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| A '''strangelet''' is a hypothetical particle consisting of a ] of roughly equal numbers of ], ], and ] ]s. Its size would be a minimum of a few ] across (with the mass of a light nucleus). Once the size becomes macroscopic (on the order of metres across), such an object is usually called a ] or "strange star" rather than a strangelet. An equivalent description is that a strangelet is a small fragment of ]. The term "strangelet" originates with Edward Farhi and ].<ref name = 'Farhi and Jaffe'>E. Farhi and R. Jaffe, "Strange Matter", </ref> Strangelets have been suggested as a ] candidate.<ref name='Witten'/> | |||
| ==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, so strange particles, such as the ], which contains an up, down, and strange quark, always lose their strangeness, by decaying via the ] to lighter particles containing only up and down quarks. But states with a larger number of quarks might not suffer from this instability. This is the "strange matter hypothesis" of Bodmer <ref name='Bodmer'>A. Bodmer "Collapsed Nuclei" </ref> and ].<ref name='Witten'>E. Witten, "Cosmic Separation Of Phases" </ref> According to this hypothesis, when a large enough number of quarks are collected 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 ]; 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 (]s and ]s). 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. {{Citation needed|date=April 2015}} | |||
| ===Size=== | |||
| 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.<ref>H. Heiselberg, "Screening in quark droplets", </ref> | |||
| The surface tension of strange matter is unknown. If it is smaller than a critical value (a few MeV per square femtometer<ref name='screen'>M. Alford, K. Rajagopal, S. Reddy, A. Steiner, "The Stability of Strange Star Crusts and Strangelets", Phys. Rev. D73 114016 (2006) </ref>) 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 ] 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 (]s). 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 with very low charge to mass ratio could have already registered strangelets.<ref>Shibaji Banerjee, Sanjay K. Ghosh, Sibaji Raha, and Debapriyo Syam, "Can Cosmic Strangelets Reach the Earth?", </ref> | |||
| * Cosmic ray impacts. In addition to head-on collisions of cosmic rays, ]s impacting on ] 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 we might make them at heavy-ion colliders. | |||
| ===Accelerator production=== | |||
| At heavy ion accelerators like the ] (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 ] has searched for strangelets produced at the RHIC,<ref>STAR Collaboration, "Strangelet search at RHIC", </ref> but none were found. The ] (LHC) is even less likely to produce strangelets,<ref name="LSAGreport">Ellis J, Giudice G, Mangano ML, Tkachev I, Wiedemann U (LHC Safety Assessment Group) (5 September 2008). "" (PDF, 586 ]). '''']''. 35, 115004 (18pp). {{doi|10.1088/0954-3899/35/11/115004}}. {{arxiv|0806.3414}}. .</ref> but searches are planned<ref>A. Angelis ''et al.'', "Model of Centauro and strangelet production in heavy ion collisions", Phys. Atom. Nucl. 67:396-405 (2004) {{arXiv|nucl-th/0301003}}</ref> for the LHC ] detector. | |||
| ===Space-based detection=== | |||
| The ] (AMS), an instrument that is mounted on the ], could detect strangelets.<ref>J. Sandweiss, "Overview of strangelet searches and Alpha Magnetic Spectrometer: When will we stop searching?" </ref> | |||
| ===Possible seismic detection=== | |||
| In May 2002, a group of researchers at ] reported the possibility that strangelets may have been responsible for seismic events recorded on October 22 and November 24 in 1993.<ref>D. Anderson ''et al.'', "Two seismic events with the properties for the passage of strange quark matter through the earth" </ref> 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.<ref></ref> | |||
| It has been suggested that the ] being set up to verify the ] (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 {{convert|1|ktonTNT|lk=on}} 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 metorite mass, would puncture planets and other solar system objects, leading to impact (exit) craters which show characteristic features.<ref>Lance Labun, Jeremey Birrell, Johann Rafelski, "Solar System Signatures of Impacts by Compact Ultra Dense Objects",{{arxiv|1104.4572}}</ref> | |||
| ==Dangers== | |||
| If the strange matter hypothesis is correct ''and'' its surface tension is larger than the aforementioned critical value, 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.<ref name='DDH'/><ref name='BJSW'/><!-- original suggestion may have been Glashow and De Rujula in Nature--> This "]"-like disaster scenario is as follows: one strangelet hits a nucleus, catalyzing its immediate conversion to strange matter. This liberates energy, producing a larger, more stable strangelet, which in turn hits another nucleus, catalyzing its conversion to strange matter. In the end, all the nuclei of all the atoms of Earth are converted, and Earth is reduced to a hot, large lump of strange matter. | |||
| 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.<ref>J. Madsen, "Intermediate mass strangelets are positively charged", Phys. Rev. Lett. 85 (2000) 4687-4690 (2000) </ref><ref>J. Madsen "Strangelets in Cosmic Rays", for Proceedings of 11th Marcel Grossmann Meeting, Germany, Jul 2006, </ref> But high-energy collisions could produce negatively charged strangelet states which live long enough to interact with the nuclei of ordinary matter.<ref>J. Schaffner-Bielich, C. Greiner, A. Diener, H. Stoecker, "Detectability of strange matter in heavy ion experiments", Phys. Rev. C55:3038-3046 (1997), </ref> | |||
| The danger of catalyzed conversion by strangelets produced in heavy-ion colliders has received some media attention,<ref></ref><ref>], an episode of the ] ] ]</ref> and concerns of this type were raised<ref name='DDH'>A. Dar, A. De Rujula, U. Heinz, "Will relativistic heavy ion colliders destroy our planet?", Phys. Lett. B470:142-148 (1999) </ref><ref>W. Wagner, "Black holes at Brookhaven?" and reply by F. Wilzcek, Letters to the Editor, Scientific American July 1999</ref> at the commencement of the ] (RHIC) experiment at Brookhaven, which could potentially have created strangelets. A detailed analysis<ref name='BJSW'>W. Busza, R. Jaffe, J. Sandweiss, F. Wilczek, "Review of speculative 'disaster scenarios' at RHIC", Rev. Mod. Phys.72:1125-1140 (2000) </ref> concluded that the RHIC collisions were comparable to ones which naturally occur as ] 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 ]<ref name='NYT'>Dennis Overbye, Asking a Judge to Save the World, and Maybe a Whole Lot More, NY Times, 29 March 2008 </ref> but such fears are dismissed as far-fetched by scientists.<ref name='NYT'/><ref>{{cite web|url=http://public.web.cern.ch/Public/en/LHC/Safety-en.html|title=Safety at the LHC}}</ref><ref>J. Blaizot ''et al.'', "Study of Potentially Dangerous Events During Heavy-Ion Collisions at the LHC", </ref> | |||
| In the case of a ], 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 could convert a small region of it, and that region would grow to consume the entire star, creating a ].<ref>{{cite journal|last-author-amp=yes|journal=Astrophys. J.|volume=310|page=261|date=1986|url=http://adsabs.harvard.edu/full/1986ApJ...310..261A|doi=10.1086/164679|title=Strange stars|last1=Alcock|first1=Charles|last2=Farhi|first2=Edward|last3=Olinto|first3=Angela|bibcode = 1986ApJ...310..261A }}</ref> | |||
| ==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 (see references in earlier sections). If any of the objects we call 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. But there is no strong evidence for strange matter surfaces on neutron stars (see below). | |||
| Another argument against the hypothesis is that if it were true, all neutron stars should be made of strange matter, and otherwise none should be.<ref>J. Friedman and R. Caldwell, "Evidence against a strange ground state for baryons", Phys. Lett. B264, 143-148 (1991)</ref> Even if there were only a few strange stars initially, violent events such as collisions would soon create many strangelets flying around the universe. Because one strangelet will convert a neutron star to strange matter, by now all neutron stars would have been converted. This argument is still debated,<ref>J. Madsen, "Strangelets as cosmic rays beyond the GZK-cutoff", Phys. Rev. Lett. 90:121102 (2003) </ref><ref>S. Balberg, "Comment on 'strangelets as cosmic rays beyond the Greisen-Zatsepin-Kuzmin cutoff'", Phys. Rev. Lett. 92:119001 (2004), </ref><ref>J. Madsen, "Reply to Comment on Strangelets as Cosmic Rays beyond the Greisen-Zatsepin-Kuzmin Cutoff", Phys. Rev.Lett. 92:119002 (2004), </ref><ref>J. Madsen, "Strangelet propagation and cosmic ray flux",Phys. Rev. D71, 014026 (2005) </ref> but if it is correct then showing that one 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 ]s, which is well-explained in terms of a nuclear matter crust,<ref>A. Heger, A. Cumming, D. Galloway, S. Woosley, "Models of Type I X-ray Bursts from GS 1826-24: A Probe of rp-Process Hydrogen Burning", </ref> and from measurement of seismic vibrations in ]s.<ref>A. Watts and S. Reddy, "Magnetar oscillations pose challenges for strange stars", MNRAS, 379, L63 (2007) </ref> | |||
| ==In fiction== | |||
| *An episode of '']'' featured an attempt to destroy the planet by intentionally creating negatively charged strangelets in a ].<ref>'']: '', an episode of the Canadian science fiction television series '']'' by Manny Coto (2002)</ref> | |||
| *The ] ] '']'' features a scenario where a particle accelerator in ] explodes, creating a strangelet and starting a catastrophic chain reaction which destroys Earth. | |||
| *The story ''A Matter most Strange'' in the collection '']'' by ] deals with the making of a strangelet in a ]. | |||
| *'']'', published in 2010 and written by ], 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 ] as the third and final part of his ''Domain'' trilogy, presents a fictional story where strangelets are unintentionally created at the ] and escape from it to destroy the Earth. | |||
| *In ], strangelets are used as a method to create a ]. | |||
| *In the 1992 black-comedy novel ''Humans'' by ], an irritated God sends an angel to Earth to bring about ] by means of using a strangelet created in a particle accelerator to convert the Earth into a quark star. | |||
| *In comic book ], the manipulation of strangelets is described as the hypernatural power of Shoal to reinforce mass and find ways out from tight spots. | |||
| *In the 2010 film '']'', a strangelet approaches the Earth from space. | |||
| *In the novel '']'' by ] and the rest of the trilogy, strangelets are mostly used as weapons, but during an early project to ] Mars, one was used to convert ] into an additional "sun." | |||
| ==See also== | |||
| *] | |||
| *] | |||
| ==References== | |||
| {{Reflist|30em}} | |||
| ==Further reading== | |||
| *{{cite web|url=http://www.physics.rutgers.edu/~jholden/strange/strange.html|title=The Story of Strangelets|last=Holden|first=Joshua|date=May 17, 1998|publisher=]|accessdate=2010-04-01}} | |||
| *{{cite journal|author1=Fridolin Weber|title=Strange Quark Matter and Compact Stars|date=2004|doi=10.1016/j.ppnp.2004.07.001|journal=Progress in Particle and Nuclear Physics|volume=54|pages=193–288|arxiv=astro-ph/0407155|bibcode = 2005PrPNP..54..193W }} | |||
| *{{cite journal|author1=Jes Madsen|title=Hadrons in Dense Matter and Hadrosynthesis|date=1998|doi=10.1007/BFb0107314|chapter=Physics and astrophysics of strange quark matter|series=Lecture Notes in Physics|isbn=978-3-540-65209-0|volume=516|pages=162–203|journal=Lect.Notes Phys.|arxiv=astro-ph/9809032}} | |||
| ] | |||
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| ] | |||
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