowing) channels must be present. The static nonlinear behavior of the system, and the small and large signal dynamics, have been studied. The system displays Q multiplication at low signal levels, bandwidth increase with signal level, a compressive nonlinearity at CF, and continuous oscillations if the parameters are slightly misadjusted—all well-documented physiological phenomena.