In the late 1940s, the physicists George Rochester and Clifford Butler1 observed something unusual in their charged-particle detector. They were studying the interactions between high-energy cosmic rays and a lead plate in the detector when they spotted V-shaped particle tracks (Fig. 1a). The small gap between the lead plate and the vertex of the tracks indicated that an invisible neutral particle had been produced in the plate, had travelled for a short distance and had then decayed into two visible charged particles. The mass of the neutral particle was about 1,000 times that of an electron, implying that it must be a previously unreported type of particle. This discovery paved the way for many puzzles and surprises in particle physics in the decades that followed.

Figure 1 | Particle detection that led to a better understanding of fundamental physics. a, In 1947, Rochester and Butler1 analysed the particles produced when high-energy cosmic rays hit a lead plate (the broad central stripe) in a charged-particle detector. In certain photographs, they spotted evidence of a previously undetected, invisible neutral particle decaying into two visible charged particles, which were identified by tracks (labelled with arrows). b, The discovery of many more particles following Rochester and Butler’s work led to a model12,13 in which all of the known mesons and baryons (two classes of particle) consist of elementary particles called up (u), down (d) and strange (s) quarks, along with their antiparticles (denoted by overbars). The η, η′ and π0 mesons comprise mixtures of quark pairs. The mesons and baryons are arranged by their strangeness (a quantity that is related to the presence of strange quarks) and electric charge.

At the time of Rochester and Butler’s work, protons, neutrons, electrons and particles called pions (short for π mesons) had been identified, and were known to be sufficient to form atoms. Pions were proposed2 in 1935 to explain how protons and neutrons are held together in small atomic nuclei by the strong nuclear force, and were found experimentally3,4 in 1947.

While searching for a pion in cosmic rays, scientists discovered a different particle5, which is now called a muon. A heavy charged particle was then found6 in 1944, followed by Rochester and Butler’s unstable neutral particle. But the discovery of unexpected particles did not stop there. Then came the τ meson, which decays into three pions; the θ meson, decaying into two pions; the κ meson, decaying into a muon and an invisible particle; the Λ0 particle, decaying into a proton and a pion; and the list goes on.

In the early 1950s, researchers began producing these rare particles in large quantities by firing protons at targets in particle accelerators. The τ, θ and κ mesons and Λ0 particle were peculiar, because, although they were generated by the strong force, their decay times were much longer than those expected for this force. To explain these observations, physicists proposed a quantity, known as strangeness (S), that is conserved by the strong force7,8.

The paper: Evidence for the Existence of New Unstable Elementary Particles

Protons and neutrons have S = 0, and through the strong force, can produce a pair of strange particles that have S = –1 and S = +1, so that total strangeness is conserved. However, a strange particle that has S = –1, for example, cannot decay into particles that have S = 0 through the strong force, because strangeness would not be conserved. Instead, this decay must occur much more slowly through the weak nuclear force, which allows total strangeness to change.

As the accuracy of accelerator-based measurements increased, it became clear that the τ and θ mesons had extremely similar masses and lifetimes. Scientists concluded that these mesons must be the same particle, which is able to decay into two or three pions. The mess of strange mesons was finally cleaned up into four particles dubbed kaons (short for K mesons): K+ and K0 and their antiparticles K– and K—0.

However, accepting that the τ and θ mesons were the same particle raised another problem. A state of two pions has even parity, which means that its wavefunction does not change sign under a parity transformation (in which spatial coordinates are flipped). By contrast, a state of three pions has odd parity. If the same particle could decay into two or three pions, did that mean that, contrary to all conventional wisdom, parity is not conserved by the weak force? This question, known as the τ–θ puzzle, led to the discovery, in 1957, of such parity-symmetry breaking in cobalt-60 decays9 and in pion decays10.

150 years of Nature — an anniversary collection

A consequence of parity-symmetry breaking by the weak force is that elementary particles called neutrinos can be only left-handed, which means that their motion and intrinsic angular momentum are in opposite directions. Under a parity transformation, a left-handed neutrino becomes a right-handed neutrino, which does not exist. However, if one then applies a charge-conjugation transformation (in which particles are replaced by their antiparticles), the right-handed neutrino becomes a right-handed antineutrino, which does exist. The weak force therefore seemed to conserve CP symmetry (symmetry under a combined charge-conjugation and parity transformation), until such symmetry was found to be broken in neutral-kaon decays.

A neutral kaon is a mixture of K0 and K—0 states, and can exist as the CP-even state K even or the CP-odd state K odd . The lifetime of K odd is much longer than that of K even , so these particles were named K L (for ‘K-long’) and K S (for ‘K-short’), respectively. A useful consequence of such lifetimes is that, if neutral kaons are produced by firing protons at a target, the CP-even K S component quickly decays, leaving only the CP-odd K L component. In 1964, such K L particles were observed11 to decay into the CP-even state of two oppositely charged pions (π+π–). Therefore, despite expectations, CP symmetry was shown to be broken.

In that same year, physicists proposed a model12,13 to explain all of the known mesons and baryons — a family that includes protons, neutrons and the Λ0 particle. In the model, these mesons and baryons consist of elementary particles known as quarks, which come in three types: up, down and strange (Fig. 1b).

In 1973, a theoretical model14 showed that the breaking of CP symmetry could be explained by introducing three more quarks: charm, top and bottom. In this framework, K L can have a small component of K even that can decay into the CP-even π+π– state. But unlike other theoretical models, this framework also allows K odd to decay into the CP-even state (direct CP violation).

Many generations of experiments then were carried out to see whether direct CP violation exists. The measurement required extremely high precision, and after many improvements over 25 years, direct CP violation was finally confirmed15,16. Together with the observation of CP-symmetry breaking in B mesons (mesons that contain a bottom quark)17,18, the theoretical model was confirmed, and helped to establish the standard model of particle physics, which is the current explanation of the Universe’s particles and forces.

However, the standard model is not complete. For instance, it cannot explain why the Universe contains so little antimatter, nor what the mysterious substance called dark matter is. Researchers are therefore trying to search for a hint of particle physics beyond that of the standard model. For example, experiments in Japan19 and Europe20 are using extremely rare kaon decays to search for such a hint.