LHCb discovers three new exotic particles

[malawi_glenn]

I thought pentaquarks were already considered discovered?

https://phys.org/news/2022-07-lhcb-exotic-particles-pentaquark-first-ever.html

 

[fresh_42]

I thought pentaquarks were already considered discovered?

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.
Observation of a strange pentaquark, a doubly charged tetraquark and its neutral partner.

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.

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 $(uudcc).$

 

[Orodruin]

the answer lies in the actual paper, not the pop-science article

… as usual …

 

[malawi_glenn]

Fooled again I guess

 

[malawi_glenn]

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.

Is it just my browswer, or did that just link to the phys.org article in my OP?

What wiki article?

 

[fresh_42]

Is it just my browswer, or did that just link to the phys.org article in my OP?

Corrected. Always watch what is in the buffer ... Why don't they invent a preview for buffer content?

What wiki article?

https://de.wikipedia.org/wiki/Pentaquark#Die_Entdeckung

 

[ohwilleke]

For the entire LHC Run 1 and Run 2 there have been 63 new hadrons discovered so far (some expected with almost exactly the predicted properties and others that were less certain to be predicted, although none that are not permitted in the Standard Model).
https://www.physicsforums.com/threads/new-hadron-naming-rules.1016538/

It is possible to exactly count how many different kinds of particles are permitted in the Standard Model for the most part (so long as you set some boundary like maximum number of valence quarks or minimum mean lifetime or maximum mass or up to how many iterations of excitation will be considered), although the exact theory of scalar mesons and axial vector mesons remains not fully resolved after about forty years of seriously trying to explain them, so it is harder to know how many of them were are missing.

So, in principle, you can evaluate a program of searching for new hadrons on a "percentage complete" basis. I've never seen a paper that has done that comprehensively can counted all possibilities and checked them against what we have (except for ground state pseudoscalar and vector mesons and for ground state spin 1/2 and spin 3/2 baryons with three valence quarks) , but it would be a nice background reference to have.

 

[Vanadium 50]

I've never seen a paper that has done that comprehensively

Because what would be the point?

We don't even have such a thing for nuclei, which is a much smaller and more easily understood sample. And it would provoke endless arguing: is ⁸Be a real nucleus or not? ⁴H?

Rutherford said "All science is either physics or stamp collecting." Why on Earth would we want to turn physics into stamp collecting?

 

[ohwilleke]

Because what would be the point?

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.

The point would be similar to the point of Particle Data Group review articles. It would authoritatively do the relatively straightforward task of enumerating what is possible in a way that explains any judgment calls or definitions involved in doing so, on a background issue of potentially broad relevance to anyone doing HEP hadron physics (and presumably, would update data about those that are discovered as is done to a great extent already).

* It would be a convenient place for PDG to collect predictions of the properties of undiscovered particles from the physics literature.
* It would provide a quick checklist for HEP physicists and people interested in HEP physics regarding which hadrons are likely "just around the corner". This could also be one component of many of the task of evaluating the physics case for new experiments and colliders by gauging which kinds of hadrons and how many of them are likely to be made accessible to discover/observe in any particular proposal..
* It would turn the task of assigning unclassified hadron resonances to theoretically predicted hadrons into a multiple choice question where there are plausible options. This would also simplify the through process of determining what kind of experimental data is more important to discriminate between which theoretically predicted hadron is the best fit to an unclassified hadron resonance.
* It would call attention to hadrons that seemingly should have been discovered in current experiments already but haven't be seen, as an otherwise easily overlooked sign of BSM physics
* It would highlight any cases where none of the theoretically predicted hadrons are a good fit to an unclassified hadron resonance as the anomalies that they are in a more straightforward way flagging them as BSM physics candidates in a very logical way.
* It would display more prominently, much like the periodic table does, the concept that the number of possible hadrons is finite and follows ordered patterns.
* It would better communicate to lay people on the things that the Standard Model predicts globally (the spectrum of hadrons) as opposed to on a particle by particle one at a time basis.
* It would help policy makers to evaluate when we don't need to keep looking for new SM hadrons.
* It would help new graduate students evaluate the prospects they can expect from pursuing a career/academic specialty in HEP hadron physics, something that is usually done when they aren't very knowledgeable about what's out there in HEP physics yet.

 

[Vanadium 50]

 

[ohwilleke]

View attachment 303795

When I lived in New Zealand, I went to Rutherford High School.

 

[snorkack]

The second kind is a doubly electrically charged tetraquark. It is an open-charm tetraquark composed of a charm quark, a strange antiquark, and an up quark and a down antiquark,

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.

Note that they constantly say "molecule-like" - not "nucleus-like".
The doubly-charged tetraquark might be grouped into diquarks in two ways:
D⁺_s+π⁺ 1968,3 MeV+139,6 MeV=2107,9 MeV
D⁺+K⁺ 1869,6 MeV+493,7 MeV=2363,3 MeV

The binding energy of the tetraquark against D⁺_s+π⁺ is about -800 MeV. So it is purely a resonance. It is not a strong-force-stable bound system decaying by weak interaction alone.

 

[ohwilleke]

Note that they constantly say "molecule-like" - not "nucleus-like".

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.

A "molecule-like resonance" includes any composite particle with subsystems in which the subsystems (which are basically hadrons of their own) are bound directly by the strong force, as distinguished from simple hadrons that are composite systems of particles bound directly by the strong force alone.

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.

Neither a hadron nor a nucleus has to be "stable" (the only truly stable hadron is the proton, although free neutrons are metastable and are stable when bond in nuclei, and many atomic nuclei are unstable), and hadrons need not decay by weak interactions alone, as there are other forms of hadron decay.

 

[snorkack]

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.

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!

 

[malawi_glenn]

Exotic quarks

Are defined as quarks from the second and third generation? Strange, charm, bottom?

 

[snorkack]

Are defined as quarks from the second and third generation? Strange, charm, bottom?

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.

Strictly speaking anything with upper generation quarks in them has a cap on half-life, but that´s in the order of 10^-13 s for charm and longer for beauty... which is long enough for even atoms to exist.

 

[malawi_glenn]

Could be.

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]

I see. I was using the words loosely here - short for "a quark which has rarely been observed in that context". And when the context is "molecule"... "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". Like the contrast between He (3 particles, but because 2 are light, it is atom) and H₂⁺ (also 3 particles, but because 1 is light 2 heavy, it is a molecule, or rather molecular ion). So when you have an electromagnetically bound system of a tauon and 2 protons, is it still a molecule (as a heavy analogue of H₂⁺) or is it an atom, because tauon is heavier than either proton alone and therefore the nucleus which they orbit?

Returning to more common systems - a reasonably common one is pionium. Lifetime 3 fs - short compared to lone charged pions with 26 000 000 fs, but long compared to 0,08 fs of π⁰. And even experimentally measured.
Does any wording of the new nomenclature exclude pionium from being a tetraquark?

 

[malawi_glenn]

Either way, what we call them is not that important I think. Similar goes to if we should call Pluto a planet or not (dwarf planet)... what is interesting is the underlying physics, not the semantics

 

[snorkack]

Yes. And we are dealing with different types of physics.
Ordinary matter has simple, unambiguous distinction between atoms and molecules on one hand (held together exclusively by electromagnetic forces, with tiny effects of weak interaction) and nuclei on the other (held together exclusively by strong force), because the nucleons have only one charge, and like charges repel. And electrons do not participate in strong interaction. No number of neutrons or protons are attracted by electric forces. However, when you accept exotic hadrons in the mix - such as pions, sigma hyperons or antiprotons - then you can have opposite charged hadrons and do have long distance electric attraction ensuring an infinite number of bound states including high angular momentum ones. You are having a system where electromagnetic and strong forces are both important participants.

 

[ohwilleke]

"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".

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.

One common definition of the term mesonic molecule is that:

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.

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.

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. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms).") rather than as a molecule.

To the extent that pionium is bound solely by the electromagnetic force, by definition, it is only an exotic atom and is not a meson molecule.

But, it does not appear to me that pionium is bound solely by the electromagnetic force as an atomic nucleus and the charged leptons associated with it are in a an atom, since its properties require consideration of both QED and QCD terms, and since it does not decay to positively and negatively charged particles as it would have to in the absence of QCD terms. So, it isn't obvious to me that pionium isn't both an exotic atom and a meson molecule. I acknowledge, however, that I don't have an in depth understanding of the forces at work binding charged pions together into pionium.

Does any wording of the new nomenclature exclude pionium from being a tetraquark?

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 :

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.

In particular, pionium is a subset of exotic atoms known as hadronic atoms (in which the substitute particles are hadrons).

Pionium is also part of a subset of exotic atoms known as onium:

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.

The preference for describing pionium as an atom rather than a molecule reflects my observation in Post #13 in this thread that:

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.

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 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.

 

[snorkack]

One common definition of the term mesonic molecule is that:

So, all atomic nuclei are also hadron molecules,

See the problem? There already is term "nucleus", and now the term "molecule" is stretched to include nuclei.

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.

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.

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.

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?

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.

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.

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"

 

[Melbourne Guy]

Always watch what is in the buffer ... Why don't they invent a preview for buffer content?

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.

 

[ohwilleke]

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"

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.