Our current understanding of the universe includes 4 forces: gravity,
electromagnetism, the strong force, and the weak force. Particle physics has had
considerable success at understanding these forces, however so far it is unable
to coherently include gravity. The electromagnetic force is carried (propagated)
by the familiar photon; the strong force is carried by gluons, and the weak
force is carried by Z and W bosons.
Events with a Z and a photon were studied by both the DZero and CDF collaborations using Run I data colleced between 1992 and 1995. Theory predicts that Z bosons and photons may be produced together in a proton-antiproton collision, however the Z boson and the photon do not directly interact with each other. If an excess of collisions resulting in Z+photon events is observed, it could be a sign of unexpected interactions (couplings) between the Z and photon. Run I studies specifically measured the cross section, which is the probability of a proton anti-proton collision producing a Z+photon event, and set limits on coupling between the Z and photon.
New measurements of the cross section and limits on the coupling between the Z and photon have been made. The transverse energy (energy perpendicular to the beam) of the photons from these events are shown in the figure. These current results are much better than the previous results, partly because the much larger Run II data sample yields ten times as many Z+photon candidate collisions as found in Run I. DZero detects the Z+gamma events in final states where the Z boson decays into a pair of electrons or muons (the heavy cousin of the electron). DZero observes 290 candidate events, with a background expectation of 46 events. The data were searched for events containing a high energy photon, which might signal unexpected physics. Observations are in good agreement with Standard Model theory predictions (as shown in the figure). Using the spectrum of the photons, limits were set on the coupling of the photon to the Z boson.