Cosmic ray research spanning a century has led us to a new era where these rays hold the potential to shed light on unresolved topics such as dark matter and dark energy, thanks to their energies surpassing those achievable by accelerators on Earth.
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Early History: 1900 - 1927
The start of cosmic ray research was intertwined with two famous discoveries in the year 1900: X-rays by Roentgen and radioactivity by Becquerel. The study of radioactivity led to the discovery of cosmic rays. Pitch-blende, a mineral discovered in 1896 by Madame Curie and Schmidt, was crucial for studying the properties of radioactivity as it exhibited strong radioactivity. When the mineral was placed on a table and the intensity measured as a function of distance, researchers noticed that the flux did not drop to zero even at long distances from the sample. Theodor Wulf carried an electrometer up the Eiffel Tower, but the meter did not show zero, as expected.
Three scientists, Hess, Kolhoerster, and Gockel, used balloon flights to study the difference in ionization as a function of altitude. They flew over 5,300m, carrying an instrument called an electrometer. They repeated the measurement of the ionization and found that the ionization increased with higher altitudes. Hess named these radio activities ”high altitude rays” (Hoehen Strahlung).
The name ”cosmic rays” was given to these high-altitude rays by a US scientist, Millikan, who found the unit of electric charge. Millikan carried an electroscope to the Rocky Mountains and sank a detector in Muir Lake (3,600m), located near the highest peak of the Rocky Mountains, Mt. Whitney (4,418m). He then sank the detector into another lake of lower altitude (2,000m) and compared the difference in ionization.
He found that the difference purely came from absorption by the atmosphere and concluded that these activities were induced by ”rays” coming from the top of Earth: the universe. In 1925, he named them ”cosmic rays”.
In Milikan’s paper to the American Academy, he wrote that the penetrating rays were of cosmic origin and that he sank the detector in a beautiful snow-fed lake. This experiment using the water of the lake as the detector located at high altitudes was the first of its kind. It is worth noting that Millikan had a picture of the early universe as a big nucleus, so it would be natural to imagine that high-altitude rays come from a large blob of nucleus.
After the discoveries of the electron and radioactivity just before the turn of the century, researchers investigated the old problem of charge leakage from a conductor in air in terms of the new concept of ionization and ionizing radiation, some of which was found to be highly penetrating and of extraterrestrial origin. Initially thought to consist only of ultrahigh-energy gamma rays, cosmic ray primaries are now known to be mainly charged particles.
The modern period of cosmic ray research began in 1927 when individual particles were studied using cloud-chamber and coincidence counting techniques.
The Modern Era
Anderson discovered the positron in a cloud chamber, which had been predicted by Dirac several years prior. Cloud chambers and nuclear emulsions yielded many new results, including the discovery of the muon by Anderson and Neddermeyer.
The muon was initially considered a candidate for the Yukawa particle responsible for nuclear binding. However, the discovery of charged pions in cosmic rays by Lattes, Powell, Occhialini, and Muirhead clarified the situation.
Rochester and Butler found V's, which were short-lived neutral kaons decaying into a pair of charged pions. Nuclear emulsions were also used to discover Λ's, Σ's, and Ξ's in cosmic rays. The use of accelerators and storage rings became more prominent after this period.
However, the renaissance of cosmic rays began with the search for solar neutrinos and the observation of the supernova 1987A and other accelerators in the sky. Observations of neutrino oscillations have led scientists to look beyond the standard model of elementary particles.
Studying Single Particles in Cosmic Ray Research
Throughout the course of cosmic ray research, numerous particles have been discovered. The first significant discovery was made in 1932 by Anderson, who identified positrons, an anti-matter.
Notably, Millikan provided funding for Anderson's experiment. Muons were subsequently discovered in 1937 by Anderson and Nedermyer, leading to Yukawa's speculation that these particles could be the force keeping nuclei stable. However, the meson theory faced difficulties, as the mesons found in cosmic rays were hypothesized to differ from Yukawa mesons.
The confirmation of the Yukawa meson theory came in 1947 when Lattes, Occhialini, and Powell discovered it in emulsions exposed at a high-altitude weather station. Yukawa was awarded the Nobel Prize in 1948 and Powell in 1950 for this achievement.
After this, the search for new particles in cosmic rays continued until 1970, when short-lived particles, now known as charmed particles, were found in emulsion experiments at high altitudes by balloon flight. Many particles were subsequently produced by accelerators at BNL and SLAC in 1974, with their properties extensively investigated.
Around 1980, the Grand Unified Theory (GUT) was proposed, unifying not only the electromagnetic force and weak force but also the strong force. This theory predicted that protons would decay into positrons and neutral pions, but the anticipated proton decay was not observed.
However, in 1987, a neutrino burst produced by a supernova explosion near the Large Magellanic Cloud was detected in large-volume water tanks constructed in underground mines. This discovery led to Koshiba being awarded the Nobel Prize. These tanks have also been used to search for WIMPs (Weakly Interacting Massive Particles), a type of dark matter.
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References and Further Reading
Grupen, C. (2013). The History of Cosmic Ray Studies after Hess. Nuclear Physics B. doi.org/10.1016/j.nuclphysbps.2013.05.003
Muraki, Y. (2011). A snapshot from the history of cosmic ray research: a Japanese scientist's view. Nuclear Physics B. doi.org/10.1016/j.nuclphysbps.2011.03.004.