Hunting Solar Axions in Liquid Xenon express
Inside the Sun, reactions can produce light, almost undetectable particles called axions. One such reaction yields axions with an energy of 5.49 MeV, as if each one carried the energy of a tennis ball moving at 100 km/h. These particles pass freely through Earth, but occasionally they might collide with a xenon atom.
When an axion enters a tank of liquid xenon, it can transform into a pair: an electron and its antiparticle, a positron. The probability of this transformation rises sharply if the axion mass is close to twice the electron mass. Xenon is chosen for a reason: the heavier the atom, the better the chance of catching an axion, because the probability scales as the square of the nuclear charge.
Future liquid xenon detectors, such as XENONnT and LZ, could accumulate enough data over several years of operation. With an exposure of 200 ton·years, the sensitivity to the combination of coupling constants would reach 1.59×10⁻¹⁴, and with 1000 ton·years, 7.12×10⁻¹⁵. This is better than current limits from the neutrino detector Borexino and comparable to future giants like JUNO and Hyper-Kamiokande.
If axions exist and couple to photons, the sensitivity to the combination of constants could reach 6.76×10⁻¹² GeV⁻¹ at 200 ton·years and 3.02×10⁻¹² GeV⁻¹ at 1000 ton·years. This would allow testing a wide class of theoretical models and possibly seeing the imprint of the axion field for the first time.
Original abstract
as written by the authors arXiv:2608.15699v1
The monochromatic 5.49 MeV solar axions induced by the isovector coupling $$g_{3aN}$$ can be searched for at the dark matter direct detection experiments. In this paper we estimate the prospects of the relevant axion couplings for axion mass $$m_{a} <$$ 1 MeV with xenon targets. Given the axion-electron coupling $$g_{ae}$$, the signal is dominated by the axion-induced $$e^+ e^-$$ pair production whose cross section is largely enhanced when the axion mass approaches twice of the electron mass. Furthermore, the cross section depends on the atomic number squared $$Z^2$$. This allows the next-generation xenon experiments to surpass the current Borexino constraints and provide sensitivities competitive with those of the large neutrino detectors such as JUNO and Hyper-Kamiokande. With an exposure of 200 and 1000 ton$$\cdot$$yr, the couplings $$|g_{3aN} g_{ae}|$$ can be probed down to $$1.59\times10^{-14}$$ and $$7.12\times10^{-15}$$ at 90% C.L., respectively. If the axion couples to photons, the projected sensitivities on $$|g_{3aN}g_{a\gamma}|$$ can touch down to $$6.76\times10^{-12}$$ GeV$$^{-1}$$ and $$3.02\times10^{-12}$$ GeV$$^{-1}$$, respectively.
The axion is named after a brand of laundry detergent because it was supposed to 'clean up' a problem in the theory.
In the novel 'The Three-Body Problem', Liu Cixin mentions axions as a possible channel for alien communication — almost undetectable but all-pervading.