With IceCube steadily taking data and providing insights into high-energy neutrinos, the collaboration is looking ahead to detector extensions that will enable physicists to develop a deeper understanding of mysterious neutrinos. Enter PINGU, or the Precision IceCube Next Generation Upgrade. PINGU is run by the IceCube Collaboration and, like IceCube, is a multinational effort.
Primarily designed to measure the currently unknown mass ordering of the three neutrino types—tau neutrinos, muon neutrinos, and electron neutrinos—PINGU will ultimately reveal much more than that.
The collaboration appointed Darren Grant from the University of Alberta, Canada, and Doug Cowen from Penn State as conveners to represent PINGU given their involvement in developing the idea. In the interview below, Grant and Cowen talk about PINGU’s capabilities, where the idea came from, and where it is headed.
Q: PINGU stands for Precision IceCube Next Generation Upgrade. What does this mean? Why do we need such a precision extension?
Darren Grant: It was called the Precision IceCube Next Generation Upgrade because this upgrade, or addition, to IceCube will operate at a new level of precision compared to the current detector. We want to develop a detector that has reduced systematic uncertainties in order to measure effects that are a few percent in size. For example, by improving calibration, we can make a better ice model. Overall, the primary goal is to measure the neutrino mass hierarchy.
Doug Cowen: In the last few decades, our learning curve about neutrinos has been very steep, and physicists’ knowledge about the neutrino sector has grown dramatically. However, given that that there are 300 neutrinos in every cubic centimeter of the universe, and that the three different kinds of neutrinos represent fully 1/4 of the highly exclusive fundamental particle club membership, the things we still don’t know about neutrinos keep us from resting on our laurels.
One thing we are really interested in is determining the mass of neutrinos. Although we know that all three kinds of neutrinos have really low mass, we don’t know any of their actual masses. We do know something about their relative masses, but even here our knowledge is incomplete, because we only know the relative masses of two of the neutrinos—we don’t know whether the third neutrino is heavier or lighter than the other two. It is this relative ordering of the masses that is referred to as the “neutrino mass hierarchy,” and it is what we anticipate PINGU will be able to determine.
Q: Doug, will PINGU have upgraded hardware and software? How much will the design of the modules match those in IceCube?
DC: PINGU sensors are going to look very similar to IceCube sensors. The outer pressure vessel, the connector and mechanical assembly, and the photomultiplier tube will all be identical or nearly so. The main change will be the internal module electronics. These need to be replaced because a number of key electronic components in the sensors are no longer commercially available.
Q: The main goal of IceCube was to explore the origins of cosmic rays. IceCube has actually turned out to be a multipurpose detector. Is this also the case for PINGU?
DG: When you design a detector that can measure the neutrino mass hierarchy, you naturally have an instrument that will also have improved sensitivity to extracting the atmospheric neutrino mixing parameters, and can perform indirect dark matter searches at reduced dark matter particle masses, detect supernova neutrinos, and test the chemical composition of the Earth’s core (Earth tomography). Each of those analyses is planned as a part of the PINGU physics program.
DC: PINGU extends the energy range of IceCube down to energies that allow it to use low-energy atmospheric neutrinos to determine the neutrino mass hierarchy. IceCube is optimized for much higher energies, and it does not have enough sensitivity at lower energies to make this determination. Similarly, PINGU will not have enough sensitivity for the high-energy astrophysical neutrinos recently discovered by IceCube.
Q: How did the idea of PINGU come up?
DG: It started at the Maryland collaboration meeting in Spring 2010. A few of us went to dinner to celebrate the IceCube DeepCore strings coming online officially. It was the last evening of the meeting, and we had yet to find blue crab for dinner so we went to a crab shack at the harbor. On the large piece of paper they use to protect their tables during the meal, we started to sketch out what it could look like if we added more modules inside the DeepCore region. That was when the concept developed.
The name came about while Doug and I were traveling to Ohio State University for a workshop. The idea of PINGU was already starting to be discussed but it desperately needed a name. Doug and I both had been thinking of a name based on an appropriate mascot and recalled a claymation penguin from Switzerland named Pingu. He was on a popular children’s TV show. We then spent the next six hours developing an acronym that could work with the goals of the project—and managed to make it work.
DC: I was driving at the time, and we were so engrossed in our naming effort that I blasted past a state trooper stationed at the beginning of a construction zone. I got a speeding ticket, but we got a memorable acronym—a good trade.
Q: Could you please highlight how PINGU can improve or complement other projects beyond IceCube?
DG: Using atmospheric neutrinos, PINGU complements the plans of planned reactor neutrino projects like JUNO and RENO-50 and long baseline accelerator neutrinos projects like T2K, NOvA, and LBNE. Together with these projects, we should be sensitive to the mass hierarchy in a relatively short timescale. Like in the early 2020s.
Q: What’s the status of PINGU?
DC: We recently presented PINGU to the particle physics decadal review known as “P5.” This stands for “Particle Physics Project Prioritization Panel,” and its members are charged with evaluating the state of the field of particle physics and recommending which projects to fund over the next 10–20 years. We hope that P5 will recommend PINGU for funding. It doesn’t guarantee funding, but it’s a crucial step in the right direction. We then would have to apply to various agencies for funding.
Q: Finally, could you try to predict news for the future? What do you think could be the first important results published by the PINGU Collaboration once the detector is built?
DG: Our first result, with the first year of data, will almost certainly be the precision measurement of the atmospheric mixing parameters. These will be used for the mass hierarchy measurement, which we anticipate after approximately three years of data. After about a decade of data, PINGU might be able to make a direct measurement of the composition of the earth’s core by discerning differences in the pattern of low-energy atmospheric neutrino oscillations. This pattern depends on the density of electrons in the core of the earth, and that in turn depends on the proportions of various chemical elements in the core.
Update: The PINGU Letter of Intent has been published. “Letter of Intent: The Precision IceCube Next Generation Upgrade (PINGU),” IceCube-PINGU Collaboration. arXiv:1401.2046