NuTeV - A High Energy Physics Experiment

The second Stop on a Tour for the Non-Physicist


Now that you have some idea how this detector detects the passage of a charged particle, and what we can measure when it does so, it is time to talk about the types of event for which we wish to collect data. NuTeV is a deep inelastic neutrino scattering experiment; this means that a high energy neutrino interacts with a single quark in a proton or neutron (nucleon), exchanging lots of energy, and blowing the nucleon to bits in the process. Neutrinos interact only by way of the weak force, and can do so in two distinct ways, called "charged current" and "neutral current". I will show you an example of each and explain a bit about them. First, a charged current event:

What you are looking at here on the left is a computer generated event display of our detector (top and side views), and on the right, a Feynman diagram, a sort of physics cartoon of what happens in a charged current interaction. First lets look at the event display. (If you would like to see a larger, clearer picture of an event, click here.) A neutrino enters the detector from the left, but we don't see anything! This is because neutrinos are electrically neutral, and scintillation counters and drift chambers detect only charged particles. The neutrino travels nearly half way through the target before interacting. As the interaction occurs, the energy exchanged creates a shower of charged particles as the neucleon is blown apart, and we see this energy in a few downstream scintillation counters. The bar graph above the detector in the event display represents this energy. We also see several "hits" in the drift chambers, represented by x's in the display. The hadronic debris from the neucleon does not travel far through all this steel, however you see that one particle leaves a long track through the rest of the detector. This is a muon, and the signature of a charged current event.

The Feynman diagram tells us more about what is going on. The neutrino and muon are shown in the upper part of the diagram, the nucleon in the lower part, and the wiggly line connecting the two parts represents the exchange boson, or force-carrying particle. In this diagram, the exchange boson is a W, and is itself charged. Since the neutrino is neutral, and charge must always be conserved, a "flavor change" takes place, turning the neutral neutrino into a charged muon, thus the name - charged current event.

In this event we can measure the energy of the hadron shower, the momentum of the muon as its track is bent in the toroid, and the angle of the muon track with respect to the neutrino beam. By measuring these three quantities for a large number of events, and applying some conservation laws and some mathematical manipulation, physicists can then make statements regarding the structure of nucleons. This is one of the major goals of NuTeV.

The second type of neutrino interaction we observe is called a neutral current event:

Again we see nothing entering the detector, but now nothing is seen leaving either. All we see is the burst of energy and short tracks of a hadron shower. The Feynman diagram makes this clear; a neutral Z particle is the exchange boson in this event, so the neutrino does not undergo a flavor change, but leaves without making a track. The only thing we can measure in this event is the hadron energy but we can recognise it as a neutral current event, and that is enough to help accomplish a second major goal of NuTeV. The ratio of neutral current to charged current events is related to an electro-weak parameter called the weak mixing angle, so by counting the two types of event we can determine a value for this parameter.

Now that you know something about the types of event we are looking for, and what they look like in our detector, let's go to the Trigger Room and see how the trigger electronics can automatically recognize these events and send the data to the data acquisition computer.

Continue the tour.

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Len Bugel: bugel@miranda.fnal.gov