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Short description: Type of hypothetical particle

A strangelet (pronounced /ˈstrn.lɪt/) is a hypothetical particle consisting of a bound state of roughly equal numbers of up, down, and strange quarks. An equivalent description is that a strangelet is a small fragment of strange matter, small enough to be considered a particle. The size of an object composed of strange matter could, theoretically, range from a few femtometers across (with the mass of a light nucleus) to arbitrarily large. Once the size becomes macroscopic (on the order of metres across), such an object is usually called a strange star. The term "strangelet" originates with Edward Farhi and Robert Jaffe in 1984. Strangelets can convert matter to strange matter on contact.[1] Strangelets have been suggested as a dark matter candidate.[2]

Theoretical possibility

Strange matter hypothesis

The known particles with strange quarks are unstable. Because the strange quark is heavier than the up and down quarks, it can spontaneously decay, via the weak interaction, into an up quark. Consequently, particles containing strange quarks, such as the Lambda particle, always lose their strangeness, by decaying into lighter particles containing only up and down quarks.

However, condensed states with a larger number of quarks might not suffer from this instability. That possible stability against decay is the "strange matter hypothesis", proposed separately by Arnold Bodmer[3] and Edward Witten.[4] According to this hypothesis, when a large enough number of quarks are concentrated together, the lowest energy state is one which has roughly equal numbers of up, down, and strange quarks, namely a strangelet. This stability would occur because of the Pauli exclusion principle; having three types of quarks, rather than two as in normal nuclear matter, allows more quarks to be placed in lower energy levels.

Relationship with nuclei

A nucleus is a collection of a large number of up and down quarks, confined into triplets (neutrons and protons). According to the strange matter hypothesis, strangelets are more stable than nuclei, so nuclei are expected to decay into strangelets. But this process may be extremely slow because there is a large energy barrier to overcome: as the weak interaction starts making a nucleus into a strangelet, the first few strange quarks form strange baryons, such as the Lambda, which are heavy. Only if many conversions occur almost 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 because their lifetime would be longer than the age of the universe.[5]


The stability of strangelets depends on their size. This is because of (a) surface tension at the interface between quark matter and vacuum (which affects small strangelets more than big ones), and (b) screening of charges, which allows small strangelets to be charged, with a neutralizing cloud of electrons/positrons around them, but requires large strangelets, like any large piece of matter, to be electrically neutral in their interior. The charge screening distance tends to be of the order of a few femtometers, so only the outer few femtometers of a strangelet can carry charge.[6]

The surface tension of strange matter is unknown. If it is smaller than a critical value (a few MeV per square femtometer[7]) then large strangelets are unstable and will tend to fission into smaller strangelets (strange stars would still be stabilized by gravity). If it is larger than the critical value, then strangelets become more stable as they get bigger.

Natural or artificial occurrence

Although nuclei do not decay to strangelets, there are other ways to create strangelets, so if the strange matter hypothesis is correct there should be strangelets in the universe. There are at least three ways they might be created in nature:

  • Cosmogonically, 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 may provide enough energy to overcome the energy barrier and create strangelets from nuclear matter. Some identified exotic cosmic ray events, like the Price's event[clarification needed] with very low charge-to-mass ratio could have already registered strangelets.[8]
  • Cosmic ray impacts. In addition to head-on collisions of cosmic rays, ultra high energy cosmic rays impacting on Earth's atmosphere may create strangelets.

These scenarios offer possibilities for observing strangelets. If there are strangelets flying around the universe, then occasionally a strangelet should hit Earth, where it would appear as an exotic type of cosmic ray. If strangelets can be produced in high-energy collisions, then they might be produced by heavy-ion colliders.

Accelerator production

At heavy ion accelerators like the Relativistic Heavy Ion Collider (RHIC), nuclei are collided at relativistic speeds, creating strange and antistrange quarks that could conceivably lead to strangelet production. The experimental signature of a strangelet would be its very high ratio of mass to charge, which would cause its trajectory in a magnetic field to be very nearly, but not quite, straight. The STAR collaboration has searched for strangelets produced at the RHIC,[9] but none were found. The Large Hadron Collider (LHC) is even less likely to produce strangelets,[10] but searches are planned[11] for the LHC ALICE detector.

Space-based detection

The Alpha Magnetic Spectrometer (AMS), an instrument that is mounted on the International Space Station, could detect strangelets.[12]

Possible seismic detection

In May 2002, a group of researchers at Southern Methodist University reported the possibility that strangelets may have been responsible for seismic events recorded on October 22 and November 24 in 1993.[13] The authors later retracted their claim, after finding that the clock of one of the seismic stations had a large error during the relevant period.[14]

It has been suggested that the International Monitoring System being set up to verify the Comprehensive Nuclear Test Ban Treaty (CTBT) after entry into force 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 (4.2 TJ) energy release or less, and could be able to track strangelets passing through Earth in real time if properly exploited.

Impacts on Solar System bodies

It has been suggested that strangelets of subplanetary (i.e. heavy meteorite) mass would puncture planets and other Solar System objects, leading to impact craters which show characteristic features.[15]


If the strange matter hypothesis is correct, and if a stable negatively-charged strangelet with a surface tension larger than the aforementioned critical value exists, then a larger strangelet would be more stable than a smaller one. One speculation that has resulted from the idea is that a strangelet coming into contact with a lump of ordinary matter could convert the ordinary matter to strange matter.[16][17]

This is not a concern for strangelets in cosmic rays because they are produced far from Earth and have had time to decay to their ground state, which is predicted by most models to be positively charged, so they are electrostatically repelled by nuclei, and would rarely merge with them.[18][19] On the other hand, high-energy collisions could produce negatively charged strangelet states, which could live long enough to interact with the nuclei of ordinary matter.[20]

The danger of catalyzed conversion by strangelets produced in heavy-ion colliders has received some media attention,[21][22] and concerns of this type were raised[16][23] at the commencement of the RHIC experiment at Brookhaven, which could potentially have created strangelets. A detailed analysis[17] concluded that the RHIC collisions were comparable to ones which naturally occur as cosmic rays traverse the Solar System, so we would already have seen such a disaster if it were possible. RHIC has been operating since 2000 without incident. Similar concerns have been raised about the operation of the LHC at CERN[24] but such fears are dismissed as far-fetched by scientists.[24][25][26]

In the case of a neutron star, the conversion scenario seems much more plausible. A neutron star is in a sense a giant nucleus (20 km across), held together by gravity, but it is electrically neutral and so does not electrostatically repel strangelets. If a strangelet hit a neutron star, it would initially convert only a small region of it, but that region would grow and eventually consume the entire star, creating a strange star.[27]

Debate about the strange matter hypothesis

The strange matter hypothesis remains unproven. No direct search for strangelets in cosmic rays or particle accelerators has seen a strangelet. If any of the objects such as neutron stars could be shown to have a surface made of strange matter, this would indicate that strange matter is stable at zero pressure, which would vindicate the strange matter hypothesis. However, there is no strong evidence for strange matter surfaces on neutron stars.

Another argument against the hypothesis is that if it were true, essentially all neutron stars should be made of strange matter, and otherwise none should be.[28] Even if there were only a few strange stars initially, violent events such as collisions would soon create many fragments of strange matter flying around the universe. Because collision with a single strangelet would convert a neutron star to strange matter, all but a few of the most recently formed neutron stars should by now have already been converted to strange matter.

This argument is still debated,[29][30][31][32] but if it is correct then showing that one old neutron star has a conventional nuclear matter crust would disprove the strange matter hypothesis.

Because of its importance for the strange matter hypothesis, there is an ongoing effort to determine whether the surfaces of neutron stars are made of strange matter or nuclear matter. The evidence currently favors nuclear matter. This comes from the phenomenology of X-ray bursts, which is well explained in terms of a nuclear matter crust,[33] and from measurement of seismic vibrations in magnetars.[34]

In fiction

  • An episode of Odyssey 5 featured an attempt to destroy the planet by intentionally creating negatively charged strangelets in a particle accelerator.[35]
  • The BBC docudrama End Day features a scenario where a particle accelerator in New York City explodes, creating a strangelet and starting a catastrophic chain reaction which destroys Earth.
  • The story A Matter most Strange in the collection Indistinguishable from Magic by Robert L. Forward deals with the making of a strangelet in a particle accelerator.
  • Impact, published in 2010 and written by Douglas Preston, deals with an alien machine that creates strangelets. The machine's strangelets impact the Earth and Moon and pass through.
  • The novel Phobos, published in 2011 and written by Steve Alten as the third and final part of his Domain trilogy, presents a fictional story where strangelets are unintentionally created at the LHC and escape from it to destroy the Earth.
  • In the 1992 black-comedy novel Humans by Donald E. Westlake, an irritated God sends an angel to Earth to bring about Armageddon by means of using a strangelet created in a particle accelerator to convert the Earth into a quark star.
  • In the 2010 film Quantum Apocalypse, a strangelet approaches the Earth from space.
  • In the novel The Quantum Thief by Hannu Rajaniemi and the rest of the trilogy, strangelets are mostly used as weapons, but during an early project to terraform Mars, one was used to convert Phobos into an additional "sun".

See also

Further reading


  1. Farhi, Edward; Jaffe, R. L. (1984). "Strange matter". Physical Review D 30 (11): 2379–2390. doi:10.1103/PhysRevD.30.2379. Bibcode1984PhRvD..30.2379F. 
  2. Witten, Edward (1984). "Cosmic separation of phases". Physical Review D 30 (2): 272–285. doi:10.1103/PhysRevD.30.272. Bibcode1984PhRvD..30..272W. 
  3. Bodmer, A.R. (1971-09-15). "Collapsed Nuclei". Physical Review D 4 (6): 1601–1606. doi:10.1103/PhysRevD.4.1601. Bibcode1971PhRvD...4.1601B. 
  4. Witten, Edward (1984-07-15). "Cosmic separation of phases". Physical Review D 30 (2): 272–285. doi:10.1103/PhysRevD.30.272. Bibcode1984PhRvD..30..272W. 
  5. Norbeck, E.; Onel, Y. (2011). "The strangelet saga". Journal of Physics 316 (1): 012034–2. doi:10.1088/1742-6596/316/1/012034. Bibcode2011JPhCS.316a2034N. 
  6. Heiselberg, H. (1993). "Screening in quark droplets". Physical Review D 48 (3): 1418–1423. doi:10.1103/PhysRevD.48.1418. PMID 10016374. Bibcode1993PhRvD..48.1418H. 
  7. Alford, Mark G.; Rajagopal, Krishna; Reddy, Sanjay; Steiner, Andrew W. (2006). "Stability of strange star crusts and strangelets". Physical Review D 73 (11): 114016. doi:10.1103/PhysRevD.73.114016. Bibcode2006PhRvD..73k4016A. 
  8. Banerjee, Shibaji; Ghosh, Sanjay K.; Raha, Sibaji; Syam, Debapriyo (2000). "Can Cosmic Strangelets Reach the Earth?". Physical Review Letters 85 (7): 1384–1387. doi:10.1103/PhysRevLett.85.1384. PMID 10970510. Bibcode2000PhRvL..85.1384B. 
  9. Abelev, B. I.; Aggarwal, M. M.; Ahammed, Z.; Anderson, B. D.; Arkhipkin, D.; Averichev, G. S.; Bai, Y.; Balewski, J. et al. (2007). "Strangelet search in Au+Au collisions at sNN=200 GeV". Physical Review C 76 (1): 011901. doi:10.1103/PhysRevC.76.011901. Bibcode2007PhRvC..76a1901A. 
  10. Ellis, John; Giudice, Gian; Mangano, Michelangelo; Tkachev, Igor; Wiedemann, Urs; LHC Safety Assessment Group (2008). "Review of the safety of LHC collisions". Journal of Physics G: Nuclear and Particle Physics 35 (11): 115004 (18pp). doi:10.1088/0954-3899/35/11/115004. Bibcode2008JPhG...35k5004E.  CERN record .
  11. Sadovsky, S. A.; Kharlov, Yu. V.; Angelis, A. L. S.; Gładysz-Dziaduš, E.; Korotkikh, V. L.; Mavromanolakis, G.; Panagiotou, A. D. (2004). "Model for describing the production of Centauro events and strangelets in heavy-ion collisions". Physics of Atomic Nuclei 67 (2): 396–405. doi:10.1134/1.1648929. Bibcode2004PAN....67..396S. 
  12. Sandweiss, J. (2004). "Overview of strangelet searches and Alpha Magnetic Spectrometer: When will we stop searching?". Journal of Physics G: Nuclear and Particle Physics 30 (1): S51–S59. doi:10.1088/0954-3899/30/1/004. Bibcode2004JPhG...30S..51S. 
  13. Anderson, D. P.; Rajagopal, Krishna; Reddy, Sanjay; Steiner, Andrew (2003). "Unexplained Sets of Seismographic Station Reports and a Set Consistent with a Quark Nugget Passage". The Bulletin of the Seismological Society of America 93 (6): 2363–2374. doi:10.1785/0120020138. Bibcode2003BuSSA..93.2363A. 
  14. Herrin, Eugene T.; Rosenbaum, Doris C.; Teplitz, Vigdor L.; Steiner, Andrew (2006). "Seismic search for strange quark nuggets". Physical Review D 73 (4): 043511. doi:10.1103/PhysRevD.73.043511. Bibcode2006PhRvD..73d3511H. 
  15. Rafelski, Johann; Labun, Lance; Birrell, Jeremiah; Steiner, Andrew (2013). "Compact Ultra Dense Matter Impactors". Physical Review Letters 110 (11): 111102. doi:10.1103/PhysRevLett.110.111102. PMID 25166521. Bibcode2011arXiv1104.4572R. Retrieved 2011-11-13. 
  16. 16.0 16.1 Dar, A.; De Rujula, A.; Heinz, Ulrich; Steiner, Andrew (1999). "Will relativistic heavy-ion colliders destroy our planet?". Physics Letters B 470 (1–4): 142–148. doi:10.1016/S0370-2693(99)01307-6. Bibcode1999PhLB..470..142D. 
  17. 17.0 17.1 Jaffe, R. L.; Busza, W.; Wilczek, F.; Sandweiss, J. (2000). "Review of speculative disaster scenarios at RHIC". Reviews of Modern Physics 72 (4): 1125–1140. doi:10.1103/RevModPhys.72.1125. Bibcode2000RvMP...72.1125J. 
  18. Madsen, Jes; Rajagopal, Krishna; Reddy, Sanjay; Steiner, Andrew (2000). "Intermediate Mass Strangelets are Positively Charged". Physical Review Letters 85 (22): 4687–4690. doi:10.1103/PhysRevLett.85.4687. PMID 11082627. Bibcode2000PhRvL..85.4687M. 
  19. Madsen, Jes; Rajagopal, Krishna; Reddy, Sanjay; Steiner, Andrew (2006). "Strangelets in Cosmic Rays". arXiv:astro-ph/0612784.
  20. Schaffner-Bielich, Jürgen; Greiner, Carsten; Diener, Alexander; Stöcker, Horst (1997). "Detectability of strange matter in heavy ion experiments". Physical Review C 55 (6): 3038–3046. doi:10.1103/PhysRevC.55.3038. Bibcode1997PhRvC..55.3038S. 
  21. Robert Matthews (28 August 1999). "A Black Hole Ate My Planet". New Scientist. Retrieved 25 April 2019. 
  22. Horizon: End Days, an episode of the BBC television series Horizon
  23. Wagner, Walter L. (1999). "Black Holes at Brookhaven?". Scientific American 281 (1): 8. 
  24. 24.0 24.1 Dennis Overbye (29 March 2008). "Asking a Judge to Save the World, and Maybe a Whole Lot More". New York Times. 
  25. "Safety at the LHC". 
  26. J. Blaizot et al., "Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC", CERN library record CERN Yellow Reports Server (PDF)
  27. Alcock, Charles; Farhi, Edward; Olinto, Angela (1986). "Strange stars". Astrophysical Journal 310: 261. doi:10.1086/164679. Bibcode1986ApJ...310..261A. 
  28. Caldwell, R.R.; Friedman, John L. (1991). "Evidence against a strange ground state for baryons". Physics Letters B 264 (1–2): 143–148. doi:10.1016/0370-2693(91)90718-6. Bibcode1991PhLB..264..143C. 
  29. Alford, Mark G.; Rajagopal, Krishna; Reddy, Sanjay; Steiner, Andrew (2003). "Strangelets as Cosmic Rays beyond the Greisen-Zatsepin-Kuzmin Cutoff". Physical Review Letters 90 (12): 121102. doi:10.1103/PhysRevLett.90.121102. PMID 12688863. Bibcode2003PhRvL..90l1102M. 
  30. Balberg, Shmuel; Rajagopal, Krishna; Reddy, Sanjay; Steiner, Andrew (2004). "Comment on Strangelets as Cosmic Rays beyond the Greisen-Zatsepin-Kuzmin Cutoff". Physical Review Letters 92 (11): 119001. doi:10.1103/PhysRevLett.92.119001. PMID 15089181. Bibcode2004PhRvL..92k9001B. 
  31. Madsen, Jes; Rajagopal, Krishna; Reddy, Sanjay; Steiner, Andrew (2004). "Madsen Replies". Physical Review Letters 92 (11): 119002. doi:10.1103/PhysRevLett.92.119002. Bibcode2004PhRvL..92k9002M. 
  32. Madsen, Jes (2005). "Strangelet propagation and cosmic ray flux". Physical Review D 71 (1): 014026. doi:10.1103/PhysRevD.71.014026. Bibcode2005PhRvD..71a4026M. 
  33. Heger, Alexander; Cumming, Andrew; Galloway, Duncan K.; Woosley, Stanford E. (2007). "Models of type I X-ray bursts from GS 1826-24: A probe of rp-process hydrogen burning". The Astrophysical Journal 671 (2): L141. doi:10.1086/525522. Bibcode2007ApJ...671L.141H. 
  34. Watts, Anna L.; Reddy, Sanjay (2007). "Magnetar oscillations pose challenges for strange stars". Monthly Notices of the Royal Astronomical Society 379 (1): L63. doi:10.1111/j.1745-3933.2007.00336.x. Bibcode2007MNRAS.379L..63W. 
  35. Odyssey 5: Trouble with Harry , an episode of the Canadian science fiction television series Odyssey 5 by Manny Coto (2002)

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