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Further measurements and reassessment of the magnetic-monopole candidate

1978; American Physical Society; Volume: 18; Issue: 5 Linguagem: Inglês

10.1103/physrevd.18.1382

1538-4500

P. B. Price, E. K. Shirk, W. Z. Osborne, L. Pinsky,

Dark Matter and Cosmic Phenomena

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.