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Jet substructure of light and heavy flavor jets at RHIC

This paper presents a PYTHIA8-based Monte Carlo study of jet substructure observables, including angularities and the primary Lund Plane with a focus on the dead cone effect, for light and heavy flavor jets in 200 GeV $p+p$ collisions at RHIC to test non-perturbative physics models and enable feasible measurements at sPHENIX.

Original authors: Zhuoheng Yang, Oleh Fedkevych, Roli Esha

Published 2026-08-18

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CC BY 4.0

Original authors: Zhuoheng Yang, Oleh Fedkevych, Roli Esha

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Inside every atom, protons and neutrons are held together by a force so powerful it binds the very fabric of matter. This force, carried by particles called gluons, operates in a realm where the rules of everyday physics give way to a chaotic, energetic dance of subatomic particles. When scientists smash particles together at nearly the speed of light, they create tiny, fleeting sprays of these particles known as jets. These jets are not random clouds; they are structured streams that carry a detailed history of the violent collision that created them. By studying the internal architecture of these sprays, researchers can test the fundamental laws that govern how matter behaves under extreme pressure and temperature, conditions that mimic the earliest moments of the universe.

A team of researchers has now turned its attention to a specific, lower-energy version of these collisions to see if they can spot a subtle but crucial difference between heavy and light particles. While the world's most powerful particle colliders have long studied these jets at incredibly high energies, this new work focuses on the Relativistic Heavy Ion Collider, where collisions occur at a more modest energy level of 200 billion electron volts. The goal is to understand how the mass of the original particle affects the way the jet sprays out. Specifically, the team wanted to see if the heavy mass of particles like bottom and charm quarks creates a "dead cone," a region around the particle where radiation is suppressed, much like a shadow cast by a heavy object.

To investigate this, the researchers used a sophisticated computer simulation to model what happens when protons collide at these specific energies. They generated millions of virtual collisions, creating jets that were seeded by different types of particles: light quarks, which are the building blocks of ordinary matter, and heavy quarks, which are much more massive. They also compared these to jets created by gluons, the carriers of the strong force. The team then applied a set of digital filters to these simulated jets, mimicking the capabilities of the sPHENIX detector, a large instrument designed to catch these particles in real experiments. By stripping away the soft, messy outer layers of the jets, they could focus on the core structure and measure how the particles were distributed within the spray.

The results of these simulations revealed clear distinctions between the different types of jets. When the researchers looked at the jets created by light quarks versus those created by gluons, they found that the gluon jets were generally wider and more spread out, while the light quark jets were tighter and more focused. This difference allowed them to successfully distinguish between the two types of jets with a high degree of accuracy, even at these lower energies. More importantly, when they examined the jets seeded by heavy quarks in their simulation, they observed a distinct change in the pattern. The heavy quarks seemed to suppress the emission of other particles in a narrow cone directly around their path. This suppression, known as the dead cone effect, was clearly visible in the simulated data, showing that the mass of the quark fundamentally alters how the jet forms.

The study also highlighted how the decay of heavy particles influences what is seen. Heavy quarks often turn into unstable particles called B-hadrons, which then break apart into other particles. The researchers found that this decay process adds a layer of complexity to the jet's structure, shifting the distribution of particles in a way that is distinct from the simple suppression caused by mass alone. While the computer models used in this study are not a perfect replacement for real-world data, they provide a strong foundation for what the sPHENIX experiment hopes to measure. The simulations suggest that the detector is capable of observing these subtle effects, offering a new way to test our understanding of the strong force and the behavior of matter in extreme conditions.

By confirming that these heavy-particle effects can be seen at lower energies in simulations, the work opens a new door for exploration. It suggests that the dead cone effect is not just a phenomenon of the highest-energy collisions but a fundamental property that persists even when the energy is reduced. This finding gives physicists a new tool to probe the boundaries of their theories, separating the effects of particle mass from the messy background of the collision. As the sPHENIX experiment begins to collect real data, these simulations serve as a crucial guide, helping scientists know exactly what to look for in the spray of particles that emerges from the heart of the collision.

Technical Summary: Jet Substructure of Light and Heavy Flavor Jets at RHIC

Problem and Motivation
Jet substructure studies have matured at the Large Hadron Collider (LHC), serving as tools to constrain parton distribution functions (PDFs), test perturbative QCD, and probe the quark-gluon plasma (QGP). However, investigations at the Relativistic Heavy Ion Collider (RHIC) remain sparse. The authors posit that extending these studies to lower energies (200 GeV) at RHIC offers a crucial platform for testing non-perturbative QCD models and separating contributions from hadronization and the soft underlying event. A primary scientific motivation is the study of the "dead cone effect"—the suppression of collinear QCD radiation around massive quarks—which alters the radiation pattern and manifests in jet substructure. While theoretical studies exist, direct experimental measurements of jet substructure observables for heavy-flavor jets are limited. This study aims to evaluate the potential of the sPHENIX experiment at RHIC to observe these effects, specifically utilizing the Primary Lund Plane (pLP) projection.

Methodology
The study employs a Monte Carlo simulation using the PYTHIA 8 event generator with a Detroit tune, configured for $p + p$ collisions at $\sqrt{s} = 200$ GeV.

Event Selection: Samples include jets seeded by light flavors ( $u, d, s$ ), charm ( $c$ ), and bottom ( $b$ ) quarks, as well as gluon jets. Events are generated via $2 \to 2$ LO QCD di-jet production.
Kinematic Cuts: Selection criteria are adapted for sPHENIX acceptance: leading jet $p_{T1} > 30$ GeV, subleading jet $p_{T2} > 20$ GeV, and pseudorapidity $|\eta_{jet}| < 0.7$ . A minimum transverse momentum scale ( $p_{T}^{HatMin}$ ) of 20 GeV is enforced.
Jet Reconstruction: Final-state particles are clustered using the anti- $k_t$ algorithm with radius $R=0.4$ and the $E$ -scheme recombination.
Grooming: Two versions of jets are analyzed: ungroomed and groomed. Grooming utilizes the SoftDrop algorithm (parameters $z_{cut}=0.2, \beta=0$ ) with the Cambridge-Aachen (C/A) reclustering algorithm to mitigate non-perturbative effects like hadronization and the underlying event.
Tagging: Jets are categorized by matching the jet axis to the initial hard parton in the Pythia record, requiring an angular distance $\Delta < 0.4$ .
Observables:
1. Jet Angularities ( $\lambda_\alpha$ ): Defined as $\lambda_\alpha = \sum_{i \in jet} z_i (\Delta_i/R)^\alpha$ . The study analyzes $\alpha = 1/2$ (LHA), $\alpha = 1$ (Jet Width), and $\alpha = 2$ (Jet Thrust).
2. Primary Lund Plane (pLP): A 2D projection mapping emissions based on the logarithm of the transverse momentum fraction ( $\ln(k_T)$ ) and the logarithm of the inverse emission angle ( $\ln(1/\Delta)$ ).

Key Results

Quark-Gluon Discrimination: The study confirms that jet angularities can distinguish between quark and gluon jets at RHIC energies. Gluon-initiated jets exhibit distributions shifted toward larger angularity values compared to quark jets due to different emission patterns. Receiver Operating Characteristic (ROC) curves indicate efficient separation, with an optimal operational point at $\lambda_{cut} = 0.3$ yielding a quark efficiency ( $\epsilon_q$ ) of 0.43 and a gluon efficiency ( $\epsilon_g$ ) of 0.85.
Impact of Grooming: For quark-gluon discrimination, SoftDrop grooming reduces discrimination performance compared to ungroomed jets. This is attributed to the removal of soft emissions, which partially contribute to the structural differences between quark and gluon jets.
Heavy Flavor vs. Light Flavor:
- Angularities: $b$ -jet distributions shift toward larger angularity values compared to light jets, achieving discrimination efficiencies similar to quark-gluon taggers.
- Dead Cone Effect: The pLP analysis reveals a significant suppression of collinear radiation for $b$ -jets within the logarithmic range $\ln(1/\Delta) \approx 2.5$ to $3$, a feature less pronounced in light and charm jets. This suppression is concurrent with low values of $\ln(k_T)$ .
- Grooming Sensitivity: Unlike the quark-gluon case, the ROC curves for heavy-light discrimination remain similar between groomed and ungroomed jets. The authors attribute this to the fact that the differences between heavy and light jets (radiation suppression and $B$ -hadron decays) occur in the collinear region, which is less sensitive to SoftDrop grooming.
B-Hadron Decays: The study notes that at RHIC energies, the probability of producing a jet from a single $b$ -quark is non-negligible. While stable $B$ -hadrons would result in $\lambda=0$ , the instability and subsequent decay of $B$ -hadrons modify the substructure, shifting events from the first bin to larger angularity bins. The authors note this requires careful future investigation.

Significance and Claims
The paper claims to provide a detailed PYTHIA-based framework for measuring jet substructure at RHIC using the sPHENIX detector. The primary contribution is establishing the feasibility of experimentally observing the dead cone effect in $b$ -jets at 200 GeV. The results align with theoretical predictions regarding the suppression of collinear radiation around massive quarks and demonstrate that sPHENIX can distinguish between light, charm, and bottom jets, as well as quark and gluon jets. These predictions are intended to serve as a basis for future QCD-inspired observables for tagged-jet studies at RHIC energies, complementing higher-energy LHC analyses to enhance the understanding of QGP properties and non-perturbative QCD regimes.

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