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Scientists Narrow Down Where Dark Matter Could Be Hiding · The Quark Side - Quantum Physics Podcast

• 0:01
  Speaker 1: Welcome to the quart Side. Quantum Physics podcast, an exploration
• 0:05
  Speaker 1: of the fundamental structure of reality, where quantum laws govern matter, energy,
• 0:11
  Speaker 1: and information. Here, uncertainty is a feature, not a flaw,
• 0:16
  Speaker 1: and understanding begins at the smallest scales.
• 0:27
  Speaker 2: Right now, thousands of feet beneath an Italian mountain, scientists
• 0:31
  Speaker 2: are actively hiding from the universe so they can finally
• 0:35
  Speaker 2: see it. You know, you step outside on a clear night,
• 0:37
  Speaker 2: you look up at the sweep of the Milky Way,
• 0:39
  Speaker 2: and it feels like you are looking at the totality
• 0:42
  Speaker 2: of the cosmos. But the reality is that the vast,
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  Speaker 2: overwhelming majority of the matter out there, the actual scaffolding
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  Speaker 2: that gives the universe its structure, is completely invisible to you.
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  Speaker 3: Yeah, it's invisible to your eyes, and until very recently,
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  Speaker 3: it's been practically invisible to our most advanced technology too.
• 1:00
  Speaker 3: We are dealing with an astrophysics problem where we can
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  Speaker 3: only see the frosting, but the entire cake itself is
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  Speaker 3: utterly dark. Right.
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  Speaker 2: I like to think of it as trying to find
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  Speaker 2: an invisible elephant in your living room. You can't see
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  Speaker 2: the elephant, you can't hear it, but you know, for
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  Speaker 2: an absolute mathematical fact that it is there, just because
• 1:16
  Speaker 2: you can watch its mass affecting everything else. You see
• 1:19
  Speaker 2: the sofa cushion compressing, you know, you see the coffee
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  Speaker 2: table getting nudged across the rug, but the entity itself
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  Speaker 2: remains a total ghost.
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  Speaker 3: Okay, let's unpack this, because the hunt for that invisible
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  Speaker 3: elephant just hit an unprecedented milestone.
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  Speaker 2: It really did.
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  Speaker 3: The xion Te detector has effectively pushed the boundaries of
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  Speaker 3: human detection to a sensitivity level that changes the entire
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  Speaker 3: landscape of the search.
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  Speaker 2: What's fascinating here is the physical extremity required to even
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  Speaker 2: attempt this. I mean, if we don't map dark matter,
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  Speaker 2: we remain fundamentally blind how our universe actually works. We
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  Speaker 2: are trying to finalize the laws of physics while eighty
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  Speaker 2: five percent of the mass is just off the books.
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  Speaker 3: Wow, eighty five percent. That is a massive blind spot exactly,
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  Speaker 3: and without the gravitational glue of dark matter, spinning galaxies
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  Speaker 3: would literally tear themselves apart, just flinging stars out into
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  Speaker 3: the intergalactic void. So capturing a direct kinetic signal from
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  Speaker 3: this substance isn't just some academic exercise in particle physics.
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  Speaker 3: It is the crucial missing keystone of the standard model.
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  Speaker 2: But to capture that kinetic signal, to even attempt to
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  Speaker 2: observe that kind of gravitational ghost, you have to build
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  Speaker 2: an environment utterly divorced from the chaotic physics of the surface,
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  Speaker 2: which leads us to the Grand Sasso National Laboratory, buried
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  Speaker 2: deep beneath the Dolomite Mountains in Italy. The xion Tee
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  Speaker 2: detector isn't sitting in a traditional observatory.
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  Speaker 3: No, not at all. It relies on a central time
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  Speaker 3: projection chamber containing five point nine metric tons of ultrapure
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  Speaker 3: liquid xeon And that's sitting inside a larger system containing
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  Speaker 3: nearly nine metric tons of the stuff total.
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  Speaker 2: Right, And here is where I think a lot of
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  Speaker 2: people might get tripped up. If dark matter is everywhere
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  Speaker 2: in space, pouring through our solar system constantly, why on
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  Speaker 2: Earth do we need to bury a giant tank of
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  Speaker 2: liquid xenon under four and a half thousand feet of
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  Speaker 2: solid rock to find it?
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  Speaker 3: Well, it is entirely about eliminating cosmic noise, specifically the
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  Speaker 3: muon flucks right now on the surface of the Earth.
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  Speaker 3: Everything is constantly being bombarded by cosmic radiation. High energy
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  Speaker 3: protons from distant supernovae hit our upper atmosphere, creating these
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  Speaker 3: showers of secondary particles, largely muons, that just rain down relentlessly.
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  Speaker 2: Now, if you put a detector on the surface, it
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  Speaker 2: would just be overwhelmed by that radiation.
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  Speaker 3: Oh absolutely. If you place a highly sensitive liquid noble
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  Speaker 3: gas detector up here, the baseline atomic noise would be deafening,
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  Speaker 3: the ionization trails from everyday background radiation would completely drown
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  Speaker 3: out any subtle low energy interaction. So the rock shields
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  Speaker 3: the detector. Over four thousand feet of limestone and dolomite.
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  Speaker 3: Rock absorbs the overwhelming majority of that cosmic debris.
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  Speaker 2: It'sok a massive physical low pass filter.
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  Speaker 3: Exactly. By the time you get deep into those subterranean caps,
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  Speaker 3: the muon flux is reduced by a factor of a million.
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  Speaker 3: It creates an environment that is eerily perfectly quiet on
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  Speaker 3: a subatomic level. Yeah, the only standard particles that can
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  Speaker 3: easily penetrate that far are standard neutrinos, which leaves the
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  Speaker 3: environment pristine enough to wait for a hypothetical dark matter
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  Speaker 3: particle to slip through the rock and actually strike a target.
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  Speaker 2: In the tank, and that target is the liquid xenon itself,
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  Speaker 2: and we're not just pouring raw industrial xenon into a vat.
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  Speaker 2: The mechanics of the liquid and gas phases are critical.
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  Speaker 3: Here.
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  Speaker 2: You have a dense, highly purified liquid target on the
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  Speaker 2: bottom and a layer of gases xenon at the top,
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  Speaker 2: equipped with a ris of photo multiplier tubes.
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  Speaker 3: Right, because liquid xenon is incredibly dense, roughly three times
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  Speaker 3: denser than water, which provides a massively concentrated array of
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  Speaker 3: atomic targets in a relatively confined space. Furthermore, xenon has
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  Speaker 3: a large nucleus. For a long time, the prevailing theoretical
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  Speaker 3: models assumed we were hunting for something that would interact
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  Speaker 3: directly with that heavy nucleus.
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  Speaker 2: But relying on that specific physical collision assumes we know
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  Speaker 2: what we are hunting.
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  Speaker 3: Right.
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  Speaker 2: For decades, the focus was entirely on WIMPs weakly interacting
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  Speaker 2: massive particles. The assumption was that a wimp, being relatively heavy,
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  Speaker 2: would occasionally crash directly into the dense nucleus of a
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  Speaker 2: xenon atom, and this setup would produce two very distinct signals.
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  Speaker 3: Yeah, the dual signal mechanism was the cornerstone of dark
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  Speaker 3: matter searches for thirty years. If a heavy wimp strikes
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  Speaker 3: a xenon nucleus, it causes a nuclear recoil. This immediately
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  Speaker 3: produces a prompt flash of scintillation light. Physicists call this
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  Speaker 3: the S one signal.
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  Speaker 2: Okay, so S one is that first flash of light exactly.
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  Speaker 3: Then the collision also knocks electrons loose. Electric fueld applied
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  Speaker 3: across the tank drifts these free electrons upward into the
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  Speaker 3: gases xenon layer at the top, where they produce a
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  Speaker 3: second delayed flash of light known as the S two signal.
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  Speaker 2: So you get a flash and then a delayed pulse
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  Speaker 2: S one and S two.
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  Speaker 3: Right, And the ratio between the S one light and
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  Speaker 3: the S two charge is the holy grail of background rejection.
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  Speaker 3: It allows researchers to look at an event and definitively
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  Speaker 3: separate a nuclear recoil, which could be a whimp, from
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  Speaker 3: an electron recoil, which is usually just everyday background radiation
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  Speaker 3: like a stray gamma ray. The S one flash acts
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  Speaker 3: as an absolute.
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  Speaker 2: Timestamp, but those whimb searches came up completely empty. The
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  Speaker 2: heavy bowling ball never hit the nucleus, which forced a
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  Speaker 2: massive paradigm shift in the physics community. The folks pivoted
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  Speaker 2: away from these heavy whimps toward lighter, far more elusive candidates,
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  Speaker 2: primarily axion like particles and dark photons.
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  Speaker 3: Yes, but hunting for these lighter particles fundamentally breaks the
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  Speaker 3: physical mechanism the detector relies on. It renders the traditional
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  Speaker 3: background rejection technique almost completely obsolete.
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  Speaker 2: Because the kinematics change entirely. They are too light to
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  Speaker 2: bump the nucleus exactly.
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  Speaker 3: Axion like particles and dark photons are theorized to be
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  Speaker 3: so incredibly light that they don't have the kinetic energy
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  Speaker 3: to cause a heavy xeno nucleus to recoil. Instead, they
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  Speaker 3: interact via the axio elect effect. They're absorbed by the
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  Speaker 3: bound electrons orbiting the nucleus.
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  Speaker 2: They deposit their rest mass directly into the electron.
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  Speaker 3: Shell right, which frees the electron, creating an S two
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  Speaker 3: charge signal. But crucially, it deposits far too little energy
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  Speaker 3: to produce that prompt S one flash of light. You
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  Speaker 3: just get a single, faint, isolated trickle of electrons drifting
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  Speaker 3: to the top of the tank.
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  Speaker 2: It's like we spent thirty years hunting for a bear
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  Speaker 2: using a trip wire that sets off a massive physical
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  Speaker 2: alarm bell. Now we realize we might actually be hunting
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  Speaker 2: her from mosquitos and they are too light to trip
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  Speaker 2: the wire. We've lost our alarm bell. We only have
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  Speaker 2: this ambiguous secondary signal.
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  Speaker 3: That is a great way to put it. Without that
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  Speaker 3: prompt s one flash to act as a definitive times
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  Speaker 3: stamp and a filter. Distinguishing a true dark matter signal
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  Speaker 3: from the ambient radioactive noise of the detector itself becomes
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  Speaker 3: incredibly difficult because, even buried under a mountain, the materials
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  Speaker 3: constructing the detector, the steel, the teflon, the photo multiplier tubes,
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  Speaker 3: they all contain microscopic trace amounts of radioactive isotopes.
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  Speaker 2: Right, And I want to drill into that because it
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  Speaker 2: sounds like an absolute nightmare for a data scientist. If
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  Speaker 2: you only get a tiny, isolated S two pulse of electrons,
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  Speaker 2: how can you possibly look at the data and separate
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  Speaker 2: a dark matter axion from say a stray electron or
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  Speaker 2: a trace atom decaying in the liquid xenon?
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  Speaker 3: Well, you have to map the noise with unprecedented resolution.
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  Speaker 3: To overcome the loss of the S one filter, the
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  Speaker 3: ex non collaboration built a highly detailed, deeply complex empirical
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  Speaker 3: model of exactly what every conceivable background event should look like.
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  Speaker 2: So they mapped the specific energy deposition rates, the decay
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  Speaker 2: paths of isotopes. All of that. They couldn't filter the
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  Speaker 2: noise out, so they had to completely mathematically define it.
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  Speaker 3: Exactly, and they used advanced machine learning algorithms to characterize
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  Speaker 3: these predictions with extreme precision. The model mapped the exact
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  Speaker 3: parameter space of the background noise down to the most
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  Speaker 3: granular statistical level.
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  Speaker 2: Okay, here's where it gets really interesting. Because they didn't
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  Speaker 2: just train this machine learning model and then feed the
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  Speaker 2: operational data through it to see what popped out. They
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  Speaker 2: conducted a blind test. They finalized this model without being
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  Speaker 2: able to see the actual real world data in the
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  Speaker 2: specific energy region where a dark matter signal would hypothetically appear.
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  Speaker 3: Blinding the signal region is the gold standard for this
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  Speaker 3: level of high stakes physics.
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  Speaker 2: Wait, I'm struggling to understand the logic here. They purposely
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  Speaker 2: didn't look at the data while building the model. Isn't
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  Speaker 2: looking at the data The entire point of an experiment.
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  Speaker 2: How does hiding the data solve the problem? Are you
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  Speaker 2: just flying blind?
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  Speaker 3: It solves the human problem, specifically the danger of unconscious bias.
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  Speaker 3: When you are looking for an anomaly that is virtually
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  Speaker 3: identical to a statistical fluctuation in the background noise, the
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  Speaker 3: temptation to tweak your algorithm is immense. If the researchers
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  Speaker 3: can see the live data while they are setting the
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  Speaker 3: boundaries of their machine learning model, they might see a
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  Speaker 3: slight random bump in a specific energy bin.
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  Speaker 2: Oh I see, and unconsciously they might tighten a parameter
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  Speaker 2: or adjust a data cut to isolate that bump.
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  Speaker 3: Exactly artificially inflating a random fluctuation into a massive discovery.
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  Speaker 3: By flying blind, they entirely remove the human capacity to
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  Speaker 3: tune the results. They lock in the algorithms, freeze the parameters,
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  Speaker 3: and only after the model is mathematically airtight do they
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  Speaker 3: unblind the data. They essentially encrypt the box, declare their
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  Speaker 3: exact filtering parameters, and then finally open it to see
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  Speaker 3: if anything survived the cuts.
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  Speaker 2: You call your shot before you even look at the board,
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  Speaker 2: so the box gets opened. The unblinded results from the
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  Speaker 2: exion detector were published in August twenty twenty six in
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  Speaker 2: Physical Review Letters, and the data showed well, no unexplained signals,
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  Speaker 2: nothing beyond the predicted background model, no axion like particles
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  Speaker 2: jumping out of the low energy ranges. So what does
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  Speaker 2: this all mean?
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  Speaker 3: Well, if we connect this to the bigger picture, a robust,
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  Speaker 3: highly sensitive null result like this is a massive scientific win.
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  Speaker 3: Falsifiability is how the field advances.
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  Speaker 2: Right, It's like tuning an inelau radio. They didn't find
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  Speaker 2: this space a broadcasts they were looking for, but by
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  Speaker 2: running this insanely sensitive, blinded analysis, they successfully eliminated a
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  Speaker 2: huge chunk of the static frequencies on the dial.
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  Speaker 3: Yes, they set unprecedented limits on the properties of axions
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  Speaker 3: and dark photons, creating the most detailed map yet of
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  Speaker 3: exactly where dark matter cannot be found. Theoretical physicists now
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  Speaker 3: have hard experimental data, forcing them to abandon certain mathematical
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  Speaker 3: models and refine others.
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  Speaker 2: But in narrowing that space, in pushing the liquid xenon
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  Speaker 2: detector to this extreme level of sensitivity, Exinon has essentially
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  Speaker 2: crashed headfirst into a new monumental wall in physics, a
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  Speaker 2: wall called the neutrino fog.
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  Speaker 3: And the neutrino fog is a fundamental threshold in physics.
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  Speaker 3: Until now, no detector had ever reached this limit. The
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  Speaker 3: background noise we were worried about was localized radioactive isotopes
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  Speaker 3: in the tank cosmic muons that slipped through the mountain.
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  Speaker 3: But Exion has pushed the limits so far that they
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  Speaker 3: are edging into standard model inutrinos.
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  Speaker 2: Because we are now hitting the solar neutrino floor right
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  Speaker 2: the detector is so sensitive it's catching the standard model
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  Speaker 2: neutrino scatter from the nuclear fusion happening inside our own
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  Speaker 2: sun exactly.
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  Speaker 3: And this is where the mechanics of the physics become
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  Speaker 3: incredibly cruel. The solar neutrinos interact with the liquid xenon,
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  Speaker 3: and the energy deposition from a standard solar neutrino perfectly
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  Speaker 3: mimics the exact theoretical signal of the low mass dark
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  Speaker 3: matter candidags we are currently hunting.
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  Speaker 2: It produces the exact same tiny isolated pulse. So the
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  Speaker 2: detector is now so spectacularly engineered that it can literally
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  Speaker 2: feel the sun shining right through four and a half
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  Speaker 2: thousand feet of solid mountain rock. And that solar signal
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  Speaker 2: is stepping right on top of the dark matter frequency.
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  Speaker 3: It is a cosmic camouflage. The neutrino fog isn't just
• 12:44
  Speaker 3: a technological hurdle where we can simply invent a purer
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  Speaker 3: liquid or build a better sensor. It is an intrinsic
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  Speaker 3: background dictated by nature. The natural particle emissions from our
• 12:54
  Speaker 3: own star are perfectly masking the interactions we are desperate
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  Speaker 3: to measure.
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  Speaker 2: Which makes the future of this particular physical hunt deeply
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  Speaker 2: fascinating because the scientific community is clearly not stopping here.
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  Speaker 2: The immediate plan is to build a successor detector, scaling
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  Speaker 2: up to something roughly six times larger than eccceine on
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  Speaker 2: tee yes.
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  Speaker 3: But a detector that large is going to be so
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  Speaker 3: deeply submerged in that solo neutrino fog that it will
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  Speaker 3: require vastly more sophisticated analytical tools just to attempt to
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  Speaker 3: untangle the standard model solar background from a potential dark
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  Speaker 3: matter signal.
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  Speaker 2: The engineering scale up is inevitable. But this push to
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  Speaker 2: understand the invisible universe, you know, the sheer joy of discovery,
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  Speaker 2: and this relentless human drive, it really forces us to
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  Speaker 2: confront some massive questions.
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  Speaker 3: It really does. This raises an important question regarding the
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  Speaker 3: fundamental limits of observation. If we are entering the neutrino fog,
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  Speaker 3: maybe the universe isn't just incidentally making dark matter hard
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  Speaker 3: to find. Maybe dark matter is fundamentally camouflaged by the
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  Speaker 3: very stars that give us light. What happens to our
• 13:57
  Speaker 3: understanding of the universe if we reach a point where
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  Speaker 3: where we literally cannot untangle the invisible web of dark
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  Speaker 3: matter from the blinding noise of our own sun.
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  Speaker 2: It's an incredible thought. We bury ourselves under mountains to
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  Speaker 2: escape the universe, only to find our own sun casting
• 14:12
  Speaker 2: a shadow over the subatomic data. Next time you step
• 14:15
  Speaker 2: outside and look up at the sun today, take a
• 14:17
  Speaker 2: moment to realize what it's actually doing. It isn't just
• 14:20
  Speaker 2: providing warmth and light. It is actively throwing a veil
• 14:23
  Speaker 2: of neutrinos over the greatest mystery in the cosmos, and
• 14:26
  Speaker 2: somewhere thousands of feet beneath an Italian mountain, scientists are
• 14:30
  Speaker 2: still trying to peer right through it.