Challenging Spontaneous Quantum Collapse with XENONnT

Using low-energy electronic recoil data from the first science run of the XENONnT detector and a novel model accounting for charge cancellation effects, researchers established world-leading constraints on spontaneous quantum collapse models that exclude the original parameters of the Continuous Spontaneous Localization theory for the first time.

The Gist

The Big Mystery: Why Don't Cats Exist in Two Places at Once?

Imagine you have a coin. In the quantum world (the world of tiny atoms), that coin can spin so fast it is effectively both heads and tails at the same time. This is called a "superposition." We know this happens with tiny particles; we've proven it in labs.

But in our everyday world, if you flip a coin, it lands on either heads or tails. It never stays in a blurry mix of both. Even a cat (like Schrödinger's famous thought experiment) is never both dead and alive at the same time.

The Question: How does the universe decide when to stop being "quantum" (blurry) and start being "classical" (definite)?

The Theory: The Universe Has a "Self-Correction" Mechanism

Some physicists think the answer isn't just "we don't look at it." They propose a theory called Dynamical Collapse Models.

Think of the universe like a giant, noisy room where everyone is whispering.

• The Quantum View: The whispers are so quiet that a single person (an atom) can hear two different songs at once (superposition).
• The Collapse View: There is a hidden, random "static noise" in the universe. For a single atom, the static is too quiet to matter. But for a big object (like a cat or a coin), the static gets louder and louder. Eventually, the static is so loud that it forces the object to "snap" into one state or the other.

This snapping process is called spontaneous collapse.

The Prediction: The "Snap" Should Make a Sound

Here is the tricky part. The theory says that when this "snap" happens, it's not silent. Just like snapping your fingers creates a sound, or a static shock creates a spark, the universe snapping a particle into place should release a tiny bit of energy in the form of X-rays.

If these theories are true, every atom in the universe should be constantly, very slowly, emitting a faint, random X-ray signal.

The Experiment: The Giant Fish Tank

To catch this faint signal, the scientists used the XENONnT detector.

• The Tank: Imagine a massive, ultra-pure tank filled with 6 tons of liquid xenon (a noble gas). It's buried deep underground in Italy, shielded by 3,600 meters of rock to block out cosmic rays and other noise.
• The Goal: They are waiting for a single atom in the tank to "snap" and emit an X-ray.
• The Sensitivity: This tank is so sensitive it can detect the energy of a single photon (a particle of light) hitting a single atom. It's like trying to hear a single drop of water falling in a stadium while a jet engine is running nearby.

The New Twist: The "Cancellation Effect"

In the past, scientists looked for these X-rays using a simple rule: "More atoms = More X-rays."

But this paper introduces a clever new idea. Inside a xenon atom, there are protons (positive charge) and electrons (negative charge).

• The Analogy: Imagine a band playing music. The protons are the brass section, and the electrons are the strings.
• The Old View: We thought the whole band just played louder together.
• The New View: The scientists realized that because protons and electrons have opposite charges, their "music" can actually cancel each other out, like noise-canceling headphones. If the X-ray wavelength is just right, the positive and negative parts of the atom might silence each other, making the signal much weaker than expected.

The team built a new mathematical model to account for this "silencing" effect, making their search much more accurate.

The Results: Silence is Golden

After running the detector for a long time, they looked at the data.

• Did they find the X-rays? No.
• Did they find the "snap"? No.

The tank remained perfectly quiet. There was no evidence of the universe spontaneously snapping atoms into place.

What This Means

Because they didn't find the signal, they can now say with great confidence that the "snap" theories are wrong (or at least, much weaker than we thought).

1. Ruling out the "Classic" Theory: They have experimentally proven that the specific version of the theory proposed by Ghirardi, Rimini, and Weber (GRW) in the 1980s is incorrect. The universe does not snap that often.
2. Setting New Limits: They have set the strictest rules in history for how "loud" this background noise can be. It's like saying, "We know the universe isn't whispering this loudly."
3. The Future: The door is still open for other, more complex versions of the theory, but the "easy" answers have been closed.

The Takeaway

The XENONnT experiment acted like a giant, ultra-sensitive microphone listening to the universe. They were listening for a specific "crack" that would prove the universe forces reality to choose a side. They heard nothing but silence.

This silence tells us that the transition from the quantum world to our everyday world is even more mysterious and subtle than we previously thought. The universe isn't snapping; it's holding its breath.

Technical Summary

1. Problem Statement

The paper addresses the measurement problem in quantum mechanics: the unresolved question of how classical behavior emerges from the quantum realm, specifically why macroscopic superpositions are not observed. To resolve this, Dynamical Collapse Models (DCMs) have been proposed as phenomenological modifications to the Schrödinger equation. These models introduce non-linear and stochastic terms that cause wavefunction collapse, with a rate that scales with system size.

Two primary benchmark models are investigated:

• Continuous Spontaneous Localization (CSL): Characterized by a collapse strength parameter ($\lambda$) and a correlation length ($r_C$). It assumes a Gaussian spatial correlation.
• Diósi-Penrose (DP): A gravity-induced collapse model where the collapse rate is linked to the gravitational self-energy of the superposition. It is characterized by a spatial cutoff parameter ($R_0$).

A key prediction of DCMs is the emission of spontaneous electromagnetic radiation (specifically X-rays) due to the Brownian-like diffusion of charged particles during the collapse process. While previous searches (e.g., using Germanium detectors) have set limits, they relied on high-energy approximations that neglected complex atomic effects in the X-ray regime.

2. Methodology

The authors utilized data from the XENONnT dark matter detector, a dual-phase liquid xenon (LXe) Time Projection Chamber (TPC), to search for this spontaneous radiation.

• Dataset: The analysis used data from the first science run (SR0), collected between July 6, 2021, and November 10, 2021. The exposure was 1.16 tonne-years within a fiducial mass of 4.37 tonnes.
• Signal Channel: The search focused on Electronic Recoils (ER) in the energy range of 1–140 keV. The detector distinguishes ER from Nuclear Recoils (NR) based on the charge-to-light ratio.
• Theoretical Framework (Key Innovation):
  • Unlike previous high-energy approximations (where protons emit coherently and electrons incoherently), this analysis employs a low-energy generalization derived in Ref. [23].
  • This framework accounts for cancellation effects arising from the opposing charges of electrons and protons within Xenon atoms.
  • It explicitly models the interference between electron pairs and the electron-proton cancellation using atomic orbital mean radii ($\rho_o$) calculated via Density Functional Theory (DFT).
  • The emission rate formula includes a $\text{sinc}$ function term representing these interference effects, which significantly alters the spectral shape in the 1–100 keV range.
• Background Modeling: The analysis considered nine background components, including:
  • Radioactive decays ($^{136}\text{Xe}$, $^{214}\text{Pb}$, $^{133}\text{Xe}$, $^{85}\text{Kr}$, $^{124}\text{Xe}$, $^{83m}\text{Kr}$).
  • Solar neutrino scattering (improved with a stepping approximation for electron binding energies).
  • Accidental coincidences (ACs) and gamma rays from detector materials.
  • The background model was refined with updated uncertainties (e.g., reducing $^{85}\text{Kr}$ uncertainty from ~65% to ~22%).
• Statistical Analysis: An unbinned maximum likelihood fit was performed on the reconstructed energy spectrum. The signal was modeled as a spectral shape scaled by a rate multiplier.

3. Key Contributions

• First Application of Low-Energy Atomic Physics to DCMs: This is the first search to incorporate the full atomic structure of Xenon, including electron-proton cancellation and electron-electron interference, into the prediction of spontaneous radiation spectra. This corrects previous overestimations of signal rates in the X-ray band.
• Superior Sensitivity: Leveraging the low energy threshold (~1 keV), low background, and multi-tonne target mass of XENONnT, the experiment achieves unprecedented sensitivity to collapse models.
• Refined Efficiency Modeling: The analysis corrected previous underestimations of acceptance loss due to accidental pairings of light and charge signals and included event-building efficiency projections.

4. Results

The analysis found no significant excess of events over the expected background. The local discovery significance was $0.3\sigma$ for CSL and $0.2\sigma$ for DP. Consequently, the authors set new world-leading upper limits on the model parameters:

• CSL Model:

  • Derived an upper limit on the parameter combination: $\lambda/r_C^2 < 3.0 \times 10^{-3} \, \text{s}^{-1}\text{m}^{-2}$ (90% C.L.).
  • Significance: This result excludes for the first time the original parameter values proposed by Ghirardi, Rimini, and Weber (GRW) ($\lambda_{GRW} \approx 10^{-16} \, \text{s}^{-1}$, $r_{GRW} \approx 10^{-7} \, \text{m}$) at a significance of 9.1$\sigma$.
  • This improves upon the previous best limit (from the Majorana Demonstrator) by a factor of ~135.

• Diósi-Penrose (DP) Model:

  • Derived a lower limit on the spatial cutoff parameter: $R_0 > 4.9 \times 10^{-10} \, \text{m}$ (90% C.L.).
  • Significance: This improves the previous world-leading limit by a factor of ~5.

5. Significance

• Constraining Quantum Foundations: The results place the most stringent experimental constraints to date on the parameter space of dynamical collapse models. By ruling out the original GRW parameters, the paper challenges the viability of the simplest white-noise CSL models as a solution to the measurement problem.
• Validation of Detector Capabilities: The study demonstrates the immense physics reach of dual-phase xenon TPCs beyond dark matter searches, proving their capability to probe fundamental quantum mechanics with high precision.
• Future Directions: While the standard white-noise CSL and DP models are tightly constrained, the paper notes that "colored" or dissipative extensions of these models remain viable and require further investigation. The availability of more XENONnT data promises even greater sensitivity in the future.

In conclusion, this work represents a major milestone in experimental tests of quantum mechanics, utilizing the unique properties of liquid xenon to rule out long-standing theoretical proposals regarding the mechanism of wavefunction collapse.