Peculiar high energy cosmic ray stratospheric event reveals a heavy primary origin particle above the knee region of the cosmic ray spectrum

We wish to put forward an explanation for a peculiar cosmic ray event with energy $\Sigma E_{\gamma }\ge 2\times {10}^{15}$ eV detected in $1975$ by the balloon borne emulsion chamber experiment performed in the stratosphere, at the altitude $\ge 30$ km above sea level. For almost 30 years the event has been described as unusual, invoking new exotic mechanisms or models. In our opinion there is no need for an extraordinary explanation. Contrary to the widespread belief, the event gives us an example of “unrecognized standard physics”. At the same time this event revealed a variety of features which are of considerable interest for cosmic rays, nuclear physics, and astrophysics. Here we show that the observed family is most likely to be a result of a heavy nucleus interaction with an air nucleus. In this case a primary particle would originally have been in the energy region above “the knee” of the cosmic ray spectrum.

Figure 1

A schematic view of the topology of the event. Secondary particles produced in the interaction of a heavy nucleus in the atmosphere (a star with rays) are detected as a family of gamma ray showers (a circle with dots surrounded by the dash line) in the chamber (solid line box). The family was detected near the edge of the calorimeter, so showers at the periphery (an area of gray color) were not detected. The inset I shows an incoming particle interacting with an air nucleus. Participant nucleons are presented by black circles and thick arrows, and spectator nucleons $-$ by white circles and thin arrows. The inset II shows, that if a set of circles (tens of mm in radii) have been drawn over the target map, then one could search for specific configurations in the distribution of showers (dots). For instance, some previous studies [8] recognized ringlike structures. The inset III is representing an image of a dark diffused area (a halo) of a few mm radius observed in x-ray emulsion film in the center of the event. The halo is formed by superposition of showers. It was possible to identify some showers (white dots) within the halo area. Images are not to scale and for illustration purposes only.

Figure 2

The pseudorapidity distributions. Correction for the edge of the detection has been made. Circles: experimentally observed showers of the event. Lines: the analytical distributions for spectator nucleons ($A_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}}=36$, solid line (a), $A_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}}=20$, dash line (b), $A_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}}=10$, dash-dotted line (c)), projectile participant nucleons ($A_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}=20$, dash line (d), $A_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}=16$ dotted line (e), $A_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}}=10$, dash-dotted line (f)), and charged pions and gamma rays originated by projectile participant nucleons ($A_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}=20$, dash line (g), $A_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}=16$ dotted line (h), $A_{\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{t}}=10$, dash-dotted line (i)), detected in the chamber with detector efficiency $ϵ$ $\sim 0.4$.

Figure 3

The pseudorapidity density distribution (normalized to the size of the bin) of tracks observed [7, 8, 12, 13] in the interaction of the leading particle. Solid line shows the simulated (by the QCD inspired models, from[13]) distribution of tracks originated by two nucleons with $E\sim 100$ TeV.