High-energy physics research has demonstrated a decrease in the jet transport coefficient with rising temperatures in quark-gluon plasmas, using data from major colliders. This finding enhances understanding of parton behavior in early universe conditions. Credit: SciTechDaily.com
New findings show that parton energy loss in quark-gluon plasma decreases with temperature, providing new insights into early universe physics and jet quenching processes.
Researchers in high-energy physics have identified how high-energy partons lose energy in nucleus-nucleus collisions, an essential process in studying quark-gluon plasma (QGP). This finding could provide new insights into the early universe moments after the Big Bang.
Relativistic heavy-ion collisions produce a high density of partons with strong final-state interactions and lead to the formation of the quark-gluon plasma (QGP). Experimental evidence at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) indicates that QGP is a strongly coupled system and can be described well by the relativistic hydrodynamics with a surprisingly small specific shear viscosity. When high-energy partons propagate through the color deconfined QGP medium, they encounter multiple scatterings and lose energy through medium-induced gluon radiation. The strength of the jet energy loss is controlled by the jet transport coefficient (q), which is proportional to the medium gluon number density and is defined as the average transverse momentum broadening squared per unit length for a jet propagating inside the medium. Phenomenological values of the scaled dimensionless q/T3 have been extracted through model-to-data comparisons using hadron suppression data and indicated an additional temperature (T) dependence. Credit: Han-Zhong Zhang
Understanding Temperature Effects on Jet Energy
The research shows that the jet transport coefficient over temperature cubed, a critical factor in parton energy loss in QGP, decreases with increasing medium temperature. This discovery, supported by a significant enhancement of the elliptic flow parameter (v2(pT)) for large transverse momentum (pT) hadrons, provides a more in-depth understanding of jet quenching in high-energy collisions.
The suppression and azimuthal anisotropy of the high transverse momentum (pT) hadrons are both consequences of jet quenching or energy loss. The suppression strength is given by the nuclear modification factor RAA(pT) defined as the ratio of single hadron production yield in A + A collisions to that in p + p collisions scaled by the average nuclear overlap function. Azimuthal anisotropy of large-pT hadrons owing to the path length and gluon density dependence of the jet energy loss, is characterized by elliptic flow coefficient v2(pT). Within a next-to-leading-order perturbative QCD model, the medium-temperature dependence of jet energy loss was investigated by comparing theoretical calculations with experimental data for both RAA and v2 at large pT at RHIC and LHC. Credit: Han-Zhong Zhang
Insights into Quark-Gluon Plasma Characteristics
High-energy collisions create a hot, dense state of matter known as the QGP. As partons pass through this medium, they lose energy. This process, known as jet quenching, leads to the suppression of high pT hadrons, measured by the nuclear modification factor (RAA(pT)), and the azimuthal anisotropy, measured by the v2(pT).
Under the assumption that the scaled jet transport coefficient q/T3 depends on the medium temperature in linear or Gaussian form, the single hadron nuclear modification factor RAA(pT) and elliptic flow parameter v2(pT) were calculated and compared with experimental data to constrain the q/T3 temperature dependence forms. The numerical results indicate that RAA(pT) and v2(pT) are both more sensitive to the value of q/T3 near the critical temperature (Tc) than near the initial highest temperature (T0). Moreover, the single hadron suppression is a consequence of total jet energy loss, while elliptic flow parameter favors more energy loss near Tc. Credit: Han-Zhong Zhang
Analytical Techniques in Particle Physics
The team used a next-to-leading-order perturbative QCD parton model to analyze data from the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC). By fitting their models to the experimental data, they found that the jet transport coefficient’s scaled value (q^/T3) decreases with temperature. This novel approach provides a more accurate description of how jets lose energy in these extreme conditions.
Owing to the medium-temperature evolution, different T dependencies of the jet transport coefficient can result in an identical total energy loss, but different energy-loss distributions for jet propagation. Compared with the constant case for a given total energy loss, the linearly-decreasing T dependence of q/T3 causes the energy loss to redistribute, leading to more energy loss near the critical temperature, and therefore, a stronger azimuthal anisotropy for hadron production. Finally, the jet energy loss in the hadronic phase was also considered. Because of the first-order phase transition in the current model, the hadronic phase contribution occurs mainly near Tc, which strengthens the azimuthal anisotropy of the system and thus enhances the elliptic flow parameter. Credit: Han-Zhong Zhang
Significance of Recent Discoveries
“This discovery helps us understand the behavior of partons in the quark-gluon plasma more accurately,” says Prof. Han-Zhong Zhang, the corresponding author. “It shows that partons lose more energy near the critical temperature, which could explain the enhanced azimuthal anisotropy observed in high-energy collisions.”
The findings suggest that as partons travel through the QGP, they lose more energy near the transition from QGP to hadron phase, strengthening the azimuthal anisotropy by approximately 10% at RHIC and LHC.
The numerical result of stronger jet quenching in the near-Tc region is consistent with that of the previous theoretical study. Compared with the case of constant q/T3, the going-down T dependence of q/T3 causes a hard parton jet to lose more energy near the critical temperature Tc and therefore strengthens the azimuthal anisotropy for large pT hadron production. As a result, the elliptic flow parameter v2(pT) for large pT hadrons was enhanced by approximately 10% to better fit the data at RHIC/LHC. Considering the first-order phase transition from QGP to hadron and the hadron phase contribution to the jet energy loss, v2(pT) is again enhanced by 5-10% at RHIC/LHC. Credit: Han-Zhong Zhang
Future Directions in High-Energy Physics
“In the future, we hope to refine our model and enrich the information on qˆ, allowing us to better describe RAA(pT) and v2(pT) simultaneously for both RHIC and LHC energies,” Prof. Zhang says.
This study advances high-energy nuclear physics, providing deeper insights into jet energy loss in high-energy collisions. These findings could enhance our understanding of the quark-gluon plasma and pave the way for future research into the fundamental properties of matter under extreme conditions.
The accompanying picture features the authors, as members of Professor EnKe Wang’s joint research group from the Institute of Quantum Matter at South China Normal University and the Institute of Particle Physics at Central China Normal University. The group primarily focuses on various aspects of particle physics and nuclear physics, with a special emphasis on the hard probes of quark-gluon plasma under extreme conditions. Credit: Han-Zhong Zhang
This research is a collaborative effort between South China Normal University and Central China Normal University.
Reference: “The medium-temperature dependence of jet transport coefficient in high-energy nucleus–nucleus collisions” by Man Xie, Qing-Fei Han, En-Ke Wang, Ben-Wei Zhang and Han-Zhong Zhang, 16 July 2024, Nuclear Science and Techniques. DOI: 10.1007/s41365-024-01492-4
