Science is about learning about the world we live in. Particle physics focuses on the the building blocks of Nature and how they interact. What is the smallest, undivisible part of matter around us? The question came up over 2000 years ago and intrigued Greek philosophers. This most fundamental thing was called the atom by Democritus. That was just a philosophical idea. No actual tests were tried. In early 1800s, John Dalton formulated a more precise description of Democritus's idea. There were still no machinery to test the idea.
The electrons were discovered experimentally by J.J. Thomson in 1897. His study of the mysterious rays in the cathode tubes led him to conclude that the rays consisted of something much smaller than atoms. His atomic model has the electrons distributed uniformly over the positively-charged sphere. (There are no positive protons in Thomson's model.) Then in 1911 we had the nucleus from Ernest Rutherford's study of alpha particles scattering from thin gold foils. The surprising pattern of the scattered alphas suggested that there is a concentration of positive charge in the center of the atom. In 1932, James Chadwick proved the existence of the neutron from his alpha-beryllium reaction experiment. The reaction yields carbon and a neutral particle which then had been assumed to be a gamma ray. Chadwick found that this particle could knock off protons from paraffin wax. Gamma ray's energy would have to be incredibly high to do that. The job can be done much more easily by a particle of similar mass: the neutron. Chadwick published this finding and won the 1935 Nobel Prize.
So are protons, neutrons, and electrons the three fundamental particles? That is, can we build anything in the world using just these three ingredients? Well, for normal everyday matter, yes. We can build any atom from these 3 particles, and any molecule from the atoms. However, there are a bunch of other particles in the universe that cannot be simply put together by these 3 subatomic particles. There are evidences that the proton and the neutron are not fundamental, and while the electron is point like and indivisible, it has "siblings."
Protons and neutrons can be furthered subdivided into smaller particles. The evidence comes from the experiment called "deep inelastic scattering (DIS)," where protons are bombarded with high-energy (small-wavelegth) electrons. If protons were fundamental (unbreakable, unpenetrable), there would be only elastic scattering. Electron would bounce off the protons and no new particles would be created. However, that's not the case. The sum of kinetic energy (T) before and after the collision are different. This missing energy has to go into changing the structure of either electron or proton. Electrons are found intact while protons are not. Energetic electron interact with one the proton's constituents electromagnetically. As a results, the proton disintegrates into new, lighter particles. These constituents are called quarks. There is another type of constituent called gluons. Gluons, however, are chargeless and massless, thus do not interact with electrons. DIS confirms the existence of quarks. DIS can also be thought of as elastic scattering of electrons and quarks.
At the present, there is not just one fundamental type, like the atom used to be. Even three types are not enough. There are different families of fundamental particles. quarks, leptons, and force carriers. Quarks have never been observed individually. They are always bound inside particles. The name "hadron" is used to call any particle that have quarks in them. Protons and neutrons are both hadrons. Leptons are the electron family, which also includes the muon, the tau, and their corresponding neutrinos. There are also force carriers which are used to explain how the particles interact. There are four fundamental forces in nature: electromagnetic, gravity, strong, and weak forces.
Electromagnetic forces act between charged particles. Photons mediate these forces. When we say an electric field, we mean field of photons originating from a charged body. Gravity acts on anything that has mass and is inversely proportional to the square of the distance between the masses. The carrier of gravitational force is called the graviton. Strong and weak force only relevant at a very small distance--that is, a nuclear scale. Strong force acts on quarks. Strong force carriers are called the gluons. Then, finally, weak force acts on leptons and has three mediators: W^+, W^-, and Z^0.
Quarks are particles with fractional charges. There are six members of the quark family: up, down, charm, strange, top, bottom. They are summarized in the following table:
| quark | electric charge (e) |
| up (u) | 2/3 |
| down (d) | -1/3 |
| charm (c) | 2/3 |
| strange (s) | -1/3 |
| top (t) | 2/3 |
| bottom (b) | -1/3 |
(e = 1.6 x 10^{-19} C)
Subsequently, hadronic matter can be divided into two groups: baryons and mesons. If we put three quarks together (uds, uuu, udc, ... etc), we form a baryon. A meson is formed by putting a quark and an antiquark together (u ubar, u dbar, d cbar, ... etc). Individual quarks have never been detected alone. Experiments at RHIC and LHC may be able to produce substance called quark-gluon plasma (QGP) which consist of free quarks.
Lepton family is summarized below:
| lepton | electric charge (e) |
| electron | -1 |
| electron neutrino | 0 |
| muon | -1 |
| muon neutrino | 0 |
| tau | -1 |
| tau neutrino | 0 |
Neutrinos are curious little particles. They can originate from outside the Earth, such as from solar fusion and supernovae; we call them cosmic neutrinos. This group also includes neutrinos from the Big Bang over 10 billion years ago. Those that come from the interaction between cosmic rays and Earth's atmosphere are called atmospheric neutrinos. On the Earth itself, accelerators and reactors can also generate a number of neutrinos. In an accelerator, a very energetic proton may interact with a metal target. The interaction will result in many pions being generated. A pion in turn decays into a muon and a muon antineutrino.
The neutrinos can "mix," that is, they are superposition states of different neutrino flavors (e, mu, tau) and can change from one flavor to another. This has an important implication that neutrinos are not massless as originally thought.
Moreover, neutrinos are not electrically charged therefore they do not feel electromagnetic force. They do not participate in strong force, which only affects quarks, either. Therefore the neutrinos should be great to probe weak interactions.
The quarks and leptons are normally grouped as:
| quarks | leptons | |||||
| up | charm | top | electron | muon | tau | |
| down | strange | bottom | electron nu | muon nu | tau nu | |
Force mediating particles are listed below. Only the graviton has not yet been observed experimentally.
| force | carrier particle | electric charge (e) | mass |
| gravity | graviton | 0 | 0 |
| electromagnetism | photon | 0 | 0 |
| strong | gluons | 0 | 0 |
| weak | W^+ | 1 | 80 GeV |
| W^- | -1 | 80 GeV | |
| Z^0 | 0 | 91 GeV |
These force carriers have integral spins and do not follow Pauli's exclusion principle. They are called "gauge bosons." It means that the interactions which these bosons mediate are described by "gauge theory." A gauge is a choice of local coordinates. Gauge theory describes how particles transform when coordinates are changed. Fundamental interactions obey gauge symmetry or local symmetry. In physics, symmetry means that something stays the same after a change is made. Gauge symmetry means that if the force carrier is transformed locally (that is, it is changed at any point in space), the interaction does not change, i.e., the physics is invariant.
A lingering question in particle physics is about where the mass comes from. How the masses associated with different particles get to be what they are? What makes an electron possess a 511 keV mass? If quarks are super light and gluons are massless, how do atoms become massive? Theorists believe that the answer will lie in a particle called Higgs. They believe that at a high enough energy, such as in an accelerator at the Large Hadron Collider (LHC), these Higgs particles will emerge in particle detectors for us to see.
roppon picha @ ucd npg