Hen’s hunt for neutrinos
MIT’s Hen Lab finds creative ways to study nature’s most elusive particle
The neutrino: a tiny, nearly-massless white rabbit that has haunted physics for years. Originally postulated by Wolfgang Pauli in 1930, the neutrino was an accounting tool used to satisfy certain conservation laws in processes like beta decay, where, for example, a neutron becomes a proton by emitting an electron and an electron antineutrino, the counterpart of a neutrino.
The neutrino is a type of lepton, a class of fundamental particles in the Standard Model. Leptons come in two types: electrically charged leptons, which consist of the familiar electron and its heavier cousins, the muon and the tau; and electrically neutral leptons — neutrinos — which are the electron neutrino, muon neutrino, and tau neutrino, named according to their charged leptonic partners. For instance, a neutron decays into a proton, and electron, and an electron neutrino.
Since Pauli’s postulation, various efforts have been made to detect the neutrino and thus prove its existence, but since it is electrically neutral and its mass is incredibly small — originally thought to be zero — physicists have had a very difficult time studying neutrinos. Only in 2015 was it discovered that neutrinos have mass at all, and now high-energy physicists are chomping at the bit to learn more about this mysterious little particle.
The 2015 Nobel Prize in Physics was awarded for the discovery of neutrino oscillation, which describes how neutrinos can switch among the three flavors. Perhaps at some point, a physicist will detect a beam of electron neutrinos, then at a later time they will find that some of the electron neutrinos are now tau neutrinos, or muon neutrinos. This is one of the only experimentally observed phenomena of neutrinos which is hard to explain in the context of the Standard Model. However, for the Hen Lab, interesting observations are not daunting enough — they actually measure these oscillations to learn new physics about neutrinos.
The Hen Lab's neutrino group is one of the Institute’s most unique groups; not only is the group made up entirely of women — except for Or Hen, the lab’s namesake — but it also studies nature’s most elusive particle in a very special way. The Hen Lab is an experimental nuclear physics group, and they study the structure of the nucleus through scattering experiments. “We’re basically like five-year-old kids,” says the group's primary investigator, Or Hen. “We take an atomic nucleus, we throw stuff at it to break it apart, and then use big detectors that measure the stuff that comes out to understand how it's built.” From these scattering experiments, the group reconstructs energy and other conserved quantities and aims to learn how the nucleus behaves. The group uses the scattering information to study neutrinos.
One of the group’s many exciting projects is the “Electrons for Neutrinos” project. This project takes advantage of the fact that electrons and neutrinos behave similarly in scattering experiments without having to rely solely on neutrinos, which are notoriously hard to detect. In these scattering experiments, they allow an electron to interact with a nucleus, and they detect the particles that shoot out from this interaction. From these final-state particles, they use energy conservation to reconstruct the energy of the initial particle and use data to uncover the “black box” interactions inside the nucleus.
However, beams of neutrinos are hard to control and often contain neutrinos with a range of energies, which adds more complications to the reconstruction process. Particle accelerators nowadays allow scientists to “tune” the energies of electrons in order to precisely conduct experiments, a method that is not feasible for neutrinos.
There is a very useful fact, known as the nuclear environment effect, that makes it easy to substitute electrons for neutrinos in scattering experiments: when a particle like a proton is scattered out from a nucleus, there is no information that tells the observer what type of interaction made the proton fly out from the nucleus. So if it was a neutrino or a proton, the experimenter wouldn't be able to tell — up to this point, the physics is the same. Taking advantage of not only this useful fact, but also the ability for scientists to “tune” electron energies, the Hen Lab is able to effectively study neutrinos by studying electrons instead. This allows the group to study the physics that neutrinos and electrons have in common without only relying on the headache of detecting neutrinos. Moreover, it bridges the gap between the nuclear physics community and the neutrino community, a rare occurrence.
The road to finally running a nuclear or particle physics experiment is long. Typically, these types of experiments use big accelerators like the Large Hadron Collider, and these accelerators take decades to develop and construct. And for experimentalists to even use these accelerators, they have to wait in line behind other scientists who hope to use the accelerators for their own projects, so it can be quite a while until a lab is able to actually perform their experiments. To overcome this obstacle, the Hen Lab, which relies on data from Jefferson Lab’s CEBAF Large Acceptance Spectrometer (CLAS), is approaching the problem from a data-driven standpoint. “In neutrino experiments we rely on good simulations of neutrino interactions with the nuclei. Those need to be constrained using external data,” says postdoc Adi Ashkenazi, a member of the Hen Lab. Over its many iterations, CLAS has produced lots of data that is ready to use. “Even though the CLAS experiment was constructed and operated a long time ago, its data is most relevant for this purpose.” Equipped with knowledge that was unavailable when the data was initially taken, the Hen Lab can analyze the data using their new methods, which in turn encourages the neutrino community to support further experiments in the field.
Anjali Nambrath ’21, one of the Hen Lab’s UROP students, is using deuterium, a hydrogen isotope, to improve the group's methods. “Deuterium is very simple, [...] so nuclear interactions can be abstracted away and you can look at the energy reconstruction technique in as close to a vacuum as you can get,” she says. This is an example of the group’s unique approach, unifying methods used by both experimentalists and theorists. By approaching the “two-body problem” of deuterium, the group can improve the reconstruction methods employed in experiments that use more complicated targets like argon.
As the Hen Lab’s methods get more precise, the group is looking forward to a promising future. “This next decade will be the most exciting for neutrino physics,” Ashkenazi says, as she believes the decades-long process of planning and constructing experiments is coming to an end, and finally the plentiful epoch of data collection and analysis is approaching. Afroditi Papadopoulou G, a graduate student in the group, is excited to improve the analysis of data from experiments like MicroBooNE, and she looks forward to the summer of 2021 when Jefferson Lab will be dedicating half of its beam time to producing data that will support the Electrons for Neutrinos project. Nambrath will be leading research like this at just the right time, when experiments have been built and tested and are producing lots of data to study.
Hen himself has seen the Electrons for Neutrinos project grow from a small group to a large collaboration. “There are very few significant collaborations in the field. We are in the process of becoming one, and that’s super exciting,” he says, and he is excited to see the project continue and unite the many interdisciplinary subfields of physics. He has high hopes for students and their ability to contribute to diverse collaborations. He encourages students to do what Anjali did as a first year: take a leap, reach out to a group you’re interested in — no matter how scary — and get involved. It has reached this far already, and the Hen Lab is certainly one to watch in the coming decade.
Update 9/1/20: This article was updated to indicate that the Hen Lab's neutrino group's researchers are all women. A previous version stated that all of the Hen Lab's researchers were women.
Update 9/16/20: This article was updated to state that in beta decay, when a neutron becomes a proton, it emits an electron and an electron antineutrino. A previous version wrote that a neutron emits an electron and a neutrino.