Re: Strangelets?

[Rob Davies <rodavies@DU.EDU>]

Katherine Todd asked:

> With the recent talk about stranglets and the heavy ion accelerator, I have
> to admit, I am not sure what I stranglet really is. Can anyone tell me, or
> point me to an appropriate link?

"Strangelets" is the name of a class of particles which have not yet been observed. To say much more about them I will need to use some ideas from particle physics. For the uninitiated let me start with a bit of background. Those familiar with particle physics may want to skip, skim or critique the next several of paragraphs.

The "standard model" of particle physics maintains that particles like protons and neutrons, more generally "hadrons" or "hadronic matter", contain constituent particles called quarks and their antimatter cousins, the antiquarks. For completeness, there are other particles (e.g. electrons, neutrinos, photons) which are not made up of quarks or antiquarks. In this note, however, it will be sufficient to restrict the discussion to hadronic matter.

The 6 "flavors" of quarks, in order of increasing mass, are: up, down, strange, charm, bottom, top. Up and down are by far the lightest having approximately 1/20 of the mass of a strange quark. For this reason up and down are collectively referred to as the "light" quarks and the others as the "heavy" quarks.

Quarks have electric charge. Up is positively charged. The magnitude of its charge is 2/3 that of an electron. Down and strange are negatively charged. The magnitude of their charge is 1/3 of the electron's.

Protons and neutrons are comprised of the light quarks.* The proton contains 2 up and 1 down quark. The neutron has 1 up and 2 down. Note that each contains three quarks. One interesting fact that is important to the search for strangelets is that all particles that have been observed to date consist of either three quarks or one quark and one antiquark. One might think of nuclei as large collections of quarks, but note that the quarks are grouped into threes. A given quark does not move among nucleons.

Particles which contain one or more of the heavy quarks are created routinely in cosmic rays collisions, and in high energy physics experiments. However, as noted above, the hadronic component of the matter around us contains only the light quarks (grouped as protons and neutrons.)  This is due to the fact that the heavy quarks decay with time constants which are typically measured in nanoseconds.

Now back to the original question. Strangelets are a class of theoretical particles containing more than three quarks of which approximately one third are up, 1/3 are down and 1/3 are strange. To emphasize the unusual character of strangelets, lets look at an example, viz. the "quark alpha". This particle is comprised of 6 up, 6 down and 6 strange quarks. Unlike any particle previously detected, it would contain more than 3 quarks. It's mass would approximately equal that of a Lithium nucleus and yet it would be electrically neutral. It is expected that small strangelets, like the quark alpha, would be short lived. At most, they might live for 10's of nanoseconds.

In the discussion of whether Earth is being put at risk with the start up of the RHIC collider, it is important to distinguish strangelets from Strange Quark Matter (SQM).  The notion of SQM arises when one asks, "What form would be taken by a single quantum system consisting of a large number of quarks?"  One answer that has been proposed is that such a system would contain roughly equal numbers of up, down and strange quarks. The strange quarks in this case would add to the stability of the system rather than detract from it as is the case in small systems. One reason for this has to do with the Pauli exclusion principle. Quarks are fermions, and therefore, no two quarks in a single system can occupy the same quantum state. So in spite of their large mass, it eventually becomes favorable to add strange quarks to the lowest energy level of a system rather than adding up and down quarks to higher energy levels. Another reason that strange quarks contribute to the stability of large systems is their electric

charge. Because the magnitude of the positive charge on an up quark (+2/3e) is greater than the magnitude of the negative charge on a down quark (-1/3e), the size of systems of roughly equal numbers of up quarks and down quarks is limited by coulomb repulsion. This is the idea of nuclear fission. In contrast, the addition of strange quarks with their -1/3e charge would reduce the coulomb repulsion and make the system more stable. It is important to note, however, that because of the strange quark's mass there would be slightly more up and down quarks than strange quarks; thus, the system would have a slight positive charge. A system of a large number of quarks which achieves stability through the presence of strange quarks is SQM.

It has been suggested that nuclear matter which comes into contact with an SQM system would decay and be incorporated into the SQM. But note that there would be a coulomb barrier to overcome as both SQM and nuclear matter are positively charged. This is one argument against the doomsday scenario. But a more important one follows.

One critical question remaining is "Will the number of quarks, strange and other-flavored, in a collision at RHIC be sufficient to create a stable SQM system?" Although the details of the competing theories and models vary considerably, none that I have seen suggest that RHIC will be anywhere near the mass and strangeness content thresholds for the creation of stable SQM. (Sorry to be so vague here. Ideally one could quote how many orders of magnitude away from these thresholds RHIC will be. However, the details of the predictions change, so I'd rather the theorists speak for themselves. I am an experimentalist. I have read papers in this area with interest, but have not contributed to the theories and modeling.)

This brings me back to the notion of strangelets, which can be thought of as small, short lived chunks of SQM. It is these that may be produced at RHIC. If they are, I will not be fearing for Earth's existence, as their short lifetimes will render them innocuous.


* Experts will recognize that I am referring to the valence quarks only.


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*   Rob Davies, Ph.D.
*   Lecturer and Laboratory Manager
*   University of Denver
*   Department of Physics and Astronomy
*   Denver, CO.  USA
*   rodavies@du.edu
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