What’s a Hadron?

The Large Hadron Collider is a wonderful and exciting machine.  But first things first — what’s a hadron??!!

A hadron is any particle that is made from quarks, anti-quarks and gluons.  (If you want to learn more about quarks and gluons, start here.)  The most famous example of a hadron is a proton, which I have described in detail here, and I would suggest you read this first if you are interested in hadrons.  Because once you understand the proton, then you understand almost everything there is to know about a hadron…

In particular, you will understand that a proton is made of two up quarks, a down quark, and a large number of gluons and of quark-antiquark pairs.

A neutron is basically the same as a proton except that it has one up quark and two down quarks in addition to its large number of gluons and of quark-antiquark pairs.  Unlike a proton, which has charge +1 (in fact it defines what it means to have charge +1), the neutron is electrically neutral — has charge 0 — hence its name

A pion-plus, of charge +1, differs from a proton in that it has an up quark and a down anti-quark in addition to its large number of gluons and of quark-antiquark pairs.

A Kaon-plus is like a pion-plus except that it has an up quark and a strange anti-quark (in addition to its… ok, ok.)

Get the point?  Each hadron has large number of gluons and of quark-antiquark pairs, plus something else.  The something else may include

  • Three quarks of various types
  • Three anti-quarks of various types
  • A quark and an anti-quark, possibly of different types
  • Extra energy, distributed among the many quarks, anti-quarks and gluons.

An example of the last is provided by the Delta-plus particle.  It has the same quark content as a proton, but it has extra energy.  It decays very rapidly (in a trillionth of a trillionth of a second) to other hadrons, for instance a pion-plus and a neutron.

Atomic nuclei are made from protons and neutrons, so they too are made from quarks, anti-quarks and gluons.  And they also are often called hadrons.  One month a year, the Large Hadron Collider, which mostly hosts collisions of protons, is used to create collisions of atomic nuclei (in particular, nuclei of lead.)  So that’s why it isn’t called the Large Proton Collider!

8/9/11

35 responses to “What’s a Hadron?

  1. Pingback: Standard Model Tutorials for the Masses (…er, sorry about the pun…) « Whiskey…Tango…Foxtrot?

  2. So, if atomic nuclei are hadrons, can (for instance) a helium atom (one proton + one neutron) be envisioned as a uniform (spherical?) volume with (net) 3 up quarks, 3 down quarks, and zillions of gluons+ quark/anti-quark pairs? And any other atom be (net) P*(2 up 1 down) + N*(1 up 2 down) quarks + gluons + quark/anti-quark pairs (with p being the number of protons in the nucleus and N being the number of neutrons)? Am I following you correctly?
    Pat

    • Actually, although your question makes perfect sense, it turns out that’s not the way it works. The quarks, antiquarks and gluons are tightly bound inside a proton, or inside a neutron… the strong nuclear force acts very strongly on them. But the protons and neutrons are bound together rather weakly inside a nucleus, even though the strong nuclear force is again responsible.

      There is a pretty strong analogy here with the fact that the electromagnetic force captures electrons inside of atoms AND, through a secondary effect, brings atoms together into molecules. Usually the forces inside a molecule are much weaker than those that bind an atom together. So it is typically the case that much less force is needed to break a molecule apart than an atom: often you can destroy certain molecules in chemistry class at temperatures that are not far above room temperature (or even lower!), but to break atoms apart requires heating them to many thousands of degrees.

      For the same reason, you can break atomic nuclei apart at temperatures of something like a billion degrees, but to break a proton or neutron apart requires temperatures a thousand times higher.

      So you should think of atomic nuclei as loosely assembled collections of sedate and decorous protons and neutrons, and think of the protons and neutrons as vaguely spherical, tightly wound balls full of quarks, antiquarks and gluons rushing madly around in all directions. In the same way, you can think of most molecules as made from loosely assembled groups of atoms that aren’t moving much, whereas the electrons inside the atoms are zipping around at a few thousandths of the speed of light.

  3. Very engaging and clear explanation. Pardon my ignorance, but WHERE are these other hadrons (aside from the nucleus, proton, and neutron) you mention typically found? And is it simply–which it sounds like nothing about a hadron can be–the different formula of up and down quarks that causes the proton and neutron to vary in charge? Thanks.

    • Good questions.

      All of those other hadrons decay within a tiny fraction of a second — so they are only found in the immediate aftermath of high-energy particle collisions, either the artificial ones that we create in accelerators, or in natural ones, such as cosmic rays. Why do all those particles decay? I’ve written about that here: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/most-particles-decay-why/ . When they decay, they fall apart into lighter particles. The details depend on the decaying particle, but the decay debris can in general include photons, electrons, anti-electrons, neutrinos, muons, anti-muons and/or lighter hadrons.

      As for the different electric charges of the proton and neutron: YES! that IS simple. The proton has gluons, quarks and antiquarks, but it has two more up quarks than up antiquarks, and one more down quark than down antiquarks. The gluons have no charge, and the charges of quarks and antiquarks cancel, but the extra up quarks each have 2/3 of the charge of the proton (4/3 altogether), while the extra down quark adds charge of -1/3 of the proton’s charge. Thus 2/3 + 2/3 – 1/3 = 1 gives the proton’s charge. The neutron is just like the proton except that it has ONE extra up quark and TWO extra down quarks. It’s charge is therefore given by 2/3 – 1/3 – 1/3 = 0. There you go!

      • HI Prof, I am here because my tennage daughter asked me yesterday what a proton was and I was aware that I no longer knew! And as an engineer of some decades now may I say that this is what is wrong with the universe? Mass and energy I think I just about understand enough to be humble and quiet but charge confuses me greatly. It just cannot be this simple. (I hope.)

        If (net) charge is about adding up the charges of quarks and anti-quarks – and I absolutely take your word that the model so works – what can anyone say but that the quark, antiquark, gluon model is just a mathematical simplification just like protons, neutrons and electrons were? And before that earth, wind, fire and water? Just how far are we going to go before we find out what stuff is really made of? It is a question isn’t it – as mentioned on a different thread by someone of more insight than me – of an artist drawing the hand of an artist drawing the hand of… Is there any hope of understanding or delving somehow down below these layers to the core of nature’s onion? Or is that what you all are about and I should go away and be patient? Or is that anyway a fruitless idea? The telescope can only be made of the level above that at which we wish to look.

        A seriously wonderful and helpful website btw. Thank-you.

  4. Which theory offers the above picture of a hadron? Is it the quark theory? Thanks in advance.

    • It is the theory known as quantum chromodynamics, the theory of quarks, antiquarks and gluons that became widely accepted in 1973. The original quark theory of people such as Gell-Mann, from the 60s, is wrong, as far as the internal structure of hadrons; it does not have gluons or antiquarks in the proton, and it treats the quarks as much slower and much heavier than is in fact the case.

      • Prof Strassler,
        Thank you for answering all my three questions. The Reply link is missing in the other two responses. I wanted to ask a follow up question for one of them. I will ask it here. If you prefer, I can retype it there.
        In the equation E^2 – p^2 = m^2 c^4, the momentum p is mass times velocity. Just to make sure, this mass is the rest mass, right? It has to be since relativistic mass was not defined by Einstein and later he was against it. I just want to make sure. Thank you in advance.

  5. I’ve studied that pions decay to give muons..That means particles frm one category(mesons) decays to give leptons(muons)

    • That kind of thing can happen in many contexts. The real issue is to learn the right categorizations.

      To really understand the rules for how things do and do not change, you have to study the quarks, not the hadrons that contain them. It is the quarks and leptons that interact directly with the photon, W and Z particles, and gluons. See http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-apparently-elementary-particles/ for some insights.

      What is actually happening is that a quark and an antiquark are annihilated, and in their place a lepton and antilepton are created. A positively-charged pion’s disappearance involves one of its up quarks combining with one of its down anti-quarks, turning into a disturbance in the W field (often called a W virtual particle, but these are not really particles), and then this disturbance turns into a neutrino of muon-type and an anti-muon.

      However, a neutral pion does something else. One of its quarks and one of its anti-quarks annihilate, through a disturbance in the quark fields, and become two photons.

      And a muon itself will soon enough turn into a neutrino of muon-type, an electron and and an anti-neutrino of electron-type.

  6. Wikepedia lists the decay products for all particles except the proton, which is stable. I have read that in the LHC, proton collisions can actually smash protons apart; if this is true can you tell me what happens to the bits? I find quark theory somewhat similar to Greek mythology, just not quite so convincing; therefore I would appreciate an answer in terms of things like photons, neutrinos or muons, which are proven to exist.

    • 1) Don’t trust Wikipedia for detailed information about particle physics. Some of it is right, some of it is not.

      2) You should read the articles on this website for an understanding of a proton and what particle physics is for. We do not smash protons together in order to study their bits; that’s not what it’s about at all. We smash them together to create new things out of the collisions of quarks, antiquarks and gluons. Start with

      http://profmattstrassler.com/articles-and-posts/particle-physics-basics/particle-physics-why-do-it-and-why-do-it-that-way/

      Then try

      http://profmattstrassler.com/articles-and-posts/the-higgs-particle/the-standard-model-higgs/production-of-the-standard-model-higgs-particle/

      3) You know very little about quark theory, so you will excuse me for observing that the fact that you find it similar to Greek mythology reflects more on you than on the theory.

      In fact, the theory of quarks (and gluons — you must include them as well, or you don’t have a consistent theory), called QCD, is a remarkably successful theory for which there are no known conflicts with any of hundreds of experiments. The theory of quarks and gluons can be simulated on a computer and correctly gives the hadron mass ratios for a wide variety of hadrons. And the theory of quarks and gluons correctly predicts hundreds of measurements at the Large Hadron Collider (and hundreds more at its predecessors, the Tevatron, SLC and LEP). For example, the quark/gluon theory was correctly used to predict the rate at which W particles (https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2010-02/fig_12.png), Z particles and top quarks would be produced at the Large Hadron Collider; the predictions were correct to within the accuracy of the prediction (5% – 10%). It also predicts how often two “jets” (sprays of hadrons) would be produced at the Large Hadron Collider as a function of the energies of the jets (strictly speaking, their momentum transverse to the collision axis) over a factor of 10 in energy and a factor of a hundred thousand in the rate (https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2010-03/fig_13.png). I could go on for pages and pages and pages about all the successes of this theory over the past 40 years. Meanwhile, I wish you luck using Greek Mythology to predict anything in advance. Let me know if you succeed thousands of times in a row. If you really think quarks don’t exist, then you have a lot of explaining to do — you have thousands of detailed measurements (not just numbers but complex plots) to explain, which the quark and gluon theory already explains very well.

      • To the followers of Greek mythology, the predictive power was extraordinary; for instance whenever there was a storm at sea, it was invariably shown to be caused by the wrath of Neptune. Similarly quark theory can easily account for the mass of any particle imaginable; for instance in a proton the 3 quark masses account for about 1% of the proton-mass and the rest is made up by the activities of an unknown quantity of massless gluons. This is a world away from a proper scientific theory like atomic theory; where for instance the mass of the carbon isotopes is proved to result from the numbers of protons and neutrons, less about 1% of binding energy in each case. I am still trying to find out if it is possible to destroy a proton, I do not think Wikipedia answers this.

        • No, I disagree; as you say, the storm at sea was POST-DICTED to be caused by the wrath of Neptune. But hundreds of detailed results at the Large Hadron Collider were PRE-DICTED — i.e., IN ADVANCE — and with precision of typically 10% or better. All of these — every one — used the quark/gluon theory that is generally accepted in particle physics today.

          If you can’t tell the difference between these two things, you’re going to have trouble making intelligent remarks about science.

          If the only thing you accept as a proper scientific theory is one in which binding energies are small compared to the physics of interest, I can see why you would hate the theory of quarks. Sorry to disappoint you, but nature is not interested in your opinion.

          Essentially, what you reveal by your complaints is your ignorance of well-established theoretical results about strongly bound systems. Are you familiar for instance with the exact results in supersymmetric field theories that show — with mathematical rigor — that a theory of MASSLESS gluons and quark-like particles may in the end have only MASSIVE bound states? This is an extremely famous result that is over 25 years old.

          Meanwhile – it would be extremely difficult to destroy a proton — that we know. Whether it is impossible is not known.

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  9. who discovered the hadron and when? & how does it work?

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