Measurement of jet quenching in O+O collisions at $\sqrt{s_\mathrm{NN}}=200$ GeV by the STAR experiment at RHIC

The STAR experiment at RHIC provides strong evidence for jet quenching in oxygen-oxygen collisions at $\sqrt{s_\mathrm{NN}}=200$ GeV by observing a significant 20% suppression of high-transverse-momentum hadron and jet yields in high-event-activity collisions, indicating the formation of a quark-gluon plasma even in small collision systems.

The Gist

The Big Picture: Smashing Tiny Oranges to Find a "Super-Soup"

Imagine you are trying to understand how a giant, hot, gooey soup (called Quark-Gluon Plasma or QGP) behaves. Scientists know this soup existed just a split second after the Big Bang. To study it today, they smash heavy atoms together at near-light speed. Usually, they smash big atoms like Gold (Au) or Lead (Pb).

But here's the mystery: Scientists have seen signs of this "soup" forming even when they smash much smaller things, like protons or tiny nuclei. However, they couldn't prove it was really soup because the "smoke" (the evidence) was too faint to see clearly.

This paper is about a new experiment where the STAR team at the Relativistic Heavy Ion Collider (RHIC) smashed together Oxygen nuclei (O+O). Think of Oxygen nuclei as tiny, lightweight oranges compared to the heavy gold balls they usually use.

The big question was: Can you make a "soup" out of something this small? And if you do, does it still "eat" the energy of particles flying through it?

The Experiment: The "Trigger" and the "Recoil"

To answer this, the scientists didn't just look at the mess of debris. They set up a specific game of "tag":

1. The Trigger (The Shot): They waited for a very high-energy particle (a "trigger") to be shot out of the collision. This is like firing a high-powered bullet.
2. The Recoil (The Return): In a normal collision without any soup, if you fire a bullet, you expect to see a matching bullet shooting out in the exact opposite direction (back-to-back).
3. The Soup Effect (Jet Quenching): If there is a thick, hot soup in the middle, the bullet has to fight its way through it. It loses energy, slows down, or gets scattered. The "return bullet" (the recoil) will be weaker or missing. This loss of energy is called Jet Quenching.

The Challenge: Sorting the "Party" from the "Quiet Room"

The tricky part with these tiny Oxygen collisions is that they are chaotic. Sometimes the collision is a "wild party" (high activity), and sometimes it's a "quiet room" (low activity).

• The Problem: In the past, it was hard to tell if a particle lost energy because of the soup or just because the collision geometry was different.
• The Solution: The scientists used a clever trick. They looked at the "wild party" collisions (high activity) and compared them to the "quiet room" collisions (low activity).
  • If the "wild party" has a soup, the return bullets should be weaker there than in the "quiet room."
  • If there is no soup, the return bullets should look the same in both.

The Results: The "Missing Energy" is Real

The scientists found something exciting:

1. The "Near Side" is Fine: The particles shooting out in the same direction as the trigger bullet looked normal. This means the initial shot wasn't affected.
2. The "Recoil Side" is Weak: The particles shooting out in the opposite direction were significantly weaker (about 20% less energy) in the "wild party" collisions compared to the "quiet room."

The Analogy: Imagine throwing a tennis ball at a wall.

• In a quiet room, the ball hits the wall and bounces back with full force.
• In a room filled with thick fog (the soup), the ball hits the wall, but when it bounces back, it's sluggish and weak because the fog slowed it down.

The data showed the "fog" was definitely there. The energy loss was so significant that the chance of it being a fluke is less than 1 in a million (statistically speaking).

Why This Matters

This is a huge deal because:

• It's the Smallest Soup Yet: This is the first time scientists have definitively seen this "energy eating" effect in such a small system (Oxygen + Oxygen).
• It Changes the Rules: It suggests that even tiny collisions can create a state of matter that behaves like a fluid, not just a gas.
• It's Not Just a Calculation Error: The team checked if the loss was just due to the way nuclei are built (nuclear effects). They found that standard physics calculations couldn't explain the energy loss. It had to be the soup.

The Bottom Line

The STAR experiment successfully proved that even when you smash together tiny Oxygen nuclei, you can create a microscopic drop of the "primordial soup" that existed right after the Big Bang. This soup is thick enough to slow down high-speed particles, proving that the laws of fluid dynamics apply even in the tiniest, most energetic collisions in the universe.

In short: They found the "fog" in the "tiny orange" collision, confirming that the universe's most extreme state of matter can form in the smallest packages imaginable.

Technical Summary

1. Problem Statement and Scientific Context

• The Core Question: While the formation of Quark-Gluon Plasma (QGP) is well-established in large heavy-ion collisions (e.g., Au+Au, Pb+Pb), its existence and properties in small collision systems (such as p+p, p+A, and light ions like O+O) remain a subject of intense debate.
• The Challenge: In small systems, long-range correlations (often attributed to QGP flow) have been observed, but definitive evidence of jet quenching (the energy loss of high-momentum partons traversing the medium) has been elusive.
• Methodological Limitation: Traditional measurements of inclusive jet or hadron yields in small systems rely on the Glauber model to estimate the number of binary collisions ($N_{coll}$). However, in small systems, the correlation between event activity (multiplicity) and collision geometry is weak, leading to large systematic uncertainties in $N_{coll}$ that obscure jet quenching signals.
• Goal: The STAR Collaboration aims to provide the first robust evidence of jet quenching in O+O collisions at $\sqrt{s_{NN}} = 200$ GeV by utilizing correlation observables that are independent of Glauber modeling.

2. Methodology

The analysis utilizes a dataset of approximately 500 million O+O collisions recorded in 2021 at the Relativistic Heavy Ion Collider (RHIC).

• Event Selection & Activity Quantification:

  • Events are classified by Event Activity (EA), defined by the number of Minimum Ionizing Particles (MIPs) detected in the forward Event Plane Detector (EPD, $2.14 < |\eta| < 5.09$).
  • To avoid self-correlations, the EA measurement is taken in the forward region, while the jet/hadron measurements are in the central barrel ($|\eta| < 1.5$).
  • Reference Baseline: The 40–60% EA interval is used as the reference (low-activity) population. High-activity populations (0–10%, 10–20%, 20–40%) are compared against this baseline.

• Observables:

  1. Di-hadron Correlations (h-h): Correlations between a high-$p_T$ trigger hadron ($7 < p_T < 30$ GeV/c) and associated charged hadrons ($3 < p_T < 7$ GeV/c).
  2. Semi-Inclusive Hadron-Jet Correlations (h-jet): Correlations between a trigger hadron and recoiling charged-particle jets reconstructed using the anti-$k_T$ algorithm with radii $R=0.2$ and $R=0.5$.

• Background Subtraction & Unfolding:

  • Uncorrelated Background: Estimated by fitting the correlation function in the region $0.8 < |\Delta\phi| < 1.6$ (away from the near-side peak and recoil peak) and by using Event Mixing (ME) techniques for jets.
  • Combinatorial Jets: A mixed-event dataset is constructed to estimate the combinatorial background, which is then normalized and subtracted from the same-event (SE) distribution.
  • Unfolding: Iterative Bayesian unfolding is applied to correct for detector effects (TPC tracking efficiency, resolution) and $p_T$ smearing.

• Quantification Metric ($I_{CP}$):

  • The suppression is quantified using the ratio $I_{CP}$, defined as the yield in high-EA events divided by the yield in the reference (40–60%) EA events, normalized by the number of trigger particles.
  • Key Advantage: Since $I_{CP}$ is a ratio of two hard production cross-sections, the $N_{coll}$ factors cancel out, eliminating the large systematic uncertainties associated with Glauber modeling in small systems.

3. Key Results

• Di-hadron Correlations:

  • Near-side ($\Delta\phi \approx 0$): The $I_{CP}$ is consistent with unity ($\approx 1.0$), indicating that the trigger particle (and the jet fragmenting into it) experiences negligible energy loss.
  • Recoil side ($\Delta\phi \approx \pi$): A significant suppression is observed. For the 0–10% EA interval, $I_{CP} = 0.836 \pm 0.021 (\text{stat}) \pm 0.032 (\text{syst})$.
  • Significance: The suppression is statistically significant (3.3$\sigma$ to 5.2$\sigma$ depending on the baseline used).

• Semi-Inclusive Jet Correlations (h-jet):

  • Recoil jets ($p_{T,jet} > 8$ GeV/c) show similar suppression. For $R=0.5$ in the 0–10% EA interval, $I_{CP} = 0.806 \pm 0.037 (\text{stat}) \pm 0.025 (\text{syst})$.
  • The suppression is consistent for both $R=0.2$ and $R=0.5$.
  • Significance: The suppression is significant (3.5$\sigma$ to 5.0$\sigma$).

• Exclusion of Initial-State Effects:

  • Theoretical calculations incorporating nuclear Parton Distribution Functions (nPDFs) (EPPS21, TUJU21, nNNPDF3.0) predict either no suppression or a slight enhancement ($I_{AA} > 1$) due to shadowing/anti-shadowing effects.
  • The observed strong suppression ($I_{CP} < 1$) is incompatible with nPDF-only models, confirming that the effect arises from final-state interactions (jet quenching).

• Energy Redistribution:

  • The suppression corresponds to a shift in the recoil jet $p_T$ spectrum.
  • For $R=0.5$, the effective $p_T$ shift is $0.70 \pm 0.15 (\text{stat}) \pm 0.10 (\text{syst})$ GeV/c.
  • This quantifies the energy redistribution of the jet due to interactions with the medium.

4. Significance and Impact

• First Evidence in Light Ions: This work presents the first observation of jet quenching in O+O collisions, the smallest collision system where this phenomenon has been definitively measured.
• System Size Dependence: It establishes that QGP-like behavior (specifically parton energy loss) persists even in very small, light-ion collision systems, challenging previous assumptions that jet quenching requires large, macroscopic media.
• Methodological Breakthrough: By utilizing semi-inclusive correlation observables ($I_{CP}$), the analysis successfully bypasses the limitations of Glauber modeling in small systems, providing a robust baseline for future studies in p+p and p+A collisions.
• Insight into QGP Formation: The results suggest that the mechanisms of isotropization, thermalization, and collective hydrodynamic behavior (which lead to jet quenching) can be triggered in systems as small as Oxygen-Oxygen collisions, offering new constraints on the minimum size required for QGP droplet formation.

Conclusion

The STAR Collaboration has provided strong evidence (up to 5.2$\sigma$ significance) for jet quenching in O+O collisions at $\sqrt{s_{NN}} = 200$ GeV. By observing a $\sim 20\%$ suppression of recoil hadrons and jets in high-activity events relative to low-activity events—and ruling out initial-state nuclear effects—the study confirms that energetic partons lose energy via final-state interactions even in small collision systems. This quantifies the energy redistribution ($\sim 0.7$ GeV/c shift) and deepens the understanding of the system-size dependence of QGP formation.