Chapter 01   What is a particle? (10:17)
Chapter 02   What is a model? (11:28)
Chapter 03   What is particle physics? (11:57)
Chapter 04   What are charges? (11:13)

Chapter 05   What are interactions? (10:27)
Chapter 06   What is matter? (13:48)
Chapter 07   What are conservation laws? (17:57)
Chapter 08   What are particle transformations? (14:54)

Chapter 09   What is anti-matter? (15:45)
Chapter 10   What is the Standard Model of particle physics? (15:42)
Chapter 11   What is the Higgs boson? (15:26)
Chapter 12   What is beyond the Standard Model of particle physics? (10:19)

Chapter 13   What is a particle accelerator? (23:13)
Chapter 14   What is a particle detector? (19:23)
Chapter 15   What is a cloud chamber? (14:42)
Chapter 16   What is CERN? (13:07)

TOTAL: 3 hours 50 minutes

The development of this particle physics course for high-school students was based on a thorough review of educational theory, empirical research, and years of experience in particle physics education. Below, we provide insights into the rationale behind specific content decisions and how they can support student learning.

Key Messages
Key messages are used throughout the course. They comprise elementary ideas of the science content formulated in a way accessible to the intended audience and their order supports students’ learning progression. Key messages were derived as the result of an educational reconstruction of the subject matter (Duit et al., 2012). This process included an in-depth analysis of the science content and previous research about students’ conceptions that might hinder learning. The key messages also provide testable concept knowledge for the quiz questions that accompany each chapter. However, it is important to note that these messages should be considered in the context of each chapter and should not be mistaken as stand-alone summaries of a chapter that answer the guiding question of a chapter.

Learning Progression
The course structure gradually introduces new terms and concepts to avoid overwhelming students. For example, we only introduce six elementary particles in the first four chapters to allow students to build a strong foundation before tackling more abstract ideas and more types of particles and particle systems later.
Cognitive load theory suggests that students’ ability to absorb and understand new information is dependent on how well it is integrated into their existing mental models, which are frameworks built from prior knowledge (Sweller et al., 1998). This is because prior knowledge reduces cognitive load, allowing students to engage more deeply with complex ideas and improving their learning outcomes (van Merriënboer & Sweller, 2005).
In light of the low retention and very low completion rates of online courses (Formanek et al., 2019), we regularly integrated design features aimed at catching and holding students’ interest in the course. Some of the course chapters end with open questions and the promise to get back to these later on. Indeed, creating suspense was an intentional design feature to increase students' motivation to explore the course’s content further. This approach is supported by research that has shown that cognitive dissonances and perceived knowledge gaps are powerful teaching tools. Indeed, even if perceived knowledge gaps are associated with somewhat negative emotions, they can increase students’ curiosity and interest – and consequently foster students’ drive to learn more (Litman & Spielberger, 2003; Litman et al., 2010).

Nature of Science
One of the biggest challenges when it comes to teaching particle physics is its abstractness. Hence, it does not come as a surprise that this topic is only rarely introduced in the physics classroom. Due to the inconceivable dimensions involved, graphical representations fail to convey a realistic image. However, this allows the model aspect of particle physics to stand out. Indeed, this course strongly focuses on conveying the idea that the use of models is essential in science, particularly in particle physics. The rationale of this approach is to highlight the key process of model-building since it is argued that thinking in and with models is an essential component of appropriate scientific knowledge (Gilbert, 2004).
For instance, the guiding question of the very first chapter, “What is a particle?”, reflects the need of learners to construct a mental model of these abstract entities. Is it a point-like object, a wave, a string, or something else? What does it “look like” in reality? This is a commonly asked question by high-school students, which is surprisingly difficult to give a correct answer to, as detailed in this Quantamagazine article. That is why instead of framing particles as definitively "being" something, the course emphasises that particles are "described as" having certain properties. To paraphrase Weinberg: We don’t know what a particle “is”, but we know exactly how we can describe it. (cf. Weinberg, 1996). This aligns with the pedagogical recommendation to avoid conflating models with reality. Studies show that introducing models as descriptions rather than definitive representations reduces the risk of misconceptions and helps students grasp the tentative nature of scientific knowledge (Ornek, 2008).
Even though scientific knowledge is reliable and durable, it is never absolute or certain (Abd-El-Khalick, 2012). Indeed, emphasising the tentative and model-based, rather than definitive, nature of science has been demonstrated to be important for a meaningful introduction to science (McComas, 2020). This approach encourages students to understand that science relies on models to describe phenomena, but these models are not perfect representations of reality. Studies have shown that students ranging from elementary students (Khishfe & Abd-El-Khalick, 2002), middle students (Kang et al., 2004), high school students (Dogan & Abd-El-Khalick, 2008), and even college students (Ibrahim et al., 2007), often hold naïve views of the nature of science. Addressing these misconceptions early in science education improves students' understanding of how science works. That is why the importance of empirical evidence in shaping scientific understanding is emphasised throughout the course. By showcasing how scientific models are revised based on new evidence, students are encouraged to appreciate science as an evolving field.

Linguistic Accuracy
Another challenge in particle physics is how best to talk about particles and atoms in general. Indeed, decades of empirical research have shown that students have significant difficulties in establishing an adequate understanding of a particle model ( Adbo & Taber, 2009; Andersson, 1990; Ferk et al., 2003; Harrison & Treagust, 1996; Novick & Nussbaum, 1981). Here, careful definitions of key terms and the rephrasing or avoidance of misleading terms are required.
For instance, the rapid pace of discovery in the early days of particle physics led to the establishment of key terms which now convey an outdated description of modern particle physics and should, therefore, be avoided in the classroom. Here, the so-called “particle zoo", which was used to describe the dozens of newly discovered “elementary particles” is a prominent example. This unfortunate term originates from a time when, in the absence of a complete quark theory, each newly discovered combination of quarks was classified as an elementary particle. Nowadays, following the modern description of only leptons and quarks as elementary particles, the notion of a 'particle zoo' can be seen as anachronistic and thus detrimental to students' understanding.
Hence, we consider linguistic accuracy to be a very important aspect of this online course. For this reason, the course relies on an empirically tested reconstruction of the subatomic structure of matter that addresses students’ difficulties early on by providing a clear distinction between elementary particles and composite particle systems. In particular, only elementary particles are denoted as particles, while baryons and mesons are introduced as particle systems that are made of particles (Wiener et al., 2015; Wiener et al., 2017a).

Typographic Representations
Since education research shows that visual illustrations are essential to communicate scientific ideas in the classroom (Carney & Levin, 2002; Cook, 2006), special care was taken in how particles, particle systems, and transformation processes are represented within this course. Specifically, typographic representations of particles that are commonly used in physics to denote particles in, for instance, Feynman diagrams, are used consistently throughout the course. These illustrations aim at visualising subatomic objects while avoiding triggering any misconceptions about their potential appearance. Therefore, instead of using spheres or any other misleading symbols, we represent particles and particle systems by using their respective letters. This approach has been empirically validated with high-school students (Budimaier & Hopf, 2023; Budimaier & Hopf, 2024; Wiener et al., 2017b; Wiener et al., 2017c), and we consider it to be a promising and elegant way of representing particles and particle systems within the context of this course.

Core Curriculum
The pilot version of this particle physics course for high-school students is by no means exhaustive. It covers central models of particle physics that allow students to learn about modern particle physics research and, consequently, the research conducted at CERN. It is foreseen to expand the course in the future based on the feedback we are collecting from students completing the pilot version of the course. However, there are at least two aspects that were excluded on purpose. These are the field of quantum physics and the concept of spin.
The decision not to cover quantum physics as part of this course was greatly influenced by the ongoing research into the teaching of quantum physics at the middle- and high-school levels. Here, already in 1992, Fischler and Lichtfeldt illustrated the difficulties students have when learning about quantum physics and advocated for an introduction to quantum physics that avoids references to classical physics (Fischler & Lichtfeldt, 1992). Since then, numerous articles have been published to find “a route through the ‘minefield’ of quantum phenomena” (Ireson, 2000: 20) and, more recently, the research project ReleQuant led to several publications showcasing research-based design principles for an appropriate introduction of quantum physics (cf. Bungum et al., 2015; Henriksen et al., 2014). Based on these results, it was decided to circumvent the topic of quantum physics within this particle physics course.
With the decision to omit spin when introducing high-school students to particle physics, we follow the recommendations of the educational reconstruction by Lindenau & Kobel (2019). Indeed, it is a very advanced and abstract concept, and without a university-level mathematics education, students face significant difficulties in forming suitable mental models of spin. For instance, a study by Özcan (2013) confirmed that even the majority of pre-service physics teacher students wrongly associate spin with the rotation of particles around their axis. Such a mechanical conception is detrimental to a meaningful introduction to the quantum nature of particles and should thus be avoided. Instead, the core curriculum of this course focuses on the interplay between elementary particles, charges, and fundamental interactions.