25 Days of Particles: Day 2

Proton

Classification: hadron, baryon, fermion
Fundamental: no
Mass: 938.3 MeV = 1.67e-27 kg
Interactions: Electromagnetic (charge +1), Weak, Nuclear/Strong, Gravity (all due to constituents)
Spin: 1/2

Our tour continues with the proton, the positively charged component of the atom. Unlike the highly mobile electron, the proton more or less stays put in the nucleus of the atom. The attraction between the positively charged protons in the nucleus and the negatively charged electrons is what holds the electrons in the atom. The proton share space in the nucleus with the neutron. Most atoms have equal numbers of the two, but hydrogen (the lightest element) has only a single proton in its nucleus in the most common stable isotope. Very heavy elements tend to have more neutrons than protons. It takes a lot of energy to remove a proton from the nucleus, making them the most noticeable and permanent feature of the atom. The atomic number used to classify elements is the number of protons found in the nucleus.

Since protons are so difficult to pull out of atoms, they were discovered to be particles over fifteen years later than electrons. At the same time that it was discovered you could pull negatively charged stuff out of metals, it was discovered that you could pull positively charged stuff. However, the positive charges did not have a constant charge to mass ratio, unlike the well-behaved electrons. Thomson developed the idea that the newly discovered electrons in atoms were scattered through a cloud of diffuse positive charge. In 1911, Ernest Rutherford discovered that atoms had a hard nucleus in the middle using a scattering experiment: he shot helium nuclei at gold atoms, and they bounced off. In the thinking of Thomson's model, that was like firing a gun at a tissue and having the bullet bounce back at you. Less than ten years later, Rutherford found he could kick hydrogen nuclei out of nitrogen atoms, implying that the hydrogen nucleus was a constituent of the nitrogen nucleus. He dubbed this particle the proton.

But wait! How do a bunch of positively charged particles stay bunched up together in the nucleus? This puzzling question led to the conclusion that there were more forces in the world than gravity and electromagnetism, and eventually led to the discovery that protons are not fundamental. They are made up of smaller parts called quarks. The interactions of protons we see can be traced to what the quarks inside the protons are doing. However, it is physically impossible to break a proton into its component quarks.

Protons also currently hold the special distinction of being the particles to smash in most of the world's highest energy accelerators and colliders. HERA, part of the DESY facilities in Germany, collides protons with either electrons or positrons to explore proton structure. The Tevatron at Fermilab in the US has been colliding protons with antiprotons for almost thirty years. The Large Hadron Collider here at CERN collides two beams of protons to hunt for never-before-seen particles. A salute to the proton, for leading the way into new knowledge at accelerators around the world!