Hadrons, Baryons, and Mesons

"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!"

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"Like social elephants, quarks only exist in groups with other quarks and are never found alone. Composite particles made of quarks are called Hadrons.
Although individual quarks have fractional electrical charges, they combine such that hadrons have a net integer electric charge. Another property of hadrons is that they have no net color charge even though the quarks themselves carry color charge (we will talk more about this later).
There are two classes of hadrons (try putting your mouse on the elephants):

...are any hadron which is made of three quarks (qqq). Because they are made of two up quarks and one down quark (uud), protons are baryons. So are neutrons (udd).

...contain one quark (q) and one antiquark (). One example of a meson is a pion (+), which is made of an up quark and a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (-) is made of a down quark and an up antiquark. Because a meson consists of a particle and an antiparticle, it is very unstable. The kaon (K-) meson lives much longer than most mesons, which is why it was called "strange" and gave this name to the strange quark, one of its components.

A weird thing about hadrons is that only a very very very small part of the mass of a hadron is due to the quarks in it."

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