Unveiling the jet angular broadening with photon-tagged jets in high-energy nuclear collisions

This study employs a transport approach to demonstrate that photon-tagged jets effectively mitigate selection bias, thereby revealing medium-induced angular broadening in PbPb collisions that contrasts with the narrowing observed in inclusive jets.

The Gist

Imagine a high-energy particle collision as a massive, chaotic mosh pit inside a stadium. When two heavy nuclei smash together, they create a super-hot, super-dense soup of particles called the Quark-Gluon Plasma (QGP). It's so hot that even the tiny building blocks of matter (quarks and gluons) melt into a fluid state.

In this mosh pit, scientists are trying to understand how a "jet" of particles behaves. A jet is like a high-speed bullet fired from a gun (the collision) that tries to punch its way through the crowd.

The Mystery: Narrowing vs. Broadening

For a long time, scientists observed something puzzling. When they fired these "bullets" (jets) through the crowd, the spray of particles seemed to get narrower (more focused) than expected. It was as if the crowd squeezed the bullet, making it tighter.

However, a new experiment using a special "tag" (a photon, or a particle of light) suggested the opposite: the spray was getting broader (more spread out). This created a conflict in the scientific community. Was the crowd squeezing the bullet, or was it scattering it?

The Solution: The "Photon Tag"

The authors of this paper act like detectives solving this mystery. They realized the problem wasn't the physics, but how they were picking the bullets to study.

Think of it like this:

• Inclusive Jets (The Old Way): Imagine you are looking for fast runners in a marathon. You decide to only count runners who finish in under 2 hours. But, if the crowd (the QGP) slows a runner down too much, they might finish in 2 hours and 10 minutes and get disqualified. So, your list of "fast runners" is secretly biased. You are mostly seeing the runners who were already fast enough to survive the crowd's slowing effect. You are missing the ones who got slowed down the most.
• Photon-Tagged Jets (The New Way): Now, imagine you pair every runner with a specific, unchangeable reference point, like a drone flying alongside them at a fixed speed. Even if the runner slows down, the drone stays with them. By looking at the ratio of the runner's speed to the drone's speed, you can catch all the runners, even the ones who got slowed down significantly.

The paper shows that using this "photon tag" removes the bias. When they do this, they see that the jets do actually get broader, just as the theory predicted. The "narrowing" seen in previous studies was an illusion caused by missing the slowest, most affected jets.

How They Did It

The researchers used a computer simulation (a "transport approach") to recreate the mosh pit. They simulated:

1. The Bullet: A jet of particles shooting through the plasma.
2. The Crowd: The hot, dense QGP.
3. The Interaction:
   • Radiation: As the jet moves, it loses energy by shedding "gluons" (like sparks flying off a grinding wheel). This spreads the jet out.
   • The Wake: The jet pushes the crowd aside, creating a wake (like a boat moving through water). This "medium response" also affects the shape of the jet.

They found that when they included the "photon tag" and looked at the jets that survived the journey, the sparks (gluons) and the wake clearly made the jet wider.

The Key Takeaways

1. Selection Bias is a Trap: If you only look at the jets that pass a strict speed limit, you miss the ones that got slowed down the most. This makes the data look like the jets are getting narrower, when they are actually getting wider.
2. The Photon Tag Works: By using a photon to tag the jet, scientists can select a group of jets that includes those that have been significantly slowed down (quenched). This reveals the true nature of the interaction: the jet spreads out.
3. Two Different Stories:
   • Inclusive Jets (No tag): Appear to get narrower because the slowest ones are filtered out.
   • Photon-Tagged Jets: Show the true physics, getting broader because the medium scatters the particles.

Conclusion

This paper explains that the "narrowing" of jets was a trick of the selection process. By using a smarter way to pick which jets to study (the photon tag), the authors confirmed that the hot, dense nuclear matter actually causes jets to spread out and broaden. This helps scientists better understand the properties of the Quark-Gluon Plasma, the state of matter that existed just moments after the Big Bang.

Technical Summary

Technical Summary: Unveiling the Jet Angular Broadening with Photon-Tagged Jets in High-Energy Nuclear Collisions

Problem Statement
Recent experimental measurements of jet substructure in heavy-ion collisions have presented a puzzling contradiction. While measurements of inclusive jets in nucleus-nucleus (AA) collisions consistently indicate an angular narrowing compared to proton-proton (pp) collisions, recent results from the CMS collaboration for photon-tagged jets ($\gamma$+jets) suggest evidence of angular broadening. This discrepancy is attributed to "selection bias": in AA collisions, jets that undergo significant energy loss (quenching) often fail to meet transverse momentum ($p_T$) selection thresholds, leading to a sample dominated by less-quenched jets. Theoretical explanations are required to reconcile these observations and determine the intrinsic medium modification of jet angular structure.

Methodology
The authors employ a hybrid transport approach to simulate the evolution of jets in the Quark-Gluon Plasma (QGP).

• Baseline Generation: Initial $pp$ events are generated using PYTHIA8 (Monash Tune).
• Jet Evolution in QGP: The simulation incorporates both radiative and collisional energy loss mechanisms.
  • Radiative Loss: Modeled using the higher-twist formalism, accounting for medium-induced gluon radiation and the Landau-Pomeranchuk-Migdal (LPM) effect.
  • Collisional Loss: Estimated via pQCD calculations within the Hard-Thermal-Loop (HTL) approximation.
  • Medium Response: The excitation of quasi-particles in the medium by the jet is modeled using a perturbative approach based on the Cooper-Frye formula.
• Hydrodynamics: The background medium is simulated using the CLVisc hydrodynamic model, with jet partons fragmenting into hadrons via the Colorless Hadronization prescription when the local temperature drops below $T_c = 0.165$ GeV.
• Jet-by-Jet (JBJ) Matching: To isolate intrinsic modifications from selection bias, the authors utilize a JBJ matching procedure. This method tracks individual jets from their initial state in $pp$ to their final state in $PbPb$, matching pairs based on the smallest angular distance ($\Delta R$) between their axes. This allows for a direct comparison of "unquenched" and "quenched" jets, identifying those that fall below experimental $p_T$ cuts due to energy loss.

Key Results

1. Agreement with CMS Data: The theoretical calculations for the jet girth ($g$) distribution of $\gamma$+jets in $0-30\%$ central $PbPb$ collisions at $\sqrt{s_{NN}} = 5.02$ TeV show good agreement with recent CMS measurements.
2. Role of Selection Bias ($x_{j\gamma}$): The study demonstrates that the observed modification pattern (broadening vs. narrowing) is highly sensitive to the selection cut $x_{j\gamma} = p_T^{jet}/p_T^{\gamma}$.
   • For $x_{j\gamma} > 0.4$, the model reproduces the observed broadening at larger girth values.
   • For stricter cuts ($x_{j\gamma} > 0.8$), the model predicts a narrowing effect, consistent with the suppression of the broadening signal due to the exclusion of highly quenched jets.
3. Quantification of Bias: Using JBJ matching, the authors quantify the selection bias. For $x_{j\gamma} > 0.8$, approximately $58.4\%$ of initially selected jets are rejected in $PbPb$ due to energy loss, whereas only $23.9\%$ are rejected for $x_{j\gamma} > 0.4$. This confirms that $\gamma$+jets with lower $x_{j\gamma}$ cuts effectively select a sample of sufficiently quenched jets.
4. Inclusive vs. Photon-Tagged Jets:
   • Inclusive Jets: Standard experimental selections lead to a narrowing of the girth distribution, consistent with ALICE data. However, JBJ matching reveals that inclusive jets intrinsically broaden in the medium; the observed narrowing is an artifact of selection bias excluding the most quenched jets.
   • $\gamma$+Jets: These jets exhibit a higher survival rate ($\sim 80\%$ vs. $<40\%$ for inclusive jets) because their initial $p_T$ spectrum peaks near the photon $p_T$ (100 GeV), well above the jet cut (40 GeV). Consequently, $\gamma$+jets retain a larger fraction of highly quenched events, allowing the intrinsic broadening signal to be observed.
5. Contributions to Broadening: The study identifies medium-induced gluon radiation as the primary driver of angular broadening, with medium response providing a slight enhancement at larger girth values.
6. Other Observables: The analysis extends to the groomed jet radius ($R_g$) and the angle between jet axes ($\Delta R_{axis}$). Similar to girth, both observables show intrinsic broadening for both jet types when selection bias is removed via JBJ matching. However, experimental selections result in narrowing for inclusive jets and slight broadening for $\gamma$+jets.

Significance and Claims
The paper claims to provide a theoretical resolution to the apparent contradiction between inclusive jet narrowing and $\gamma$+jet broadening. By quantitatively demonstrating that selection bias is the dominant factor obscuring the intrinsic angular broadening in inclusive jets, the authors argue that $\gamma$+jets serve as a superior channel for studying medium-induced broadening. The study asserts that $\gamma$+jets significantly reduce selection bias, allowing for the effective collection of sufficiently quenched jets in $PbPb$ collisions. These findings offer a framework for interpreting current LHC measurements and suggest that future studies focusing on $\gamma$+jet substructure with optimized kinematic cuts (specifically lower $x_{j\gamma}$) are crucial for uncovering the true nature of jet-medium interactions.