A Philatelic Story of the Hypernucleus Discovery
by Jerzy H. Rutkowski

In 1947 the general opinion was that atomic nuclei are build of protons and neutrons (nucleons) which are bound together by nuclear forces. The theory of nuclear forces was proposed earlier, in 1935, by Hideki Yukawa who predicted the existence of mesons and invented the meson theory of nuclear forces. According to the theory the nuclear interactions arise due to the exchange process of a meson from one nucleon to another. In an exchange of a charged meson, an electric charge is transferred and a proton is converted into a neutron and the neutron becomes a proton. Similarly, neutral mesons mediate interactions between two protons or two neutrons. The attractive force resulting from such exchange interaction is short-ranged and stronger than the Coulomb repulsion. Yukawa was honoured with the Nobel Prize in Physics "for his prediction of existence of mesons on the basis of theoretical work on nuclear forces" in 1949. After muons had been discovered by Anderson in 1937, they were at first thought to be the particles predicted by Yukawa to explain nuclear forces. But it was soon established that muons did not interact with nucleons and therefore could not take part in the exchange interaction.

The year 1947 brought some very important discoveries which initiated elementary particle physics as a separate branch. The first discovery was the one of pions which were recognized as Yukawa particles responsible for the exchange process governing nuclear forces. Next were the observations of strange particles V^o. Some of them, denoted originally as V^o₁ and later called L-hyperons are closely related to our topic. They have masses larger than the nucleon mass and decay into nucleon and pion.

In those days most of data concerning new particles were obtained by cosmic rays observations. Special photographic films, invented for this purpose and called nuclear emulsions, were lifted by means of balloons and exposed at high altitudes to primary cosmic rays. Then, after the developing process, the emulsions were scanned by optical microscopes and the tracks left by charged particles measured, the results analysed and interpreted.

At those times Bristol was the mayor center of new elementary particle physics. It was there, that Cecil Powell developed the nuclear emulsion technique. The pion discovery and the Nobel Prize in physics, in 1950 "for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with this method" were Powell's main achievements [1].

In 1952 Marian Danysz came back to Warsaw University after a long research stay in Bristol. He was determined to create an elementary particle research group. Soon he persuaded Jerzy Pniewski, (their friendship had begun during their stay in in England), to join the group. In 1950 Pniewski returned to Poland from Liverpool where he was involved in beta spectroscopy.

At the end of 1952, examining the nuclear emulsion (brought by him from Bristol) under a microscope, Danysz came across a surprising case of two stars connected by a thick track. Detailed analysis of this event performed together with Pniewski, allowed them to interpret it as a hypernucleus, in this particular case, a nucleus built of a L-hyperon and nucleons [2,3].

To commemorate their discovery the above postcard was issued by the Polish Post in May 1993 (designer - Maciej Jedrysik, quantity issued - 200 000 postcards, the stamp face value 1500 zl, the postcard price - 2000 zl). The event observed by Danysz is presented on the stamp (compared to papers [2] and [3] some details have been omitted). A singly charged cosmic ray particle of energy about 30 GeV enters the emulsion from the top at an approximate angle of 15^o to the vertical (the track p). Interacting with a bromine or silver nucleus the particle creates an upper star. The star consists of twenty-one tracks. Nine of them belong to alpha particles, eleven to singly charged particles (protons, deuterons or tritons) and the last one ending in the centre of the bottom star, is attributed to the hypernucleus. The charge of the hypernucleus is about 5 and its initial kinetic energy is roughly 60 MeV. Having passed the distance of 90 m during approximately 3 ps the hypernucleus gradually loses its velocity. Finally, being almost at rest, it spontaneously disintegrates initiating the bottom star. This star consists of four tracks. Two of them belong to protons, deuterons, tritons, or alpha particles, one to pion, proton, deuteron, or triton, and the last track is the recoil particle. The total energy released in the bottom star is at least 140 MeV.
On the left hand side of the postcard Professors Marian Danysz (smoking a pipe) and Jerzy Pniewski are featured during a conversation. The caption can be translated as "The 40th Anniversary of the Hypermatter Discovery".

A few years earlier, in 1989, the postmark designed on the basis of the first hypernucleus observation was used 20th International Physics Olympiad at the Warsaw post office no. 64.

Just after this first observation, many other cases of hypernuclei were announced (a survey of the hypernuclei detected up to 1955 can be found in [4]). These new results suggested the following fundamental questions: What are the main principles of creation and disintegration of hyperons and heavy mesons? Why is the L- hyperon bound to nucleons? These questions were beautifully answered with the help of a new quantum number, called "strangeness", introduced by M. Gell-Mann and K. Nishijima. Later, in 1969, Gell-Mann was honoured with the Nobel Prize in Physics "for his contributions and discoveries concerning the classification of elementary particles and their interactions".

The first double hypernucleus was discovered again in Warsaw, in 1962 [5], in a nuclear emulsion irradiated by a beam of K^--mesons (kaons) at CERN. The second double hypernucleus case was found at Brookhaven National Laboratory in 1966 [6]. Further search for double hypernuclei was not really successful [1]. Finally, in 1991, (almost thirty years after the discovery!), a spectacular experiment performed with K^- beam provided by the KEK Proton Synchrotron in Tokyo, revealed the next double hypernuclei [7]. In this experiment, a hybrid technique of nuclear emulsions and Cherenkov detectors as well as other kinds of detectors was successfully applied. This experiment has opened a new area for hypernuclei investigations.

Acknowledgments

The Author is very grateful to Mrs. Teresa Rychtelska for some technical help in preparing this page.

[1] As for the historical background the Author often follows the article by J. A. Zakrzewski, Postepy Fizyki 44 (4), 399-410 (1993) (in Polish)
[2] M. Danysz, J. Pniewski, Bull. Acad. Pol. Sci. III 1, 42 (1953)
[3] M. Danysz, J. Pniewski, Phil. Mag. 44, 348 (1953)
[4] M. Danysz, Hyperfragments, Nuovo Cimento Suppl. 4, 609 (1956)
[5] M. Danysz, at al., Phys. Rev. Lett. 11, 29 (1963); Nucl. Phys. 49, 450 (1964)
[6] D. J. Prowse, Phys. Rev. Lett. 17, 782 (1966)
[7] S. Aoki, at al., Prog. Theor. Phys. 85, 1287 (1991)

All citations have been from The Official Website of the Nobel Foundation.