Properties of the
W and Z Bosons

By: Professor Sarah Eno

There are only four known fundamental forces: the strong force, gravity, electromagnetism,and the weak force. Electromagnetism is responsible for most of the forces we see in everyday life, from the binding of atoms into molecules, to static electricity and friction. The strong force binds protons and neutrons into nuclei. Gravity causes apples to fall onto people's heads. But, what does the weak force do? And why would the Department of Energy pay Professor Sarah Eno, of the University of Maryland, and the scientists in her group, to jet to Chicago and Geneva Switzerland to study it? And, why are the labs and experiments there sooooo big?

Aerial photograph of Fermi National Accelerator Laboratory (http://www.fnal.gov) , near Chicago, Illinois. The circumference of a ring is 4 miles.	Experimentalists in France near CERN (European Organization for Nuclear Research, http://www.cern.ch) in Geneva Switzerland start to assemble the CMS detector (http://cmsdoc.cern.ch). This detector will go into the beam line in 2007.

The answer lies in what people in her field, experimental particle physics, call the "Standard Model" of particles and their interactions. This theory attempts to be a complete theory of all possible particles and their interactions, though at this point does not include gravity. You may have heard that there are 6 types of quarks, but electromagnetism only cares about the charge of the quark, and there are only 2 possible charges, 1/3rd of the electron charge, and 2/3rd s of the electron charge. To the strong force, there are only 3 charges, called "red", "green", and "blue". It's the weak force that "knows" there are 6 types, the up, down, strange, charm, top, and bottom quarks. It's the weak force that allows one type of quark to turn into another type (and thus the radioactive decays of atoms). It's also the reason that the mass of the top quark is about 200x that of the proton, while the masses of the up and down quarks, that make up the proton, are of order 1/3rd the proton mass. During the 1960's, there was a revolution in the study of the fundamental forces. Theorists such as Abdus Salaam, Sheldon Glashow, and Stephen Weinberg realized that electromagnetism and the weak force could be described with the same mathematics, unifying them in the sense that Maxwell unified electricity and magnetism in the 1860's. Some of the apparent differences between electromagnetism and the weak forces come not because of any fundamental difference in their mathematical description, but because the carrier of the force, the photon in the case of electromagnetism, the W and Z bosons in the case of the weak force, have very different masses. While the photon is massless, the W and Z bosons have a mass around 100x the proton mass. Because the W and Z are so heavy, very large accelerators are needed to accelerate "normal" particles (in the case of FNAL, protons and antiprotons, in the case of CERN, protons) to high enough energy to create these particles in their collisions.

The Tevatron at FNAL is currently the highest energy particle accelerator in the world. Sarah and her colleagues Professors Hadley and Baden work on the DØ experiment (http://www-d0.fnal.gov) and travel there approximately every other week, to meet with their students and postdocs who live permanently at the lab. Sarah's postdoc, Marco Verzocchi, is the head of the group studying W and Z bosons at the Tevatron. We currently have a sample of approximately 30,000 Z's and 400,000's W's, and expect to collect a factor 4 more by 2007. We hope to measure properties of the W, like its decay width, mass, and production properties to high precision. This is important, because if we can measure the W mass to 0.3%, we can predict the mass of a particle that is predicted to exist by the standard model, but that as of yet has not been discovered, the Higgs boson. This particle is the only particle in the standard model predicted to have a spin of zero. In some ways, it resembles the old "ether" theory, in that the Higgs is supposed to create a field that permeates the entire universe. Through interactions with the standard model particles, the Higgs creates mass. Discovering (or ruling out) the existence of the Higgs particle is one of the primary goals of experimental high energy physics today, and is the primary goal of the CMS experiment, which Sarah also works on, along with Professors Skuja and Baden. Our group has sent undergraduate students to both labs during the summer to help us with our research.

This is an exciting time in high energy physics. The top quark was discovered (at the Tevatron, by DØ) in the early 90's. Neutrinos were unexpectedly found to have mass in the late 90's. With the Tevatron running now, and a new machine at CERN coming on line soon, we expect more exciting discoveries in the coming decade!

Its fun to work with researchers from all over the world.	The resonance at 90 GeV/c2 in the di-electron invariant mass spectra from D0 data is due to the Z boson.