[Paper Review] We need lab experiments to look for axion-like particles

Joerg Jaeckel, Eduard Massó+3 more|ArXiv.org|May 29, 2006

Dark Matter and Cosmic Phenomena1 references16 citations

TL;DR

This paper proposes that axion-like particles (ALPs) could evade astrophysical bounds by suppressing their production in the Sun through environment-dependent couplings—such as density, temperature, or momentum transfer—thereby reconciling the PVLAS signal with stellar energy loss and CAST constraints. Laboratory experiments, particularly light-shining-through-walls setups, are essential to conclusively test this hypothesis.

ABSTRACT

The PVLAS signal has renewed the interest in light bosons coupled to the electromagnetic field. However, astrophysical bounds coming from the lifetime of the sun and the CAST experiment are seemingly in conflict with this result. We discuss effective models that allow to suppress production of axion-like particles in the sun and thereby relax the bounds by some orders of magnitude. This stresses the importance of laboratory searches.

Motivation & Objective

• To resolve the conflict between the PVLAS experiment's reported axion-like particle signal and stringent astrophysical bounds from solar energy loss and the CAST experiment.
• To explore whether ALP production in the Sun can be suppressed via environmental dependencies (e.g., density, temperature, momentum transfer) to relax astrophysical constraints.
• To argue that laboratory experiments are the only conclusive way to test whether the PVLAS signal originates from new physics.
• To demonstrate that suppression factors as low as 10⁻²⁰ can still allow the PVLAS signal while evading astrophysical bounds, under conservative assumptions.

Proposed method

• Constructing effective models where the ALP coupling $ g $ depends on environmental parameters such as density $ \rho $, temperature $ T $, or momentum transfer $ \langle q \rangle $, using a step-function suppression at critical thresholds.
• Calculating the suppression factor $ S(R_0) = \text{production}(R > R_0) / \text{production}(\text{full sun}) $ to quantify reduced ALP emission in the solar outer shell.
• Using the BP2000 solar model to compute ALP production rates and comparing them with astrophysical bounds, including the solar luminosity limit and CAST experiment constraints.
• Applying the suppression factor to modify the effective coupling in energy loss ($ L_{\text{ALP}} \propto g^2 $) and CAST detection rate ($ \text{rate} \propto g^4 $), deriving $ g_{\text{supp}} $ from $ S_{\text{loss}} $ and $ S_{\text{CAST}} $.
• Evaluating critical environmental thresholds (e.g., $ \rho \sim 0.003\, \text{g cm}^{-3} $, $ T \sim 12\, \text{eV} $) for suppression factors of $ 10^{-20} $, showing they are physically plausible.
• Proposing that light-shining-through-walls experiments—like APFEL at DESY—offer the only controlled environment to test the PVLAS signal definitively.

Experimental results

Research questions

• RQ1 Can the PVLAS signal be reconciled with astrophysical bounds if ALP production in the Sun is suppressed by environmental dependencies?
• RQ2 What are the required suppression factors and environmental thresholds (density, temperature, momentum transfer) to evade solar energy loss and CAST constraints?
• RQ3 Is it possible to construct effective models where $ g $ depends on $ \rho $, $ T $, or $ \langle q \rangle $, such that the PVLAS signal remains viable?
• RQ4 Why are laboratory experiments the only definitive way to test the particle interpretation of the PVLAS signal?
• RQ5 What level of environmental suppression is needed to allow the PVLAS coupling parameters while remaining consistent with astrophysical observations?

Key findings

• A suppression factor of $ S = 10^{-4} $ at $ R_0 = 0.79 R_\odot $, corresponding to $ \rho_0 \approx 0.1\, \text{g cm}^{-3} $ and $ T_0 \approx 120\, \text{eV} $, allows the PVLAS signal to be consistent with astrophysical bounds.
• Even a suppression factor as low as $ 10^{-20} $—requiring $ \rho_0 \approx 0.003\, \text{g cm}^{-3} $ and $ T_0 \approx 12\, \text{eV} $—is sufficient to evade constraints, though it lies at the edge of physical reasonableness.
• The PVLAS coupling parameters ($ m_\phi \approx 1-1.5\, \text{meV}, \, g \approx (1.7-5) \times 10^{-6}\, \text{GeV}^{-1} $) are compatible with a suppression mechanism that turns off ALP production in the Sun’s core.
• The CAST experiment’s null result is evaded if the effective coupling is suppressed by $ g^4 $-dependent suppression, requiring $ S_{\text{CAST}} \sim g^4_{\text{supp}} / g^4_{\text{loss}} $, which is achievable with modest environmental thresholds.
• Laboratory experiments such as APFEL, which test photon regeneration in magnetic fields, are uniquely suited to confirm or rule out the PVLAS signal due to their controlled environment.
• The paper concludes that only laboratory experiments can provide a conclusive test of the PVLAS signal, as astrophysical bounds can be evaded by exotic environmental dependencies.