The D-Zero Experiment - May 7, 2005
The W and Z bosons are particles that carry the weak force, one of the four fundamental forces found in nature. According to our standard model the W and Z bosons will interact with each other due to their weak charge. In contrast, photons, the carriers of the electromagnetic force, do not carry electromagnetic charge, and do not interact with each other.
At Fermilab, we can study the strength of the interaction of the W boson with the Z boson by identifying and studying collisions in which both are produced at the same time. By counting the number of occurrences and measuring the properties of the WZ events, we can test the strength of the interaction between the bosons.
These particles are very massive. The W boson weighs in at 85 times the mass of a proton. The Z boson is even heavier. Shortly after they are produced, they disintegrate into lighter types of matter and we detect these "decay products". The Figure shows one of our WZ events.
Fermilab's proton-antiproton collider, the Tevatron, is the only particle accelerator in the world that has ever produced both a W and a Z boson in the same collision. They are very rare collisions, indeed, and nobody has ever claimed to have seen them before this. We scoured approximately 14 trillion collisions produced between April, 2002 and June, 2004 and found three events with both a W and a Z boson in them. We estimate that processes which look like, but aren't WZ production, will provide us three or more such events 3.5% of the time.
With these three candidates we are able to measure the rate the Tevatron produces WZ events. Also, we set the best constraints on the strength of the interaction between the W and Z bosons involving only this process.
An event display picture can be seen here.
Figure 1. End-on view of one of three WZ events. The collision occured at the center of the detector. A Z boson decayed to two muons (two of the green tracks). A W boson candidate decayed to a muon and a neutrino (a green track and purple arrow opposite to it). Other lower energy charged-particles were produced in the collision and are represented as grey tracks. The straighter the track, the more energetic the particle. The red and blue towers in between the dark circles represent energy deposited in our calorimeter by the particles created in the collisions.
This result was sent to Physics Review Letters on April 11, 2005. If you are interested in reading more about this physics please see our paper.