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

TL;DR

The paper addresses the search for solar axions with energy $5.5$ MeV produced in $p+d\rightarrow {^3He}+A$ using the complete Borexino dataset, exploiting four detection channels ($A\rightarrow 2\gamma$, $A+Z\rightarrow\gamma+Z$, $A+e\rightarrow e+\gamma$, $A+e+Z\rightarrow e+Z$). It performs a model-independent analysis to constrain $|g_{A\gamma}\times g_{3AN}|$, $|g_{Ae}\times g_{3AN}|$, and, via axion-mass relations, $|g_{A\gamma}\times m_A|$ and $|g_{Ae}\times m_A|$ for $m_A<1$ MeV, including flux and decay effects. The results yield tighter bounds than previous Borexino studies and carve out new regions of axion parameter space within the KSVZ and DFSZ frameworks, providing complementary constraints to other laboratory and astrophysical searches. Overall, the work demonstrates Borexino’s sensitivity to heavy, weakly coupled axions in the MeV range and strengthens the landscape of laboratory limits on axion-like particles.

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 $({\rm A}\rightarrow2γ)$, inverse Primakoff conversion on nuclei $({\rm A}+Z\rightarrowγ+Z$), the Compton conversion of axions to photons $({\rm A}+e\rightarrow e+γ)$ and the axio-electric effect $({\rm A}+e+Z\rightarrow e+Z$). Model-independent limits on axion-photon ($g_{Aγ}$), axion-electron ($g_{Ae}$), and isovector axion-nucleon ($g_{3AN}$) couplings are obtained: $|g_{Aγ}\times g_{3AN}| \leq 2.3\times 10^{-11} \rm{GeV}^{-1}$ 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 $g_{Aγ}$, and $g_{Ae}$ coupling constants and axion masses $m_A$, and leads to constraints on the products $|g_{Aγ}\times m_A|$ and $|g_{Ae}\times m_A|$ for the KSVZ- and the DFSZ-axion models.

Figures (8)

• Figure 1: The expected fluxes of the solar axions emitted in the $p + d \rightarrow {^3{\rm{He}}} + A$ reaction. 1 - KSVZ, 2 - DFSZ ($\rm{tan}\beta^*=140$), 3 - DFSZ ($\rm{tan}\beta^*=0.25$), 4, 5, 6 - the decay $A\rightarrow2\gamma$ is taken into account, in the KSVZ- and DFSZ-models, respectively. At $m_A \geq 2m_e$ the decay $A\rightarrow e^+e^-$ is possible (dot line).
• Figure 2: The cross-section of axion reactions and axion lifetime vs axion mass. 1 - axion lifetime for $A\rightarrow 2\gamma$ decay (the vertical scale shows $\tau\times10^{-15}$ s), 2 - inverse Primakoff conversions on $^{12}\rm{C}$ nuclei, 3 - Compton conversion and 4 - axioelectric effect on carbon atoms. The lifetime and cross-section of 1 and 2 were calculated for $g_{A\gamma}=1~\rm{GeV^{-1}}$, the cross-sections of 3 and 4 with $g_{Ae}=1$.
• Figure 3: Simulated responses to axion interactions in the Borexino IV: 1 - axioelectric effect ($5.49$ MeV electrons), 2 - Compton axion to photon conversion (electrons and $\gamma$-quanta), 3 - decay $A\rightarrow 2\gamma$, 4 - Primakoff conversion ($5.49$ MeV $\gamma$-quanta). The response functions are normalized to single axion interaction or decay in $5$ m radius sphere.
• Figure 4: Energy spectra of the events and effect of the selection cuts. From top to bottom: 1 - raw spectrum; 2 - with $2$ ms muon veto cut; 3 - final spectrum after application of the cosmogenic veto and FV cut. The bumps at $0.4$, $1.3$, $2.4$, and $3.3$ MeV are caused by decays of $^{210}$Po, $^{11}$C, external $\gamma$'s from $^{208}$Tl and internal $^{208}$Tl, respectively.
• Figure 5: Fit of the Borexino energy spectrum in the ($4.4-6.8$) MeV range. Curve 1 is the detector response function for axio-electric effect on carbon atoms and corresponds to $90$% c.l. upper limit ($S = 5.6$ events). The fit (curve 2) is complementary to the radial fit on Fig. \ref{['Figure:Fit_R3']}.