A bestiary of exotic hadrons

[whatshisface]

>The dynamics of quarks and gluons can be described perturbatively in hard processes thanks to the smallness of the strong coupling constant at short distances, but the spectrum of stable hadrons is affected by non-perturbative effects and cannot be computed from the fundamental theory. Though lattice QCD attempts this by discretising space–time in a cubic lattice, the results are time consuming and limited in precision by computational power. Predictions rely on approximate analytical methods such as effective field theories.

I'm glad this was mentioned, non-perturbative effects are not well understood and this is a big part of why it's worthwhile to study bound states of the strong force.

[evanb]

Give LQCD practitioners resources on the scale of the experiment, the computations will get faster!

I'm not sure what they mean by "Predictions rely on approximate analytical methods such as effective field theories." The predictions of LQCD are ab initio. Sometimes we fit EFTs to LQCD results, that's true. But EFTs are under control and have quantifiable uncertainties, they're not just willy-nilly approximations.

[munchler]

I assume that if we ever unify QCD with General Relativity, the resulting theory would be able to predict these hadrons from first principles?

[addaon]

Are there a lot of missing overbars in this article, or some other typographic marker for antiquarks? I assume the hexaquark descriptions early on are supposed to be (using Q for q-overbar) "QQQqqq or qqqqqq", where it reads to me as "qqqqqq or qqqqqq".

[dukwon]

They are there in the print version (page 26) https://cerncourier.com/wp-content/uploads/2024/12/CERNCouri...

[cwillu]

“Other manifestly exotic hadrons followed, with two exotic hadrons Tcccc(6600) and Tcccc(6900) observed by LHCb, CMS and ATLAS in the J/ψJ/ψ spectrum. They can be interpreted as a tetraquark made of two charm and two anti-charm quarks – a fully charmed tetraquark.”

Not sure if it was deliberate or not, but yeah.

[gtoast]

I really read this title wrong

[ceejayoz]

When the LHC got built I kept waiting for a newspaper to make that mistake. One eventually did.

[noufalibrahim]

I'd wager that most people did but then corrected themselves.

[timthorn]

As I wrote somewhere else, I rather like the cuddly hadrons from The Particle Zoo: https://www.particlezoo.net/collections/particle-packs

[addaon]

"also implies the existence of a Tbb state, with a bbud quark content, that should be stable except with regard to weak decays"

Can someone explain this to me?

Tcc(3875)+ can decay to a D0 and a D+, yes? And this is a strong decay?

I guess the reason Tbb doesn't have an equivalent strong decay to B mesons because of the sign difference -- that is, B0 and B+ would have anti-bs, not bs; and anti-B0 and anti-B+ would have negative charge?

And so the only major decay pathway is for the b itself to decay to a K+ (plus lepton noise), giving a temporary bu\s\u\d pentaquark, that then has uninhibited decays?

I guess what I'm asking is... is this the right way to think about this?

[adrian_b]

In strong decays, the products will contain the same quarks and antiquarks that have existed in the original particle, possibly with the addition of one or more quark-antiquark pairs that have been generated during the decay.

In weak decays, one or more of the original quarks or antiquarks will be converted in a quark or antiquark with a different flavor, which is a process that has a low probability of happening, so the weak decays happen less frequently, therefore the hadrons that can decay only through weak decays have a lifetime that is many orders of magnitude greater than the hadrons that can decay through strong decays (or electromagnetic decays, i.e. annihilation of quarks with the corresponding antiquarks).

D+ is c quark + d antiquark, D0 is c quark + u antiquark

Tcc(3875)+ is 2 c quarks + d antiquark + u antiquark

Therefore the 4 quarks/antiquarks in Tcc(3875)+ are the same as the 4 quarks/antiquarks in D0 + D+.

So this is a strong decay, because no quark or antiquark is converted into another kind of quark or antiquark.

For the Tbb- tetraquark, its composition would allow a similar strong decay into two b-quark + u or d antiquark hadrons, except that its binding energy is so great that the mass of the Tbb- tetraquark is smaller than the sum of the masses of the hadrons that would be produced during a strong decay (it is also smaller than the sum of masses of the hadrons that could be produced by an electromagnetic decay, see https://www.sciencedirect.com/science/article/pii/S037026931... ).

This forbids the strong decay and the electromagnetic decay, so the only admissible decay must be weak, where one of the b quarks will be converted into another kind of quark.

[dukwon]

The strong decay would just be forbidden from conservation of energy. If the mass of the Tbb state is less than the sum of the B+ and the B0 masses, then that decay isn't allowed.

[Crazyontap]

Whenever I come across such news, it seems like we are still far from grasping the complete picture. It's akin to gazing at the sky without a telescope and assuming we have seen all the stars in the universe.

I speculate that in the coming decades or centuries, a new instrument may enable us to delve deeper into the atom and reveal that what we perceive now is merely a minuscule fraction of the whole picture.

Perhaps the notion that the subatomic world is as vast as the universe, as stated by Richard Feynman when he said "There’s plenty of room at the bottom.", holds more truth than we realize.

[elashri]

> Perhaps the notion that the subatomic world is as vast as the universe, as stated by Richard Feynman when he said "There’s plenty of room at the bottom.", holds more truth than we realize.

That's true and he knew this even at the time of this famous lecture. He was talking about that there is a plenty of room at the room for us to explore how can we use atoms in synthetic chemistry not into exploring the fundamental particles inside them . When we are talking about particle physics we are basically talking about the successor field of nuclear physics. It studies the interactions and particles inside the sub-atomic structure. Feynman's most interesting work - parton model- was one of the first innovative theoretical work in QCD and was one of milestone of development and validation of the quark model.

The idea that protons, neutrons, and other hadrons are composed of fundamental particles called quarks that come in six -flavors- (up, down, strange, charm, top, and bottom) and possess fractional electric charges. These quarks are bound together by the strong nuclear force, mediated by particles called gluons, and must combine in specific ways to form observable particles (mesons or baryons). One day this was a wild theory and needed a lot of work on validating this model experimentally.

[akomtu]

> The challenge of understanding how quarks are bound inside exotic hadrons is the greatest outstanding question in hadron spectroscopy.

They must be more like knots: https://en.wikipedia.org/wiki/Knot_(mathematics)

Quarks are small masses, gluons are strings connecting them, and the whole thing is in a rapid periodic motion.

> Like Mendeleev and Gell-Mann, we are at the beginning of a new field, in the taxonomy stage, discovering, studying and classifying exotic hadrons.

The chemistry of matter that's smaller than protons and larger than electrons is indeed a missing piece. But the real breakthru will be discovering a membrane that's impenetrable to those multiquarks.

[schaefer]

gratuitously suggestive title is gratuitous. :P

[FabHK]

Do we have anomalies accumulating here that indicate the early phase of a scientific revolution in Thomas Kuhn's terminology, that is, a replacement of the standard model/QCD? Or is it still "so far, so good"?

[drpossum]

Do you feel like those two options would cover all possible scenarios for "the state of the field"?

[ggm]

Since the way we find these is to smash the larger atomic constructs with (relatively) huge amounts of energy I do wonder how much we can know of their ground state, motion & behaviour absent those forces.

[cmason]

Can anyone recommend a book or other resource for a lay person to understand this?

[hodapp]

I thought that "The Particle Zoo: The Search for the Fundamental Nature of Reality" by Hesketh did an excellent job of explaining without dumbing down to the point of meaninglessness.

[m3kw9]

I shouldn’t have skimmed the tittle

[Mistletoe]

“All science is either physics or stamp collecting.” -Ernest Rutherford

Is this stamp collecting? Do these exotic hadrons mean anything?

[iterance]

That quote isn't real; it was a metaphor Rutherford purportedly once used, posthumously recalled by John Bernal. It was incorrectly converted into a direct quotation by later writers. But even then, you're misunderstanding the quote, by which is meant that physics has supremacy and all other sciences are collecting specific instances of physics; the LHC is decidedly doing physics.

However, even if you take the quote to mean what you imagined, it is unnecessarily cynical. LHC has advanced our understanding of physics.

[ur-whale]

>Do these exotic hadrons mean anything?

Given their horribly short lifespans, probably not much other than the fact that they manage to exist for however short a time might vindicate QFT a tad more (I'm assuming that QFT somewhat predicts their likelihood to show up).

Or maybe they'll bring a deeper understanding of the strong force.

But generally speaking, I feel you: lots of work and energy spent to create these exotic things, but that may or may not have an actual use or even meaning.

A lot of science is like this these days, it looks like we're hitting exponentially diminishing returns (as in: useful applications) in some areas of science.

[whatshisface]

Learning about the properties of exotic hadrons clarifies our understanding of nuclear forces.

[olooney]

Might be useful someday:

https://en.m.wikipedia.org/wiki/Femtotechnology

[neilellis]

Now that's a headline that you don't want to type wrong.

[linsomniac]

[flagged]

[thih9]

There is a nonzero chance this was intentional. There is a long tradition; an article about the “Queen Mary” vessel being cleaned was allegedly titled “Queen Mary Having Bottom Scraped”.