The New Phenomena Group: a guide for non-physicists

What are we made of?

How many fundamentally different kinds of particles exist in nature? How many different kinds of forces? These are the questions the D0 New Phenomena group is trying to answer. Our current answers to these questions evolved in the 1970's. Our model, called the "Standard Model", contains 3 forces, the electroweak force, the strong force, and gravity, and two types of particles, fermions and bosons. Fermions are matter particles, and make up the "stuff" of the universe. Bosons are particles which are exchanged between the fermions and cause the forces between them. There are two types of fermions, quarks and leptons. The proton is made of quarks. The electron is a lepton. There are one or more bosons for each force. One aspect of the electroweak force is electricity, the force most noticable in our day-to-day lives. Electricity is responsible for the binding of atoms into molecules, and thus for the chemical properties of all the materials we observe on earth. The boson for the electric force is light, which physicists call a photon. The weak force causes the radioactive decay of some atoms by allowing one type of fermion to change into another by emitting a weak boson. For example, an "down" quark inside a proton can change into a "up" quark by emitting a weak-type boson. The weak boson can then change into an electron and another type of lepton called a neutrino. This is called beta decay. The strong force is responsible for binding 3 quarks into a proton. Gravity is the force made famous by Newton and Einstein.

How do we know this?

The "Standard Model" has been tested in a wide range of experiments since its conception. Particle "factories", large accelerators with high- enough energy to produce all the known types of particles, have produced them in copious quantities, and have studied all their properites to high precision. The LEP accelerator in Europe produces 5 types of quarks, the strong bosons, and 2 types of weak bosons. The Fermilab accelerator here in Chicago produces the same particles, plus the heaviest quark, the "top" quark. Specialized accelerators also exist. An accelerator in Ithaca NY produces the "tau" lepton and particles containing the "b" quark (and other particles as well). Brookhaven lab in Long Island and Fermilab produces particles containing the "strange" quark. The results from the experiments located at these accelerators so far are in good agreement with the "Standard Model".

So, is everything already known? No!

Is the "Standard Model" a complete picture? Most physicists think not. Two important building blocks of our theories, symmetry and unification, hint that a more complete theory may exist. Unification has always played an important role in theory-building. The electric and magnetic force were united by Maxwell in the 1860's into the electromagnetic force. The Weak force and the electromagnetic force were united by Glashow, Salam, and Weinberg, amoung others, in the 1960's. Can the electroweak force be unified with gravity and and the strong force? Several "Grand Unified" theories exist that are compatible with all known experimental results. The other building block of the "Standard Model" is symmetry. As you know, atoms are made up of protons, neutrons and electrons. The electron is very much lighter than the proton or the neutron, and does not participate in the strong force. However, the neutron and the proton are almost exactly alike, except that one is electrically charged, the other not. The neutron can also turn into a proton by emitting a weak boson. We say there is a symmetry between them, which is the symmetry of the weak force. The neutron and the proton are in a way two different aspsects of the same particle, related by the Weak symmetry. A new proposed symmetry is called, unimaginatively, Super symmetry. This symmetry would relate the bosons and the fermions. So, for each particle that currently exists, there would have to be a "super" partner, a particle which would be exactly like a known particle, but would have the opposite fermion/boson-ness. Both models which unify the forces, and models that introduce new symmetries, predict new heavy particles. If we collider ordinary matter at high-enough energy, we can produce these exotics particles and study their properties. Fermi National Accelerator Laboratory has the highest energy in the world. It has a center of mass energy of 1.8 TeV. Most of the new theories predict that particles should exist that are light enough to be produced at this collider. If so, the D0 new phenomena group will find them!