The Weak Mixing Angle & The NuTeV Anomaly

The weak mixing angle is one of the parameters of the "Standard Model," our theory of particle interactions. This angle is related to the probability that a neutrino will interact with matter to produce a scattered neutrino plus the remains of the target. In particle physics, we describe interactions as exchanges of particles. In the case just described, the neutrino is exchanging a Z-boson with the target when it scatters.


The diagram at the top of the page illustrates a neutrino exchanging a Z-boson with a proton containing three quarks. The neutrino (purple) exchanges a Z-boson (orange) with one of the quarks (blue). This causes the neutrino to scatter. The proton may bounce off in the other direction or it may break up into many particles.


This weak mixing angle can be measured in many different ways. Any interaction where a Z-boson is exchanged will be sensitive to this parameter. Neutrinos do not have to be involved. But no matter what particle is involved in the interaction, this parameter should always have the same measured value, within errors.


Therein lies the problem. The weak mixing angle measured by the NuTeV neutrino experiment is not in agreement with the weak mixing angle measurement from other types of interactions. This is called the NuTeV anomaly.


The NuTeV anomaly has generated more than 150 theoretical papers and a lot of excitement. The systematic errors on the experiment have been carefully studied and the explanation for the deviation has not been identitified. So now we need a new experiment to probe further into the question: is new physics causing the NuTeV anomaly?

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