Hadron

• 1. Introduction

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  A hadron is a composite subatomic particle, made of two or more quarks held together by the strong nuclear force. Almost everything you can touch owes its weight to two of them. Most of the mass of ordinary matter comes from the proton and the neutron, both hadrons. Yet most of that mass is not the quarks themselves. It comes from the binding energy of those quarks, the energy of the strong force lashing them together. The name itself was coined in a single talk. So why did a physicist feel he had to invent a word at a conference in 1962? Why are nearly all of these particles believed to be unstable, doomed to decay into something else? And how can a particle weigh far more than the parts it is supposedly made of? The answers run through quarks that flicker in and out of existence, exotic states found only in this century, and a force that grows weaker, not stronger, the harder you push.

• 2. A Word Born At CERN

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  L. B. Okun introduced the term "hadron" in a plenary talk at the 1962 International Conference on High Energy Physics at CERN. He opened with a complaint about language. Although his report dealt with weak interactions, he said, he would frequently have to speak of strongly interacting particles. That phrase, he argued, was a very clumsy term that did not yield itself to the formation of an adjective. The workaround physicists used was to call certain decays "non-leptonic." Okun objected that this was not exact, because "non-leptonic" might also signify photonic. So he proposed calling strongly interacting particles "hadrons" and their decays "hadronic." The new Greek word, he explained, signifies "large" or "massive," in contrast to a term meaning "small" or "light." A naming problem about adjectives, then, gave physics one of its foundational categories. The deeper puzzle Okun named was strength itself: what does the strong force actually do to the particles it grips?

• 3. The Rule Of Color

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  Color confinement is the law that decides which combinations of quarks are allowed to exist. Quarks carry a property called color charge, but a hadron must have zero total color charge. It must be, in the physicists' phrasing, "colorless" or "white." There are two simple ways to satisfy this rule. One is a quark of a single color paired with an antiquark of the matching anticolor. The other is three quarks, each carrying a different color. These two arrangements are not interchangeable; they define two separate families of particle. The first arrangement produces a meson, the second a baryon. The quark model also explains a hadron's charge through its valence quarks. A proton, for instance, holds two up quarks and one down quark, and adding their charges yields a total of plus one. Color confinement is also why quarks are never seen wandering alone, which raises the question of what fills the space between them.

• 4. Mass From Almost Nothing

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  Massless virtual gluons make up the overwhelming majority of the particles inside a hadron. They are also the major source of its mass, with the exception of the heavy charm and bottom quarks. The top quark never gets the chance to take part, because it vanishes before it has time to bind into a hadron at all. Inside this churning interior, short-lived pairs of virtual quarks and antiquarks are continually forming and vanishing again. They are not stable wave packets but an irregular and transient phenomenon. It is therefore meaningless to ask which quark is real and which is virtual. Only the small excess of quarks over antiquarks is apparent from the outside. So when a hadron is described as made of two or three quarks, that count really refers to this constant excess. The mass of a hadron has very little to do with the mass of its valence quarks. Because of mass-energy equivalence, most of that mass is simply the energy of the strong interaction. The same force that supplies this mass also has a stranger habit, one that depends on how hard you hit it.

• 5. Asymptotic Freedom

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  Asymptotic freedom is the principle that the strong interaction grows weaker, not stronger, as energy rises. Quantum chromodynamics, the theory of the strong force, predicts that at very high temperature and pressure quarks and gluons will no longer stay confined inside hadrons. This unbinding requires sufficiently many flavors of quarks to be present. The reason, in the theory's own words, is that "the strength of the strong interaction diminishes with energy." This counterintuitive behavior is not just theory. It has been experimentally confirmed across the energy range between 1 GeV and 1 TeV. In these other phases of matter, the hadrons can simply disappear. Hadrons also have their own internal restlessness in the form of excited states known as resonances. Each ground state hadron may have several, and experiments have observed several hundred different resonances. These resonances decay extremely quickly via the strong nuclear force. That fragility points to a wider truth: almost nothing in this family lasts.

• 6. Living And Dying Particles

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  Free protons appear to be stable, the one possible exception to a family of doomed particles. If they decay at all, it takes immense amounts of time, on the order of ten to the thirty-fourth years or more. Almost all other free hadrons and antihadrons, meaning those in isolation and not bound within a nucleus, are believed to be unstable and to eventually decay. The longest-lived unstable particle among them is the free neutron. It decays with a half-life of about 611 seconds and has a mean lifetime of 879 seconds. To study these particles, physicists collide hadrons such as protons with one another or with the nuclei of dense, heavy elements like lead or gold. They then detect the debris in the showers of particles that result. Nature runs a similar experiment on its own. In the extreme upper atmosphere, cosmic rays strike rarefied gas particles, producing muons and mesons such as pions. The same pions born in that high-altitude collision also do quieter work closer to home, helping hold atomic nuclei together.

• 7. Baryons And Mesons

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  Baryons are hadrons built from an odd number of valence quarks, at least three. The proton and neutron, with three quarks each, are the familiar examples. Because they hold an odd number of quarks, all baryons are fermions with half-integer spin, and they carry a baryon number of one. Every baryon has an antiparticle counterpart in which each quark is swapped for its antiquark. The antiproton, for example, is made of two up antiquarks and one down antiquark. Mesons take the other path, holding an even number of valence quarks, at least two. Most are a quark-antiquark pair. Because their quark count is even, mesons are bosons with integer spin, and they carry a baryon number of zero. Pions and kaons are common examples produced in particle physics experiments. Beyond these tidy pairs lie hypothetical exotic mesons that escape the standard quark model, including glueballs and hybrid mesons bound by excited gluons. The hunt for the genuinely exotic has already produced confirmed finds in this century.

• 8. The Exotic Frontier

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  The Z(4430) is a tetraquark state, a kind of exotic meson, first discovered in 2007 by the Belle Collaboration. Its existence was put on firmer ground when the LHCb collaboration confirmed it as a resonance in 2014. Then came the baryon side of the frontier. As of August 2015, two known pentaquarks, P(4380) and P(4450), had been found, both discovered in 2015 by the LHCb collaboration. A pentaquark carries five quarks: three quarks of different colors plus one extra quark-antiquark pair. That extra pair does not change its identity, because the additional quark's and antiquark's baryon numbers cancel, leaving the pentaquark with a baryon number of one, just like an ordinary baryon. Other candidates wait in line. Possible hexaquarks, made of six quarks as either a dibaryon or three quark-antiquark pairs, are being investigated to confirm their nature. Each new colorless combination that survives confirmation extends a list that began, decades ago, with nothing more than a physicist's wish for a better adjective.