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{{short description|Degenerate matter made from strange quarks}}
'''Strange matter''' (also known as '''quark matter''') is an ultra-dense ] that is theorized to form inside particularly massive ]s. It is theorized that when the ] which makes up a neutron star is put under sufficient pressure due to the star's ], the individual ]s break down and their constituent ]s form strange matter. The star then becomes known as a "]" or "quark star". Strange matter is composed of ]s bound to each other directly, in a similar manner to how neutronium is composed of neutrons; a strange star is essentially a single gigantic ]. A strange star lies between neutron stars and ]s in terms of both mass and density, and if sufficient additional matter is added to a strange star it will collapse into a black hole as well.
{{Refimprove|date=October 2017}}


'''Strange matter''' (or '''strange quark matter''') is ] containing ]s. In extreme environments, strange matter is hypothesized to occur in the core of ]s, or, more speculatively, as isolated droplets that may vary in size from ]s (]s) to kilometers, as in the hypothetical ]. At high enough density, strange matter is expected to be ].{{citation needed|date=February 2019}}
Some theories suggest that strange matter, unlike neutronium, may be stable outside of the intense pressure that produced it; if this is so, then small substellar pieces of strange stars (sometimes called "strangelets") may exist in space in a wide range of sizes all the way down to atomic scales. There is some concern that ordinary matter, upon contacting a strangelet, would be compressed into additional strange matter by its gravity; strangelets would therefore be able to "eat" any ordinary matter it came into contact with, such as planets or stars. Mainstream physicists, while acknowledging this possibility, tend to downplay its probability.


Ordinary ], also referred to as atomic matter, is composed of atoms, with nearly all matter concentrated in the atomic nuclei. ] is a liquid composed of ]s and ]s, and they are themselves composed of ] and ]. Quark matter is a ] composed entirely of ]s. When quark matter does not contain strange quarks, it is sometimes referred to as non-strange quark matter.
Strangelets are thought to have a net positive charge, which is neutralized by the presence of ] ]s extending slightly beyond the edge of the strangelet, a kind of electron "atmosphere." If a normal matter ] encounters a strangelet, it will approach until it begins penetrating this negatively charged atmosphere. At that point it will start to see the positive electrical potential and be repelled from the strangelet. Sufficiently energetic nuclei, or neutrons (which are unaffected by electrical charges), can reach the strangelet and be absorbed; the up/down/strange quark ratio would then readjust by ].


==Context==
Strange matter is one candidate for the hypothetical ] that is a feature of several ] theories.
In ] and ], the term 'strange matter' is used in two different contexts, one broader and the other more specific and hypothetical:<ref name="Madsen:1998">{{Cite book |last1=Madsen |first1=Jes |title=Hadrons in Dense Matter and Hadrosynthesis |year=1999 |isbn=978-3-540-65209-0 |series=Lecture Notes in Physics |volume=516 |pages=162–203 |chapter=Physics and astrophysics of strange quark matter |doi=10.1007/BFb0107314 |arxiv=astro-ph/9809032 |s2cid=16566509 }}</ref><ref name="Weber">{{Cite journal |last1=Weber |first1=F. |year=2005 |title=Strange quark matter and compact stars |journal=Progress in Particle and Nuclear Physics |volume=54 |issue=1 |pages=193–288 |arxiv=astro-ph/0407155 |bibcode=2005PrPNP..54..193W |doi=10.1016/j.ppnp.2004.07.001 |s2cid=15002134 }}.</ref>


# In the broader context, our current understanding of the ] predicts that strange matter could be created when nuclear matter (made of ] and ]) is compressed beyond a critical density. At this critical pressure and density, the protons and neutrons dissociate into quarks, yielding quark matter and potentially strange matter.
Strange matter is largely theoretical at this point, but observations by the ] in ] detected two candidate strange stars, designated ] and ], which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than they should, suggesting that they are composed of material denser than neutronium. However, these observations have been under attack by researchers who say the results were not conclusive; it remains to be seen how the question of strange star existence will play out.
# A more specific hypothesis is that quark matter is the true ground state of all matter, and thus more stable than ordinary nuclear matter. This idea is known as the "strange matter hypothesis", or the ]–] assumption.<ref name="Bodmer">{{Cite journal |last1=Bodmer |first1=A. R. |date=September 1971 |title=Collapsed Nuclei |url=https://ui.adsabs.harvard.edu/abs/1971PhRvD...4.1601B/abstract |journal=Physical Review D |volume=4 |issue=6 |pages=1601–1606 |bibcode=1971PhRvD...4.1601B |doi=10.1103/PhysRevD.4.1601 |access-date=2022-03-22 |archive-date=2022-01-20 |archive-url=https://web.archive.org/web/20220120042524/https://ui.adsabs.harvard.edu/abs/1971PhRvD...4.1601B/abstract |url-status=live }}</ref><ref name="Witten">{{Cite journal |last=Witten |first=Edward |author-link=Edward Witten |date=July 1984 |title=Cosmic separation of phases |url=https://ui.adsabs.harvard.edu/abs/1984PhRvD..30..272W |journal=Physical Review D |volume=30 |issue=2 |pages=272–285 |bibcode=1984PhRvD..30..272W |doi=10.1103/PhysRevD.30.272 |access-date=2022-03-22 |archive-date=2022-01-25 |archive-url=https://web.archive.org/web/20220125070503/https://ui.adsabs.harvard.edu/abs/1984PhRvD..30..272W |url-status=live }}</ref> Under this hypothesis, the nuclei of the atoms we see around us are only ], even when the external critical pressure is zero, and given enough time (or the right stimulus) the nuclei would decay into stable droplets of strange matter. Droplets of strange matter are also referred to as strangelets.


== Stability of strange matter only at high pressure ==
There has also been some evidence that quark matter may have been produced in ]s at ] in ].


In the general context, strange matter might occur inside neutron stars, if the pressure at their core is high enough to provide a sufficient gravitational force (i.e. above the critical pressure). At the sort of densities and high pressures we expect in the center of a neutron star, the quark matter would probably be strange matter. It could conceivably be non-strange quark matter, if the effective mass of the strange quark were too high. ] quarks and heavier quarks would only occur at much higher densities.
In May ], a group of researchers at the ] reported the possibility that strange matter may have been responsible for two unexplained seismic events recorded on ] and ] in ]; they proposed that two strangelets of unknown mass moving at roughly 400 km/s had passed through Earth, generating seismic shock waves along its path. The members of the group were Vidgor Teplitz, Eugene Herrin, David Anderson and Ileana Tibuleac. It has been suggested that the ] being set up to verify the ] (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.


Strange matter comes about as a way to relieve ]. The ] forbids fermions such as quarks from occupying the same position and energy level. When the particle density is high enough that all energy levels below the available thermal energy are already occupied, increasing the density further requires raising some to higher, unoccupied energy levels. This need for energy to cause compression manifests as a pressure. Neutrons consist of twice as many ]s (charge −{{sfrac|1|3}}&nbsp;]) as ]s (charge +{{sfrac|2|3}}&nbsp;''e''), so the degeneracy pressure of down quarks usually dominates electrically neutral quark matter. However, when the required energy level is high enough, an alternative becomes available: half of the down quarks can be transmuted to strange quarks (charge −{{sfrac|1|3}}&nbsp;''e''). The higher rest mass of the strange quark costs some energy, but by opening up an additional set of energy levels, the average energy per particle can be lower,{{r|Madsen:1998|p=5}} making strange matter more stable than non-strange quark matter.
== External links ==


A neutron star with a quark matter core is often{{r|Madsen:1998|Weber}} called a hybrid star. However, it is difficult to know whether hybrid stars really exist in nature because physicists currently have little idea of the likely value of the critical pressure or density. It seems plausible that the transition to quark matter will already have occurred when the separation between the ]s becomes much smaller than their size, so the critical density must be less than about 100 times nuclear saturation density. But a more precise estimate is not yet available, because the ] that governs the behavior of quarks is mathematically intractable, and numerical calculations using ] are currently blocked by the fermion ].
* http://www.wired.com/wired/archive/11.02/matter.html
* http://www.msfc.nasa.gov/news/news/releases/2002/02-082.html
* http://user.web.cern.ch/Chronological/Announcements/CERNAnnouncements/2000/NewStateMatter/Story.html
* http://www.space.com/scienceastronomy/astronomy/new_matter_020410.html
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One major area of activity in neutron star physics is the attempt to find observable signatures by which we could tell whether neutron stars have quark matter (probably strange matter) in their core. <!-- See ] -->
⚫ ]

During the merger of two neutron stars, strange matter may be ejected out into the space around the stars, which may allow for the studying of strange matter. However, the rate at which strange matter decays is unknown, and there are very few binary pairs of neutron stars nearby to the Solar System, which could make the official discovery of strange matter very difficult.

==Stability of strange matter at zero pressure==

If the "]" is true, then nuclear matter is metastable against decaying into strange matter. The lifetime for spontaneous decay is very long, so we do not see this decay process happening around us.<ref name='Witten' /> However, under this hypothesis there should be strange matter in the universe:
# Quark stars (often called "strange stars") consist of quark matter from their core to their surface. They would be several kilometers across, and may have a very thin crust of nuclear matter.<ref name='Weber' />
# ] are small pieces of strange matter, perhaps as small as nuclei. They would be produced when strange stars are formed or collide, or when a nucleus decays.<ref name='Madsen:1998' />

==See also==
* {{annotated link|Exotic matter}}
* {{annotated link|Quark star}}
* ] &ndash; subatomic signature
* {{annotated link|Strangelet}}
* {{annotated link|Quark}}
* {{annotated link|QCD matter}}

==References==
{{reflist}}

{{Phase of matter}}
{{Stellar core collapse}}

⚫ ]
]
]
]
]

Latest revision as of 13:28, 26 December 2023

Degenerate matter made from strange quarks
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Strange matter (or strange quark matter) is quark matter containing strange quarks. In extreme environments, strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers (strangelets) to kilometers, as in the hypothetical strange stars. At high enough density, strange matter is expected to be color superconducting.

Ordinary matter, also referred to as atomic matter, is composed of atoms, with nearly all matter concentrated in the atomic nuclei. Nuclear matter is a liquid composed of neutrons and protons, and they are themselves composed of up and down quarks. Quark matter is a condensed form of matter composed entirely of quarks. When quark matter does not contain strange quarks, it is sometimes referred to as non-strange quark matter.

Context

In particle physics and astrophysics, the term 'strange matter' is used in two different contexts, one broader and the other more specific and hypothetical:

  1. In the broader context, our current understanding of the laws of nature predicts that strange matter could be created when nuclear matter (made of protons and neutrons) is compressed beyond a critical density. At this critical pressure and density, the protons and neutrons dissociate into quarks, yielding quark matter and potentially strange matter.
  2. A more specific hypothesis is that quark matter is the true ground state of all matter, and thus more stable than ordinary nuclear matter. This idea is known as the "strange matter hypothesis", or the Bodmer–Witten assumption. Under this hypothesis, the nuclei of the atoms we see around us are only metastable, even when the external critical pressure is zero, and given enough time (or the right stimulus) the nuclei would decay into stable droplets of strange matter. Droplets of strange matter are also referred to as strangelets.

Stability of strange matter only at high pressure

In the general context, strange matter might occur inside neutron stars, if the pressure at their core is high enough to provide a sufficient gravitational force (i.e. above the critical pressure). At the sort of densities and high pressures we expect in the center of a neutron star, the quark matter would probably be strange matter. It could conceivably be non-strange quark matter, if the effective mass of the strange quark were too high. Charm quarks and heavier quarks would only occur at much higher densities.

Strange matter comes about as a way to relieve degeneracy pressure. The Pauli exclusion principle forbids fermions such as quarks from occupying the same position and energy level. When the particle density is high enough that all energy levels below the available thermal energy are already occupied, increasing the density further requires raising some to higher, unoccupied energy levels. This need for energy to cause compression manifests as a pressure. Neutrons consist of twice as many down quarks (charge −⁠1/3⁠ e) as up quarks (charge +⁠2/3⁠ e), so the degeneracy pressure of down quarks usually dominates electrically neutral quark matter. However, when the required energy level is high enough, an alternative becomes available: half of the down quarks can be transmuted to strange quarks (charge −⁠1/3⁠ e). The higher rest mass of the strange quark costs some energy, but by opening up an additional set of energy levels, the average energy per particle can be lower, making strange matter more stable than non-strange quark matter.

A neutron star with a quark matter core is often called a hybrid star. However, it is difficult to know whether hybrid stars really exist in nature because physicists currently have little idea of the likely value of the critical pressure or density. It seems plausible that the transition to quark matter will already have occurred when the separation between the nucleons becomes much smaller than their size, so the critical density must be less than about 100 times nuclear saturation density. But a more precise estimate is not yet available, because the strong interaction that governs the behavior of quarks is mathematically intractable, and numerical calculations using lattice QCD are currently blocked by the fermion sign problem.

One major area of activity in neutron star physics is the attempt to find observable signatures by which we could tell whether neutron stars have quark matter (probably strange matter) in their core.

During the merger of two neutron stars, strange matter may be ejected out into the space around the stars, which may allow for the studying of strange matter. However, the rate at which strange matter decays is unknown, and there are very few binary pairs of neutron stars nearby to the Solar System, which could make the official discovery of strange matter very difficult.

Stability of strange matter at zero pressure

If the "strange matter hypothesis" is true, then nuclear matter is metastable against decaying into strange matter. The lifetime for spontaneous decay is very long, so we do not see this decay process happening around us. However, under this hypothesis there should be strange matter in the universe:

  1. Quark stars (often called "strange stars") consist of quark matter from their core to their surface. They would be several kilometers across, and may have a very thin crust of nuclear matter.
  2. Strangelets are small pieces of strange matter, perhaps as small as nuclei. They would be produced when strange stars are formed or collide, or when a nucleus decays.

See also

References

  1. ^ Madsen, Jes (1999). "Physics and astrophysics of strange quark matter". Hadrons in Dense Matter and Hadrosynthesis. Lecture Notes in Physics. Vol. 516. pp. 162–203. arXiv:astro-ph/9809032. doi:10.1007/BFb0107314. ISBN 978-3-540-65209-0. S2CID 16566509.
  2. ^ Weber, F. (2005). "Strange quark matter and compact stars". Progress in Particle and Nuclear Physics. 54 (1): 193–288. arXiv:astro-ph/0407155. Bibcode:2005PrPNP..54..193W. doi:10.1016/j.ppnp.2004.07.001. S2CID 15002134..
  3. Bodmer, A. R. (September 1971). "Collapsed Nuclei". Physical Review D. 4 (6): 1601–1606. Bibcode:1971PhRvD...4.1601B. doi:10.1103/PhysRevD.4.1601. Archived from the original on 2022-01-20. Retrieved 2022-03-22.
  4. ^ Witten, Edward (July 1984). "Cosmic separation of phases". Physical Review D. 30 (2): 272–285. Bibcode:1984PhRvD..30..272W. doi:10.1103/PhysRevD.30.272. Archived from the original on 2022-01-25. Retrieved 2022-03-22.
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