Electrons for neutrinos

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Neutrinos are ethereal particles which are extremely difficult to detect. They have a high abundance in the universe (around a trillion neutrinos pass through the average person every second), but having masses less than a millionth that of an electron and only interacting via the weak force and gravity, their probability of interaction is very small. In fact, neutrinos can pass through the entire diameter of the earth without interaction!So how one can observe and study these ethereal particles at all in terrestrial experiments? The solution is to construct a detector that is made up of as large a volume of material as possible and to produce neutrino beams with incredibly intense flux. Such facilities are operational and exciting new facilities are planned. For example, in the Deep underground Neutrino Experiment (DUNE) under construction, neutrinos produced at Fermilab will pass 800 miles through the earths mantle to another laboratory in Sanford, where detectors comprise 40 tonnes of liquid Argon! The rare interactions of neutrinos in detectors are obtained by examining the reaction products produced (usually with detection of a knocked-out proton) when a neutrino interacts with an atomic nucleus. The energy of the neutrino that produced the reaction is then inferred by nuclear reaction theoretical models. The problems stem from the fact that the atomic nuclei used for this purpose need to be large in order to get enough neutrino induced events; for example the Argon at Dune has 40 protons and neutrons in it's nucleus. A large nucleus is a very complicated object and the modelling of the reaction processes is very difficult with lots of outstanding questions: Can we suppress events where the knocked out nucleon scattered on it's way out of the nucleus? How well can we suppress contributions where the neutrino is absorbed on more than one nucleon and we only detect one? Producing a pion from a nucleon has a similar probability to knockout, but how well can we suppress these erroneous events in the experimental data? Can we get rid of processes where a pion (or pions) are initially produced in the nucleus and then reabsorbed to knock out nucleons? What about if we excite a nucleon in the reaction mechanism, and how do such excited nucleons behave in the nucleus? These are merely a small set of outstanding questions that directly impact the determination of the incident neutrino energy and flux.In our programme we will use an analogous reaction to the neutrino-nucleus interactions: we will scatter electrons off the nuclei rather than neutrinos. This has the advantage that we get many orders of magnitude more events to test the models and very importantly, in a controlled way! By knowing the incident electron energy, all the assumptions in getting from nuclear fragments to the beam energy for a whole host of reactions and a wide range of nuclei becomes accessible. This data set is urgently needed to reduce errors in the theoretical modelling; these errors typically produce the largest systematic error for the neutrino experiments. By using common theoretical models for the neutrino- and electron-induced reactions we can challenge the models and improve the modelling of various processes at a level of details that was previously impossible. This then reduces significantly any systematic errors in extracting physics from the next generation neutrino facilities. Progress requires a major programme of analysis of existing, as well as planned electron scattering experiments from nuclei with complex detector systems at Jefferson Laboratory (USA). We will construct a new analysis framework, which will be used to analyse and archive data in a form that makes it readily accessible and flexible for use by nuclear and particle communities for decades to come. The work will be carried out as part of a new collaborative network including colleagues at MIT, ODU, Jefferson Lab and Tel Aviv University.