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A strangelet or "strange nugget" is a hypothetical object, consisting of a bound state of roughly equal numbers of up, down, and strange quarks. The size could be anything from a few femtometers across (with the mass of a light nucleus) to something much larger. Once the size becomes macrosopic (of order meters across), such an object is usually called a quark star or "strange star" rather than a strangelet. An equivalent description is that a strangelet is a small fragment of strange matter. Strangelets have been suggested as a Dark matter candidate .

Theoretical possibility of strangelets

The strange matter hypothesis

The main question about strangelets concerns their stability. The strange quark is heavier than the up and down quarks, so the known particles containing strange quarks (such as the Lambda particle, which contains an up, down, and strange quark) always lose their strangeness, decaying, via the weak interaction to lighter particles containing only up and down quarks. However this might cease to be true for states with a sufficiently large number of quarks. This is the "strange matter hypothesis" of Bodmer and Witten . According to this hypothesis, when you collect a large enough number of quarks together, the lowest energy state is one that has roughly equal numbers of up, down, and strange quarks, namely a strangelet.

Strangelets vs nuclei

At first glance, the strange matter hypothesis seems to have been experimentally ruled out, since we already know what you get when you collect a large number of quarks together: a nucleus, which consists of only up and down quarks, confined into triplets (neutrons and protons). If strangelets were more stable than nuclei, wouldn't nuclei quickly decay into strangelets? Actually, this need not be a quick process. There is a large barrier because if the weak interaction starts trying to make a nucleus into a strangelet, the first few strange quarks will just form strange baryons, such as the Lambda, which are heavy. Only if a large number of conversions occur simultaneously will the number of strange quarks reach the critical proportion required to achieve a lower energy state. This is very unlikely to happen, so even if the strange matter hypothesis were correct, nuclei would never be seen to decay to strangelets: their lifetime would be longer than the age of the universe.

Occurrence in nature

Even though nuclei do not decay to strangelets, there are other ways to make strangelets, so if strange matter hypothesis is correct there should be strangelets in the universe. There are two ways they could be created:

• Cosmologically, i.e. in the early universe when the QCD confinement phase transition occurred. It is possible that strangelets were created along with the neutrons and protons that form ordinary matter.
• High energy processes. The universe is full of very high energy particles (cosmic rays). It is possible that when these collide with each other or with neutron stars they might provide enough energy to overcome the energy barrier and create strangelets from nuclear matter.

Both these scenarios offer possibilities for observing strangelets. If there are strangelets flying around the universe, then occasionally a strangelet should hit the planet Earth, where it would appear as an exotic type of cosmic ray, and we should be able to observe them. If they can be produced in high energy collisions, then we might make them at heavy-ion colliders.

Accelerator production

The STAR collaboration has searched for strangelets produced at the Relativistic Heavy Ion Collider .

Space-based detection

The Alpha Magnetic Spectrometer (AMS), an instrument that is planned to be mounted on the International Space Station, could detect strangelets .

Possible seismic observation

In May 2002, a group of researchers at Southern Methodist University reported the possibility that strangelets may have been responsible for two seismic events recorded on October 22 and November 24 in 1993 . Most seismologists, however, consider the events to be normal deep earthquakes.

IMS Strangelet 'observatory'

It has been suggested that the International Monitoring System being set up to verify the Comprehensive Nuclear Test Ban Treaty (CTBT) may be useful as a sort of "strangelet observatory" using the entire Earth as its detector; the IMS will be designed to detect anomalous seismic disturbances down to 1 kiloton of TNT's equivalent energy release or less, and could be able to track strangelets passing through Earth in real time if properly exploited.

Danger of strangelets: catalyzed conversion to strange matter

If the strange matter hypothesis is correct then when a strangelet from space hits the Earth (or any other lump of ordinary matter) it could convert it to strange matter. The disaster scenario is this: one strangelet hits a nucleus, catalyzing its immediate conversion to strange matter. This liberates energy, and sends pieces (more strangelets) flying in all directions. These merge with other nuclei and convert them, leading to a chain reaction, at the end of which all the nuclei of all the atoms have been converted, and Earth has been reduced to a hot cloud of strangelets.

The general belief is that this would not happen, because most models predict that strangelets, like nuclei, are positively charged, so they are electrostatically repelled by nuclei, and would rarely merge with them. However, the idea has received some media attention , , and concerns of this type were raised at the commencement of the Relativistic Heavy Ion Collider (RHIC) experiment at Brookhaven, which could potentially have created strangelets. A detailed analysis concluded that the RHIC collisions were comparable to ones that naturally occur as cosmic rays traverse the solar system, so we would already have seen such a disaster if it were possible.

In the case of a neutron star, however, the conversion scenario seems much more plausible. A neutron star is in a sense one giant (20 km across) nucleus, held together by gravity. If a strangelet hit a neutron star, it could convert a small region of it, and that region would grow to consume the entire star.

Is the "strange matter hypothesis" true?

The strange matter hypothesis is generally regarded as a radical idea. Because one strangelet can convert a neutron star to a strange star, it seems likely that if the strange matter hypothesis were correct, all the objects we observe as neutron stars would actually have to be strange stars. But there is good evidence that at least some of them are not strange stars, and have fairly thick crusts of nuclear matter. There is an ongoing debate among experts on this question.

Strangelets in Fiction

An episode of Odyssey 5 featured an attempt to destroy the planet by intentionally creating strangelets in a particle accelerator.

The BBC docudrama End Day features a scenario where a New York City based particle accelerator explodes, starting a catastrophic chain reaction that destroys Earth.

External links

• The Story of Strangelets

References

1. ^ E. Witten, "Cosmic Separation Of Phases" Phys. Rev. D30, 272 (1984)
2. A. Bodmer "Collapsed Nuclei" Phys. Rev. D4, 1601 (1971)
3. STAR Collaboration, "Strangelet search at RHIC", arXiv:nucl-ex/0511047
4. J. Sandweiss, "Overview of strangelet searches and Alpha Magnetic Spectrometer: When will we stop searching?" J. Phys. G30:S51-S59 (2004)
5. D. Anderson et al, "Two seismic events with the properties for the passage of strange quark matter through the earth" arXiv:astro-ph/0205089
6. J. Madsen, "Intermediate mass strangelets are positively charged" Phys. Rev. Lett. 85 (2000) 4687-4690 (2000)
7. New Scientist, 28 August 1999: "A Black Hole Ate My Planet"
8. Horizon: End Days, an episode of the BBC television series Horizon
9. W. Busza, R. Jaffe, J. Sandweiss, F. Wilczek, "Review of speculative 'disaster scenarios' at RHIC", Rev. Mod. Phys.72:1125-1140 (2000)
10. C. Alcock, E. Farhi and A. Olinto, "Strange stars", Astrophys. Journal 310, 261 (1986)
11. J. Madsen, "Strangelets as cosmic rays beyond the GZK-cutoff", Phys. Rev. Lett. 90:121102 (2003)
12. S. Balberg, "Comment on 'strangelets as cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff'", Phys. Rev. Lett. 92:119001 (2004)
13. J. Madsen, "Reply to Comment on Strangelets as Cosmic Rays beyond the Greisen-Zatsepin-Kuzmin Cutoff", Phys. Rev.Lett. 92:119002 (2004)
14. J. Madsen, "Strangelet propagation and cosmic ray flux" Phys. Rev. D71, 014026 (2005)

• Quark matter
• Nuclear physics