We have searched for hadronic axions which may be produced in the Sun by a bremsstrahlung-like process, and observed in the HPGe detector by an axioelectric effect. A conservative upper limit on the hadronic axion mass m_a < 334 eV at 95% C.L. is obt
ained. Our experimental approach is based on the axion-electron coupling and it does not include the axion-nucleon coupling, which suffers from the large uncertainties related to the estimation of the flavor-singlet axial-vector matrix element.
In this experiment we aim to detect Kaluza-Klein (KK) excitations of the bulk gauge field, emitted in a bremsstrahlung process on solar plasma constituents, by looking at a process analogous to the photoelectric effect inside the HPGe detector. Using
a generic feature of the underlying effective theory that the unknown 4-dimensional gauge coupling is independent of the number of extra large dimensions delta, we show that the expected number of events in the detector is insensitive to the true scale of quantum gravity for delta=2. With the entire data collection time of 202 days in the energy interval 1.7 - 3.8 keV, the number of events detected was as low as 1.1x10^6, compared to 2.7x10^6 from the expected high multiplicity of the solar KK excitations for delta =2. Hence, our bound from the presumed existence of new forces associated with additional gauge bosons actually conforms with very stringent bounds set from various astrophysical considerations. Baring any modifications of the infrared part of the KK spectrum, this bound would therefore rule out the possibility of observing any signal at the LHC for delta=2. Although a dependence on the fundamental scale referring to 4+delta-dimensional gravity turns on again for delta=3, the experimental sensitivity of the present setup proves insufficient to draw any constraint for delta>2.
