  We present highly sensitive measurements taken with MeerKAT at 1280 MHz as
well as archival GBT, MWA and VLA images at 333, 88 and 74 MHz. We report the
detection of synchrotron radio emission from the infrared dark cloud (IRDC)
associated with the halo of the Sgr B complex on a scale of ~60 pc. A strong
spatial correlation between low-frequency radio continuum emission and dense
molecular gas, combined with spectral index measurements, indicates enhanced
synchrotron emission by cosmic-ray electrons. Correlation of the FeI 6.4 keV
Kalpha line and synchrotron emission provides compelling evidence that the low
energy cosmic-ray electrons are responsible for producing the Kalpha line
emission. The observed synchrotron emission within the halo of the Sgr B cloud
complex has mean spectral index alpha -1+/-1 gives the magnetic field strength
~100 muG for cloud densities nH = 10^4-10^5 cm-3 and estimate cosmic-ray
ionization rates between 10^-13 and 10^-14 s^-1. Furthermore, the energy
spectrum of primary cosmic-ray electrons is constrained to be E^-3 +/-1 for
typical energies of few hundred MeV. The extrapolation of this spectrum to
higher energies is consistent with X-ray and gamma-ray emission detected from
this cloud. These measurements have important implications on the role that
high cosmic-ray electron fluxes at the Galactic center play in production of
radio synchrotron emission, the FeI Kalpha line emission at 6.4 keV and ~GeV
gamma-ray emission throughout the central molecular zone (CMZ).
