The G-Stack experiment was conceived as a high-altitude balloon experiment using an enormous photographic emulsion stack to capture the tracks of cosmic-ray particles and their decay products at an altitude of approximately 30 kilometers. It was a landmark scientific endeavor undertaken in the early 1950s with the primary goal of investigating the nature and decay properties of K mesons, a class of unstable particles whose behavior was poorly understood at the time. The initiative was part of a broader effort in post-war European physics to rejuvenate the field through international collaboration, reflecting a transition from "small science" to "big science" and from cosmic-ray-based research to the emerging dominance of particle accelerators.

The collaboration involved six main laboratories from four European countries: Bristol in the UK, Milan and Padua in Italy, Dublin in Ireland, and Copenhagen in Denmark. Central figures in this collaboration included Cecil Powell, Giuseppe Occhialini, and Michelangelo Merlin, whose longstanding personal and professional relationships facilitated the project's coordination. The Italian groups provided critical technical expertise in emulsion and balloon technologies and were deeply invested in the experiment's success, both scientifically and symbolically, as part of the post-war reconstruction of Italian physics.

The scientific goals of the G-Stack were multifold. The first was to obtain precise measurements of the masses of K mesons through the observation of their decay products within the emulsion. The second aim was to determine the relative frequencies of various decay modes (the branching ratios), thereby addressing discrepancies between results obtained using cloud chambers and nuclear emulsions. A third objective was to measure the energy distributions of decay products in three-body decays, as these spectra contained valuable information about the interaction dynamics and the rest mass of the neutral particles involved.

The G-Stack itself was a giant emulsion block consisting of 250 Ilford G5 photographic plates, each measuring 37 x 27 cm and 600 microns thick. Assembled into a solid stack approximately 15 cm thick, the emulsion had a total volume of 15 liters and weighed around 63 kg. The stack was formed of stripped emulsions-layers of sensitive material removed from their glass bases and stacked together to create a continuous detection medium, which significantly improved the tracking of particle paths. This approach allowed for detailed analysis of the decay sequences of particles captured in the emulsion. Processing of the plates was done in stages, primarily in Bristol and Milan, with more than 90% of the emulsion successfully recovered after the flight, despite the parachute failing to deploy during descent. Recovery efforts were aided by the fortuitous landing of the payload in a tree near the village of Berceto.

The balloon used for the flight had a volume of approximately 4,000 cubic meters and was filled with hydrogen. The payload, including the emulsion stack and associated equipment, weighed 140 kg. The balloon ascended at a rate of 200 meters per minute, reaching an altitude of 27,000 meters in about two hours. The launch took place on October 12, 1954, from Novi Ligure in northern Italy after numerous delays due to poor weather. The flight lasted six hours, and the balloon was tracked with radio telemetry and visual observations from a theodolite on Mount Righi.

The tracking and communication equipment included radar reflectors, three radio transmitters, and a time-triggered payload release mechanism consisting of two rugged kitchen clocks. The stack was exposed to cosmic radiation during its time at high altitude, and after landing, the emulsions were quickly recovered, processed, and distributed among the participating laboratories.

Once distributed, the scanning of the plates began. This labor-intensive process involved more than fifty microscope operators, many of them students or assistants. By the end of the initial scanning phase, over 400 heavy meson and hyperon decays had been catalogued. Special emphasis was placed on identifying events where a single charged particle decayed into another single charged particle, enabling precise mass calculations based on range and decay dynamics.

The data were rapidly analyzed in time for presentation at key international conferences, including the Rochester and Pisa conferences in early and mid-1955. The G-Stack results were published collectively under the name "G-Stack Collaboration" in several high-profile journals. These results included mass measurements confirming that the Kµ and Kp decay modes were two-body decays with masses consistent with that of the t meson, supporting the hypothesis that all observed modes originated from a single particle. The G-Stack also provided estimates of the branching ratios, finding Kµ and Kp modes to account for roughly 67% and 20% of decays respectively, with the remaining modes (such as K beta, x, and t') accounting for the remainder.

The experiment also yielded additional insights into the production of K mesons and hyperons, observed several K- mesons and jets, and provided energy spectra of decay modes. The data discredited the earlier hypothesis of a 1400 me particle, proposed by Daniel and Perkins, and corrected misestimates of K meson masses by both emulsionists and cloud chamber physicists.

While the G-Stack was lauded by many of its participants and supporters as a definitive experiment, particularly for resolving the uniqueness of K mesons, it did not fully achieve its goal of precisely determining branching ratios. Furthermore, it was soon overshadowed by experiments using particle accelerators like the Berkeley Bevatron, which were able to produce higher event rates under controlled conditions, offering superior statistics and systematic precision. The G-Stack, though successful in many respects, marked the twilight of cosmic-ray-based particle physics research. Nonetheless, it left a lasting legacy as a pioneering international collaboration and a training ground for a new generation of European physicists.