- #1
- 6,735
- 2,457
I thought pentaquarks were already considered discovered?
https://phys.org/news/2022-07-lhcb-exotic-particles-pentaquark-first-ever.html
https://phys.org/news/2022-07-lhcb-exotic-particles-pentaquark-first-ever.html
I guess, the answer lies in the actual paper, not the pop-science article.drmalawi said:I thought pentaquarks were already considered discovered?
https://phys.org/news/2022-07-lhcb-exotic-particles-pentaquark-first-ever.html
The international LHCb collaboration at the Large Hadron Collider (LHC) has observed three never-before-seen particles: a new kind of pentaquark and the first-ever pair of tetraquarks, which includes a new type of tetraquark.
Wikipedia said:On July 13, 2015, researchers at the LHCb detector of CERN's Large Hadron Collider in Geneva reported the discovery of two pentaquark charmonium states (pentaquarks involving charm and anti-charm quarks) in the lambda-b baryon decay ##{\displaystyle \Lambda_{b}^{0}\ }## into the kaon ##{\displaystyle \mathrm{K}^{-}}## and the pentaquark ##(uudcc).##
… as usual …fresh_42 said:the answer lies in the actual paper, not the pop-science article
fresh_42 said:I guess, the answer lies in the actual paper, not the pop-science article.
Observation of a strange pentaquark, a doubly charged tetraquark and its neutral partner.
Corrected. Always watch what is in the buffer ... Why don't they invent a preview for buffer content?drmalawi said:Is it just my browswer, or did that just link to the phys.org article in my OP?
https://de.wikipedia.org/wiki/Pentaquark#Die_Entdeckungdrmalawi said:What wiki article?
Because what would be the point?ohwilleke said:I've never seen a paper that has done that comprehensively
There are tables of isotopes for atomic nuclei. I have one as part of a big bound scientific reference book in my living room. I've referred to it quite a few times.Vanadium 50 said:Because what would be the point?
When I lived in New Zealand, I went to Rutherford High School.Vanadium 50 said:
Note that they constantly say "molecule-like" - not "nucleus-like".While some theoretical models describe exotic hadrons as single units of tightly bound quarks, other models envisage them as pairs of standard hadrons loosely bound in a molecule-like structure. Only time and more studies of exotic hadrons will tell if these particles are one, the other or both.
This is something of a deceptive wording convention in particle physics and a false friend.snorkack said:Note that they constantly say "molecule-like" - not "nucleus-like".
Note that exotic elementary particles can easily form atoms and/or molecules bound to each other solely by electromagnetic forces with minimal weak force and no strong force component. Starting with muonic atoms (no strong force option), but also protonium, pionium... (would be bound by electromagnetic force, but strong force can participate). Exotic quarks are no disqualification for participation in these types of systems!ohwilleke said:This is something of a deceptive wording convention in particle physics and a false friend.
A "molecule-like" resonances include residual strong force bound composite systems, which while not hadrons that are bound directly by the strong force like protons and neutrons and pions and kaons, are more analogous to atomic nuclei bound by the residual strong force mediated by mesons, than to ordinary chemical molecules in which atoms are bound to each other solely by electromagnetic forces, without a weak force or strong force component.
Are defined as quarks from the second and third generation? Strange, charm, bottom?snorkack said:Exotic quarks
Could be. Though of these, strange is the lowest and most common, so maybe it is more useful to call just charm and beauty "exotic"? For example, for strange quarks, the whole baryon decuplet and octet is observed experimentally, including Ω- - but no baryon of charm 3 or beauty under -1 has ever been detected. On the other hand, when possible strong force or electromagnetic force bound systems are considered, hypernuclei are seen but seem far from having any complete systematic observability... so in that context strange is already exotic.drmalawi said:Are defined as quarks from the second and third generation? Strange, charm, bottom?
I mean, the nomenclature "exotic quark" - is unknown to me - "exotic hadrons" is at least, to my understanding - a more well defined concept. The only place where I can recall that "exotic quarks" have been used is within the context of 4th generation models.snorkack said:Could be.
This is not what the term "molecule" means in the context of systems of four or more quarks that have sub-system structure. As I explained above, while this expectation is a natural one for the what the term should mean, a hadronic molecule uses the term molecule in a manner that is a false friend.snorkack said:"Molecule" implies a bound system which, for the first, is bound largely by electrostatic force - but so are atoms - and which, in contrast to atoms, contains at least three bound particles of which at least two are "heavy".
So, all atomic nuclei are also hadron molecules, even though they, like many or all meson molecules, they are bound by the "residual strong force" rather than directly by gluonic interactions with each other that way that quarks and gluons within a simple hadron are bound.A mesonic molecule is a set of two or more mesons bound together by the strong force.[1][2] Unlike baryonic molecules, which form the nuclei of all elements in nature save hydrogen-1, a mesonic molecule has yet to be definitively observed.[3] The X(3872) discovered in 2003 and the Z(4430) discovered in 2007 by the Belle experiment are the best candidates for such an observation.
While one could argue that pionium should count as a meson molecule, a 2011 paper on the subject states that it is an exotic atom :snorkack said:Does any wording of the new nomenclature exclude pionium from being a tetraquark?
In particular, pionium is a subset of exotic atoms known as hadronic atoms (in which the substitute particles are hadrons).Pionium (A2π) is the π +π − hydrogen-like atom, with 378 f m Bohr radius, which decays predominantly into π0π0 . The alternative γγ [i.e. diphoton] decay accounts for only ∼ 0.4% of the total rate.
The preference for describing pionium as an atom rather than a molecule reflects my observation in Post #13 in this thread that:An onium (plural: onia) is the bound state of a particle and its antiparticle. The classic onium is positronium, which consists of an electron and a positron bound together as a metastable state, with a relatively long lifetime of 142 ns in the triplet state. Positronium has been studied since the 1950s to understand bound states in quantum field theory. A recent development called non-relativistic quantum electrodynamics (NRQED) used this system as a proving ground.
Pionium, a bound state of two oppositely-charged pions, is useful for exploring the strong interaction. This should also be true of protonium, which is a proton–antiproton bound state. Understanding bound states of pionium and protonium is important in order to clarify notions related to exotic hadrons such as mesonic molecules and pentaquark states. Kaonium, which is a bound state of two oppositely charged kaons, has not been observed experimentally yet.
The true analogs of positronium in the theory of strong interactions, however, are not exotic atoms but certain mesons, the quarkonium states, which are made of a heavy quark such as the charm or bottom quark and its antiquark. (Top quarks are so heavy that they decay through the weak force before they can form bound states.) Exploration of these states through non-relativistic quantum chromodynamics (NRQCD) and lattice QCD are increasingly important tests of quantum chromodynamics.
Muonium, despite its name, is not an onium containing a muon and an antimuon, because IUPAC assigned that name to the system of an antimuon bound with an electron. However, the production of a muon–antimuon bound state, which is an onium (called true muonium), has been theorized.
But the distinction between a 2011 paper's use of the term "atom" and the frequent use of the term meson molecule in some more recent papers, may also reflect, in part, an evolution in the terminology in common use between 2011, when meson molecules were largely hypothetical while exotic atoms like muonium and muonic hydrogen were well known were more familiar, and the present, when there are numerous tetraquarks and pentaquarks that have been observed, some of which are meson molecule candidates.The term "molecule-like" really primarily reflects the visual appearance of a diagram of its structure, rather than reflecting the exact nature of that structure.
See the problem? There already is term "nucleus", and now the term "molecule" is stretched to include nuclei.ohwilleke said:One common definition of the term mesonic molecule is that:
So, all atomic nuclei are also hadron molecules,
Muonium also has only two charged particles. A separate reason to call it "atom" not molecule. "Leptons only" is a criterion that can get silly... what would you call a bound system of two positive tauons and two electrons? Positive tauon looks like simply a heavy isotope of hydrogen.ohwilleke said:In contrast, the term "molecule" is not applied to a purely electromagnetically bound system made up purely of leptons, such as muonium which is commonly described as kind of exotic atom (i.e. "an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge.
Which is to be deplored. For systems of two or more (so far in practice more than two, but I can offer theoretical speculations to hope for two) baryons, there is a perfectly established term "hypernucleus". Wouldn´t it be logical to likewise term "mesonucleus" - a nucleus where some or all constituent hadrons are mesons?ohwilleke said:But the distinction between a 2011 paper's use of the term "atom" and the frequent use of the term meson molecule in some more recent papers, may also reflect, in part, an evolution in the terminology in common use between 2011, when meson molecules were largely hypothetical while exotic atoms like muonium and muonic hydrogen were well known were more familiar, and the present, when there are numerous tetraquarks and pentaquarks that have been observed, some of which are meson molecule candidates.
ohwilleke said:The proposed new naming rules for hadrons (mostly tetraquarks and pentaquarks) from the LHCb aren't particular clear about how one distinguishes a tetraquark or pentaquark from a hadronic molecule on their face. One needs to either look in the interpretative fine print of the full paper, or resort to rules about what counts as a hadron that predate the new naming rules.
My understanding has been that a true (simple?) hadron is a composite particle directly by gluonic interactions with each other that way that quarks and gluons, while a hadron molecule is one that is bound instead by the "residual strong force", in which case pionium is not a tetraquark even if it is a meson molecule, since the terms tetraquark and pentaquark are usually reserved only for true simple hadrons as distinct from hadron molecules. But, I can't at this time find an authority to cite or quote that is squarely on point regarding this distinction.
It seems clearly implied that the proposed rules propose to not speculate whether a tetraquark is a "mesonic molecule"/"mesonucleus" or a "bag tetraquark". By implication, it also would include states clearly bound mainly by electromagnetic force... which would require renaming pionium into T something. It would be reasonable to exclude mainly electromagnetically bound exotic atoms while including strong force bound mesonic molecules, but I cannot see language purporting to do so - only excluding "speculation about the degrees of freedom within the hadron"1. As in the current scheme, symbols are assigned based on measured quantum numbers, rather than speculation about the degrees of freedom within the hadron.
Very late to this, @fresh_42, off topic vis-à-vis the OP, and not sure whether you were joshing, but Windows has a buffer preview, it's the Windows-V key, though you do need to enable the clipboard buffer in Settings before it is useful.fresh_42 said:Always watch what is in the buffer ... Why don't they invent a preview for buffer content?
Terminology is a largely descriptive rather than prescriptive matter. Regardless of what would make more sense in some logical way, what matters is understanding others are saying in their papers.snorkack said:See the problem? There already is term "nucleus", and now the term "molecule" is stretched to include nuclei.
Muonium also has only two charged particles. A separate reason to call it "atom" not molecule. "Leptons only" is a criterion that can get silly... what would you call a bound system of two positive tauons and two electrons? Positive tauon looks like simply a heavy isotope of hydrogen.
Which is to be deplored. For systems of two or more (so far in practice more than two, but I can offer theoretical speculations to hope for two) baryons, there is a perfectly established term "hypernucleus". Wouldn´t it be logical to likewise term "mesonucleus" - a nucleus where some or all constituent hadrons are mesons?It seems clearly implied that the proposed rules propose to not speculate whether a tetraquark is a "mesonic molecule"/"mesonucleus" or a "bag tetraquark". By implication, it also would include states clearly bound mainly by electromagnetic force... which would require renaming pionium into T something. It would be reasonable to exclude mainly electromagnetically bound exotic atoms while including strong force bound mesonic molecules, but I cannot see language purporting to do so - only excluding "speculation about the degrees of freedom within the hadron"
LHCb stands for Large Hadron Collider beauty experiment. It is one of the four main experiments at the Large Hadron Collider (LHC) located at CERN. LHCb's main focus is studying the properties of particles containing the beauty quark, also known as the bottom quark.
Exotic particles are particles that do not fit into the standard model of particle physics. They have properties that are not predicted by the standard model and are not made up of the usual combinations of quarks and leptons.
The LHCb experiment uses a large particle accelerator to collide protons at high speeds. These collisions produce a large number of particles, some of which are exotic particles. The LHCb detector then records and analyzes the particles produced in the collisions, allowing scientists to identify and study new particles.
The three new exotic particles are named X(6900), X(6970), and X(7000). The numbers in parentheses represent the mass of the particles in megaelectronvolts (MeV).
The discovery of these new exotic particles provides further evidence for the existence of particles beyond the standard model. It also opens up new possibilities for studying the properties and behavior of these particles, which can help us better understand the fundamental laws of the universe.