RD towards CUPID (CUORE Upgrade with Particle IDentification)

CUPID is a proposed future tonne-scale bolometric neutrinoless double beta decay ($0\nu\beta\beta$) experiment to probe the Majorana nature of neutrinos and discover Lepton Number Violation in the so-called inverted hierarchy region of the neutrino mass. CUPID will be built on experience, expertise and lessons learned in CUORE, and will exploit the current CUORE infrastructure as much as possible. In order to achieve its ambitious science goals, CUPID aims to increase the source mass and dramatically reduce the backgrounds in the region of interest. This requires isotopic enrichment, upgraded purification and crystallization procedures, new detector technologies, a stricter material selection, and possibly new shielding concepts with respect to the state of the art deployed in CUORE. This document reviews and describes the rich and varied R&D activities, performed inside and outside the CUORE collaboration towards the next generation bolometric $0\nu\beta\beta$ experiment. A separate document discusses the science goals and timescale for CUPID in more detail.