Search for high energy 5.5 MeV solar axions with the complete Borexino dataset

Abstract: A search for solar axions and axion-like particles produced in the $p+d\rightarrow\rm{^3He}+A~(5.5\rm{ ~MeV})$ reaction was performed using the complete dataset of the Borexino detector (3995 days of measurement live-time). The following interaction processes have been considered: axion decay into two photons $({\rm A}\rightarrow2\gamma)$, inverse Primakoff conversion on nuclei $({\rm A}+Z\rightarrow\gamma+Z$), the Compton conversion of axions to photons $({\rm A}+e\rightarrow e+\gamma)$ and the axio-electric effect $({\rm A}+e+Z\rightarrow e+Z$). Model-independent limits on axion-photon ($g_{A\gamma}$), axion-electron ($g_{Ae}$), and isovector axion-nucleon ($g_{3AN}$) couplings are obtained: $|g_{A\gamma}\times g_{3AN}| \leq 2.3\times 10^{-11} \rm{GeV}^{-1}$ and $|g_{Ae}\times g_{3AN}| \leq 1.9\times 10^{-13}$ at $m_A <$ 1 MeV (90\% c.l.). The Borexino results exclude new large regions of $g_{A\gamma}$, and $g_{Ae}$ coupling constants and axion masses $m_A$, and leads to constraints on the products $|g_{A\gamma}\times m_A|$ and $|g_{Ae}\times m_A|$ for the KSVZ- and the DFSZ-axion models.

An Overview of Borexino's Search for Solar Axions

The Borexino collaboration has conducted an extensive search for solar axions emanating from the $p+d\rightarrow \rm{^3He}+A$ ($5.5\ \rm{MeV}$) reaction using data collected over $3995$ days. This investigation forms part of the broader effort to detect axions, hypothetical pseudoscalar particles proposed to resolve the strong CP problem in Quantum Chromodynamics (QCD) and contribute to dark matter research.

Solar Axion Production and Detection

Axions can be produced in the Sun and other stars through various processes, with their interactions typically being highly suppressed due to their weak coupling constants—$g_{A\gamma}$, $g_{Ae}$, and $g_{AN}$—in the context of models like Kim-Shifman-Vainshtein-Zakharov (KSVZ) and Dine-Fischler-Srednicki-Zhitnitsky (DFSZ). The paper explores axion interactions such as decay into two photons $\rm{(A}\rightarrow2\gamma)$, inverse Primakoff conversion on nuclei $\rm{(A}+Z\rightarrow\gamma+Z)$, Compton conversion $\rm{(A}+e\rightarrow e+\gamma)$, and the axio-electric effect $\rm{(A}+e+Z\rightarrow e+Z)$. These processes are linked to axion-photon, axion-electron, and axion-nucleon coupling constants.

Methodology

The Borexino detector, located at the Gran Sasso National Laboratory, relies on low-background scintillation for neutrino detection, offering a promising environment for axion studies. The detector response to axion-induced events was modeled using Monte Carlo simulations, considering the distinct energy signatures of axion conversions and decays. This simulation provided the basis for identifying potential axion-related events amidst data drawn from a fiducial mass of approximately $145$ tons of pseudocumene.

Results and Constraints

The analysis did not observe statistically significant evidence of solar axions, enabling the derivation of new limits on axion coupling constants:

• $|g_{A\gamma} \times g_{3AN}| \times m_A^2 \leq 1.6 \times 10^{-11} \ \rm{eV}$
• $|g_{A\gamma} \times g_{3AN}| \leq 2.3 \times 10^{-11} \ \rm{GeV^{-1}}$
• $|g_{Ae} \times g_{3AN}| \leq 1.9 \times 10^{-13}$

These results exclude significant portions of the parameter spaces for axion couplings and masses and improve upon boundaries established by older experimental attempts.

Implications and Future Perspectives

The Borexino detector has demonstrated enhanced sensitivity to solar axion interactions, refining constraints on axion models. These findings assist in narrowing the search field for axion properties compatible with both the DFSZ and KSVZ frameworks. While current limits are stringent, advancing this research necessitates further improvements in detector technology and background discrimination methods. The derived constraints may influence the theoretical modeling of axions, potentially guiding efforts in dark matter physics and the quest for clarifying the strong CP problem in QCD.

Concluding Remarks

This study exemplifies the continuing push within particle physics to uncover elusive particles like axions. Although direct detection remains uncertain, establishing rigorous bounds on these parameters is crucial for informing theory and directing future experimental efforts in the search for axions and other candidates of dark matter.