If you thought finding Nemo was hard, think again. Imagine a trillion (that’s one followed by 12 zeros) little Nemos traveling through your body each second without you even noticing them. Give it a day, a month, or even a year, and you wouldn’t catch one. This is happening to you right now with neutrinos, coming from the sun. Even though they are everywhere, they rarely interact with anything due to their tiny mass and lack of charge.
Physicists to the rescue! Fortunately, there is a way to overcome neutrinos’ elusiveness by creating intense neutrino beams and building large dense detectors, like MINERvA, made of heavy atoms packed with protons and neutrons.
Great! Now we are ready to make measurements of how neutrinos interact, right? But it’s not quite that easy. For one, we can detect neutrinos only indirectly, through some telltale signature of their presence. And then there are the fakers: Having these big atomic nuclei poses complicated issues, as several different effects come into play. Some of these effects can mimic what the experimenter is actually looking for. Physicists therefore need to take great care in understanding what fakes their signal and to constrain it so that they can precisely measure their interaction of interest.
Cheryl Patrick and the MINERvA collaboration did just this, taking it one step further with a double differential cross section. At the June 17 Wine and Cheese Seminar, Cheryl presented results on the latest antineutrino measurement at MINERvA. By searching for events with a single antimuon, any number of neutrons and only low-energy protons, she was able to look for a particular type of action that experimenters are interested in and present how various models compared with data in two dimensions.
Why are two dimensions better than one? Think of being in a restaurant and finishing your meal with a coffee. You love the flavor but want to make it a little more sweet. On your table are two identical pots that look like sugar, but while one contains sugar, the other has salt. You don’t want to spoil your coffee by adding the wrong white powder, but the test using just your eyes is not enough to tell them apart. To get a definitive answer, you can use your tongue to taste which is right. This is how we use multiple dimensions to add information and better understand what’s happening. In neutrino physics these dimensions can be different aspects of the final particles that we can measure in our detector.
The work reported by Cheryl Patrick looked at variables in two dimensions to not only isolate signal events from those faking it, but also to enable a better understanding of how these results compare to theoretical models of the nucleus. These models will then be used as early as when NOvA starts looking at antineutrino interactions this month.
David Coplowe is a physicist at Oxford University.