Observation of nuclear suppression in coherent $\Upsilon$(1S) photoproduction off heavy nuclei at the LHC

The CMS experiment at the LHC reports the first observation of nuclear suppression in coherent $\Upsilon$(1S) photoproduction off heavy nuclei, measuring a nuclear gluon suppression factor of $R_\text{g}^\text{Pb} \approx 0.55$ at a high energy scale of $\mu^2 = 22.4$ GeV$^2$ and $x \approx 10^{-3}$.

The Gist

The Big Picture: Smashing "Ghost" Balls to See Inside the Atom

Imagine you have a giant, incredibly dense ball of yarn (a heavy lead nucleus). You want to know how the threads (gluons, the particles that hold atoms together) are packed inside it. But the ball is so dense and the threads are so small that you can't just look at it with a microscope.

Instead, the scientists at CERN's CMS experiment decided to use a very specific trick: The "Ghost" Flashlight.

They didn't smash the balls together head-on (which would be like crashing two cars). Instead, they fired two lead nuclei past each other at incredibly high speeds, but just close enough that they didn't touch. Because they are moving so fast, they create a massive, intense flash of light (photons) around them, like a sonic boom made of light.

This flash of light hits the other nucleus. It's like shining a super-bright, ultra-fast flashlight into the yarn ball. The light bounces off the threads inside, creating a new, heavy particle called a Upsilon (Υ). By studying how this new particle is created, the scientists can figure out how the yarn (gluons) is packed inside the nucleus.

The Discovery: The "Shadow" Effect

The scientists were looking for a specific type of interaction called Coherent Photoproduction.

• The Analogy: Imagine you are trying to take a photo of a crowd of people.
  • Incoherent: You take a picture of just one person. You see their individual details, but you don't see the crowd's overall shape.
  • Coherent: You take a picture of the entire crowd at once. They act as one giant unit.

In this experiment, the "flashlight" (photon) hit the entire lead nucleus at once, treating it as a single giant object.

The Surprise:
When the scientists calculated how many Upsilon particles should be created if the nucleus was just a bag of loose, free particles, they got a certain number. But when they actually counted the particles in the detector, they found far fewer than expected.

It was as if the nucleus was wearing a dark cloak or casting a shadow that blocked the light. The light couldn't penetrate as deeply as they thought. This is called Nuclear Suppression. The nucleus is "shadowing" itself, meaning the gluons inside are so crowded that they hide each other from the incoming light.

Why This is a Big Deal: The "Deep Dive"

Previous experiments had looked at lighter particles (like the J/ψ or the ϕ meson).

• The Analogy: Imagine trying to see the bottom of a swimming pool.
  • Previous experiments used a flashlight that could only see the surface or the shallow end.
  • This experiment used a flashlight with a much higher energy (because the Upsilon particle is much heavier). This allowed them to see much deeper into the "pool" of the nucleus.

They probed the nucleus at a scale of 22.4 GeV². This is a massive scale, about 90 times deeper than previous studies with the ϕ meson.

The Result:
Even though they looked much deeper (where the physics should be different), they found that the "shadow" (suppression) was almost the same strength as it was for the lighter particles.

• The Metaphor: It's like diving deep into the ocean. You might expect the water pressure to change drastically as you go deeper. But here, the "pressure" (the density of the gluons) felt almost the same as it did near the surface. This suggests that the "gluon soup" inside the nucleus is incredibly dense and uniform, even at these extreme depths.

The "Glue" Factor

The scientists calculated a number called $R_g$ (the nuclear gluon suppression factor).

• If the number is 1.0, the nucleus is just a bag of free particles.
• If the number is less than 1.0, the nucleus is "shadowing" the light.

They found the number to be 0.55. This means the nucleus is only about half as "transparent" to this light as a single free proton would be. The gluons are so packed together that they are effectively hiding from the probe.

Why Should You Care?

1. Testing the Rules of the Universe: This experiment tests the fundamental laws of how matter holds itself together (Quantum Chromodynamics, or QCD). The fact that the suppression is so strong and consistent at such high energies challenges our current computer models. It's like finding a crack in the foundation of a building that everyone thought was solid.
2. The Future of Physics: This result helps prepare us for the Electron Ion Collider (EIC), a future machine designed specifically to take "CT scans" of the nucleus. This paper tells the EIC team exactly what kind of "glue" density they are likely to find.
3. The "Gluon Saturation" Mystery: Physicists suspect that at very high densities, gluons stop splitting and start merging (saturation). This experiment didn't find the "smoking gun" of saturation yet (because the scale was so high that nonlinear effects should be small), but it set a very strict boundary. It tells us: "If saturation is happening, it's happening in a very subtle way that our current theories haven't fully captured."

In a Nutshell

The CMS team used a high-speed "ghost flashlight" to shine through a lead nucleus. They expected to see a certain amount of light passing through, but the nucleus was much darker than predicted. Even when they used a flashlight powerful enough to see 90 times deeper than before, the darkness remained the same. This proves that the "glue" holding the nucleus together is incredibly dense and crowded, acting like a thick fog that blocks our view, challenging our current understanding of how the universe's building blocks are packed together.

Technical Summary

1. Problem and Physics Motivation

The paper addresses the fundamental question of how the internal structure of nucleons changes when they are bound within a heavy nucleus, specifically regarding the behavior of gluons at high densities and low momentum fractions ($x$).

• Gluon Saturation: At very small $x$ ($x \lesssim 10^{-3}$), gluon densities in nucleons rise rapidly. Theoretical models predict that nonlinear Quantum Chromodynamics (QCD) effects, such as gluon recombination, will eventually moderate this growth, leading to a state known as gluon saturation (often described by the Color Glass Condensate framework).
• The Probe: Ultraperipheral collisions (UPCs) of heavy ions provide a clean environment to study photon-nucleus interactions. In these events, a quasireal photon from one nucleus fluctuates into a quark-antiquark dipole, which then interacts with the target nucleus via two-gluon exchange to form a vector meson.
• The Gap: While coherent photoproduction of lighter vector mesons ($\rho$, $J/\psi$, $\psi(2S)$, and $\phi$) has been extensively studied, coherent $\Upsilon(1S)$ photoproduction off heavy nuclei had not been measured. The $\Upsilon(1S)$ (bottomonium) offers a unique probe because its large mass ($M_{\Upsilon} \approx 9.46$ GeV) corresponds to a much higher hard scale ($\mu^2 \approx 22.4$ GeV$^2$) compared to charmonium ($J/\psi$) or light mesons. This high scale minimizes nonlinear QCD effects, allowing for a cleaner test of perturbative QCD (pQCD) and nuclear parton distribution functions (nPDFs).

2. Methodology

The analysis utilizes data collected by the CMS experiment during the 2018 lead-lead (PbPb) run at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{NN}} = 5.02$ TeV.

• Data Sample: An integrated luminosity of $1.66 \pm 0.03$ nb$^{-1}$.
• Event Selection:
  • Trigger: Dedicated trigger requiring at least one muon candidate coincident with a PbPb bunch crossing, with no explicit $p_T$ requirement.
  • Veto: Events with energy deposits in forward calorimeters (HF) above noise thresholds are rejected to suppress hadronic interactions.
  • Reconstruction: Events must contain a primary vertex and exactly two tracks within $|\eta| < 2.4$. Both tracks must be identified as high-quality muons ($p_T > 3.5$ GeV).
  • Signal Definition: $\Upsilon(1S)$ candidates are reconstructed via the $\mu^+\mu^-$ decay channel with an invariant mass in the range $8.0 < m_{\mu\mu} < 12.0$ GeV.
• Background Suppression & Separation:
  • Coherent vs. Incoherent: Coherent production (photon interacts with the whole nucleus) results in low transverse momentum ($p_T < 0.2$ GeV). Incoherent production (interaction with individual nucleons) yields higher $p_T$.
  • Fit Strategy: The raw yield is extracted by fitting the invariant mass ($m_{\mu\mu}$) and dimuon $p_T$ distributions simultaneously.
    • The signal shape is modeled using a double-sided Crystal Ball function.
    • The dominant background ($\gamma\gamma \to \mu^+\mu^-$) is modeled with a polynomial.
    • Contributions from incoherent processes and feed-down from excited states ($\Upsilon(2S)$, $\Upsilon(3S)$) are constrained using Monte Carlo (STARLIGHT and PYTHIA 8) templates.
• Kinematic Probing:
  • Measurements are performed in two rapidity intervals: central ($|y| < 1$) and forward ($1 < |y| < 2.4$).
  • Midrapidity ($|y| < 0.2$) corresponds to a photon-nucleon center-of-mass energy $W_{\gamma N} \approx 200$ GeV and probes gluons at $x \approx 10^{-3}$.

3. Key Contributions

• First Measurement: This is the first observation of coherent $\Upsilon(1S)$ photoproduction off a heavy nucleus (Pb).
• High-Scale Probe: It probes the nuclear gluon structure at an unprecedented scale of $\mu^2 = 22.4$ GeV$^2$ for coherent vector-meson photoproduction, roughly 90 times higher than that of $\phi$ mesons and 9 times higher than $J/\psi$.
• Quantification of Suppression: The paper provides the first quantitative measurement of the nuclear gluon suppression factor ($R_g^{Pb}$) at this specific kinematic point.

4. Results

• Observation of Suppression: The measured cross sections are significantly lower than predictions from the Impulse Approximation (IA), which treats the nucleus as a collection of free nucleons (neglecting nuclear effects).
  • Suppression factor $S_{\Upsilon(1S)} = \text{Data}/\text{IA}$:
    • At $|y| < 1$: $0.25 \pm 0.06 (\text{stat}) \pm 0.02 (\text{syst})$.
    • At $1 < |y| < 2.4$: $0.32 \pm 0.09 (\text{stat}) \pm 0.06 (\text{syst})$.
• Nuclear Gluon Suppression Factor ($R_g^{Pb}$):
  • Extracted at $x \approx 10^{-3}$ and $\mu^2 = 22.4$ GeV$^2$.
  • Result: $R_g^{Pb} = 0.55 \pm 0.12 (\text{stat}) \pm 0.02 (\text{syst})$.
  • This indicates that the gluon density in the lead nucleus is suppressed by roughly 45% compared to a free nucleon at this scale.
• Mass Dependence: Remarkably, the suppression factor for $\Upsilon(1S)$ ($R_g \approx 0.55$) is only slightly larger than that previously measured for coherent $\phi$ photoproduction ($R_g \approx 0.18-0.20$), despite the $\Upsilon$ probing a scale $\mu^2$ that is two orders of magnitude larger. This suggests that nuclear shadowing effects persist even at high scales where nonlinear saturation effects are expected to be minimal.
• Theoretical Comparison:
  • Impulse Approximation (IA): Overpredicts the data significantly.
  • NLO pQCD Models: Models using nuclear PDFs (EPS09, EPPS21) generally overestimate the data, though large uncertainties in the nPDFs allow for consistency within errors.
  • CGC/Saturation Models: Color Glass Condensate (CGC) models (IP BFKL, IP BK) tend to overestimate the cross section, suggesting that for the high scale of $\Upsilon(1S)$, nonlinear saturation effects are not the dominant driver of the observed suppression, or that current saturation models need refinement.

5. Significance

• Validation of QCD Dynamics: The result confirms that nuclear modifications (shadowing) are a robust feature of QCD that persists even at high momentum transfer scales ($\mu^2 \approx 22.4$ GeV$^2$), challenging the assumption that nuclear effects vanish at high scales.
• Constraints on nPDFs: The measurement provides a critical new data point to constrain nuclear Parton Distribution Functions (nPDFs), particularly the gluon distribution at $x \sim 10^{-3}$.
• Future Physics: The observation of mass-dependent nuclear suppression (comparing $\phi$, $J/\psi$, and $\Upsilon$) is crucial for understanding the transition from non-perturbative to perturbative regimes in nuclear matter. These results will serve as a benchmark for future high-precision measurements at the LHC and the planned Electron-Ion Collider (EIC), which aims to map the gluon saturation regime in detail.