Elena Aprile in her lab at Columbia’s Nevis Laboratories.

Ben Sklar for Quanta Magazine

Elena Aprile in her lab at Columbia’s Nevis Laboratories.

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In a lab buried under the Apennine Mountains of Italy, Elena Aprile, a professor of physics at Columbia University, is racing to unearth what would be one of the biggest discoveries in physics.

She has not yet succeeded, even after more than a decade of work. Then again, nobody else has, either.

Aprile leads the XENON dark matter experiment, one of several competing efforts to detect a particle responsible for the astrophysical peculiarities that are collectively attributed to dark matter. These include stars that rotate around the cores of galaxies as if pulled by invisible mass, excessive warping of space around large galaxy clusters, and the leopard-print pattern of hot and cold spots in the early universe.

For decades, the most popular explanation for such phenomena was that dark matter is made of as-yet undiscovered weakly interacting massive particles, known as WIMPs. These WIMPs would only rarely leave an imprint on the more familiar everyday matter.

That paradigm has recently been under fire. The Large Hadron Collider located at the CERN laboratory near Geneva has not yet found anything to support the existence of WIMPs. Other particles, less studied, could also do the trick. Dark matter’s astrophysical effects might even be caused by modifications of gravity, with no need for the missing stuff at all.

The most stringent WIMP searches have been done using Aprile’s strategy: Pour plenty of liquid xenon — a noble element like helium or neon, but heavier — into a vat. Shield it from cosmic rays, which would inundate the detector with spurious signals. Then wait for a passing WIMP to bang into a xenon atom’s nucleus. Once it does, capture out the tiny flash of light that should result.

The XENON Experiment

The time projection chamber at the heart of the detector is filled with 3.5 metric tons of liquid xenon.

These experiments use progressively larger tanks of liquid xenon that the researchers believe should be able to catch the occasional passing WIMP. Each successive search without a discovery shows that WIMPs, if they exist, must be lighter or less prone to leave a mark on normal matter than had been assumed.

In recent years, Aprile’s team has vied with two close competitors for the title of Most-thorough WIMP Search: LUX, the Large Underground Xenon experiment, a U.S.-based group that split from her team in 2007, and PandaX, the Particle and Astrophysical Xenon experiment, a Chinese group that broke away in 2009. Both collaborators-turned-rivals also use liquid-xenon detectors and similar technology. Soon, though, Aprile expects her team to be firmly on top: The third-generation XENON experiment — larger than before, with three and a half metric tons of xenon to catch passing WIMPs — has been running since the spring, and is now taking data. A final upgrade is planned for the early 2020s.

The game can’t go on forever, though. The scientists will eventually hit astrophysical bedrock: The experiments will become sensitive enough to pick up neutrinos from space, flooding the particle detectors with noise. If WIMPs haven’t been detected by that point, Aprile plans to stop and rethink where else to look.

Aprile splits her time between her native Italy and New York City. Quanta caught up with her on a Saturday morning in her Brooklyn high-rise apartment that faces toward the Statue of Liberty. An edited and condensed version of the interview follows.

QUANTA MAGAZINE: How closely do you follow the theoretical back and forth about the nature of dark matter?

ELENA APRILE: For me, driving the technology, driving the detector, making it the best detector is what makes it exciting. The point right now is that in a couple of years, maybe four or five in total, we will definitely say there is no WIMP or we will discover something.

I don’t care much about what the theorists say. I go on with my experiment. The idea of the WIMP is clearly today still quite ideal. Nobody could tell you “No, you’re crazy looking for a WIMP.”

Ben Sklar for Quanta Magazine

What do you imagine will happen over the next few years in this search?

If we find a signal, we have to go even faster and build a larger scale detector which we are planning already — in order to have a chance to see more of them, and have a chance to build up the statistics. If we see nothing after a year or two, the same story.

The plan for the collaboration, for me and how I drive these 130 people, is very clear for the next four or five years. But beyond that, we will go almost to the level that we start really to see neutrinos. If we end up being lucky — if a supernova goes off next to us and we see neutrinos — we will not have found dark matter, but still detect something very exciting.

How did you get started with this xenon detector technology?

I started my career as a summer student at CERN. Carlo Rubbia was a professor at Harvard and also a physicist at CERN. He proposed a liquid-argon TPC — time projection chamber. This was hugely exciting as a detector because you can measure precisely the energy of a particle, and you can measure the location of the interaction, and you can do tracking. So, that was my first experience, to build the first liquid-argon ‘baby’ detector — 1977, yes, that’s when it started. And then I went to Harvard, and I did my early work with Rubbia on liquid argon. That was the seed that led eventually to the monstrous, huge liquid-argon detector called ICARUS.

Later, I left Rubbia and I accepted the position of assistant professor here at Columbia. I got interested in continuing with liquid-argon detectors, but for neutrino detection from submarines. I got my first grant from DARPA [the Defense Advanced Research Projects Agency]. They didn’t give a damn about supernova neutrinos, but they wanted to see neutrinos from the [nuclear] Russian submarines. And then we had Supernova 1987A, and I made a proposal to fly a liquid-argon telescope on a high-altitude balloon to detect the gamma rays from this supernova.

I studied a lot — the properties of argon, krypton, xenon — and then it became clear that xenon is a much more promising material for gamma-ray detection. So I turned my attention to liquid xenon for gamma-ray astrophysics.

How did that swerve into a search for dark matter?

I had this idea that this detector I built for gamma-ray astrophysics could have been, in another version, ideal to look for dark matter. I said to myself: “Maybe it’s worth going into this field. The question is hot, and maybe we have the right tool to finally make some progress.”

It’s atypical that the NSF [National Science Foundation], for someone new like me, will fund the proposal right away. It was the strength of what I had done all those years with the a liquid-xenon TPC for gamma-ray astrophysics. They realized that this woman can do it. Not because I’m very bold and I proposed a very aggressive program — which of course is typical of me — but I think it was the work that we did for another purpose which gave the strength to the XENON program, which I proposed in 2001 to the NSF.

Ben Sklar for Quanta Magazine

Video: Elena Aprile explains how she hunts for dark matter in the world’s largest underground laboratory.

What was it like to go from launching high-altitude balloons to working underground?

We had quite a few balloon campaigns. It’s something that I would do again, and I didn’t appreciate it then. You get your detector ready, you sit it on this gondola. At some point you are ready, but you can’t do anything because every morning you go and you wait for the weather guy to tell you if it’s the right moment to fly. In that scenario you are a slave to something bigger than you, which you can’t do anything about. You go on the launch pad, you look at the guy measuring, checking everything, and he says “No.”

Underground, I guess, there is no such major thing holding you from operating your detector. But there are still, in the back of your mind, thoughts about the seismic resilience of what you designed and what you built.

In a 2011 interview with The New York Times about women at the top of their scientific fields, you described the life of a scientist as tough, competitive and constantly exposed. You suggested that if one of your daughters aspired to be a scientist you would want her to be made of titanium. What did you mean by that?

Maybe I shouldn’t demand this of every woman in science or physics. It’s true that it might not be fair to ask that everyone is made of titanium. But we must face it — in building or running this new experiment — there is going to be a lot of pressure sometimes. It’s on every student, every postdoc, every one of us: Try to go fast and get the results, and work day and night if you want to get there. You can go on medical leave or disability, but the WIMP is not waiting for you. Somebody else is going to get it, right? This is what I mean when I say you have to be strong.

Going after something like this, it’s not a 9-to-5 job. I wouldn’t discourage anyone at the beginning to try. But then once you start, you cannot just pretend that this is just a normal job. This is not a normal job. It’s not a job. It’s a quest.

In another interview, with the Italian newspaper La Repubblica, you discussed having a brilliant but demanding mentor in Carlo Rubbia, who won the Nobel Prize for Physics in 1984.  What was that relationship like?

It made me of titanium, probably. You have to imagine this 23-year-old young woman from Italy ending up at CERN as a summer student in the group of this guy. Even today, I would still be scared if I were that person. Carlo exudes confidence. I was just intimidated.

He would keep pushing you beyond the state that is even possible: “It’s all about the science; it’s all about the goal. How the hell you get there I don’t care: If you’re not sleeping, if you’re not eating, if you don’t have time to sleep with your husband for a month, who cares? You have a baby to feed? Find some way.” Since I survived that period I knew that I was made a bit of titanium, let’s put it that way. I did learn to contain my tears. This is a person you don’t want to show weakness to.

Now, 30 years after going off to start your own lab, how does the experience of having worked with him inform the scientist you are today, the leader of XENON?

For a long time, he was still involved in his liquid-argon effort. He would still tell me, “What are you doing with xenon; you have to turn to argon.” It has taken me many years to get over this Rubbia fear, for many reasons, probably — even if I don’t admit it. But now I feel very strong. I can face him and say: “Hey, your liquid-argon detector isn’t working. Mine is working.”

I decided I want to be a more practical person. Most guys are naive. All these guys are naive. A lot of things he did and does are exceptional, yes, but building a successful experiment is not something you do alone. This is a team effort and you must be able to work well with your team. Alone, I wouldn’t get anywhere. Everybody counts. It doesn’t matter that we build a beautiful machine: I don’t believe in machines. We are going to get this damn thing out of it. We’re going to get the most out of the thing that we built with our brains, with the brains of our students and postdocs who really look at this data. We want to respect each one of them.

Correction: On December 20, 2016, this article was changed to reflect that Aprile was not the first female professor of physics at Columbia University. The first female professor of physics at Columbia was C.S. Wu.

This article was reprinted on Wired.com.

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  • I don't quite understand what is wrong with these people, this is reaching desperation levels. WIMPs and MOND are so over. No amount of money is going to save them.

  • This in no way detracts from Prof. Aprile and her fascinating work, but she is not the first woman to have been appointed a full professor of physics at Columbia. I would hope that some of your older readers will remember Chien-Shiung Wu, a brilliant experimenter, the "Dragon Lady" of the Columbia physics department, who was named a full professor back in 1958.

  • I disagree with Thomas Lee Elifritz's comment––WIMPs remain one of the most promising candidates for dark matter, predicted in several well developed theories beyond the standard model such as supersymmetry and extra dimensions. XENON is a very exciting (and certainly very challenging) experiment. After the detection of Higgs and gravitational wave, the detection of dark matter feels so near! Best of luck to Prof. Aprile and her team!

  • Would it be scientific if we did not research Dark Matter? The idea that extra gravity is an effect of dark energy, sounds plausible but is not even close to being definitive. Both ideas should be researched irregardless of the outcome.

  • @Jia: My (inconsequential) bet would be on Erik Verlinde being right. This story provides a different perspective on this problem.

    BTW, how much does 3.5 tons of ultrapure xenon cost? How does one do a costs-benefits analysis for such projects? At what point should they be defunded? Who decides on such a step? Let's not invoke the usual "broadening our knowledge" argument–I'm not against fundamental research at all, only against the very expensive, never ending personal quests funded by the taxpayer to the detriment of many more smaller projects. Sorry for the rant…

  • Addendum: a link to an excellent story in this magazine on Erik Verlinde:

    https://www.quantamagazine.org/20161129-verlinde-gravity-dark-matter/

  • Interesting article. But not much meat to it; I mean this is Quanta, not SciAm.

    If I had to bet, I think Dr. Lisa Randall right about her Dark Disk thing.

    This is strange stuff we are dealing with here. The more I work on it, the more I'm not convinced they are particles per se. I mean it doesn't show up on solar system searches, and we are right on the Orion Arm where the effect should be the strongest. We should be drowning in WIMPS.

    Dr. Verlinte's paper was brilliant. But let me bounce off all of you a thought I had.

    We have decades of looking at detectors like XENON and ATLAS, so we've been subconsciously kind of processing it. We know what it looks like, I mean instinctually. I side with Dark Matter over MOND. Why? Because so many astronomers, cosmologists and astrophysicists have been looking at it for so long now, that they collectively figured out: its dark matter, its just how it swirls. "I can't mathematically prove it, but its Dark Matter." Its like guessing the number beans in a jar.

    But the reason why I'm siding with Dr. Randall on this one, is that something that comes through the algorithms I'm working on is this brilliant light that reality is so much more elegant that we give it credit for. I don't think Dark Matter manifests as particles with a distinct XYZ coordinate (ie. WIMPS). I had a thought that the Dark Matter is up in the galactic bubbles and in a torus at about the galactic disk perimeter, but the gravity emanates from a disk right on the galactic plane. That Dark Matter might be that different from the forms of matter we are familiar with (ie. its gravity is where it isn't).

    My point isn't to forward my pet theory (which that wasn't), but rather to point out that maybe its that strange, that its still physics, but not a physics we are familiar with. Maybe we need to be theorizing at this point instead of experimenting.

    Thoughts?

  • "She has not yet succeeded, even after more than a decade of work. Then again, nobody else has, either."

    So it seems reasonable to say that maybe Dark Matter does not exist at all. Perhaps it's a similar case to the SUSY which also was failed recently in the LHC experiments. But unlike the SUSY, the Dark Matter seem to have promising substitutes like Dark Energy, MOND, Erilk Verlind's Gravity theory.

  • "Try to go fast and get the results, and work day and night if you want to get there. You can go on medical leave or disability, but the WIMP is not waiting for you. Somebody else is going to get it, right?"

    I wish certain academics would stop with this aberrant XIX-century sweatshop mentality. Actually, the WIMPs ARE going to be there, eternally. If they exist, they have been there long before anybody cared or even was there to see them, and will be long after we all disappear. Practically any other job can be regarded as more urgent than this one. Your life and the life of those around you though, that is certainly NOT waiting for you.

    It's certainly a race between groups, but scientific races should be for the benefit of humanity, not to detract basic labor rights in the name of a self-declared poorly-disguised exploitative efforts. Practically all forms of industry are in extremely competitive races nowadays – should we see as desirable or even normal that they propose their laborers sacrifice everything for that all-consuming, never-ending goal?

    Science is a quest, and sometimes it absorbs you to the point nothing else exists, but this situation should be something purely circumscribed to the individual feeling it at that moment in his/her life, and definitely not a general requirement or expectation for a career. This "standard" to give up your personal life for somebody else's (your usually precarious employer's no less) expectations and obsessions is **sick** and should be regarded as such, not encouraged.

    Especially when we are presented with "high rise apartments overlooking the Statue of Liberty" as an example. If anybody else, from any field, was making these claims publicly (s)he would be shunned immediately.

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