The Elementary Particles Santa Rosa Junior College Physics 4D – Younes Ataiiyan May 11 th 2006 Stephen Ngamate and Thomas Mutunga

Introduction In ancient times, people sought to organize the world around them into fundamental elements, such as earth, air, fire, and water. Today we know that there is something more fundamental than earth, water, air, and fire……THE ATOM But is the atom fundamental?

Is the Atom Fundamental? Scientists soon realized that they could categorize atoms into groups that shared similar chemical properties (as in the Periodic Table of the Elements). This indicated that atoms were made up of simpler building blocks, and that it was these simpler building blocks in different combinations that determined which atoms had which chemical properties. Various experiments helped scientists determine that atoms have a tiny but dense, positive nucleus and a cloud of negative electrons (e - ).

Is the Nucleus Fundamental? Because it appeared small, solid, and dense, scientists originally thought that the nucleus was fundamental. Later, they discovered that it was made of protons (p + ), which are positively charged, and neutrons (n), which have no charge. So, then, are protons and neutrons fundamental?

Are Protons and Neutrons Fundamental? Physicists have discovered that protons and neutrons are composed of even smaller particles called quarks. As far as we know, quarks are like points in geometry. They're not made up of anything else.

The Fundamental Particles After extensive research,scientists now have developed The Standard Model. The model classifies quarks and leptons as the fundamental particles and they are held together by fundamental interactions. Leptons and quarks are collectively called fermions.

Leptons There are six leptons, three of which have electrical charge and three of which do not. They appear to be point-like particles without internal structure. The best known lepton is the electron (e - ). The other two charged leptons are the muon (µ ) and the tau (  ), which are charged like electrons but have a lot more mass. The other leptons are the three types of neutrinos ( ). Neutrinos have no electrical charge, very little mass, and they are very hard to find.

Quarks Quarks are one type of matter particle. Most of the matter we see around us is made from protons and neutrons, which are composed of quarks. There are six quarks, but physicists usually talk about them in terms of three pairs: up/down, charm/strange, and top/bottom. (Also, for each of these quarks, there is a corresponding antiquark.) Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively. Quarks also carry another type of charge called color charge. The most elusive quark, the top quark, was discovered in 1995 after its existence had been theorized for 20 years. Quarks only exist in groups with other quarks and are never found alone.

Fundamental interactions A fundamental interaction is a mechanism by which particles interact with each other, and which cannot be explained by another more fundamental interaction. The table below illustrates the fundamental interactions and their mediating particles.

Problems With the Standard Model Although all experimental evidence confirms the predictions of the Standard Model, many physicists find this model to be unsatisfactory due to: Many undetermined parameters. Many fundamental particles. The non-observation of the Higgs boson. Too many theoretical considerations such as the hierarchy problem. There are many speculative theories beyond the Standard Model which attempt to rectify these deficiencies.

Beyond the Standard Model Grand unification - attempts to combine the strong, weak, electromagnetic and gravitational forces into one unified force in the 'grand unified theory' (GUT). Super symmetry - extends the Standard Model by adding an additional class of symmetries to the Lagrangian. These symmetries exchange fermionic particles with bosonic ones. Such a symmetry predicts the existence of super symmetric particles, abbreviated as sparticles, which include the sleptons, squarks, neutralinos and charginos. String theory - According to string theorists, each kind of fundamental particle corresponds to a different pattern of fundamental string. All strings are essentially the same, although they may be open (lines) or closed (loops). Preon theory - According to the preon theory there are one or more orders of particles more fundamental than those (or most of those) found in the Standard Model.It is not widely accepted after rules out in 1980.

References Stephen, Thornton and Andrew, Rex. Modern Physics for Scientists and Engineers. Belmont: Thomson Brooks/Cole, 2006