Reconciling the CAST and PVLAS Results
R. N. Mohapatra, Salah Nasri
TL;DR
The paper addresses the apparent tension between PVLAS's hint of an ultralight axion-like particle and CAST's null solar axion search. It proposes a low-energy effective theory with a pseudoscalar $a$ whose coupling to photons is mediated by a scalar $\phi$ whose vacuum expectation value is temperature dependent, controlled by a keV-scale field $\sigma$. At lab temperatures, $v_\phi$ yields an effective coupling with $M_a\sim 2\times10^5$ GeV, while in the solar interior $v_\sigma\to0$ drives $v_\phi\to0$, suppressing solar production and reconciling PVLAS with CAST. The model makes testable predictions such as light-by-light scattering and rare decays, remains broadly consistent with SN1987A and BBN constraints, and may be completed by a UV framework involving a heavier pseudoscalar and vector-like fermions. Overall, it offers a novel mechanism to explain the experimental tension while remaining compatible with established astrophysical and cosmological bounds.
Abstract
The PVLAS experiment has recently claimed evidence for an axion-like particle in the milli-electron-Volt mass range with a coupling to two photons that appears to be in contradiction with the negative results of the CAST experiment searching for solar axions. The simple axion interpretation of these two experimental results is therefore untenable and it has posed a challenge for theory. We propose a possible way to reconcile these two results by postulating the existence of an ultralight pseudo-scalar particle interacting with two photons and a scalar boson and the existence of a low scale phase transition in the theory.