Program: GS-2023A-Q-131

RR Lyrae stars are sensitive probe for the precision stellar astrophysics and also for the distance scale thanks to their well-defined near-infrared Period-Luminosity relations (PLRs). These horizontal branch variables together with the tip of the red giant branch (TRGB) can be used for the primary calibration of the population II distance ladder evaluating the ongoing tension between population I Cepheid-Supernovae based Hubble constant and the Planck results. Since the TRGB-based Hubble constant values are systematically smaller than other independent local measurements, TRGB calibration needs to be tested or complemented with population II RR Lyrae variables. While the pulsation models of RR Lyrae can reproduce most observables, they predict a significant metallicity effect on their JHKs PLRs that is not well-quantified in limited observational studies. Our goal is to quantify metallicity dependence in RR Lyrae PLR over a wide metallicity range (-0.4 <[Fe/H]< -2.4 dex). Therefore, we propose JHKs time-series observations of RR Lyrae in two globular clusters that fill the metallicity gap in our current sample of globular clusters at the metal-rich end. These observations will complement our previously collected data to construct a novel sample of 13 globular clusters hosting 1300 RR Lyrae stars and covering a wide metallicity range (Delta[Fe/H]~ 2 dex). Our legacy dataset will provide an absolute calibration of RR Lyrae PLRs at near-infrared wavelengths thus improving their use for distance determinations, and a stringent test of theoretical predictions from the stellar pulsation models.