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Richard Ruiz | Univ. of Pittsburgh | U.S.A.

View Blog | Read Bio

That’s Right, Count Them: 4 Quarks

Hi All,

Exciting news came out the Japanese physics lab KEK (@KEK_jp, @KEK_en) last week about some pretty exotic combinations of quarks and anti-quarks. And yes, “exotic” is the new “tantalizing.” At any rate, I generally like assuming that people do not know much about hadrons so here is a quick explanation of what they are. On the other hand, click to jump pass “Hadrons 101” and straight to the news.

Hadrons 101: Meeting the Folks: The Baryons & Mesons

Hadrons are pretty cool stuff and are magnitudes more quirky than those quarky quarks. The two most famous hadrons, the name for any stable combination of quarks and anti-quarks, are undoubtedly the proton and the neutron:

According to our best description of hadrons (Quantum Chromodynamics), the proton is effectively* made up two up-type quarks, each with an electric charge of +2/3 elementary charges**; one down-type quark, which has an electric charge of -1/3 elementary charges; and all three quarks are held together by gluons, which are electrically neutral. Similarly, the neutron is effectively composed of two down-type quarks, one up-type quark, and all the quarks are held strongly together by gluons. Specifically, any combination of three quarks or anti-quarks is called a baryon. Now just toss an electron around the proton and you have hydrogen, the most abundant element in the Universe! Bringing together two protons, two neutrons, and two electrons makes helium. As they say, the rest is Chemistry.

However, as the name implies, baryons are not the only type of hadrons in town. There also exists mesons, combinations of exactly one quark and one anti-quark. As an example, we have the pions (pronounced: pie-ons). The π+ (pronounced: pie-plus) has an electric charge of +1 elementary charges, and consists of an up-type quark & an anti-down-type quark. Its anti-particle partner, the π (pronounced: pie-minus), has a charge of -1, and is made up of an anti-up-type quark & a down-type quark.


If we now include heavier quarks, like strange-type quarks and bottom-type quarks, then we can construct all kinds of baryons, mesons, anti-baryons, and anti-mesons. Interactive lists of all known mesons and all known baryons are available from the Particle Data Group (PDG)***. That is it. There is nothing more to know about hadrons, nor has there been any recent discovery of additional types of hadrons. Thanks for reading and have a great day!


* By “effectively,” I mean to ignore and gloss over the fact that there are tons more things in a proton, like photons and heavier quarks, but their aggregate influences cancel out.

** Here, an elementary charge is the magnitude of an electron’s electron charge. In other words, the electric charge of an electron is (-1) elementary charges (that is, “negative one elementary charges”). Sometimes an elementary charge is defined as the electric charge of a proton, but that is entirely tautological for our present purpose.

*** If you are unfamiliar with the PDG, it is arguably the most useful site to high energy physicists aside from CERN’s ROOT user guides and Wikipedia’s Standard Model articles.

The News: That’s Belle with an e

So KEK operates a super-high intensity electron-positron collider in order to study super-rare physics phenomena. It’s kind of super. Well, guess what. While analyzing collisions with the Belle detector experiment, researchers discovered the existence of two new hadrons, each made of four quarks! That’s right, count them: 1, 2, 3, 4 quarks! In each case, one of the four quarks is a bottom-type quark and another is an anti-bottom quark. (Cool bottom-quark stuff.) The remaining two quarks are believed to be an up-type quark and an anti-down type quark.

The two exotic hadrons have been named Zb(10610) and Zb(10650). Here, the “Z” implies that our hadrons are “exotic,” i.e., not a baryon or meson, the subscript “b” indicates that it contains a bottom-quark, and the 10610/10650 tell us that our hadrons weigh 10,610 MeV/c2 and 10,650 MeV/c2, respectively. A proton’s mass is about 938 MeV/c2, so both hadrons are about 11 times heavier than the proton (that is pretty heavy). The Belle Collaboration presser is really great, so I will not add much more.

Other Exotic Hadrons: When Barry met Sally.

For those keeping track, the Belle Collaboration’s recent finding of two new 4-quark hadrons makes it the twelfth-or-so “tetra-quark” discovery. What makes this so special, however, is that all previous tetra-quarks have been limited to include a charm-type quark and an anti-charm-type quark. This is definitely the first case to include bottom-type quarks, and therefore offer more evidence that the formation of such states is not a unique property of particularly charming quarks but rather a naturally occurring phenomenon affecting all quarks.

Furthermore, it suggests the possibility of 5-quark hadrons, called penta-quarks. Now these things take the cake. They are a sort of grand link between elementary particle physics and nuclear physics. To be exact, we know 6-quark systems exist: it is called deuterium, a radioactive stable isotope of hydrogen (Thanks to @incognitoman for pointing out that deuterium is, in fact, stable.). 9-quark systems definitely exist too, e.g., He-3 and tritium. Etc. You get the idea. Discovering the existence of five-quark hadrons empirically establishes a very elegant and fundamental principle: That in order to produce a new nuclear isotope, so long as all Standard Model symmetries are conserved, one must simply tack on quarks and anti-quarks. Surprisingly straightforward, right? Though sadly, history is not on the side of 5-quark systems.

Now go discuss and ask questions! 🙂

Run-of-the-mill hadrons that are common to everyday interactions involving the Strong Nuclear Force (QCD) are colloquially called “standard hadrons.” They include mesons (quark-anti-quark pairs) and baryons (three-quark/anti-quark combinations). Quark combinations consisting of more than three quarks are called “exotic hadrons.”





Happy Colliding.

– richard (@bravelittlemuon)


PS, I am always happy to write about topics upon request. You know, QED, QCD, OED, etc.


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  • Xezlec

    Thanks for the great update! I have a couple questions:

    1) Does this shed any light on the mysterious X(3872) or Y(4360) particle? I recall those and their other weird friends were thought by some to be candidate exotic hadrons.

    1.5) Is a so-called “mesonic molecule” the same thing as an exotic hadron? If deuterium is an example of a hexaquark, as you say, then I would think so.

    2) Wikipedia says tetraquarks and pentaquarks are still both hypothetical. In fact, previous articles on quantum diaries site have implied as much. You say that these are about the 12th tetraquarks that have been discovered. Am I misunderstanding something here?

  • norsetto

    So, a cyan quark would imply what? A green antiquark!?

  • norsetto

    Sorry, I should have said magenta, not cyan …

  • Hi Norsetto,

    Spot on. In chromodynamics, the charge is collectively referred as “color”; this is 99% analogous to the electric charge in electrodynamics. The difference is that in chromodynamics, there are three different possible charges for a single quark, whereas for a single electron, there is only one possible charge.

    For example in electrodynamics, an electron always possess an electric charge of -1, and an anti-electron (aka a positron) will always be a charge of anti-“-1”, which is sometimes called +1. 🙂
    And in chromodynamics, a top quark, for example, can be found with a “red” charge, a “blue” charge, or a “green” charge; an anti-top quark can posses an “anti-red” charge, an “anti-blue” charge, or an “anti-green” charge. Additionally, the top quark still has a +2/3 electric charge, and the anti-top quark has a -2/3 electric charge.

    If I recall correctly, the color mapping goes as:
    anti-red = cyan
    anti-green = magenta
    anti-blue = yellow

    However, this is simply a convention and is inconsequential to the physics. Personally, I just say “anti-red/blue/green”.

  • Hi Xezlec,

    Sorry for the slow reply. I wanted to do some checking before I replied. Great questions by the way!

    1. This discovery provides evidence for the argument that X(8372) & Y(4360) should be interpreted as 4-quark systems, where each contain at least one charm and one anti-charm quark. To my understanding, it is a description that best fits the data and is *also* verifiable. I answered number 2 first and inadvertently wrote a few things relevant to this question there.

    1.5. To be honest, this is the first time I have ever heard of that term. According to Wiki’s definition, I suppose a “mesonic molecule” might qualify as an exotic hadron. However, I am not a particular fan of the phrase because, to me, the term “molecule” implies an electrodynamical bonding and not chromodynamical. Like in chemistry, molecules exist because of the bonding is due to an exchange of electrons; so chemical bonds are electrodynamical in nature. Hadronic structures exist due to the strong force and not electrodynamics. Lastly, to me the difference between “tetraquark” and “4-quark” systems is whichever sounds better in a particular sentence. 🙂

    2. Wikipedia is a great source of information; but at times it does not contain up-to-date information about a scientific discovery, if a part of it is still open to debate. Rightfully so, I might add. There is zero evidence for 5-quark systems. For 4-quark system, there is growing evidence. In fact, I decided to write this post after realizing that KEK laboratories had previously discovered 10 other exotic hadrons (http://www.kek.jp/intra-e/press/2012/011014/#term07 – The link will take you to a specific part of the press release to which I refer in the post.). The scientific consensus that is forming is that these 12 exotic hadrons are actually 4-quark systems, which is unexpected but not forbidden by current models.

    2.1 Regarding previous QD articles, the only other post that comes to mind is a great piece by Aiden (http://www.quantumdiaries.org/2011/10/10/who-ordered-that-an-x-traordinary-particle/), where he addresses this specific debate. One strong argument regarding 4-quark systems is if they exist, then they there should be other exotic hadrons that contain non-charmed quarks, e.g., bottom quarks. Belle’s discovery is precisely confirmation of this argument.

  • Xezlec

    Alright, thanks for the detailed reply!

  • What is the likelihood of genuine 6 & 8 quark particles?

    Inspired by the idea that 2, 3, 4, 6, 8, 12, 20, are all the numbers of points that can be placed on the surface of a sphere in a symmetrical arrangement. However, this need not necessarily translate into meaningful physics – as, apart from other considerations, quarks have vastly different masses and differing amount of charge – so naive geometrical arguments like this, are suspect.

    So what do the theorists have to say about particles with greater than 4 quarks?

  • Kyle Ferreira

    I’ve been contemplating the possible Hadron States that satisfy Color Confinement, and this lead me to the intriguing idea. It’s what I call the Decaquark. Ithe has two possible combinations of Quarks: 8 Quarks and 2 Antiquark, or 5 Quarks and 5 Antiquarks. I’m simply curious about wether such a state is hypothetically possible.