Further measurements and reassessment of the magnetic-monopole candidate

Abstract

Within the stack, consisting of 35 Lexan detectors and three nuclear emulsions, in which the unusual event was found, we have measured tracks of \ensuremath{\sim} 200 cosmic-ray nuclei with $26\ensuremath{\le}Z\ensuremath{\le}83$ which provide an internal calibration of the response of the detectors. Our measurements in Lexan and in emulsion together show that the unusual particle produced a knockon-electron energy distribution incompatible with any known nucleus. The track etch rate and its gradient in Lexan give the quantity $\frac{|Z|}{\ensuremath{\beta}}$ and, if the particle was a nucleus, a lower limit on its velocity. We found $\frac{|Z|}{\ensuremath{\beta}}\ensuremath{\approx}114$ at each of 66 positions in the Lexan stack extending over a range of \ensuremath{\sim} 1.4 g/${\mathrm{cm}}^{2}$. The best fit to the Lexan data alone would be for a hypothetical superheavy element with $Z\ensuremath{\approx}108$ to 114 and $\ensuremath{\beta}$ such that $\frac{Z}{\ensuremath{\beta}}\ensuremath{\approx}114$. A known nucleus with $90\ensuremath{\le}Z\ensuremath{\le}96$ would also give an acceptable fit to the Lexan data if it fragmented once in the stack with a loss of about 2 units of charge, keeping $\frac{Z}{\ensuremath{\beta}}\ensuremath{\approx}114$. A nucleus with $Z<90$ could maintain $\frac{Z}{\ensuremath{\beta}}\ensuremath{\approx}114$ only by a properly spaced set of fragmentations. A nucleus with $\ensuremath{\beta}$ as low as 0.6 could fit the Lexan data only if it fragmented at least eight times in succession, with a probability \ensuremath{\sim} ${10}^{\ensuremath{-}17}$. In the 200-\ensuremath{\mu}m G-5 emulsion, visual measurements of the track "cores" produced by relatively-low-energy electrons $\ensuremath{\lesssim}10$ keV) are consistent with the Lexan result that the unusual particle had $\frac{|Z|}{\ensuremath{\beta}}\ensuremath{\approx}114$. However, measurements of the density of silver grains at radial distances greater than \ensuremath{\sim} 10 \ensuremath{\mu}m show that the particle produced far fewer high-energy $\ensuremath{\gtrsim}50$ keV) knockon electrons in each of the three emulsions than would a known nucleus with $\frac{Z}{\ensuremath{\beta}}\ensuremath{\approx}114$. If it were a known, long-lived nucleus with $Z<96$ and therefore having $0.84>~\ensuremath{\beta}$ 0.6 in order to fit the Lexan data, its signals in the three emulsions would imply a very low $\frac{Z}{\ensuremath{\beta}}$ of only \ensuremath{\sim} 85 instead of 114. The abnormally small production rate of long-range electrons observed in all three emulsions is the essential evidence that we have found a unique particle. A monopole does not provide an acceptable fit to all of the data. A slow particle ($\ensuremath{\beta}\ensuremath{\approx}0.4$) could fit all of the observations, provided its mass were so great (${10}^{3}$ amu) that it did not slow appreciably in the 1.4-g/${\mathrm{cm}}^{2}$ stack. A fast ($0.7\ensuremath{\lesssim}\ensuremath{\beta}\ensuremath{\lesssim}0.9$) antinucleus with $\frac{Z}{\ensuremath{\beta}}\ensuremath{\approx}\ensuremath{-}114$, because of its low Mott cross section for production of high-energy knockon electrons, could fit the data, especially if it fragmented once with loss of 1 or 2 units of charge. An ultra-relativistic ($\ensuremath{\beta}\ensuremath{\gtrsim}0.99$) superheavy element with $Z\ensuremath{\approx}+110$ to +114 can also account for the data and is not in conflict with any negative searches. Our knowledge of the $Z$ and $\ensuremath{\beta}$ dependence of the response of Lexan appears sufficient to preclude values of $|\frac{Z}{\ensuremath{\beta}}|$ less than \ensuremath{\sim} 110. An explanation of the weak distant energy deposition in terms of fluctuations by a normal nucleus or locally insensitive emulsion regions appears to be unlikely. Freak occurrences such as a ${10}^{20}$-eV jet or an upward moving nucleus do not fit the data. Having achieved only an incomplete characterization of a single example of what appears to be a new particle, we emphasize the obvious-that further examples of such particles must be found before its identity can be established.