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How do today's scientists use conservation of momentum?

Top Home - Find the Mass

In the DØ experiment physicists have recorded data from a collision between a proton and an antiproton that produced a top and an antitop quark.

To find the top quark mass you need a data printout (color is best), a ruler and protractor and paper to record your analysis. (Sample Analysis Sheet)

Mass: The mass is determined from the magnitude of the momenta. (The link below explains the relationship.) While the momentum of the total system is zero, the momenta of the various particles are very different. We need to know these numbers.

Momentum: The data plot shows the direction of the momenta and the magnitude reported in GeV/c for all particle debris seen by the detector.

What's missing? You have to find all the numbers.

Make a momentum vector diagram. Remember momentum is conserved! The total momentum of the system – the proton and antiproton – before the collision is zero, and it must be zero after the collision. The vectors must add to zero.

When you make the vector diagram, measuring the angle is a bit tricky from these plots because the red and blue towers that represent the momentum of the jets are very wide. Pick the angle that you think best represents the direction of the momentum.

Ready? Get the Data

Need help or want more information? Check these links.

How To: Determine Mass - Work with Vectors

Resources: Animation - Background - Online Resources

How does your solution compare with those of your classmates? You can be more confident of your result if you average several solutions. Physicists look at the data from multiple events before they announce results.

Reflect on your work. What have you done? What have you learned?