The D-Zero Experiment - February 13, 1998

  From 1992-1995 we observed nearly 6 trillion collisions and identified 
  39 collisions in which both a Z boson and a photon were produced.  We 
  studied the properties of these events and published what we learned. 

  The Tevatron accelerates protons and anti-protons in opposite directions 
  around a circular path and collides them head-on, converting their 
  energy-of-motion into new particles.  Our detector identifies the 
  particles, determines where they went, and how much energy they had. 

  The Standard Model of electroweak interactions describes two ways for 
  proton/anti-proton collisions to produce events with a Z boson and a 
  photon in them. It also predicts characteristics of the events such as 
  the energy spectrum of the photons and the center-of-mass energy of the 
  Z boson plus the photon.  The theory predicts that there are no direct 
  interactions between the Z boson and the photon because they both have 
  neutral charge. A non-Standard Model describes what would happen if there 
  were direct interactions, called anomalous couplings, and predicts that 
  the characteristics of a collection of such Z boson + photon events would 
  differ from the Standard Model prediction.  We test this prediction. 

  We studied three decay modes of the Z boson which we can easily identify. 
  In total we found 17 candidate events where the Z decayed into a muon and 
  anti-muon, 18 candidate events where the Z decayed into an electron and 
  anti-electron (positron), and 4 candidate events (in the first 1/8 of the 
  collisions) where the Z decayed into a neutrino and anti-neutrino.  In each 
  case, we found that the number and characteristics of the events were 
  consistent  with the predictions of the Standard Model.  There were a 
  couple of electron anti-electron events where the photon had ~75 GeV/c 
  energy in the direction perpendicular to the original direction of the 
  protons and anti-protons and where the mass of the Z boson plus the photon 
  was ~200 GeV/c^2 (supposing they originated from the decay of a heavier 
  particle).  This is interesting because we know of no particles with a mass 
  that high.  However, we estimate that 1 time in 8 repeats of our experiment 
  the Standard Model would produce such a result.  Thus, this is a tantalizing
  prospect but not evidence of a new particle or interaction.  The neutrino 
  anti-neutrino measurement is particularly interesting because it is unique 
  to the D0 detector. 

  We set limits on the maximum value of anomalous couplings that could 
  reproduce the data.  They are the most restrictive limits on Z-Z-photon 
  and Z-photon-photon couplings available in the world.  We hope to improve
  the result within the next year when we finish the analysis of the remaining
  7/8 of the neutrino anti-neutrino data. 

  The results were summarized in publications in journals which are read by
  physicists around the world.  If you are interested in seeing what these 
  papers are like, see:

• Phys. Rev. Letters 75, 1028 (1995) Preprint at HEP-EX.
• Phys. Rev. Letters 78, 3640 (1997) Preprint at HEP-EX.
• Paper to be published April 1, 1998 in Phys. Rev. D Preprint at HEP-EX