[HN Gopher] A bestiary of exotic hadrons
___________________________________________________________________
A bestiary of exotic hadrons
Author : rbanffy
Score : 105 points
Date : 2024-12-20 15:29 UTC (7 hours ago)
(HTM) web link (cerncourier.com)
(TXT) w3m dump (cerncourier.com)
| whatshisface wrote:
| > _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.
| munchler wrote:
| I assume that if we ever unify QCD with General Relativity, the
| resulting theory would be able to predict these hadrons from
| first principles?
| ur-whale wrote:
| > the resulting theory would be able to predict these hadrons
| from first principles?
|
| Not sure how bringing GR into the fray would help solve what
| essentially seems to be a computational complexity problem.
| Might actually make things worse.
| whatshisface wrote:
| It's not a computational complexity problem, it's an
| undefinedness problem. Proving that the lattice simulations
| converge has been estimated as well beyond this century's
| mathematics by the pair of people (Glimm and Jaffe) that
| have done the most to study it. In any case it is beyond
| today's.
| frutiger wrote:
| No. The reason perturbation theory doesn't work as well for
| QCD as it does for QED is because of two reasons:
|
| 1. The coupling constant of QCD is much higher than QED so
| contributions to the overall result from Feynman diagrams
| that have more vertices (the multiplicative factor of each
| element in the sum is proportional to the power of the number
| of vertices) don't vanish as quickly as they do for QED
|
| 2. The gauge bosons in QCD (i.e. gluons) themselves have
| colour charge whereas those in QED (i.e. photons) do not have
| electrical charge.
| whatshisface wrote:
| You can't give a definite no to that because, since
| gravitons have stress-energy and are non-perturbative, a
| field theory advance that worked for them could also help
| with the strong force.
| frutiger wrote:
| I mean sure, since we don't know what GR + QFT could look
| like, the result could be just about anything and somehow
| give us nice closed solutions to QCD problems. But I
| don't feel like that line of reasoning is particularly
| useful.
| whatshisface wrote:
| AdS/CFT is already an example of an approach to gravity
| yielding an approach to strongly coupled field theories.
| evanb wrote:
| 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.
| trentonstrong wrote:
| May be referring not to LQCD relying on approximate
| analytical methods but some of the other non-perturbative
| methods? Example would be trying to apply homotopy analysis
| method (HAM) or a related transform to whatever field
| equations to make some semi-analytical predictions.
| addaon wrote:
| 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".
| cwillu wrote:
| "Other manifestly exotic hadrons followed, with two exotic
| hadrons Tcccc(6600) and Tcccc(6900) observed by LHCb, CMS and
| ATLAS in the J/psJ/ps 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.
| dukwon wrote:
| They are there in the print version (page 26)
| https://cerncourier.com/wp-content/uploads/2024/12/CERNCouri...
| timthorn wrote:
| As I wrote somewhere else, I rather like the cuddly hadrons from
| The Particle Zoo:
| https://www.particlezoo.net/collections/particle-packs
| addaon wrote:
| "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 wrote:
| 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 wrote:
| 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.
| FabHK wrote:
| 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 wrote:
| Do you feel like those two options would cover all possible
| scenarios for "the state of the field"?
| anyfoo wrote:
| Well, either the standard model is right, or it isn't, isn't
| it? They asked for indication of an "early phase", not that
| we're ready to throw the standard model out (which, boringly,
| held up extremely well so far).
| whatshisface wrote:
| The standard model Lagrangian is a sum of many terms, and
| changing one of them, adding a new one or even a radical
| revolution in our understanding of the results of integrals
| taken over it would not count as a Kuhnian revolution.
| Physics has not had one of those since Newton.
| Keysh wrote:
| Physics has obviously had Kuhnian revolutions since
| Newton, the emergence of relativity and quantum mechanics
| being two obvious examples.
| whatshisface wrote:
| Physics advances like geography: there's a New World in
| the Americas, but Libson is still there. Newtonian
| mechanics remains as the consequence of relativity and
| quantum mechanics where we "live," and the existence of
| other things under different conditions doesn't change
| that. Kuhnian revolutions involve the old models being
| discarded.
| shwouchk wrote:
| We did "discard" newtonian gravity and mechanics in favor
| of sr,gr and qm as fundamental theories. They still give
| good approximations over a wide range of conditions so we
| keep using them for calculations.
| whatshisface wrote:
| I wouldn't call it discarded if it's still used for
| everything it used to be used for, while also being a
| logical implication of the new theories
| linsomniac wrote:
| My dyslexic brain: "Exotic whatnow?"
| vvpan wrote:
| Same...
| thih9 wrote:
| 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".
| Mistletoe wrote:
| "All science is either physics or stamp collecting." -Ernest
| Rutherford
|
| Is this stamp collecting? Do these exotic hadrons mean anything?
| ur-whale wrote:
| >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.
| iterance wrote:
| 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.
| whatshisface wrote:
| Learning about the properties of exotic hadrons clarifies our
| understanding of nuclear forces.
| m3kw9 wrote:
| I shouldn't have skimmed the tittle
| gtoast wrote:
| I really read this title wrong
| Crazyontap wrote:
| 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 wrote:
| > 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 wrote:
| > 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.
___________________________________________________________________
(page generated 2024-12-20 23:00 UTC)