The History of Particle Physics

Science is built on ideas that must withstand experimental testing. Some become part of our understanding of nature; others, once disproved, eventually fade away—or enter the history of science. CERN’s International Symposia on the History of Particle Physics bring together scientists, historians and philosophers to retrace the history of particle physics—its successes and discoveries, as well as ideas that proved wrong but still offered insights into the fundamental nature of reality.

The 4th edition of the symposium, held from 10 to 13 November 2025, focussed on the years between 1980 and 2000. ICTP Emeritus Scientist Alexei Smirnov, who made major contributions on the physics of solar neutrinos during that period, was invited to the meeting alongside other eminent scholars.

Those decades proved particularly fruitful in advancing knowledge about particles physics. Between 1980 and 2000, scientists completed observations of all the fundamental particles predicted by the Standard Model, with the exception of the Higgs boson. “Key discoveries are related to the top quark—the heaviest of all quarks and the last to be observed, in 1995,” Smirnov says, adding, “Just a few years later, in 2000, the tau neutrino was also discovered. Key open questions about neutrinos, including the fact that they have a mass, were answered, and compelling evidence of the existence of dark matter was collected.”

The path to these achievements was discussed at the symposium by some of the very scientists who contributed to them, including Nobel Laureates, leaders of major experiments and eminent theoreticians. Smirnov worked at the Institute for Nuclear Research in Moscow in the 1980s, before joining ICTP. He devoted most of his career to studying neutrinos and between 1984 and 1985, he worked out the mechanism underlying the solar neutrino deficit, which had been puzzling physicists for almost two decades. The symposium he attended in November gave him the opportunity to retrace the steps leading to that discovery.

Solar neutrinos are produced by nuclear reactions in the Sun. In the late 1960s, Ray Davis measured the flux of electron neutrinos from the Sun with the Homestake experiment in South Dakota, USA, applying a method that had been proposed by Bruno Pontecorvo in 1946. Based on models of solar evolution, astrophysicist John Bahcall made a theoretical prediction that was three times higher than observed in experiments. Nearly twenty years later, Smirnov and Stanislav Mikheyev proposed an explanation building on earlier work by American physicist Lincoln Wolfenstein, which led to the Mikheyev–Smirnov–Wolfenstein effect.

“I started to work on this problem in 1984, when Mikheyev asked me to check Wolfenstein’s 1978 claim that interactions with matter modify neutrino oscillations,” Smirnov recounts. “To better understand Wolfenstein’s paper, I invented a graphic representation of neutrino oscillations as a precession,” he explains. The diagram helped Smirnov understand what quantities are key to describing what happens to neutrinos as they travel through matter.

“I realised that when the frequency of neutrino oscillations in vacuum equals the characteristic frequency of the medium, there is a resonance phenomenon and neutrino mixing is enhanced,” he recounts. This means that neutrinos are found in a superposition of states, including muon and tau neutrinos. The discovery was pivotal to explaining what Davis and Bahcall had found: while Davis’ detector was designed for electron neutrinos, by the time they reach Earth, the particles are in a different state as a result of their interaction with matter.

More than 30 years after this discovery, there is still a very active community of scientists trying to learn more about neutrinos. These particles have such tiny masses and interact so weakly with matter that scientists do not yet know how heavy they are, or what the relative ordering of their masses is. Increasingly large experiments have been built over the years to find out. “Now we are in the era of precision measurements,” Smirnov notes. “In particular, the JUNO experiment in China looks very promising. It is a huge 20,000-ton scintillator detector for neutrinos from atomic reactors and other sources. The experiment was set up quickly and the first publication appeared at the end of 2025. The results are in good agreement with previous ones and have higher precision,” he continues.

Yet the Sun and atomic reactors on Earth are not the only sources of neutrinos. What Smirnov is most excited about is the possibility that a rare event could occur: the collapse of a supernova inside our galaxy. “Such events release a huge number of neutrinos, and if such an event occurred, we could gather a lot of information,” he states. “Then I would easily be able to find out the mass hierarchy,” he adds with an assurance that stems from his years of studying supernova neutrinos. The last time a signal from a collapsing supernova was detected was in 1987, and the event happened in a satellite of our galaxy. This was exceptional enough that Smirnov remembers hearing the news on the radio while travelling to a conference. “I would really like to see a neutrino signal from our galaxy during my lifetime,” he concludes.