Measurement of isolated-prompt photon$-$hadron correlations in Pb$-$Pb collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}} = 5.02}$ TeV

The ALICE Collaboration reports the first measurement of isolated-prompt photon-hadron azimuthal correlations in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV down to a photon transverse momentum of 18 GeV/c, observing a strong suppression of associated hadron yields in central collisions that is compared with theoretical models and results from other experiments.

The Gist

The Big Picture: Smashing Heavy Balls to See What's Inside

Imagine you have two giant, heavy bowling balls (Lead nuclei) and you smash them together at nearly the speed of light. When they collide, they don't just shatter; for a split second, they melt into a super-hot, super-dense soup of their smallest parts (quarks and gluons). Scientists call this soup the Quark-Gluon Plasma (QGP). It's the state of matter that existed just microseconds after the Big Bang.

The goal of this experiment is to figure out how "thick" or "sticky" this soup is. Does it slow down particles moving through it, or do they zip right through?

The Experiment: A "Flashlight" and a "Bullet"

To study this soup, the ALICE team at CERN's Large Hadron Collider (LHC) used a clever trick involving two types of particles:

1. The Flashlight (The Photon): When the balls smash, they sometimes create a high-energy particle of light called a "prompt photon." Think of this as a flashlight. Because light doesn't interact with the sticky soup, it flies straight out of the collision without getting slowed down or deflected. It acts as a perfect, uncorrupted marker of the initial crash.
2. The Bullet (The Hadron): At the exact same moment, the collision usually shoots out a high-speed "bullet" (a jet of particles called hadrons) in the opposite direction. This bullet does have to travel through the sticky soup.

The Analogy:
Imagine you are in a dark room (the soup). You shine a flashlight (the photon) straight up at the ceiling. At the same time, you throw a ball (the hadron) straight down at the floor.

• If the room is empty air, the ball hits the floor with full force.
• If the room is filled with thick, sticky honey (the QGP), the ball will slow down, lose energy, and maybe break apart before it hits the floor.

By measuring how much energy the "bullet" has when it finally escapes, compared to the "flashlight" that didn't slow down, scientists can measure how much energy was lost in the soup.

What They Did

The ALICE team looked at thousands of these collisions in Lead-Lead (Pb-Pb) mode. They focused on three types of crashes:

• Central (0–30%): A head-on, hard smash. The soup is huge and thick.
• Semicentral (30–50%): A glancing blow. The soup is medium-sized.
• Peripheral (50–90%): A very light tap. The soup is small or non-existent.

They measured the "flashlight" (photons) and the "bullets" (charged particles) to see how the bullets behaved in different sizes of soup.

The Key Findings

1. The "Suppression" Effect: In the big, head-on collisions (Central), the "bullets" were significantly weaker than expected. They had lost a lot of energy. This is called jet quenching. It proves the soup is very thick and acts like a brake on high-speed particles.
2. The Comparison: In the light taps (Peripheral collisions), the bullets kept most of their energy, behaving almost like they were in a vacuum.
3. The Ratio: When they compared the Central collisions to the Peripheral ones, they found a ratio of about 0.5. This means the bullets in the thick soup had only half the punch they would have had in empty space.
4. Theory Check: They compared their results to computer models. The models that included "energy loss" (friction in the soup) matched the data perfectly. The models that ignored the soup (assuming the particles just flew through) were completely wrong.

Why This Matters

This paper is important because it uses a very specific method (isolated photons) to get a cleaner measurement than before. It confirms that the Quark-Gluon Plasma is a real, dense medium that steals energy from particles moving through it.

The authors also compared their results with other experiments (like CMS at CERN and STAR/PHENIX at RHIC). Even though they used slightly different settings, the story is the same: The soup is thick, and it slows things down.

Summary in One Sentence

By using a beam of light (photons) as a perfect ruler to measure the speed of a particle (hadron) flying through a hot, dense soup created by smashing lead atoms, the ALICE team proved that the soup is thick enough to significantly slow down and weaken high-speed particles.

Technical Summary

1. Problem and Motivation

The primary goal of this research is to probe the properties of the Quark-Gluon Plasma (QGP) created in ultra-relativistic heavy-ion collisions. Specifically, the study addresses the phenomenon of jet quenching, where high-energy partons lose energy as they traverse the dense QGP medium.

• The Challenge: In standard jet measurements, the initial energy of the parton is unknown because the parton loses energy before fragmenting. This makes it difficult to quantify the exact amount of energy lost.
• The Solution: The paper utilizes isolated-prompt photons as "trigger" particles. Photons are produced in the initial hard scattering of the collision and, being electromagnetically interacting, do not interact strongly with the QGP. Therefore, they escape the medium unmodified and serve as a precise reference for the initial energy scale of the recoiling parton (the "jet").
• The Objective: By measuring the correlation between these isolated photons and the associated charged hadrons (the recoil jet fragments), the collaboration aims to reconstruct the parton fragmentation function ($D(z_T)$) in the presence of the QGP and compare it to vacuum (proton-proton) expectations. This paper extends previous measurements to a lower photon transverse momentum ($p_T^\gamma$) range (down to 18 GeV/c) compared to other LHC experiments, probing a regime where nuclear effects and energy loss are expected to be significant.

2. Methodology

Experimental Setup and Data

• Experiment: ALICE detector at the CERN LHC.
• Collision System: Lead-Lead (Pb–Pb) collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.02$ TeV.
• Data Samples: Collected during Run 2 (2015 and 2018).
• Centrality Classes: The analysis is performed in three centrality bins:
  • Central: 0–30%
  • Semicentral: 30–50%
  • Peripheral: 50–90%

Particle Selection and Reconstruction

• Trigger (Photon): Isolated-prompt photons were reconstructed using the Electromagnetic Calorimeter (EMCal).
  • Kinematics: $18 < p_T^\gamma < 40$ GeV/c and $|\eta_\gamma| < 0.67$.
  • Isolation: A crucial selection criterion requiring the sum of transverse momentum of charged particles within a cone of radius $R=0.2$ around the photon to be below a threshold ($p_{T, iso}^{ch} < 1.5$ GeV/c). This suppresses photons from neutral meson decays ($\pi^0 \to \gamma\gamma$) and fragmentation.
  • Purity: The "ABCD method" was used to estimate the signal purity (ranging from 0.5 in peripheral to 0.65 in central collisions) and subtract the background from $\pi^0$ decays.
• Associated Particles (Hadrons): Charged hadrons were reconstructed using the Inner Tracking System (ITS) and Time Projection Chamber (TPC).
  • Kinematics: $p_T^h > 1.8$ GeV/c and $|\eta_h| < 0.9$.

Analysis Strategy

1. Azimuthal Correlation: The distribution of hadrons relative to the photon trigger in azimuthal angle ($\Delta\phi$) was constructed.
2. Background Subtraction:
   • Underlying Event (UE): Estimated using the mixed-event technique (correlating triggers from one event with tracks from different events with similar centrality and vertex position) and subtracted.
   • Photon Background: The contribution from $\pi^0$ decays was subtracted using the purity ($P$) and the correlation distributions of "narrow" (signal + background) and "wide" (background only) clusters: $C_{\gamma iso} = \frac{1}{P}C_{narrow} - \frac{1-P}{P}C_{wide}$.
3. $D(z_T)$ Extraction: The conditional yield of associated hadrons, $D(z_T)$, was calculated by integrating the azimuthal correlation in the away-side region ($3\pi/5 < |\Delta\phi| < \pi$).
   • $z_T = p_T^h / p_T^\gamma$ represents the fraction of the trigger photon's momentum carried by the associated hadron.
4. Corrections: The raw yields were corrected for detector acceptance, reconstruction efficiency, and resolution using Monte Carlo simulations (PYTHIA 8 embedded in real Pb–Pb events).

3. Key Contributions

• Extended Kinematic Reach: This measurement probes a lower $p_T^\gamma$ range (18–40 GeV/c) than previous LHC measurements (e.g., CMS, which typically starts above 60 GeV/c). This allows access to lower $Q^2$ scales where the interplay between parton virtuality and medium scales is more relevant.
• Robust Background Subtraction: The application of the isolation technique combined with the ABCD method in the high-multiplicity environment of central Pb–Pb collisions demonstrates a sophisticated approach to isolating prompt photons from the overwhelming background.
• Comprehensive Theoretical Comparison: The results are compared against:
  • NLO pQCD calculations (without energy loss) to establish a baseline.
  • NLO pQCD + Energy Loss models (Higher-Twist formalism).
  • CoLBT-hydro model: A coupled Linear Boltzmann Transport and hydrodynamics model that simulates jet transport and medium excitation event-by-event.
• Cross-Experiment Synthesis: The paper provides a qualitative comparison with results from CMS (isolated $\gamma$-jet and $Z^0$-hadron correlations at LHC) and STAR/PHENIX (direct $\gamma$-hadron correlations at RHIC), creating a unified view of jet quenching across different collision energies and experiments.

4. Results

• Suppression of Fragmentation ($I_{AA}$):
  • The ratio $I_{AA} = D(z_T)_{AA} / D(z_T)_{pp}$ (approximated here as $I_{pQCD} = D(z_T)_{AA} / D(z_T)_{pQCD}$) shows a strong suppression in central collisions (0–30%) compared to peripheral collisions.
  • In central collisions, the ratio is approximately 0.5 for intermediate $z_T$, indicating that the recoil parton loses roughly half its energy to the medium before fragmenting.
  • The suppression diminishes as collisions become more peripheral (30–50% and 50–90%), approaching unity in the most peripheral bins.
• Model Agreement:
  • The measured $D(z_T)$ distributions and the suppression ratios are in good agreement with both the NLO pQCD + $\Delta E_{loss}$ and CoLBT-hydro models.
  • Models that include only Cold Nuclear Matter (CNM) effects (using EPPS21 nPDFs) fail to reproduce the observed suppression, confirming that the effect is due to final-state interactions (jet quenching) within the QGP.
• Centrality Dependence ($I_{CP}$):
  • The ratio of central to peripheral yields ($I_{CP}$) shows a clear decrease from peripheral to central collisions, with average values of ~0.75 (30–50%) and ~0.5 (0–30%).
• Comparison with Other Experiments:
  • The ALICE results are consistent with CMS measurements of isolated $\gamma$-jet and $Z^0$-hadron correlations in the overlapping $z_T$ range.
  • Comparisons with STAR and PHENIX at RHIC show similar trends, though PHENIX reports a higher yield at low $z_T$, likely due to their lower trigger momentum allowing for the observation of enhanced soft hadrons from lower-energy partons.

5. Significance

• Validation of Jet Quenching Mechanisms: The results provide strong evidence that the suppression of high-$p_T$ hadrons is driven by parton energy loss in the QGP, as the data aligns with models incorporating energy loss but not with vacuum or CNM-only calculations.
• Precision Benchmark: By utilizing isolated photons, the study avoids the ambiguities of jet reconstruction (where the jet energy is modified by the medium), offering a cleaner probe of the parton energy loss mechanism.
• Low-$p_T$ Access: Extending the measurement to lower photon $p_T$ fills a gap in the kinematic landscape, allowing for a more complete understanding of how jet quenching depends on the energy scale of the hard scattering.
• Future Outlook: This measurement serves as a critical benchmark for Run 3 and Run 4 of the LHC. The increased statistics in future runs will allow for differential studies (e.g., event-plane dependence) and access to even lower $z_T$ and $p_T^\gamma$ values, further refining our understanding of the QGP transport properties.

In conclusion, this paper successfully demonstrates the utility of isolated-prompt photons as a standard candle for studying jet quenching in heavy-ion collisions, confirming significant energy loss of partons in the QGP and validating theoretical models that incorporate medium-induced energy loss.