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Interaction of strangelets with normal matter

Before designing an experiment to find strange matter, it is necessary to understand how it will interact with normal matter. One striking result from the models of strange matter is that the energy per baryon always decreases with A. This is in contrast to nuclei, where the increase in energy per baryon resulting from the coulomb force destabilizes nuclei of large A. It may seem that strangelets might interact with normal nuclear matter by absorbing it, since that would be energetically favorable. Although the charge per baryon of a strangelet is small, it is sufficient to produce a coulomb barrier high enough to prevent such reactions at low energies. Strangelets would be inert in contact with ordinary nuclear matter in the same way that nuclei lighter than iron do not spontaneously undergo nuclear reactions.

The coulomb barrier would not prevent interactions with neutrons, however. Indeed, this is a key ingredient in the scenario for producing strange `neutron' stars. The absorption of neutrons will be limited by the rate at which the weak interactions can equilibrate the chemical potentials. This ability to absorb neutrons coupled with the increased stability at greater A may allow strangelets to be grown to arbitrary size in the lab. This is particularly important if the minimum size of a stable strangelet is greater than can be produced directly in a collider. These reactions are of course exothermic, leading to speculation of using strangelets as an energy source. Any neutron rich environment would be appropriate for such growth, such as the interior of a conventional reactor.

The situation is somewhat different for strangelets with an overall negative charge, as allowed by the above strangelet model for large tex2html_wrap_inline264 and small tex2html_wrap_inline262 . It is not clear if a more complete calculation (e.g. to higher order in tex2html_wrap_inline258 ) would make the allowed range of parameters for negative strangelets larger of smaller. A negatively charged strangelet would have no coulomb barrier against absorption of normal matter, and would in fact attract it. The resulting exothermic reaction would simply produce a larger strangelet. Since the energy per baryon always decreases with A, a negatively charged strangelet on earth would continue to digest all of the matter it came into contact with until the earth itself was entirely strange. The only limit on such a process would again be the rate of the weak interactions in equilibrating the strangeness. From this it is clear that no negatively charged strangelets exist on earth. The possibility of producing such objects would be of interest primarily to Hollywood, the military (presumably for use against an extra-terrestrial invasion), and opponents of Brookhaven.

Gravitational considerations are most important for the possibility of finding naturally occuring strangelets. A strict limit is placed on the size of a strangelet at the Earth's surface. Typically predicted values for strangelet density are tex2html_wrap_inline526 g/cm tex2html_wrap_inline528 , compared to the density of the earth, tex2html_wrap_inline530 g/cm tex2html_wrap_inline528 . This gives a limit of tex2html_wrap_inline534 or tex2html_wrap_inline536 g before the strangelet can no longer be gravitationally supported by the earth. An astronomically produced strangelet impingent on the earth, with a typical velocity of tex2html_wrap_inline538 cm/sec, would plow straight through the earth, punching a hole .01 - 1 cm in radius. A low predicted flux of such events ( tex2html_wrap_inline390 1 - 10 per decade) may alleviate some panic, but also reduces the probability of detection. The high density of strange matter also puts a limit on total mass of a strange star; such an object would be gravitationally unstable with a mass greater than about 2 tex2html_wrap_inline542 .

A common feature of all strange matter models, regardless of parameters, is a very low charge to mass ratio. As a result this is the best signature for experimental identification of strangelets. While this should be sufficient for detection, it is difficult to be specific about the properties of strangelets. To get an idea of the possibilities, a few example configurations allowed by the models are summarized in the following table:

tabular188

The first two would appear to be just superheavy isotopes of known elements; in this case magnesium and xenon. The others are much larger in both mass and charge, and would have a Z/A drastically smaller than nuclei. The last example would be radioactive. These are properties which would be studied once the strangelets were produced, but due to their uncertainty could not be used to identify strangelet candidates.


next up previous
Next: Detection of natural sources Up: Searches Previous: Baryon number conserving decays

Joshua Holden
Sun May 17 15:37:00 EDT 1998