Introduction -- The Standard Model -- Known knowns -- Known unknowns -- Neutrino Masses -- Flavor structure -- Strong CP puzzle -- The quest for heavy neutral leptons -- Motivations to go beyond simplified scenarios -- Collider searches of HNL -- Current Status -- Beyond the single mixing assumption -- A specific example: dimuon channel -- Beyond the single HNL assumption -- Summary -- Confronting open issues in Flavor Physics -- B→K(∗)νν ̄ Decays in the SM -- Effective theory description.

This book presents a comprehensive exploration of unresolved mysteries in particle physics, delving into cutting-edge research aimed at unraveling the enigmatic facets of the Standard Model (SM). It tackles fundamental questions surrounding neutrino masses, flavor physics, and the strong CP problem, shedding light on the universe's underlying structure. The insightful analysis takes readers on a journey that begins with probing the mechanisms behind neutrino mass generation, including the existence of heavy neutral leptons and their implications for collider experiments. The book then navigates the intricate landscape of flavor physics, elucidating rare B-meson decays and their role as a window to new physics beyond the SM. Additionally, it investigates axions as potential hot dark matter candidates, offering valuable insights into their interactions with pions and cosmological implications. This work not only addresses current theoretical puzzles but also paves the way for future advancements in particle physics. Aimed at researchers, graduate students, and professionals in the field, this book serves as an indispensable resource for those seeking to delve deeper into the forefront of particle physics research.