Twenty-five years of XMM-Newton EPIC-MOS2 Unfocused Particle Background: Solar Cycle Modulation, Galactic Cosmic-Ray Correlation, and Implications for NewAthena

We present a study of the unfocused particle-induced background measured by the XMM-Newton EPIC-MOS2 detector, extending previous analyses to the end of 2025 and covering approximately twenty-five years of observations. Using out-of-field-of-view (outFOV) data, we investigate the long-term evolution of the particle background over two complete solar cycles and compare it with independent tracers of the Galactic cosmic-rays. We find an extremely tight correlation between the MOS2 outFOV count rate and the AMS proton flux, with an intrinsic scatter of only about 1.1%, confirming that variations in the unfocused background are primarily driven by changes in the Galactic cosmic-rays flux. The extended temporal baseline clearly reveals the modulation associated with the solar cycle and the characteristic signatures of the 22 year Hale cycle. A Lomb-Scargle analysis identifies a dominant periodicity at 11.28 yr, while structure-function and wavelet analyses reveal a persistent $\sim$27 day modulation associated with solar rotation. We also confirm the presence of a residual background component that does not scale with the Galactic cosmic-ray flux, consistent with previous studies and possibly associated with Compton-scattered high-energy X-ray photons. This work places the particle-induced background measured by the XMM-Newton EPIC-MOS2 detector in the broader context of Galactic cosmic-ray studies, while also providing direct implications for the development of background-monitoring and mitigation strategies for future X-ray missions such as NewAthena.