MicroBooNE ‘shines a flashlight’ on tricky neutrinos

These are boom times for Yale physicist Bonnie Fleming and the Micro Booster Neutrino Experiment (MicroBooNE).

On Oct. 27, Fleming and other leaders of the international experiment announced the first results of MicroBooNE’s search for an anomaly that could have indicated a fourth type of neutrino, a subatomic particle considered a fundamental building block of matter.

Created from the decay of radioactive elements, neutrinos carry no electric charge and travel through the universe almost entirely unaffected by natural forces. There are three known types, or “flavors,” of neutrino: electron, muon, and tau. A theorized fourth neutrino, the sterile neutrino, would exist outside the current parameters of the Standard Model of Particle Physics — and may explain certain “anomalies” in data coming out of an earlier experiment called MiniBooNE and other experiments.

The discovery of a new particle would be a transformative moment for scientific inquiry.

Launched in 2008, the MicroBooNE experiment studies how neutrinos interact and change within a distance of 500 meters. Its 40-foot-long detector, located outside of Chicago, is filled with 170 tons of liquid argon, which is 40% more dense than water. When a neutrino hits the nucleus of an argon atom in the detector, its collision creates a spray of subatomic particle debris. Tracking these particles allows scientists to reveal the type and properties of the neutrino that produced them.

According to the new findings, four separate MicroBooNE analyses found no hint of an anomaly indicating this sterile neutrino. It is the most comprehensive finding thus far for an accelerator-based search for sterile neutrinos (other searches, including Yale’s PROSPECT experiment, are based at nuclear reactors).

Fleming, a professor of physics in Yale’s Faculty of Arts and Sciences, proposed the MicroBooNE experiment. She spoke with Yale News about the experiment’s latest findings.

What is the big takeaway from the analyses?

Bonnie Fleming: We see no hint of a sterile neutrino.

There are a number of anomalies, both from the accelerator-based experiments, such as MiniBooNE, and from reactor experiments, that can be explained by one or several sterile neutrinos. But our result strongly disfavors a sterile neutrino as the sole source of the anomaly observed by MiniBooNE.

Was it a surprising result, from your perspective?

Fleming: It is perhaps surprising that we still don’t have an explanation for what is causing the MiniBooNE anomaly. The things we consider the possible “usual suspects” — an excess of electron neutrinos, for instance, or the appearance of single photons from Delta decays — are not responsible. However, there are other explanations and still a big mystery out there that MicroBooNE will address. There is more work to do.

What can we do with this information?

Fleming: We can use these first tests to guide what questions we ask going forward both with the second half of the MicroBooNE data, which has not yet been analyzed, and with the two other experiments on the short baseline program at Fermilab [where MicroBooNE is based], still under construction and just starting to take data.

It’s like we’ve taken a flashlight in a dark room and shined it in some areas to see if we can find an answer. We’ve shown that the most likely or most popular, beyond-the-Standard-Model interpretation is not there. Now we have to shine the flashlight into other areas.

What can you tell us about the technology at the heart of your work — the Liquid Argon Time Projection Chambers (LArTPCs)?

Fleming: MicroBooNE is the first in a long line of U.S.-based neutrino experiments now using this technology. It had been developed in Europe years earlier, with the photograph-like images produced by LArTPCs showing great promise.  A critical takeaway from this result is that LArTPC detectors can be used very successfully for high statistics, precision neutrino physics.

MicroBooNE showed that a long-running experiment can succeed with the technology — and that we can analyze data with a wealth of precision information provided by this detection technique.

Yale played a big role in developing the technology and subsequent work for MicroBooNE, correct?

Fleming: My group at Yale provided a first proof of principle in the U.S. for the technology. We saw the first “tracks” of neutrino interactions in a precision LArTPC chamber in the U.S. — called the “Yale TPC” back in 2007.

I was the founding spokesperson of the Argon Neutrino Teststand (ArgoNeuT) experiment, which was the precursor to MicroBooNE, and the founding spokesperson for MicroBooNE (since 2012, I have been co-spokesperson).

We built the time projection chamber for MicroBooNE here at Yale in the Wright Lab. It was then assembled with the rest of the detector at Fermilab in Illinois. Since then, our analysis team at Yale has collaborated primarily with Brookhaven National Laboratory and also with the University of California-Santa Barbara [UCSB] and the University of Michigan on the “wire cell” analysis — the most sensitive and inclusive of the new analyses exploring electron neutrinos in the experiment. Yale students and postdocs have been integral to this analysis, in particular with Brooke Russell’s dissertation work and more recently with Yale postdoc Jay Hyun Jo’s co-leadership of the Wire cell effort (along with former Brookhaven postdoc Hanyu Wei).

Other students and postdocs at Yale who have contributed to this analysis are Xiao Luo, who is now on the faculty at UCSB, and students London Cooper-Troendle and Kaicheng Li. The next phase of the experiment, searching for the source of the anomaly in single photon mode — where we have not yet shined the flashlight enough — is being driven by Yale graduate students Giacomo Scanavini and Lee Hagaman.

On a personal level, what do the latest MicroBooNE findings mean to you?

Fleming: When I started at Yale in 2004, LArTPCs were a fringe technology in the U.S. People told me that working on the technology was too risky. I used to say that no one would sit next to me at the lunch tables in the cafeteria at Fermilab.

Now, not only has MicroBooNE come to fruition with an in-depth and critical result addressing a longstanding anomaly in the field, but also the scientific community in the interim has embraced this technology so much that it will be used for the next generation of experiments, including the international Deep Underground Neutrino Experiment (DUNE).

With respect to the actual result that MicroBooNE has observed, I am most proud that we have done an excellent job addressing it.  Mother Nature is just that, so the fact that our result does not hint at sterile neutrinos just is what it is. Going forward, we have a transformational technology to help us really understand what neutrinos are telling us about the universe.