Abstract
In theories with the large extra dimensions beyond the standard 4-dimensional spacetime, axions could propagate in such extra dimensions, and acquire Kaluza-Klein (KK) excitations. These KK axions are produced in the Sun and could solve unexplained heating of the solar corona. While most of the solar KK axions escape from the solar system, a small fraction is gravitationally trapped in orbits around the Sun. They would decay into two photons inside a terrestrial detector. The event rate is expected to modulate annually depending on the distance from the Sun. We have searched for the annual modulation signature using $832\times 359$ kg$\cdot$days of XMASS-I data. No significant event rate modulation is found, and hence we set the first experimental constraint on the KK axion-photon coupling of $4.8 \times 10^{-12}\, \mathrm{GeV}^{-1}$ at 90% confidence level for a KK axion number density of $\bar{n}_\mathrm{a} = 4.07 \times 10^{13}\, \mathrm{m}^{-3}$, the total number of extra dimensions $n = 2$, and the number of extra dimensions $\delta = 2$ that axions can propagate in.
Highlights
Kamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo, Higashi-Mozumi, Kamioka, Hida, Gifu 506-1205, Japan
We have searched for the annual modulation signature using 832 × 359 kg·days of XMASS-I data
We evaluated the significance of this result by using 10 000 no-modulation dummy samples that have the same statistical and systematic errors as the data (Ref. [16])
Summary
The XMASS-I detector is located 1000 m (2700 m water equivalent) underground at the Kamioka. This cut is applied only to the events with a total number of PE less than 200 in order to remove Cherenkov events originating from 40 K decays in the photocathodes of the PMTs The second largest contribution to the systematic error is the nonlinearity of the scintillation yield (∼ ±10% on the event rate) This uncertainty is taken into account in calculating the signal expectation, and is introduced as Li in Eq (6) below. We have Ci and Ai as the constant term and the expected amplitude of the event rate in the ith energy-bin, respectively, Ai corresponds to half of the residual event rate, and Li , which is associated with the penalty term β, accounts for the uncertainty stemming from the nonlinearity of the scintillation efficiency on Ai. We define.
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