The Z° Boson:
Calculating the Mass

In this exercise we will be looking at Z0 boson decays and measuring the mass of the particle by examining it's decay products. This is necessary because the Z0 boson decays too quickly to allow us to measure it directly. Here we make use of the invariant mass introduced on the last page. By conserving energy and momentum we find that the rest mass of a decaying particle is equal to the invariant mass of it's decay products.

The applet on the right of your screen is a simulation of a detector in the D0 Experiment. In the detector you can see an 'event', showing the tracks of the particles produced in a Z0 boson decay. You can cycle through the available events with the controls at the bottom of the applet. By adding each decay particle to the invariant mass of the system and then adding the invariant mass of several systems (one per event) to the graph you will find the mass of the Z0 boson at the peak. The points on the graph are spread around the mass of the Z0 boson due to experimental error in the data provided.

In summary:

  • Click on a particle track to select the particle and see it's properties
  • Click on 'add' next to 'particle list' to add the selected particle to the invariant mass of the system
  • Click on the drop-down "particle list" to see which particles are currently included
  • Click on "add" next to 'graph' to add the invariant mass of the system to the graph
  • Add the invariant mass of several events to find the mass of the Z0 boson at the peak of your graph
  • You can also remove particles from the invariant mass and data points from the graph
  • Zoom in on an area of the graph by holding down SHIFT and drawing a box with your mouse - click on the graph to return to the original scale.

 


Last modified Fri  8 February 2002 . View page history
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