Cold Nuclear Matter Effects on Inclusive $J/\psi$ Production in $p+\text{Au}$ Collisions at $\sqrt{s_\text{NN}}$ = 200 GeV with the STAR Experiment

This paper reports that the nuclear modification factor ($R_{p\text{Au}}$) for inclusive $J/\psi$ production in $p+\text{Au}$ collisions at $\sqrt{s_\text{NN}} = 200$ GeV is consistent with unity in the $p_\text{T}$ range of 4–12 GeV/$c$, suggesting that cold nuclear matter effects are negligible in this specific kinematic region.

The Gist

The Cosmic "Cold" Mystery: A Summary of the STAR Collaboration Paper

Imagine you are trying to understand how a high-speed car crash works. To understand the "big" crash (like two massive semi-trucks colliding), you first need to understand what happens when a small pebble hits a large boulder.

In the world of particle physics, scientists at the STAR experiment are doing exactly this. They are studying how tiny particles called J/ψ (J-psi) behave when they are produced in collisions between a proton (the "pebble") and a gold nucleus (the "boulder").

Here is the breakdown of what they found, using everyday language.

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1. The Main Characters

• The J/ψ (The Messenger): Think of the J/ψ particle as a tiny, high-tech sensor. Because of how it is made, it is incredibly sensitive to its surroundings. If something "heavy" or "hot" is happening in the collision, the J/ψ will change or disappear. It’s our messenger telling us what the environment looks like.
• The QGP (The Cosmic Soup): In massive collisions (like Gold hitting Gold), the temperature gets so high that matter melts into a "Quark-Gluon Plasma"—a hot, primordial soup that existed just after the Big Bang.
• CNM (The Cold Neighborhood): In smaller collisions (Proton hitting Gold), scientists believe it isn't hot enough to make that "soup." Instead, they are looking at Cold Nuclear Matter (CNM) effects. This is like studying how a car behaves when it drives through a thick fog or a heavy rainstorm, rather than driving through a literal volcano.

2. The Experiment: "The Fog Test"

The scientists wanted to know: Does the "fog" of the gold nucleus change how many J/ψ messengers make it out alive?

To figure this out, they used a math tool called $R_{pAu}$.

• If $R_{pAu}$ is 1, it means the fog had no effect. The messengers passed through perfectly.
• If $R_{pAu}$ is less than 1, it means the fog "swallowed" or blocked the messengers.

3. The Discovery: "Clear Skies"

After crunching the numbers from their high-speed collisions at the Relativistic Heavy Ion Collider (RHIC), the results were surprising: The $R_{pAu}$ was consistent with 1.

In plain English: In the specific speed range they studied (high momentum), the "fog" of the gold nucleus didn't really stop the J/ψ particles. They zipped through almost as if the gold wasn't even there.

4. Why does this matter? (The "Big Picture")

This might sound like "nothing happened," but in science, knowing what doesn't happen is just as important as knowing what does.

By proving that the "cold fog" (CNM) doesn't significantly mess with these particles at high speeds, the scientists have cleared the path for other discoveries. They have confirmed that when we see J/ψ particles disappearing in much larger collisions (like Gold-on-Gold), it must be because of the "hot soup" (the QGP) and not just the "cold fog."

The Analogy:
If you see a runner disappear in a massive forest fire, you know it's because of the heat. But before you can be sure about that, you have to prove that the runner wouldn't have disappeared just by running through a regular, cold, misty forest. This paper just proved the "misty forest" isn't the culprit.

Summary Table

Scientific Term	Everyday Metaphor	What it means in this paper
J/ψ Particle	The Messenger	A sensitive probe used to sense the environment.
CNM Effects	The Cold Fog	The influence of the nucleus without extreme heat.
$R_{pAu} \approx 1$	Clear Visibility	The "fog" didn't block the messengers.
QGP	The Cosmic Soup	The hot, melted state of matter we are trying to study.

Technical Summary

Technical Summary: Cold Nuclear Matter Effects on Inclusive $J/\psi$ Production in $p + \text{Au}$ Collisions

1. Problem Statement

The study of the Quark-Gluon Plasma (QGP)—a deconfined state of quarks and gluons—is a central goal of high-energy nuclear physics. While $J/\psi$ meson suppression is a well-known signature of the hot medium (QGP) created in heavy-ion collisions (like $\text{Au}+\text{Au}$), it is crucial to distinguish these "hot" effects from Cold Nuclear Matter (CNM) effects.

CNM effects occur in smaller collision systems (like $p+\text{Au}$), where the energy density is generally presumed insufficient to form a QGP. These effects include nuclear modification of parton distribution functions (shadowing/anti-shadowing), gluon saturation (Color Glass Condensate), energy loss of $c\bar{c}$ pairs, and nuclear absorption. To accurately quantify QGP signatures in heavy-ion collisions, the baseline CNM effects must be precisely measured and understood.

2. Methodology

The STAR Collaboration performed a new measurement of inclusive $J/\psi$ production at $\sqrt{s_{NN}} = 200$ GeV using $p+p$ and $p+\text{Au}$ collisions.

• Experimental Setup: Data were collected using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The reconstruction of $J/\psi \to e^+e^-$ relied on the Time Projection Chamber (TPC) for tracking and particle identification (via $dE/dx$) and the Barrel Electromagnetic Calorimeter (BEMC) for high-momentum electron identification (via $E/p$ ratio).
• Kinematic Range: The study focused on the mid-rapidity region ($|y| < 1$) and a transverse momentum range of $4 < p_T < 12$ GeV/c.
• Quantification of CNM: The primary observable is the nuclear modification factor ($R_{p\text{Au}}$), defined as the ratio of the $J/\psi$ yield in $p+\text{Au}$ collisions to the $p+p$ yield, scaled by the average number of binary nucleon-nucleon collisions ($\langle N_{\text{coll}} \rangle$) derived from a Glauber model.
• Statistical Approach: The $p+p$ cross-section was improved by combining the new 2015 data with previous STAR results from 2009 and 2012 using the Best Linear Unbiased Estimator (BLUE) method to minimize total uncertainty.

3. Key Contributions

• High-Precision Baseline: The paper provides a significantly more precise measurement of the inclusive $J/\psi$ dielectron cross-section in $p+p$ collisions at $\sqrt{s} = 200$ GeV compared to previous studies.
• Improved $R_{p\text{Au}}$ Precision: By using the same detector configuration and trigger setup for both $p+p$ and $p+\text{Au}$ in the same year, the researchers minimized systematic uncertainties related to detector response and efficiency.
• Comprehensive Comparison: The study compares experimental results against a wide array of theoretical frameworks, including Improved Color Evaporation Model (ICEM), Non-Relativistic QCD (NRQCD), Color Glass Condensate (CGC), and "comover" models.

4. Results

• $p+p$ Cross-Section: The combined $p+p$ dielectron measurement is consistent with previous dimuon measurements and provides a robust baseline for $R_{p\text{Au}}$ calculations.
• $R_{p\text{Au}}$ Findings: In the measured kinematic region ($4 < p_T < 12$ GeV/c), the $R_{p\text{Au}}$ is consistent with unity. This suggests that CNM effects are negligible for $J/\psi$ production at mid-to-high $p_T$ at RHIC energies.
• Model Comparison:
  • Most models (ICEM, CGC+ICEM, Lansberg) that incorporate nuclear PDF (nPDF) effects are consistent with the $R_{p\text{Au}}$ results.
  • The "comover" model showed a discrepancy, underpredicting $R_{p\text{Au}}$ at mid-to-high $p_T$, which provides a constraint for future refinements of that model.
  • The individual invariant yields ($p+p$ and $p+\text{Au}$) were found to be less satisfactory when compared to models than the $R_{p\text{Au}}$ ratio itself, indicating that models may struggle with absolute normalization but capture the nuclear scaling correctly.

5. Significance

The finding that $R_{p\text{Au}} \approx 1$ at high $p_T$ is highly significant for the broader field of heavy-ion physics. It provides strong evidence that the significant $J/\psi$ suppression observed in $\text{Au}+\text{Au}$ collisions is indeed driven by the hot medium (QGP) rather than by cold nuclear effects. This result solidifies the use of $J/\psi$ suppression as a reliable probe for studying the properties and temperature of the Quark-Gluon Plasma.