Measurement of isolated photon plus two-jet correlations in Pb+Pb and $pp$ collisions at 5.02 TeV with ATLAS

Using ATLAS data from 5.02 TeV $pp$ and Pb+Pb collisions, this paper presents a measurement of isolated photon plus two-jet correlations that reveals significant suppression of two-jet yields in heavy-ion collisions due to parton-medium interactions, with results compared against three jet quenching models.

The Gist

Imagine a giant, high-speed billiard table inside a massive underground ring (the Large Hadron Collider). In this experiment, scientists at the ATLAS detector are studying what happens when they smash heavy lead atoms together at nearly the speed of light.

Here is the story of what they found, explained simply:

The Setup: A Flashlight and Two Billiard Balls

Usually, when these atoms smash, they create a chaotic explosion of energy. But to study this chaos clearly, the scientists needed a "flashlight" to mark the starting point.

1. The Flashlight (The Photon): They look for a specific particle called a photon (a particle of light). Because light has no "charge" (it doesn't interact with the sticky stuff created in the crash), it flies straight out of the explosion without getting slowed down. It acts like a perfect ruler, telling the scientists exactly how much energy was created at the very moment of the crash.
2. The Billiard Balls (The Jets): Opposite the flashlight, the crash creates two sprays of particles called jets. Think of these as two billiard balls shooting out in the opposite direction.

The Experiment: The "Mud Pit" vs. The "Empty Room"

The scientists ran this experiment in two different environments:

• The Empty Room ($pp$ collisions): They smashed single protons together. This is like shooting the billiard balls across a clean, empty table. They fly out exactly as expected.
• The Mud Pit (Pb+Pb collisions): They smashed heavy lead atoms together. This creates a super-hot, super-dense soup of energy called Quark-Gluon Plasma (QGP). It's like shooting the billiard balls through a thick, sticky mud pit.

The Three Questions They Asked

By comparing the "Empty Room" results to the "Mud Pit" results, they measured three specific things to see how the mud affected the billiard balls:

1. Did they lose energy? ($x_{JJ\gamma}$)

   • The Metaphor: In the empty room, the two billiard balls should have the same total energy as the flashlight. In the mud pit, do they arrive with less energy because the mud slowed them down?
   • The Result: Yes. The balls arrived with significantly less energy. The "mud" (the plasma) absorbed some of their energy. This is called "jet quenching."

2. Did they lose energy equally? ($A_{JJ\gamma}$)

   • The Metaphor: Imagine the two billiard balls are different sizes (one is a heavy lead ball, one is a lighter wood ball). Did the mud slow down the heavy one more than the light one? Or did they both get stuck in the mud equally?
   • The Result: The scientists found that the two jets (which represent different types of particles, quarks and gluons) lost energy in a way that suggests they interact with the mud differently, but the overall effect is a significant slowdown for both.

3. Did they spread out? ($\Delta R_{JJ}$)

   • The Metaphor: When the balls fly out of the mud, do they stay close together, or does the mud push them apart so they end up at a wider angle?
   • The Result: The angle between the two balls changed depending on how deep into the mud they traveled. This helps scientists understand if the "mud" can "see" the two balls as separate objects or if they act as a single unit when they are close together.

The Big Discovery

The paper reports a clear suppression. In the heavy lead collisions (the mud pit), the number of events where they found these two jets was much lower than in the proton collisions (the empty room).

• The "I_AA" Ratio: This is a score they calculated. A score of 1 means "no change." A score less than 1 means "something is missing."
• The Finding: The score was well below 1 (especially in the most violent, central collisions). This proves that the "mud" (the Quark-Gluon Plasma) is very effective at stealing energy from the particles trying to escape.

Checking the Theory

The scientists compared their real-world data with three different computer simulations (models named Jewel, Jetscape, and Lbt).

• Think of these models as three different weather forecasters trying to predict how the mud behaves.
• The data showed that while all three models got the general idea right (the balls slow down), they disagreed on the details, especially regarding the angle between the balls. This tells the scientists that their understanding of the "mud" needs to be refined.

Summary

In short, this paper is a measurement of how much energy particles lose when they try to escape a super-hot, dense soup of energy created in a nuclear crash. By using a "flashlight" (photon) to set the starting score, they proved that the soup acts like a thick, energy-sucking fluid, slowing down the escaping particles more than anyone expected. This helps us understand the fundamental nature of the universe just a fraction of a second after the Big Bang.

Technical Summary

Technical Summary: Measurement of Isolated Photon Plus Two-Jet Correlations in Pb+Pb and $pp$ Collisions at 5.02 TeV

Problem and Motivation
The study of the Quark-Gluon Plasma (QGP) relies heavily on understanding how high-energy partons lose energy as they traverse the deconfined medium, a phenomenon known as jet quenching. While significant progress has been made in measuring inclusive jet suppression and photon-tagged inclusive jets ($\gamma + 1$ jet), unresolved questions remain regarding the specific nature of parton-medium interactions. Key open questions include the magnitude of energy loss, its dependence on parton flavor (quark vs. gluon), and the existence of a minimum separation distance between color charges before the medium can resolve them as independent entities (color coherence).

Previous measurements of photon-tagged inclusive jets provided a baseline for the initial hard scattering scale, as the photon does not interact via the strong force. However, the dynamics of the recoiling system in the presence of a medium are more complex when multiple partons are involved. This paper addresses these gaps by presenting the first measurement of isolated photon plus two-jet ($\gamma + 2$ jets) correlations in Pb+Pb collisions. By analyzing events where a photon's transverse momentum is balanced by two distinct jets, the study probes the energy loss of a multiparton system, the relative energy loss between color-charge carriers, and medium-induced modifications to the opening angle of the jet pair.

Methodology
The analysis utilizes data recorded by the ATLAS detector at the Large Hadron Collider (LHC) at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.02$ TeV. The dataset comprises:

• $pp$ collisions: 2017 data with an integrated luminosity of 260 pb$^{-1}$.
• Pb+Pb collisions: 2018 data with an integrated luminosity of 1.72 nb$^{-1}$.

Event Selection and Reconstruction:

• Photons: Selected with transverse momentum ($p_T$) in the range 90–180 GeV and pseudorapidity $|\eta| < 2.37$ (excluding the transition region). Strict isolation criteria are applied to ensure purity.
• Jets: Reconstructed using the anti-$k_t$ algorithm with a small radius parameter $R=0.2$ and $p_T > 30$ GeV. The small radius is chosen to resolve jet pairs at small angular separations and reduce background contributions in heavy-ion collisions.
• Topology: Events must contain an isolated photon and at least two jets in the azimuthally opposite hemisphere ($\Delta\phi_{jet,\gamma} > \pi/2$). Additional kinematic requirements ensure the jets are well-separated from the photon and each other.

Background Subtraction and Unfolding:
A significant challenge in Pb+Pb collisions is the combinatorial background arising from uncorrelated hard scatterings and underlying event (UE) fluctuations. The paper employs a novel multijet mixing technique to correct for these contributions. This procedure involves constructing background distributions by combining the photon from a signal event with jets from minimum-bias events of similar centrality and event plane angle. A three-step subtraction and addition scheme is used to account for background-background pairs and avoid over-subtraction.

Following background subtraction, the distributions are corrected for detector resolution effects, kinematic bin migration, and reconstruction efficiency using an iterative Bayesian unfolding method. The results are reported at the particle level.

Key Observables
Three observables are measured to characterize the multiparton system:

1. $x_{JJ\gamma}$: The ratio of the magnitude of the vector sum of the two jet $p_T$ to the photon $p_T$. This characterizes the overall energy loss of the multiparton system.
2. $A_{JJ\gamma}$: The asymmetry between the two jet $p_T$ values divided by the photon $p_T$. This probes the relative energy loss between the two color-charge carriers.
3. $\Delta R_{JJ}$: The angular distance between the two jets. This is sensitive to medium-induced modifications of the jet opening angle and potential color coherence effects.

Results
The paper presents unfolded distributions for $x_{JJ\gamma}$, $A_{JJ\gamma}$, and $\Delta R_{JJ}$ in $pp$ collisions and in three centrality intervals of Pb+Pb collisions (0–10%, 10–30%, and 30–80%).

• Suppression: A significant suppression of the per-photon two-jet yields is observed in Pb+Pb collisions compared to $pp$ collisions across all three observables and all centrality intervals, indicated by the nuclear modification factor $I_{AA} < 1$.
• Centrality Dependence: The suppression is most pronounced in the most central collisions (0–10%) and decreases as the collisions become more peripheral (30–80%), consistent with the expectation that the QGP volume and temperature are larger in central events.
• Kinematic Trends: In the most central collisions, $I_{AA}$ decreases for larger values of $x_{JJ\gamma}$, $A_{JJ\gamma}$, and $\Delta R_{JJ}$, suggesting stronger suppression for events with higher energy imbalance or larger angular separation.
• Model Comparison: The experimental results are compared to three theoretical jet quenching models: Jewel, Jetscape, and Lbt.
  • All models qualitatively describe the $x_{JJ\gamma}$ distribution, predicting larger suppression at high $x_{JJ\gamma}$.
  • The $A_{JJ\gamma}$ distribution is generally well-described, though Jewel slightly over-predicts $I_{AA}$.
  • For $\Delta R_{JJ}$, the data shows less suppression for dijet events with larger angular separation. While Jetscape and Lbt describe this trend, Jewel predicts the opposite trend, showing substantial suppression for small angular separations.

Significance and Claims
The paper claims that these measurements provide new constraints on the microscopic processes of energy loss in the QGP. Specifically, the $\gamma + 2$ jet topology allows for the study of:

• The energy loss of a multiparton system rather than a single jet.
• The relative energy loss between different color charges (quark vs. gluon jets).
• The resolution of color charges by the medium at varying angular separations.

The authors state that these results offer further tests of the underlying assumptions of theoretical models, which will help refine their descriptions of parton-medium interactions. The paper does not claim to definitively resolve the nature of color coherence but notes that the data provides constraints on possible effects at larger angular separations, a phase space not previously well-constrained by such measurements. The comparison with models highlights areas where current theoretical frameworks (specifically Jewel regarding $\Delta R_{JJ}$) may require adjustment to fully capture the observed physics.