Towards a new generation axion helioscope

TL;DR

The paper proposes NGAH, a next-generation axion helioscope building on CAST by dramatically increasing the magnet cross-section, implementing full-aperture x-ray optics, and achieving ultra-low detector backgrounds. It analyzes the theoretical axion/ALP landscape, establishes target sensitivities in $g_{a\gamma}$ and $m_a$, and outlines a concrete implementation path with a toroidal magnet, Wolter-I optics, and advanced Micromegas-like detectors. The work presents a quantitative figure of merit framework and multiple realistic NGAH scenarios, showing feasibility for reaching $g_{a\gamma}\sim10^{-12}$ GeV$^{-1}$ up to $m_a\sim$ a few 10 meV, with potential to test SN1987A bounds and WD cooling hints. If realized, NGAH would probe a broad class of QCD axion models and ALPs, opening a largely uncharted low-energy frontier with significant experimental and astrophysical impact.

Abstract

We study the feasibility of a new generation axion helioscope, the most ambitious and promising detector of solar axions to date. We show that large improvements in magnetic field volume, x-ray focusing optics and detector backgrounds are possible beyond those achieved in the CERN Axion Solar Telescope (CAST). For hadronic models, a sensitivity to the axion-photon coupling of $\gagamma\gtrsim {\rm few} \times 10^{-12}$ GeV$^{-1}$ is conceivable, 1--1.5 orders of magnitude beyond the CAST sensitivity. If axions also couple to electrons, the Sun produces a larger flux for the same value of the Peccei-Quinn scale, allowing one to probe a broader class of models. Except for the axion dark matter searches, this experiment will be the most sensitive axion search ever, reaching or surpassing the stringent bounds from SN1987A and possibly testing the axion interpretation of anomalous white-dwarf cooling that predicts $m_a$ of a few meV. Beyond axions, this new instrument will probe entirely unexplored ranges of parameters for a large variety of axion-like particles (ALPs) and other novel excitations at the low-energy frontier of elementary particle physics.

Figures (11)

• Figure 1: Solar axion flux spectrum at Earth, originating from the Primakoff process (dashed line) and from processes involving electrons (solid line), bremsstrahlung and Compton processes. We have chosen illustrative values of $g_{ae}=10^{-13}$ and $g_{a\gamma}=10^{-12}~{\rm GeV}^{-1}$, corresponding to DFSZ axions with $f_a=0.85\times 10^9$ GeV, $C_e=1/6$ and $C_\gamma=0.75$. For better comparison, the Primakoff flux has been scaled up by a factor of 100.
• Figure 2: Comprehensive ALP parameter space, highlighting the three main front lines of direct detection experiments: laser-based laboratory techniques, helioscope (solar ALPs and axions), and microwave cavities (dark matter axions). The blue line corresponds to the current helioscope limits, dominated by CAST Andriamonje:2007ewArik:2008mq for practically all axion masses but for the $m_a \sim 0.85-1$ eV exclusion line from the last Tokyo helioscope resultsInoue:2008zp. Also shown are the constraints from horizontal branch (HB) stars, supernova SN1987A, and hot dark matter (HDM). The yellow "axion band" is defined roughly by $m_a f_a\sim m_\pi f_\pi$ with a somewhat arbitrary width representing the range of realistic models. The green line refers to the KSVZ model ($C_\gamma\sim-1.92$).
• Figure 3: Possible conceptual arrangement of the NGAH. On the left we show the cross section of the NGAH toroidal magnet, in this example with six coils and bores. On the right the longitudinal section with the magnet, the optics attached to each magnet bore and the x-ray detectors.
• Figure 4: LEFT: The parameter space for hadronic axions and ALPs. The CAST limit, some other limits, and the range of PQ models (yellow band) are also shown. The blue lines indicate the sensitivity of the four scenarios discussed in the text and table 1. RIGHT: The expected sensitivity regions of the four same scenarios in the parameter space of non-hadronic axions with both electron and photon coupling. In GUT models $C_\gamma$ is fixed to $0.75$ and we show the bound on the electron coupling ($C_e$) from red giants (dashed line along the diagonal) and the region motivated by WD cooling (orange band). DFSZ models lie below the horizontal line $C_\gamma C_e< 0.25$.
• Figure 5: The barrel toroid of the ATLAS experiment at CERN. The huge dimensions of the magnet can be appreciated by a comparison to the man standing at the bottom of the photo. The NGAH magnet's volume will be about 1--2 % of this enormous magnet. Courtesy of the ATLAS experiment.