Medium-Induced Modification of Groomed and Ungroomed Jet Mass and Angularities in Lead-Lead (Pb-Pb) and Proton-Proton (pp) Collisions at √sNN = 5.02 TeV: An ALICE Collaboration Study

Core Concepts

This research paper presents new measurements of jet substructure in Pb-Pb and pp collisions, revealing a narrowing effect on jet mass and angularities in Pb-Pb collisions, indicating significant modification of the jet core due to energy loss as jets traverse the quark-gluon plasma.

Abstract

• Bibliographic Information: ALICE Collaboration. (2024). Medium-induced modification of groomed and ungroomed jet mass and angularities in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. CERN.

• Research Objective: To investigate the impact of the quark-gluon plasma (QGP) on jet substructure by measuring and comparing jet mass (m_jet) and jet angularities (λ_α) in both Pb-Pb (heavy-ion) and pp (proton-proton) collisions at a center-of-mass energy of 5.02 TeV.

• Methodology: The study utilizes data collected by the ALICE detector at the Large Hadron Collider (LHC). Charged-particle jets are reconstructed using the anti-kT algorithm with a resolution parameter of R=0.2. The analysis employs both groomed and ungroomed jet observables, with soft drop grooming applied to remove soft, wide-angle radiation. The study compares experimental results with theoretical predictions from various jet quenching models, including JETSCAPE, Higher-Twist, and Hybrid models.

• Key Findings:

  • A narrowing of both the jet mass and angularity distributions is observed in Pb-Pb collisions compared to pp collisions.
  • This narrowing effect is more pronounced for groomed jets, suggesting significant modification of the jet core.
  • The study addresses previous inconsistencies in interpreting quenching measurements by using consistent jet definitions and kinematic cuts for mass and angularities.
  • The results indicate that the relationship between jet mass and jet thrust, while theoretically direct, may involve significant higher-order corrections or non-perturbative effects at low transverse momentum.

• Main Conclusions:

  • The measurements provide strong evidence for the modification of jet substructure due to interactions with the QGP, confirming the phenomenon of jet quenching.
  • The study highlights the importance of using a well-controlled baseline (pp collisions) for interpreting jet quenching measurements.
  • The findings provide valuable constraints for theoretical models of jet quenching and enhance our understanding of energy loss mechanisms in the QGP.

• Significance: This research significantly contributes to the field of heavy-ion physics by providing precise measurements of jet substructure modifications in the presence of the QGP. The findings offer crucial insights into the properties of the QGP and the mechanisms of jet-medium interactions.

• Limitations and Future Research: The study acknowledges the presence of systematic uncertainties, particularly in the tails of the distributions and at lower transverse momentum intervals. Future research could focus on reducing these uncertainties and exploring a wider range of jet transverse momenta and angularity parameters to further constrain theoretical models.

Stats

The measurements are performed for charged-particle jets with transverse momenta of 40 < pch jet T < 60 GeV/c.
Jets are groomed using soft drop with parameters zcut = 0.2 and β = 0.
The pp data sample contains 870 million events, corresponding to an integrated luminosity of 18.0 ± 0.4 nb−1.
The 0-10% centrality Pb-Pb data sample contains 91.2 million events, corresponding to an integrated luminosity of 0.119±0.003 nb−1.

Quotes

Key Insights Distilled From

Medium-induced modification of groomed and ungroomed jet mass and angularities in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

by ALICE Collab... at arxiv.org 11-06-2024

https://arxiv.org/pdf/2411.03106.pdf

Deeper Inquiries

How will future heavy-ion collision experiments with higher luminosities and collision energies further enhance our understanding of jet quenching and the properties of the QGP?

Answer: Future heavy-ion collision experiments, such as those planned at the High-Luminosity LHC (HL-LHC) and the Future Circular Collider (FCC), are poised to revolutionize our understanding of jet quenching and the properties of the Quark-Gluon Plasma (QGP). These advancements will stem from significant increases in both luminosity and collision energies, leading to a considerable expansion in the volume and quality of data collected.

Higher Luminosities: The HL-LHC, for instance, aims to increase the luminosity of Pb-Pb collisions by a factor of 10 compared to the LHC. This dramatic increase translates to a proportionally larger number of jet events being produced, especially rare high-energy jets. This abundance of data will be crucial for:

Precision Measurements: With reduced statistical uncertainties, we can perform far more precise measurements of jet substructure observables like jet mass, angularities (λ α), and groomed jet shapes. This precision will be vital in rigorously testing and refining theoretical models of jet quenching.
Rare Probes: The increased statistics will enable the study of rare probes like heavy-flavor jets (charm and bottom) and fully reconstructed jets. These probes are particularly sensitive to the QGP's transport properties and can provide unique insights into its microscopic structure.

Higher Collision Energies: The proposed FCC-hh is envisioned to collide heavy ions at energies significantly higher than the LHC. This leap in energy will:

Explore New Regimes: Higher energies allow us to probe the QGP at even higher temperatures and densities, potentially uncovering new phases of QCD matter and novel energy-loss mechanisms.
Reduce Uncertainties: Theoretical calculations of jet quenching often rely on perturbative QCD, which works best at high energies. Higher collision energies will reduce the uncertainties associated with these calculations, leading to more robust interpretations of experimental data.

Furthermore, these future experiments are expected to incorporate advanced detector technologies, further improving the resolution and particle identification capabilities. This will be essential for:

Detailed Jet Substructure: Precisely measuring the momentum and angular distributions of jet constituents will provide a more detailed picture of how the QGP modifies the internal structure of jets.
Flavor Tagging: Improved flavor tagging will allow us to distinguish between quark-initiated and gluon-initiated jets, offering insights into the different ways these fundamental particles interact with the QGP.
In conclusion, the combination of higher luminosities, increased collision energies, and advanced detector technologies in future heavy-ion collision experiments promises to usher in a new era of precision jet quenching studies. This will not only deepen our understanding of the QGP but also provide crucial tests of QCD under extreme conditions, potentially leading to breakthroughs in our understanding of the fundamental forces governing the universe.

Could the observed discrepancies between the data and some theoretical models, particularly regarding the role of in-medium elastic scattering, point to limitations in our current understanding of the QGP's transport properties?

Answer: Yes, the observed discrepancies between experimental data on jet quenching and certain theoretical models, particularly concerning the role of in-medium elastic scattering, strongly suggest potential limitations in our current understanding of the QGP's transport properties.
The paper highlights that while some models, like the Hybrid model with in-medium elastic Molière scattering, struggle to accurately describe the observed jet mass distributions, they show better agreement with the measured jet angularities. This discrepancy is particularly intriguing as it points to a potential mismatch in how effectively our models capture the interplay between different energy-loss mechanisms within the QGP.
Here's a breakdown of the potential implications:

In-Medium Elastic Scattering: The disagreement regarding the inclusion of elastic scattering processes suggests that our current theoretical framework might not be adequately capturing the microscopic details of how partons interact with the QGP constituents. It could indicate:

Oversimplification of Interactions: Current models might be oversimplifying the complex, non-perturbative interactions between the propagating parton and the QGP medium. The actual scattering processes could involve more intricate dynamics than currently accounted for.
Unaccounted Medium Response: The response of the QGP medium to the presence of the hard parton, which can influence the scattering rates and energy loss, might not be fully understood.

Transport Properties: The transport properties of the QGP, such as the shear viscosity (η/s) and jet quenching parameter (ˆq), are crucial inputs to theoretical models. Discrepancies with data could imply:

Inaccurate Parameter Estimation: The values of these transport coefficients, often extracted from other experimental observables, might not be accurate enough to make precise predictions for jet substructure modifications.
Spatial and Temporal Dependence: The QGP is not a static medium; its properties evolve both spatially and temporally. Current models often use simplified assumptions about this evolution, which might not capture the full complexity of the system.

Need for Refined Theoretical Tools: Addressing these discrepancies might require developing more sophisticated theoretical tools that can:

Go Beyond Perturbation Theory: Incorporate non-perturbative aspects of QCD more effectively, as the strong coupling constant (αS) can be large in certain regions of the QGP.
Improve Medium Modeling: Utilize more realistic models of the QGP that account for its dynamical evolution, fluctuations, and potentially even the presence of color magnetic monopoles or other exotic quasi-particles.

In conclusion, the observed discrepancies between data and models regarding in-medium elastic scattering underscore the complexity of the QGP and highlight the need for continuous refinement of our theoretical understanding. These discrepancies offer valuable clues that can guide the development of more accurate models and lead to a deeper understanding of the QGP's transport properties and the fundamental interactions governing strongly coupled systems.

What are the broader implications of studying jet quenching for understanding other areas of physics, such as astrophysical phenomena involving dense and hot QCD matter?

Answer: The study of jet quenching in heavy-ion collisions has profound implications that extend far beyond the realm of nuclear physics, offering valuable insights into a wide range of astrophysical phenomena involving dense and hot QCD matter. Here are some key connections:

Neutron Stars: These incredibly dense stellar remnants are thought to harbor some of the densest matter in the universe, potentially exceeding the densities achieved in heavy-ion collisions.

Equation of State: Jet quenching studies can help constrain the equation of state (EOS) of dense QCD matter, which governs the relationship between pressure, density, and temperature inside neutron stars. This, in turn, influences their stability, mass-radius relationship, and ultimately their fate.
Cooling Mechanisms: The energy loss of particles within a neutron star, analogous to jet quenching, plays a crucial role in their cooling processes. Understanding how quarks and gluons interact in a dense medium can shed light on the mechanisms by which neutron stars cool over time.

Supernova Explosions: The violent deaths of massive stars involve the formation of a hot and dense proto-neutron star at their core.

Explosion Dynamics: Jet quenching-like processes can influence the propagation of neutrinos and other particles through the dense matter created in a supernova, impacting the explosion mechanism and the observed neutrino signal.
Nucleosynthesis: The conditions within a supernova are ripe for the creation of heavy elements. Understanding energy loss in dense QCD matter can help us model the processes of nucleosynthesis and explain the observed abundances of elements in the universe.

Early Universe: In the microseconds after the Big Bang, the universe was filled with a hot, dense soup of quarks and gluons—the QGP.

Cosmological Phase Transitions: Jet quenching studies can provide insights into the dynamics of the QGP as it cooled and transitioned into hadrons, a process that shaped the early universe's evolution.
Primordial Magnetic Fields: The interactions of quarks and gluons in the early universe might have played a role in generating primordial magnetic fields, which are thought to have influenced the formation of large-scale structures in the cosmos.

Quark Matter in Compact Objects: Beyond neutron stars, other hypothetical objects like quark stars, composed primarily of deconfined quark matter, might exist.

Stability and Properties: Jet quenching studies can help determine the properties of quark matter and whether such exotic objects could be stable under extreme conditions.

By studying jet quenching in the controlled environment of heavy-ion collisions, we gain invaluable knowledge about the behavior of QCD matter under extreme conditions. This knowledge can then be extrapolated and applied to model and understand a wide range of astrophysical phenomena, deepening our understanding of the universe's most extreme environments and the fundamental laws governing them.