A hypernucleus is similar to a conventional atomic nucleus, but contains at least one hyperon in addition to the normal protons and neutrons. Hyperons are a category of baryon particles that carry non-zero strangeness quantum number, which is conserved by the strong and electromagnetic interactions.
A variety of reactions give access to depositing one or more units of strangeness in a nucleus. Hypernuclei containing the lightest hyperon, the lambda (Λ), tend to be more tightly bound than normal nuclei, though they can decay via the weak force with a mean lifetime of around 200 ps . Sigma (Σ) hypernuclei have been sought, as have doubly-strange nuclei containing xi baryons (Ξ) or two Λ's.
Hypernuclei are named in terms of their atomic number and baryon number, as in normal nuclei, plus the hyperon(s) which are listed in a left subscript of the symbol, with the caveat that atomic number is interpreted as the total charge of the hypernucleus, including charged hyperons such as the xi minus (Ξ−) as well as protons. For example, the hypernucleus 16
Λ O
contains 8 protons, 7 neutrons, and one Λ (which carries no charge). [1]
The first was discovered by Marian Danysz and Jerzy Pniewski in 1952 using a nuclear emulsion plate exposed to cosmic rays, based on their energetic but delayed decay. This event was inferred to be due to a nuclear fragment containing a Λ baryon. [2] Experiments until the 1970s would continue to study hypernuclei produced in emulsions using cosmic rays, and later using pion (π) and kaon (K) beams from particle accelerators. [1]
Since the 1980s, more efficient production methods using pion and kaon beams have allowed further investigation at various accelerator facilities, including CERN, Brookhaven National Laboratory, KEK, DAφNE, and JPARC. [3] [4] In the 2010s, heavy ion experiments such as ALICE and STAR first allowed the production and measurement of light hypernuclei formed through hadronization from quark–gluon plasma. [5]
Hypernuclear physics differs from that of normal nuclei because a hyperon is distinguishable from the four nucleon spin and isospin. That is, a single hyperon is not restricted by the Pauli exclusion principle, and can sink to the lowest energy level. [6] As such, hypernuclei are often smaller and more tightly bound than normal nuclei; [7] for example, the lithium hypernucleus 7
ΛLi
is 19% smaller than the normal nucleus 6Li. [8] [9] However, the hyperons can decay via the weak force; the mean lifetime of a free Λ is 263±2 ps , and that of a Λ hypernucleus is usually slightly shorter. [10]
A generalized mass formula developed for both the non-strange normal nuclei and strange hypernuclei can estimate masses of hypernuclei containing Λ, ΛΛ, Σ, and Ξ hyperon(s). [11] [12] The neutron and proton driplines for hypernuclei are predicted and existence of some exotic hypernuclei beyond the normal neutron and proton driplines are suggested. [7] This generalized mass formula was named the "Samanta formula" by Botvina and Pochodzalla and used to predict relative yields of hypernuclei in heavy-ion collisions. [13]
The simplest, and most well understood, type of hypernucleus includes only the lightest hyperon, the Λ. [6]
While two nucleons can interact through the nuclear force mediated by a virtual pion, the Λ becomes a Σ baryon upon emitting a pion, [lower-alpha 1] so the Λ–nucleon interaction is mediated solely by more massive mesons such as the η and ω mesons, or through the simultaneous exchange of two or more mesons. [15] This means that the Λ–nucleon interaction is weaker and has a shorter range than the standard nuclear force, and the potential well of a Λ in the nucleus is shallower than that of a nucleon; [16] in hypernuclei, the depth of the Λ potential is approximately 30 MeV. [17] However, one-pion exchange in the Λ–nucleon interaction does cause quantum-mechanical mixing of the Λ and Σ baryons in hypernuclei (which does not happen in free space), especially in neutron-rich hypernuclei. [18] [19] [20] Additionally, the three-body force between a Λ and two nucleons is expected to be more important than the three-body interaction in nuclei, since the Λ can exchange two pions with a virtual Σ intermediate, while the equivalent process in nucleons requires a relatively heavy delta baryon (Δ) intermediate. [15]
Like all hyperons, Λ hypernuclei can decay through the weak interaction, which changes it to a lighter baryon and emits a meson or a lepton–antilepton pair. In free space, the Λ usually decays via the weak force to a proton and a π– meson, or a neutron and a π0, with a total half-life of 263±2 ps . [21] A nucleon in the hypernucleus can cause the Λ to decay via the weak force without emitting a pion; this process becomes dominant in heavy hypernuclei, due to suppression of the pion-emitting decay mode. [22] The half-life of the Λ in a hypernucleus is considerably shorter, plateauing to about 215±14 ps near 56
Λ Fe
, [23] but some empirical measurements substantially disagree with each other or with theoretical predictions. [24]
The simplest hypernucleus is the hypertriton (3
Λ H
), which consists of one proton, one neutron, and one Λ hyperon. The Λ in this system is very loosely bound, having a separation energy of 130 keV and a large radius of 10.6 fm, [25] compared to about 2.13 fm for the deuteron. [26]
This loose binding would imply a lifetime similar to a free Λ. However, the measured hypertriton lifetime averaged across all experiments (about 206+15
−13 ps) is substantially shorter than predicted by theory, as the non-mesonic decay mode is expected to be relatively minor; some experimental results are substantially shorter or longer than this average. [27] [28]
The existence of hypernuclei containing a Σ baryon is less clear. Several experiments in the early 1980s reported bound hypernuclear states above the Λ separation energy and presumed to contain one of the slightly heavier Σ baryons, but experiments later in the decade ruled out the existence of such states. [6] Results from exotic atoms containing a Σ− bound to a nucleus by the electromagnetic force have found a net repulsive Σ–nucleon interaction in medium-sized and large hypernuclei, which means that no Σ hypernuclei exist in such mass range. [6] However, an experiment in 1998 definitively observed the light Σ hypernucleus 4
Σ He
. [6]
Hypernuclei containing two Λ baryons have been made. However, such hypernuclei are much harder to produce due to containing two strange quarks, and As of 2016 [update] , only seven candidate ΛΛ hypernuclei have been observed. [29] Like the Λ–nucleon interaction, empirical and theoretical models predict that the Λ–Λ interaction is mildly attractive. [30] [31]
Hypernuclei containing a Ξ baryon are known.[ citation needed ] Empirical studies and theoretical models indicate that the Ξ––proton interaction is attractive, but weaker than the Λ–nucleon interaction. [30] Like the Σ– and other negatively charged particles, the Ξ– can also form an exotic atom. When a Ξ– is bound in an exotic atom or a hypernucleus, it quickly decays to a ΛΛ hypernucleus or to two Λ hypernuclei by exchanging a strange quark with a proton, which releases about 29 MeV of energy in free space: [lower-alpha 2]
Hypernuclei containing the omega baryon (Ω) were predicted using lattice QCD in 2018; in particular, the proton–Ω and Ω–Ω dibaryons (bound systems containing two baryons) are expected to be stable. [35] [36] As of 2022 [update] , no such hypernuclei have been observed under any conditions, but the lightest such species could be produced in heavy-ion collisions, [37] and measurements by the STAR experiment are consistent with the existence of the proton–Ω dibaryon. [38]
Since the Λ is electrically neutral and its nuclear force interactions are attractive, there are predicted to be arbitrarily large hypernuclei with high strangeness and small net charge, including species with no nucleons. Binding energy per baryon in multi-strange hypernuclei can reach up to 21 MeV/A under certain conditions, [7] compared to 8.80 MeV/A for the ordinary nucleus 62Ni. [39] Additionally, formation of Ξ baryons should quickly become energetically favorable, unlike when there are no Λ's, because the exchange of strangeness with a nucleon would be impossible due to the Pauli exclusion principle. [40]
Several modes of production have been devised to make hypernuclei through bombardment of normal nuclei.
One method of producing a K− meson exchanges a strange quark with a nucleon and changes it to a Λ: [41]
The cross section for the formation of a hypernucleus is maximized when the momentum of the kaon beam is approximately 500 MeV/c. [42] Several variants of this setup exist, including ones where the incident kaons are either brought to rest before colliding with a nucleus. [41]
In rare cases, the incoming K− can instead produce a Ξ hypernucleus via the reaction:
The equivalent strangeness production reaction involves a π+ meson reacts with a neutron to change it to a Λ: [44]
This reaction has a maximum cross section at a beam momentum of 1.05 GeV/c, and is the most efficient production route for Λ hypernuclei, but requires larger targets than strangeness exchange methods. [44]
Electron scattering off of a proton can change it to a Λ and produce a K+: [45]
where the prime symbol denotes a scattered electron. The energy of an electron beam can be more easily tuned than pion or kaon beams, making it easier to measure and calibrate hypernuclear energy levels. [45] Initially theoretically predicted in the 1980s, this method was first used experimentally in the early 2000s. [46]
The capture of a Ξ− baryon by a nucleus can make a Ξ− exotic atom or hypernucleus. [33] Upon capture, it changes to a ΛΛ hypernucleus or two Λ hypernuclei. [47] The disadvantage is that the Ξ− baryon is harder to make into a beam than singly strange hadrons. [48] However, an experiment at J-PARC begun in 2020 will compile data on Ξ and ΛΛ hypernuclei using a similar, non-beam setup where scattered Ξ− baryons rain onto an emulsion target. [33]
This section is empty. You can help by adding to it. (December 2022) |
The K– meson can orbit a nucleus in an exotic atom, such as in kaonic hydrogen. [49] Although the K–-proton strong interaction in kaonic hydrogen is repulsive, [50] the K––nucleus interaction is attractive for larger systems, so this meson can enter a strongly bound state closely related to a hypernucleus; [6] in particular, the K––proton–proton system is experimentally known and more tightly bound than a normal nucleus. [51]
Nuclei containing a charm quark have been predicted theoretically since 1977, [52] and are described as charmed hypernuclei despite the possible absence of strange quarks. [53] In particular, the lightest charmed baryons, the Λc and Σc baryons, [lower-alpha 3] are predicted to exist in bound states in charmed hypernuclei, and could be created in processes analogous to those used to make hypernuclei. [53] The depth of the Λc potential in nuclear matter is predicted to be 58 MeV, [53] but unlike Λ hypernuclei, larger hypernuclei containing the positively charged Λc would be less stable than the corresponding Λ hypernuclei due to Coulomb repulsion. [54] The mass difference between the Λc and the
Σ+
c is too large for appreciable mixing of these baryons to occur in hypernuclei. [55] Weak decays of charmed hypernuclei have strong relativistic corrections compared to those in ordinary hypernuclei, as the energy released in the decay process is comparable to the mass of the Λ baryon. [56]
In particle physics, a baryon is a type of composite subatomic particle, including the proton and the neutron, that contains an odd number of valence quarks, conventionally three. Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classified as fermions because they have half-integer spin.
In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules, which are held together by the electric force. Most of the mass of ordinary matter comes from two hadrons: the proton and the neutron, while most of the mass of the protons and neutrons is in turn due to the binding energy of their constituent quarks, due to the strong force.
In particle physics, proton decay is a hypothetical form of particle decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron. The proton decay hypothesis was first formulated by Andrei Sakharov in 1967. Despite significant experimental effort, proton decay has never been observed. If it does decay via a positron, the proton's half-life is constrained to be at least 1.67×1034 years.
The omega baryons are a family of subatomic hadron particles that are represented by the symbol
Ω
and are either neutral or have a +2, +1 or −1 elementary charge. They are baryons containing no up or down quarks. Omega baryons containing top quarks are not expected to be observed. This is because the Standard Model predicts the mean lifetime of top quarks to be roughly 5×10−25 s, which is about a twentieth of the timescale for strong interactions, and therefore that they do not form hadrons.
The charm quark, charmed quark, or c quark is an elementary particle found in composite subatomic particles called hadrons such as the J/psi meson and the charmed baryons created in particle accelerator collisions. Several bosons, including the W and Z bosons and the Higgs boson, can decay into charm quarks. All charm quarks carry charm, a quantum number. This second generation particle is the third-most-massive quark with a mass of 1.27±0.02 GeV/c2 as measured in 2022 and a charge of +2/3 e.
A pentaquark is a human-made subatomic particle, consisting of four quarks and one antiquark bound together; they are not known to occur naturally, or exist outside of experiments specifically carried out to create them.
In particle physics, a kaon, also called a K meson and denoted
K
, is any of a group of four mesons distinguished by a quantum number called strangeness. In the quark model they are understood to be bound states of a strange quark and an up or down antiquark.
In particle physics, the hyperchargeY of a particle is a quantum number conserved under the strong interaction. The concept of hypercharge provides a single charge operator that accounts for properties of isospin, electric charge, and flavour. The hypercharge is useful to classify hadrons; the similarly named weak hypercharge has an analogous role in the electroweak interaction.
In nuclear physics and particle physics, isospin (I) is a quantum number related to the up- and down quark content of the particle. Isospin is also known as isobaric spin or isotopic spin. Isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions of baryons and mesons.
In particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm, bottom, or top quark. This form of matter may exist in a stable form within the core of some neutron stars. Hyperons are sometimes generically represented by the symbol Y.
In physics, the eightfold way is an organizational scheme for a class of subatomic particles known as hadrons that led to the development of the quark model. Both the American physicist Murray Gell-Mann and the Israeli physicist Yuval Ne'eman independently and simultaneously proposed the idea in 1961. The name comes from Gell-Mann's (1961) paper and is an allusion to the Noble Eightfold Path of Buddhism.
In high-energy physics, a pseudoscalar meson is a meson with total spin 0 and odd parity . Pseudoscalar mesons are commonly seen in proton-proton scattering and proton-antiproton annihilation, and include the pion, kaon, eta, and eta prime particles, whose masses are known with great precision.
Charmed baryons are a category of composite particles comprising all baryons made of at least one charm quark. Since their first observation in the 1970s, a large number of distinct charmed baryon states have been identified. Observed charmed baryons have masses ranging between 2300 and 2700 MeV/c2. In 2002, the SELEX collaboration, based at Fermilab published evidence of a doubly charmed baryon, containing two charm quarks) with a mass of ~3520 MeV/c2, but has yet to be confirmed by other experiments. One triply charmed baryon has been predicted but not yet observed.
In particle physics, chiral symmetry breaking generally refers to the dynamical spontaneous breaking of a chiral symmetry associated with massless fermions. This is usually associated with a gauge theory such as quantum chromodynamics, the quantum field theory of the strong interaction, and it also occurs through the Brout-Englert-Higgs mechanism in the electroweak interactions of the standard model. This phenomenon is analogous to magnetization and superconductivity in condensed matter physics. The basic idea was introduced to particle physics by Yoichiro Nambu, in particular, in the Nambu–Jona-Lasinio model, which is a solvable theory of composite bosons that exhibits dynamical spontaneous chiral symmetry when a 4-fermion coupling constant becomes sufficiently large. Nambu was awarded the 2008 Nobel prize in physics "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics".
The Xi baryons or cascade particles are a family of subatomic hadron particles which have the symbol Ξ and may have an electric charge of +2 e, +1 e, 0, or −1 e, where e is the elementary charge.
The lambda baryons (Λ) are a family of subatomic hadron particles containing one up quark, one down quark, and a third quark from a higher flavour generation, in a combination where the quantum wave function changes sign upon the flavour of any two quarks being swapped. They are thus baryons, with total isospin of 0, and have either neutral electric charge or the elementary charge +1.
The sigma baryons are a family of subatomic hadron particles which have two quarks from the first flavour generation, and a third quark from a higher flavour generation, in a combination where the wavefunction sign remains constant when any two quark flavours are swapped. They are thus baryons, with total isospin of 1, and can either be neutral or have an elementary charge of +2, +1, 0, or −1. They are closely related to the lambda baryons, which differ only in the wavefunction's behaviour upon flavour exchange.
In high-energy nuclear physics, strangeness production in relativistic heavy-ion collisions is a signature and diagnostic tool of quark–gluon plasma (QGP) formation and properties. Unlike up and down quarks, from which everyday matter is made, heavier quark flavors such as strange and charm typically approach chemical equilibrium in a dynamic evolution process. QGP is an interacting localized assembly of quarks and gluons at thermal (kinetic) and not necessarily chemical (abundance) equilibrium. The word plasma signals that color charged particles are able to move in the volume occupied by the plasma. The abundance of strange quarks is formed in pair-production processes in collisions between constituents of the plasma, creating the chemical abundance equilibrium. The dominant mechanism of production involves gluons only present when matter has become a quark–gluon plasma. When quark–gluon plasma disassembles into hadrons in a breakup process, the high availability of strange antiquarks helps to produce antimatter containing multiple strange quarks, which is otherwise rarely made. Similar considerations are at present made for the heavier charm flavor, which is made at the beginning of the collision process in the first interactions and is only abundant in the high-energy environments of CERN's Large Hadron Collider.
{{cite journal}}
: CS1 maint: multiple names: authors list (link)