This study investigates how undergraduate students conceptualize particle physics through a think-aloud concept mapping task. Participants, drawn from an online upper-division particle physics course offered across Hispanic-Serving Institutions (HSIs), constructed digital concept maps via an online diagram app using a curated set of terms related to different particles and fundamental interactions.

Building on prior work using concept maps, we developed a concept map activity to investigate how students understand what particle physics is, through organizing their core ideas using the concept map. Responses ranged from process-oriented structures highlighting particle behaviors (e.g., annihilation, scattering) to more classification-driven maps reflecting textbook frameworks. While students often correctly linked bosons with fundamental forces, many showed difficulty articulating relationships between particle families, the roles of mediators, or distinctions among categories like quarks, leptons, and mesons. Several participants referenced external resources during the task, offering insight into areas of productive struggle and conceptual uncertainty.

Our findings suggest that think-aloud concept mapping is a promising tool for surfacing students' implicit reasoning and structural understanding of complex topics. We discuss implications for supporting advanced concept development in upper-division coursework and online learning environments.