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MINERvA is a neutrino scattering experiment that prides itself on being able measure in exquisite detail the probability that a neutrino will interact: We look for many different reactions on many different nuclei. However, in order to measure those probabilities, we have to know precisely how many neutrinos are produced in the first place.
Although Fermilab’s Accelerator Division can tell MINERvA just how many protons it delivers to the target that starts the NuMI beamline, knowing how many neutrinos are actually produced after those protons hit the target is tricky. How many particles are made by the protons hitting the target? Where do those resulting particles go afterward? How many of them subsequently create neutrinos? How many of those neutrinos pass through the detector located a kilometer away?
Until now, MINERvA has had to rely on other experiments’ measurements of what happens when protons hit a target to predict how many neutrinos pass through the detector. But now MINERvA has its own way to measure how many neutrinos were produced. Although we know little about neutrino interactions with atomic nuclei, there is one reaction that is extremely well known: when a neutrino interacts with an electron. By measuring that one process, we can predict how many neutrinos must have originally passed through the detector.
This is similar to predicting the number of snowflakes that fall on Fermilab’s 27.5-square-kilometer campus by measuring the number of snowflakes that fall into a cup in the parking lot. We can’t count all of the snowflakes that fall on the campus, but it’s possible to count those in a small cup. By knowing the area covered by our cup, we can extrapolate to all of Fermilab grounds.
But here’s the trick: You need to collect enough snow in the cup to make a precise measurement.
The probability of a neutrino scattering off an electron is precisely known, but it is tiny, even by the standards of rarely interacting neutrinos! So to measure this process you need a very fine-grained detector to see a single high-energy electron that seems to appear out of nowhere and that travels in the same direction as the neutrino beam (see above figure).
After analyzing hundreds of thousands of interactions, MINERvA found 121 of these events and predicts that only 33 of them are from background processes. In the new run, MINERvA expects to see at least 10 times more events and thus hopes to measure the number of neutrinos even more precisely.