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Strangelet
Strangelet
Strangelet
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A strangelet (pronounced /ˈstreɪndʒ.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. It has been theorized that strangelets can convert matter to strange matter on contact.[1] Strangelets have also been suggested as a dark matter candidate.[2]

Theoretical possibility

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Strange matter hypothesis

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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

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A nucleus is a collection of a number of up and down quarks (in some nuclei a fairly large number), 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]

Size

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The stability of strangelets depends on their size, because of

  • surface tension at the interface between quark matter and vacuum (which affects small strangelets more than big ones). The surface tension of strange matter is unknown. If it is smaller than a critical value (a few MeV per square femtometer[6]) 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.
  • charge screening, 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.[7]

Natural or artificial occurrence

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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, such as "Price's event"—i.e., those with very low charge-to-mass ratios (as the s-quark itself possesses charge commensurate with the more-familiar d-quark, but is much more massive)—could have already registered strangelets.[8][9]
  • 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 strangelets can be produced in high-energy collisions, then they might be produced by heavy-ion colliders. Similarly, if there are strangelets flying around the universe, then occasionally a strangelet should hit Earth, where it may appear as an exotic type of cosmic ray; alternatively, a stable strangelet could end up incorporated into the bulk of the Earth's matter, acquiring an electron shell proportional to its charge and hence appearing as an anomalously heavy isotope of the appropriate element—though searches for such anomalous "isotopes" have, so far, been unsuccessful.[10]

Accelerator production

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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,[11] but none were found. The Large Hadron Collider (LHC) is even less likely to produce strangelets,[12] but searches are planned[13] for the LHC ALICE detector.

Space-based detection

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The Alpha Magnetic Spectrometer (AMS), an instrument that is mounted on the International Space Station, could detect strangelets.[14]

Possible seismic detection

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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.[15] 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.[16]

It has been suggested that the International Monitoring System be 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

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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.[17]

Potential propagation

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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 over time convert the ordinary matter to strange matter.[18][19]

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.[20][21] 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.[22]

The danger of catalyzed conversion by strangelets produced in heavy-ion colliders has received some media attention,[23][24] and concerns of this type were raised[18][25] at the commencement of the RHIC experiment at Brookhaven, which could potentially have created strangelets. A detailed analysis[19] 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[26] but such fears are dismissed as far-fetched by scientists.[26][27][28]

In the case of a neutron star, the conversion scenario may be 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 would not electrostatically repel strangelets. If a strangelet hit a neutron star, it might catalyze quarks near its surface to form into more strange matter, potentially continuing until the entire star became a strange star.[29]

Debate about the strange matter hypothesis

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The strange matter hypothesis remains unproven. No direct search for strangelets in cosmic rays or particle accelerators has yet confirmed 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.[30] 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,[31][32][33][34] 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,[35] and from measurement of seismic vibrations in magnetars.[36]

In fiction

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  • An episode of Odyssey 5 featured an attempt to destroy the planet by intentionally creating negatively charged strangelets in a particle accelerator.[37]
  • 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

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Further reading

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Strangelet

View on Grokipedia
from Grokipedia
A strangelet is a hypothetical particle composed of a small, finite lump of strange quark matter (SQM), a proposed state of deconfined quarks featuring roughly equal numbers of up, down, and in . Unlike ordinary , which consists of protons and neutrons bound by , SQM is theorized to have a lower per , potentially making strangelets stable or metastable against decay into hadrons. If negatively charged, such particles could catalytically convert surrounding ordinary into additional upon contact, though this scenario remains speculative and unconfirmed. The concept of strange quark matter originated in the early 1970s with theoretical work suggesting that adding to quark matter could reduce its compared to two-flavor (up and down) quark matter. This idea was revitalized in 1984 by , who proposed that SQM might be the ground state of baryonic matter, implying that strangelets could exist as stable remnants from the early or astrophysical processes. The term "strangelet" was coined by Edward Farhi and Robert L. Jaffe in 1984 to describe these compact, droplet-like configurations of SQM with low numbers (typically A < 10^7), whose properties are modeled using approaches like the MIT bag model, liquid-drop approximations, or shell models. Stability depends on factors such as the bag constant (confining strange matter within a "bag" of perturbative vacuum), quark masses, and surface effects; for instance, strangelets with energy per below approximately 930 MeV are considered stable, while those between 930 and 938 MeV may be metastable. Strangelets are predicted to form in high-energy environments, including the quark-gluon plasma produced in ultrarelativistic heavy-ion collisions at facilities like the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), as well as in cosmic rays or the interiors of compact stars such as neutron stars or hypothetical strange stars. Searches for strangelets have been conducted in cosmic rays, lunar samples, and accelerator experiments, yielding only upper limits on their abundance; for example, detectors like the Search for Strange Quark Matter on the International Space Station (SQM-ISS) aim to identify massive, non-relativistic strangelets among cosmic radiation. Early concerns about strangelet production at RHIC and LHC potentially triggering a global matter-conversion catastrophe were addressed in safety reviews, which concluded that such risks are negligible based on the absence of evidence for stable strange matter and the transient nature of collision conditions. Recent theoretical advances, including models favoring up-down quark matter over strange-inclusive variants at low temperatures due to vacuum energy penalties, suggest that observable strangelets—if they exist—may require baryon masses exceeding 300 proton equivalents, placing them beyond current synthesis capabilities. Despite these challenges, strangelets remain a focal point in quantum chromodynamics research, with implications for understanding the strong interaction, dark matter candidates, and the equation of state of dense matter in astrophysics.

Theoretical Foundations

Strange Matter Hypothesis

Strange matter, also known as strange quark matter, is a hypothetical phase of baryonic matter composed of roughly equal fractions of up, down, and strange quarks in a deconfined state, unbound by the usual hadron confinement predicted by . Unlike ordinary nuclear matter, where quarks are confined within protons and neutrons, strange matter would exist as a degenerate Fermi gas of free quarks, stabilized by the balance of weak interactions that maintain chemical equilibrium among the flavors. The concept of stable strange quark matter originated with A. R. Bodmer's 1971 proposal of "collapsed nuclei," a dense state where quarks from multiple nucleons merge into a single entity, potentially representing a lower-energy configuration than ordinary nuclei. This idea predated the full formulation of QCD but aligned with early quark models. The hypothesis gained renewed attention in 1984 when Edward Witten suggested that strange matter could be the absolute ground state of baryonic matter, possessing an energy per baryon lower than that of iron-56 (approximately 923 MeV versus 930 MeV for ^{56}Fe), implying that ordinary nuclei might be metastable and could convert to strange matter if exposed to it. Witten's analysis, building on the MIT bag model for quark confinement, emphasized that the inclusion of strange quarks reduces the Fermi energy, enhancing stability at zero pressure. Following the development of QCD in the 1970s, which provided a perturbative framework for strong interactions at high energies, theoretical explorations of quark matter phases intensified, setting the stage for the strange matter hypothesis. Witten further implicated strange matter in cosmology, proposing that it could have formed in the early universe during the quark epoch—a brief period of deconfined quark-gluon plasma shortly after the [Big Bang](/page/Big Bang)—through phase separation if the transition to hadronic matter was incomplete. Small fragments of such matter, known as , might persist as metastable relics.

Stability and Formation

Strange quark matter, or strangelets, achieves stability when its energy per baryon number is lower than that of ordinary nuclear matter (approximately 930 MeV for iron-56), a condition met if the strange quark mass msm_s is around 100–150 MeV. This mass range allows the chemical potentials (Fermi levels) of the up (uu), down (dd), and strange (ss) quarks to equilibrate approximately equally, minimizing the total energy density within the MIT bag model framework, where the non-perturbative QCD vacuum contribution is parameterized by the bag constant B≈50B \approx 50–100100 MeV/fm³. The MIT bag model serves as a primary theoretical tool for computing binding energies of strange matter, treating quarks as free Fermi gases confined within a spherical "bag" that enforces color neutrality and confinement. In this model, the total energy includes kinetic contributions from the degenerate quark Fermi seas, the rest mass of the strange quarks, and the positive bag energy BB, with stability requiring the binding energy to exceed that of nuclear matter for large baryon numbers AA. Weak interactions are essential for reaching this equilibrium by facilitating flavor-changing processes, such as d↔sd \leftrightarrow s, which drive the strangeness chemical potential μs≈0\mu_s \approx 0, ensuring roughly equal numbers of uu, dd, and ss quarks (strangeness fraction Ys≈1Y_s \approx 1). Formation of strangelets occurs via quark deconfinement in extreme conditions of high temperature (T≳150T \gtrsim 150 MeV) and density (nB≳1n_B \gtrsim 1 fm⁻³), as in the early universe during the quark-hadron phase transition or in ultrarelativistic heavy-ion collisions that create a quark-gluon plasma (QGP). In the QGP, initial up and down quark dominance evolves through strong interactions producing strange-antistrange pairs (e.g., gluon fusion gg→ssˉgg \to s\bar{s}), rapidly building strangeness content before hadronization or direct coalescence into bound strangelets upon cooling. If strange matter forms in a non-equilibrium state with insufficient strangeness equilibration, it may exhibit metastability, persisting as a local energy minimum and decaying slowly via weak processes or fission into smaller fragments, potentially over timescales exceeding the age of the universe.

Relation to Ordinary Nuclei

Strange matter, hypothesized as a state composed of roughly equal numbers of up, down, and strange quarks in a 1:1:1 ratio, fundamentally differs from ordinary atomic nuclei, which consist of up and down quarks in an approximately 1:1 ratio within protons and neutrons. This distinction in quark content arises because ordinary nuclei are built from nucleons—protons (uud) and neutrons (udd)—while strange matter forms a deconfined quark phase where the inclusion of strange quarks lowers the overall energy per baryon compared to nuclear matter under certain conditions. Lattice QCD simulations support the possibility of a phase transition from hadronic nuclear matter to a quark matter phase, including strange quark matter, at high densities, indicating that strangeness can stabilize matter beyond the regime of ordinary nuclei. In terms of density, ordinary nuclei exhibit a baryon number density of approximately 0.17 fm⁻³, corresponding to an energy density on the order of 2.8 × 10¹⁷ kg/m³, whereas bulk strange matter is predicted to achieve higher densities around 10¹⁸ kg/m³ due to its incompressibility and the Pauli exclusion principle acting on quarks rather than nucleons. This higher density in strange matter reflects its potential as a more compact phase, where the Fermi energy of quarks allows for tighter packing without the repulsive core interactions dominant in nuclear matter. Theoretical models propose that small strangelets, with low baryon numbers, could interact with ordinary nucleons by binding to form hybrid strange-nuclear states, akin to fusion processes that increase the strangelet size through quantum tunneling or overcoming the Coulomb barrier. In contrast, larger strangelets may catalyze the conversion of surrounding nuclear matter into strange matter by acting as seeds, progressively absorbing nucleons and releasing energy, though this requires the strange matter to be absolutely stable relative to iron nuclei. Such hybrid formations are particularly relevant in high-density environments like neutron star cores, where small strange matter droplets could nucleate the transition to a strange star, entirely composed of strange quark matter, altering the star's structure and stability.

Physical Properties

Size and Mass

Theoretical models predict that strangelets can span a broad range of sizes, parameterized by their baryon number AA, which corresponds to the number of constituent quarks divided by three. Small strangelets, typically with 6≤A≤186 \leq A \leq 18, are expected to have diameters on the order of 3 to 6 femtometers (fm), comparable to nuclear scales, due to the confinement within a quark matter droplet. For these compact objects, surface effects dominate the energy budget, arising from the interface between the strange quark matter and the vacuum. In contrast, if bulk strange quark matter is absolutely stable, larger strangelets could theoretically grow to planetary masses, with radii scaling as R∝A1/3R \propto A^{1/3} in the MIT bag model, though such macroscopic forms remain highly speculative. The mass of a strangelet is closely tied to its baryon number and the underlying quark model parameters. For small strangelets, masses are estimated to range from approximately 6 to 18 GeV/c2c^2, reflecting the contributions from quark kinetic energy, bag constant, and surface terms. In the MIT bag model, the total energy (and thus mass) per baryon is given by the semi-empirical formula: EA=ε0+csA−1/3+ccA−2/3,\frac{E}{A} = \varepsilon_0 + c_s A^{-1/3} + c_c A^{-2/3}, where ε0\varepsilon_0 is the bulk energy density (dependent on the strange quark mass msm_s and bag constant BB), cs≈100c_s \approx 100 MeV is the surface coefficient, and cc≈300c_c \approx 300 MeV accounts for curvature effects. For neutral strangelets with B1/4=145B^{1/4} = 145 MeV and ms=0m_s = 0, this yields ε0≈829\varepsilon_0 \approx 829 MeV, leading to masses that increase nearly linearly with AA for larger clusters but show deviations for small AA due to shell structure. Shell model calculations, incorporating non-relativistic approximations for quark orbitals, reveal mass gaps for A<6A < 6, where no stable bound states exist owing to insufficient filling of the lowest energy levels. The bag constant BB, representing the vacuum energy difference, significantly influences strangelet compactness. Higher values of BB (e.g., B1/4>145B^{1/4} > 145 MeV) result in smaller radii for a given AA, as the increased confining compresses the matter droplet to minimize the bag energy contribution proportional to the volume. This effect is particularly pronounced in small strangelets, where the surface-to-volume ratio amplifies the role of BB. However, the small size inherent to these objects poses stability challenges: , estimated at 5–30 MeV/fm² in bag models, elevates the energy per above that of ordinary nuclei, rendering small strangelets unstable unless mitigated by charge screening or external that reduce electrostatic repulsion.

Charge and Density

Strangelets are characterized by a small positive electric charge, with the charge-to-baryon ratio Z/AZ/A typically ranging from 0.1 to 0.3 for small strangelets (baryon number A≲100A \lesssim 100), arising from the reduced number of negatively charged strange (s) quarks compared to up (u) and down (d) quarks. This low charge fraction, much smaller than the Z/A≈0.5Z/A \approx 0.5 of ordinary nuclei, stems from the higher mass of the s quark (ms≈150m_s \approx 150 MeV), which limits its population in the quark Fermi sea to achieve energy minimization. In the simplest non-interacting approximation, the charge can be estimated as Z≈(A/3)(1−ms/EF)Z \approx (A/3) (1 - m_s / E_F), where EFE_F is the Fermi energy of approximately 200–300 MeV in the MIT bag model for strange quark matter. The of strangelets is high, on the order of 3×10173 \times 10^{17} to 101810^{18} kg/m³, which is 1 to 4 times the of ordinary (≈2.3×1017\approx 2.3 \times 10^{17} kg/m³), due to the equal filling of the Fermi seas by u, d, and s within the confined volume of the bag model. This enhanced reflects the deconfined structure, where the nB≈0.2n_B \approx 0.2–0.30.3 fm⁻³ is maintained, but the inclusion of the bag constant and masses contributes to a higher overall mass-energy compared to hadronic . Larger strangelets approach the of strange , which is only slightly above nuclear in equilibrated conditions. The low charge of small strangelets has significant implications for their detectability, as it minimizes loss in matter, allowing these particles to penetrate large distances with reduced interaction signatures and rendering them relatively "stealthy" in experimental searches. While bulk strange quark matter is electrically neutral through balanced chemical potentials, finite-size effects in strangelets lead to charge variations, with neutrality possible in the interior but positive charge accumulating at the surface due to s-quark depletion.

Production and Occurrence

In Particle Accelerators

The (RHIC) at began operations in 2000, conducting heavy-ion collisions such as gold-gold (Au+Au) at a center-of-mass energy per pair of sNN=200\sqrt{s_{NN}} = 200
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