Interviewing neutrino-hunting particle physicist Kate Scholberg
A peek into the efforts to detect particles from exploding stars!
Kate Scholberg is the Arts and Sciences Professor of Physics and Bass Fellow at Duke University. She has worked on a multitude of neutrino-detecting experiments worldwide and is a recipient of numerous awards like the DOE Outstanding Junior Investigator award for instance. She also coordinates the SuperNova Early Warning System (SNEWS) designed to give astronomers around the world an early alert of a star explosion in our galaxy.
Prof. Scholberg visited the Tata Institute of Fundamental Research (TIFR) on November 27-30, 2019 to discuss her work and get her up to speed on India’s upcoming neutrino detector. As a science communicator at TIFR, I took the opportunity to chat with Kate on her work and journey in science.
What are neutrinos and where are they found?
Neutrinos are fun little particles. They are everywhere. They are going through you right now. They’re coming from the Sun, from the ground and from the universe. But they are particles that hardly interact at all, they just go right through things without a trace most of the time, like a ninja!
So if these neutrinos are everywhere, what makes them so hard to detect?
It’s mostly because they have such rare interactions. If you want to actually detect one, you need to have a really enormous amount of mass in your detector to increase the chances of a neutrino interacting. These huge detectors are often underground to avoid noise from cosmic rays. One way you can get around not being underground is that you can have an artificial source of neutrinos from a particle collider nearby.
What would be the purpose of studying those neutrinos when we are producing them in effect?
We’re trying to understand properties of neutrinos. That includes their mass and their flavor change, which tells you about their properties. Many experiments are looking for change in neutrino flavors, which can give insights into why the Universe is made of matter and not anti matter, or if neutrinos and antineutrinos behave the same way.
Another experiment that I’m working on is not looking at flavor change. It is looking to see if the interaction with the neutrino is exactly the interaction we expect from the Standard Model of physics, which represents our best understanding of the subatomic world. We want to see if there’s new physics in the interactions between neutrinos and matter particles. And you can look for that with both types of neutrinos, natural or artificially produced ones.
How many neutrinos do you detect per day?
It depends on the sensitivity of the detector. At the Japanese detector Super-K, with whom I collaborate, we see about maybe eight neutrinos from cosmic sources per day. From the sun, it’s a few tens per day. And Super-K has 40 kilotons of pure water for detecting neutrinos!
Are you able to pinpoint where the neutrinos come from?
Pinpoint would be too strong a word. There’s some direction information we can infer from the scattering of electrons in the detector. So the neutrinos scatter an electron and the electron remembers the direction it’s kicked from. So that’s how in fact we can tell which neutrinos come from the Sun and which come from other cosmic objects. Cosmic ray neutrinos are also higher in energy, typically more than a hundred times than the ones from the Sun. So to sum up we can at least tell which part of the sky neutrinos come from roughly.
If you have enough of these detectors spread across the world, can you triangulate, like a GPS, where the neutrinos are coming from?
Sort of. That’s actually one thing that I work on. Collapse of the cores of stars makes them explode and creates a strong bust of neutrinos for about 10 seconds. When we get lots of neutrinos at once, we can get pretty good directional information by looking at their interactions in the detectors. Current detectors won’t do that great but future ones will fare much better.
But amateur astronomers actually can help us a lot, They know the sky very well. Most optical telescopes have a pretty small field of view, so it’s challenging. With help from amateur astronomers around the world, the search to map neutrinos to the star source can be sped up.
The upcoming Deep Underground Neutrino Experiment (DUNE) – which of the two kinds of detectors is that and what do we expect to learn from it?
DUNE is different from other detectors in that it’s cryogenic. It boasts 40 kilotons liquefied argon. And that is special for two reasons. One, where we’re trying to understand neutrino flavor transitions using a beam from Fermilab coming from 1300 km away. That’s a very long baseline which gives you very good sensitivity to any deviations from our current understanding of neutrino physics.
Two, in the case of exploding stars i.e. supernovae, DUNE is sensitive to what we call the electron flavor of neutrinos. Most of the other detectors like Super-K are sensitive to antineutrinos. But in a supernova, you get neutrinos and antineutrinos, and in all electron, muon and tau flavors. By detecting electron neutrinos, you can see what’s called a neutronization flash, which is what you get right at the beginning of the supernova. You can actually see the core of a star collapse!
Neutron stars formed from such collapses are also highly energetic in radiation. Do they have a neutrino emission and can we detect them?
They do. They emit neutrinos as they cool after formation. And that’d be curious. But unfortunately there’s not enough neutrinos being emitted. Unless the star is really close, most of the detectors that we have won’t be able to see it simply because of the distance.
In a supernova, neutrinos would come to us before the light because light will face a lot of hurdles right after the star explodes. Does detecting these neutrinos give enough time for a heads up to be issued to optical telescopes around the world?
We hope so, we’re trying to go as fast as possible. It depends on the nature of the star. The difference could be a very short interval of tens of seconds, which would be bad. But in some cases, it could be as much as a day too. And then our automated SuperNova Early Warning System (SNEWS) will deliver on its promise.
Have scientists detected such a neutrino flash from an exploding star before?
Yes! A star explosion was seen in 1987. But is was more like, “Oh, there’s the supernova, we better go look at the neutrino data in the detectors.” The delay between neutrinos and the first light was found to be of two and a half hours. Most supernova we detect in the future will likely have similar time gaps.
Would these stars we detect neutrinos from be inside our galaxy, outside or both?
We’re sensitive to basically our galaxy and maybe the Small and Large Magellanic Clouds just outside our galaxy. Andromeda, we might get like one neutrino or ten at best for future detectors.
Speaking of the new generation of optical telescopes coming up, you might have heard of SpaceX’s Starlink’s tiff with the Astronomy community. Is that a concern?
Yes, that’s a big problem for the astronomy community. Astronomers are very upset about the Starlink interfering in observations more prominently than satellites before. It will be very difficult to work around issues caused by it, especially for telescopes doing large surveys.
What motivated you to become a scientist and then specifically get into neutrinos and the related physics?
When I was a very small kid growing up in Canada, I wanted to be an astronomer. I was like five or so and read tons of books on space. In middle school, I learnt chemistry and loved it and I wanted to be a chemist. So I took an organic chemistry course in college but it was terrible. At which point I realized that the part of chemistry I really liked was physics. And here I am.
As for the neutrino part of particle physics, it started in my graduate school’s physics department where I worked on an experiment that was looking for magnetic monopoles. I did a neutrino related topic on the same experiment. And that’s how I got here.
As a woman in science, did you face any challenges that you’d like to share?
It’s always awkward if you’re the only woman or amongst a few in a group. It can be difficult. There are certainly occasions where I haven’t been treated well but overall it’s been okay. I’ve seen a lot of issues women face in the field so maybe I’ve been lucky to not have faced severe problems.
How do you see India’s upcoming neutrino detector aiding the worldwide efforts in understanding neutrino science?
The India-based Neutrino Observatory (INO) certainly has unique capabilities, especially for atmospheric neutrinos and related mysteries.
INO and most other neutrino detectors won’t be sensitive to the initial burst but rather to the neutrinos that come later. These latter neutrinos are of much higher energy and can point back to the star very well. Usually higher the energy, the better that particle points.
And I’ve been thinking about using that in DUNE which can see both the initial burst of neutrinos from exploding stars and the latter one. INO could do very well in the latter aspect because that’s exactly the kind of neutrinos it is sensitive to. It will help in modelling what happens after a star explodes.
These higher energy neutrinos could be coming hours or even up to several months after the stellar explosion. The delay is because it’s a different mechanism. These are not neutrinos from the collapse of a hot core of a star, but are from the shockwaves that accelerates protons and trigger high energy neutrino formation from the interactions.
So we better not keep our detectors off!
Originally published at TIFR.
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