Introduction - This section attempts to provide a minimal and concise summary of the basic particle physics relevant to enjoy the interpretation of the bubble chamber photographs curated by the project. If you're interested to learn more, the following resources are particularly recommended:
The particle physics section of the HyperPhysics project (Georgia State University)
The Particle Adventure project (LBNL)
The science section of the CERN public website
What is Particle Physics? - Particle physics attempts to understand all physical phenomena at microscopic scales (i.e. the scale of an atom and smaller) in terms of a few fundamental particles which together describe the structure of matter and its interactions through the fundamental forces of electromagnetism and the strong and weak nuclear forces. While the effects of electromagnetism are ubiquitous in everyday life, the strong and weak forces are only relevant on the scale of the atomic nucleus, though their properties lead to well known phenomena including the chemical elements and radioactivity. While the fundamental force of gravity has well known and dramatic implications for the distance scales relevant to everyday life and larger (e.g. the solar system), it is too weak to have practical relevance at the sub-atomic scale and is typically ignored in the context of particle physics. This is convenient from a practical perspective, since a satisfactory theoretical description of gravity in the picture of fundamental particles has eluded physicists to date and remains an open and actively researched topic in theoretical physics.
The Standard Model - Our modern understanding of particle physics centres around a theory known as the Standard Model (SM), which attempts to explain microscopic phenomena in terms of two classes of particle: fermions, which constitute the most fundamental form of matter and bosons, whose interactions with the fermions describe three of the fundamental forces.
Fermions - The fermions (matter particles) can be arranged into two further classes, known as quarks and leptons. The two classes are distinguished by their participation in the strong interactions: quarks "feel" the strong force while leptons do not. The SM involves two varieties of quarks, the "up" type and the "down" type, which have fractional electric charges (relative to that of the proton) of +2/3 and -1/3, respectively. One important consequence of the strong interactions between quarks is an attractive force between certain configurations which can lead to the formation of bound states of multiple quarks known as hadrons. The proton (two up quarks and a down quark) and neutron (two down quarks and an up quark), which together constitute the atomic nucleus are perhaps the most well known hadrons, though a great many more exist (see later). In fact, the properties of the strong interaction are such that quarks are not understood to exist in isolation, but rather always bound together in hadrons.
The other class of fundamental matter particle, the leptons, do not experience the strong interaction and its associated "confinement" phenomenon and so exist as free particles. Two varieties of lepton are known, distinguished by their electrical charge. The ubiquitous electron is one example of a charged lepton, while the electrically neutral neutrinos comprise the other variety.
Generations - As if this wasn't enough, there exist three "generations" of each fermion (quarks and leptons), which are apparently identical in every respect other than their intrinsic masses. The rationale behind the existence of multiple generations of fermions is an open question in particle physics. While the SM features exactly three generations, motivated by the current experimental evidence, the existence of further generations, with particles masses beyond the sensitivity of current experiments is not excluded. Each specific sub-variety of fermion (e.g. a down quark or an electron) is known as a "flavour".
Antimatter - The final feature relevant to the fermions is the concept of antimatter or antiparticles. For every fermion there exists an anti-fermion which possesses the same mass, but with opposite physical charges. For example, the anti-particle of the electron, which has negative electrical charge, is known as the positron, which has a positive electrical charge of the same magnitude as the electron. The concept is also inherited by hadrons, due to their composite nature as bound states of quarks. For example, two anti-up quarks and an anti-down quark form an anti-proton, which has the same mass as a proton, but a negative electrical charge. In fact, the notion of antiparticles is intimately linked with an important class of hadron known as a meson, which consists of a single quark and a single antiquark. For example, the bound state of a down quark and an anti-up quark constitutes a meson known as a pi-minus. See below for much more on mesons, they're very relevant to bubble chamber experiments.
Bosons - The SM involves four bosons responsible for the mediating the fundamental forces (excluding gravity). The photon is a massless particle which mediates the electromagnetic interaction and can be understood as the indivisible unit of light. All electrically charged particles (the quarks and the charged leptons) experience the electromagnetic force, through interactions the photon. The gluon is another massless particle which mediates the strong force and only interacts with the quarks. The massive W and Z bosons mediate the weak force, which allows for interactions between, and transitions among the different fermion "flavours".
The final boson of the SM, the Higgs boson, isn't directly associated with any fundamental force but rather plays an central role our theoretical understanding of why the W and Z bosons are massive, unlike the photon and gluon. The Higgs boson also provides a means to describe the masses of the fermions. There's much more to say about the Higgs boson, but it's doesn't have too much direct relevance to bubble chamber experiments, so consider taking a look at these few nice pages of the CERN website if you're interested to learn more.
Summary - While the set of particles described above may seem somewhat arbitrary and involve a rather large number of elements for a "fundamental" theory, it represents the minimal model capable of adequately describing the bulk of the current experimental evidence, a task in which it has enjoyed considerable success. However, there remain a handful of important experimental observations which the SM is entirely unable to describe, including dark matter, dark energy, neutrino masses / oscillations and the matter-antimatter asymmetry of the universe. The SM is necessarily, therefore, an incomplete theory and the resolution of this unsatisfactory situation in terms of the development of a more complete theory is one of the primary goals of contemporary experimental and theoretical particle physics.
Bubble chamber experiments in the mid-twentieth century played a central role in the development of the SM. These experiments led a prolific period of discovery during the late 1950s and early 1960s in which dozens of new and seemingly exotic hadrons (such as the appropriately named "strange" particles, see later) were identified. At the time, this "zoo" of hadrons seemed too numerous to represent a fundamental picture of particle physics and ultimately motivated the development of the quark model of hadrons, which remains a central aspect of the modern SM.
Now that you hopefully have a feel for the "big picture" of modern particle physics, take a look at [PAGE] to understand which of the particles described above you'll be able to spot in the bubble chamber photographs curated by this project.