arXiv subjects

Yacine Mehtar-Tani

Publications and source records attributed to Yacine Mehtar-Tani.

At least 19 recordsLinked to original sources

arXiv2026

The Colors of Jet Quenching

We combine inclusive jet nuclear modification factors with energy-energy correlators to perform a data-driven extraction of the quark and gluon quenching factors and the medium resolution scale from the small-angle region. Taking coherent energy loss as a null hypothesis, we find jet quenching factors incompatible with Casimir scaling. Incorporating color decoherence through antenna energy loss improves the description of the data and restores Casimir scaling, providing evidence that the quark-gluon plasma partially resolves the internal structure of jets in heavy-ion collisions.

hep-ph↗

arXiv2026

Energy-energy correlators and color decoherence from a generating functional

Using jet calculus at leading-logarithmic accuracy, we develop a generating-functional approach to describe the hard-collinear sector of jets propagating through the quark-gluon plasma. This framework yields coupled evolution equations for the jet nuclear modification factor and energy-energy correlators (EECs). We first construct the EEC evolution in the fully incoherent regime, and then derive evolution equations that account for the transition from the coherent to the incoherent limit in the large-$N_c$ limit. Furthermore, we show that combining the EEC with the jet nuclear modification factor provides complementary sensitivity to the overall jet suppression and to medium-induced modifications of the jet's internal structure. This framework therefore offers a way to constrain the magnitude and flavor dependence of jet energy loss, together with the medium resolution angle governing color decoherence.

hep-ph↗

arXiv2026

Jet Quenching Meets Gluon Saturation

We present a theoretical framework for jet fragmentation in heavy-ion collisions based on the resummation of large energy logarithms. Exploiting the hierarchy of scales characteristic of jet quenching, we show that the jet function obeys the Banfi-Marchesini-Smye evolution equation, with medium-induced energy loss and color decoherence encoded in the initial condition. This structure reveals a close correspondence with saturation physics. In particular, the coherence angle emerges as the analog of the saturation scale and exhibits the same asymptotic scaling behavior under nonlinear evolution. As a proof of principle, we compute the jet nuclear modification factor to quantify the interplay between vacuum radiation and medium-induced color decoherence. Our framework provides a unified perturbative description of vacuum-like parton showers, medium-induced radiation, and color-coherence effects, paving the way for precision studies of jet quenching at RHIC and the LHC.

hep-ph↗

arXiv2026

Color Coherence and the Soft Structure of QCD Jets in Vacuum and the QGP

We formulate a theoretical framework for the evolution of QCD jets in vacuum and in the quark--gluon plasma through the resummation of large energy logarithms. Exploiting the strong hierarchy between the hard scale of the jet and the energy scale associated with jet energy loss, we show that jet observables near threshold can be formulated in terms of Wilson-line correlators obeying Banfi-Marchesini-Smye (BMS) evolution. In this description, soft radiation resolves the internal color structure of the jet, leading to a hierarchy of non-linear evolution equations that govern the evolution of color coherence and the emergence of decoherent energy loss. For jets propagating through a QCD medium, we demonstrate that medium-induced interactions modify the boundary conditions of the evolution while leaving its ultraviolet structure unchanged. This separation of scales provides a unified description of vacuum-like radiation, medium-induced energy loss, and color coherence. In the large-$N_c$ limit, the resulting evolution is closely related to the Balitsky-Kovchegov equation of high-energy QCD, allowing concepts from saturation physics to be applied to jet quenching. In particular, the medium coherence angle plays a role analogous to the saturation scale and acquires the same asymptotic scaling behavior under evolution. Our framework establishes a perturbative foundation for the study of color coherence effects in jet quenching and provides a unified picture of soft jet evolution in vacuum and in dense QCD matter.

hep-ph↗

arXiv2026

Rotating the Color Glass Condensate

High-energy QCD evolution beyond leading order suffers from instabilities driven by large collinear logarithms. We present a framework, consistent with the standard high-energy operator product expansion (OPE), that restores perturbative stability order by order. The method involves a change of basis in the space of high-energy operators, which modifies both the evolution kernel and the coefficient functions while leaving physical observables invariant. Within this factorization scheme, we derive a next-to-leading-order renormalization-group equation whose numerical solution exhibits stable evolution up to large rapidities, paving the way for a systematic framework for precision studies of gluon saturation at current and future colliders.

hep-ph↗

arXiv2025

The Physics of Jet Quenching in Perturbative QCD

Hard processes in collider experiments typically produce QCD jets, which have long served as precision tests of QCD in the vacuum. More recently, heavy-ion programs at RHIC and the LHC have offered a novel perspective on jets, establishing them as unique probes of strongly interacting matter. Experimental observations, including the suppression of high-$p_T$ hadrons and jets, provide compelling evidence for the formation of a new state of matter and its strong coupling to energetic partons. These advances have motivated new theoretical approaches to jet quenching that go beyond standard perturbative techniques, aiming to elucidate the mechanisms of energy dissipation and thermalization of energetic partons in the quark-gluon plasma. This review highlights recent progress, beginning with a unified description of medium-induced radiation across the Landau-Pomeranchuk-Migdal regime and its role in turbulent gluon cascades. We then examine radiative corrections that renormalize the transport coefficient $\hat q$, the mechanism of color decoherence in multi-parton systems, and nonlinear QCD evolution equations for jet energy loss. Finally, we confront this framework with experimental measurements, underscoring the need for precision phenomenology to fully exploit the rich data sets from RHIC and the LHC.

hep-ph↗

arXiv2025

Nonlinear dynamics of jet quenching

We present an analytic framework for jet quenching in dense QCD matter that unifies medium-induced branching with vacuum collinear evolution. Energy transported outside the jet region is governed by a non-linear rate equation that resums arbitrary-angle gluon splittings, each enhanced by the medium length $L$. We show that for asymptotically large energies the energy-loss distribution for a single hard parton exhibits an exponential (generalized Poisson) behavior that provides the initial condition for a non-linear, DGLAP-like evolution that resums collinear logarithms associated with early in-vacuum fragmentation and with the angular resolution of the medium. This framework allows a systematic resummation of parton branching contributions to jet energy loss.

hep-ph↗

arXiv2025

Collinear Structure of Nonlinear Small-$x$ Evolution

We introduce a novel approach to high-energy QCD factorization of cross-sections for processes involving a dilute projectile and a dense target. Our method preserves the factorization between "fast" and "slow" modes in the longitudinal momentum $k^+$ for a projectile moving along the positive light-cone direction, making it compatible with higher-order loop computations done within the standard formulation of the Color Glass Condensate (CGC) effective field theory. Moreover, it eliminates the anomalous double collinear logarithms that typically hinder the convergence of perturbation theory at small-$x$. Our scheme amounts to a change of basis in the space of CGC operators, introducing an arbitrary transverse scale dependence while leaving physical cross-sections invariant. Implementing this scheme requires a modification of the high-energy renormalization group equation for the CGC operators, which we explicitly derive at next-to-leading order (NLO) and in the non-linear regime for the case of the dipole operator and the DIS impact factor. This general framework provides a consistent and systematic prescription for computing observables in the small-$x$ regime of QCD, free from collinear instabilities.

hep-ph↗

arXiv2025

Open quantum system approach to inclusive jet production in heavy-ion collisions

We derive a factorization formula for inclusive jet production in heavy-ion collisions using the tools of Effective Field Theory (EFT). We show how physics at widely separated scales in this process can be systematically separated by matching to EFTs at successively lower virtualities. Owing to a strong scale separation, we recover a vacuum-like DGLAP evolution above the jet scale, while the additional low-energy scales induced by the medium effectively probe the internal structure of the jet. As a result, the cross section can be written as a series with an increasing number of subjets characterized by perturbative matching coefficients each of which is convolved with a {\it distinct} function. These functions encode broadening, medium-induced radiations as well as quantum interference such as the Landau-Pomeranchuk-Migdal effect and color coherence dynamics to all orders in perturbation theory. As a first application of this EFT framework, we investigate the case of an unresolved jet and show how the cross section can be factorized and fully separate the jet dynamics from the universal physics of the medium. To compare to the existing literature, we explicitly compute the medium jet function at next-to-leading order in the coupling and leading order in medium opacity.

hep-ph↗

arXiv2025

Jet Suppression and Azimuthal Anisotropy from RHIC to LHC

Azimuthal anisotropies of high-$p_T$ particles produced in heavy-ion collisions are understood as an effect of a geometrical selection bias. Particles oriented in the direction in which the QCD medium formed in these collisions is shorter, suffer less energy loss, and thus, are over-represented in the final ensemble compared to those oriented in the direction in which the medium is longer. In this work we present the first semi-analytical predictions, including propagation through a realistic, hydrodynamical background, of the azimuthal anisotropies for jets, obtaining a quantitative agreement with available experimental data as function of the jet $p_T$, its cone size $R$ and the collisions centrality. Jets are multi-partonic, extended objects and their energy loss is sensitive to substructure fluctuations. This is determined by the physics of color coherence that relates to the ability of the medium to resolve those partonic fluctuations. Namely, color dipoles whose angle is smaller than a critical angle, $θ_c$, are not resolved by the medium and they effectively act as a coherent source of energy loss. We find that jet azimuthal anisotropies have a specially strong dependence on coherence physics due to the marked length-dependence of $θ_c$. By combining our predictions for the collision systems and center of mass energies studied at RHIC and the LHC, covering a wide range of typical values of $θ_c$, we show that the relative size of jet azimuthal anisotropies for jets with different cone-sizes $R$ follow a universal trend that indicates a transition from a coherent regime of jet quenching to a decoherent regime. These results suggest a way forward to reveal the role played by the physics of jet color decoherence in probing deconfined QCD matter.

hep-ph↗

arXiv2024

Non-linear dynamics of jet quenching

We develop a comprehensive analytic framework for jet quenching in QCD media, based on a medium-induced parton cascade sourced by collinear virtual splittings. We show that the energy flow out of the jet cone, driven by turbulent gluon cascades, is governed by a non-linear rate equation that resums gluon splittings at arbitrary angles and is enhanced by the medium length, $L$. The solution of this equation sets the initial condition for a non-linear DGLAP-like evolution equation, which describes the collinear early vacuum cascade resolved by the medium at angles exceeding the medium resolution angle, $θ_c$. For asymptotic jet energies, the medium-induced cascade displays an exponential behavior that generalizes the Poisson-like distribution of parton energy loss. This formulation enables the resummation of leading contributions in $α_s \ln (1/R)$, and $α_s \ln (R / θ_c)$, and powers of $α_s L$. We briefly explore the limit of strong quenching, where analytic treatments are feasible, offering insights into the impact of parton cascades on jet quenching. These results provide guidance for future numerical simulations and analytical investigations.

hep-ph↗

arXiv2024

Factorization for jet production in heavy-ion collisions

We develop an Effective Field Theory approach for jet observables in heavy-ion collisions, where the jet is treated as an open quantum system interacting with a hot and dense QCD medium. Within this framework, we derive a novel factorization formula for inclusive jet production, expressed as a series expansion with an increasing number of radiating subjet functions that encode forward scattering with the Quark-Gluon Plasma, convolved with perturbative matching coefficients. This work provides a systematic framework for computing jet observables at higher order and understanding their non-perturbative aspects, paving the way for future applications in heavy-ion phenomenology.

hep-ph↗

arXiv2024

Predictions for the sPHENIX physics program

sPHENIX is a next-generation detector experiment at the Relativistic Heavy Ion Collider, designed for a broad set of jet and heavy-flavor probes of the Quark-Gluon Plasma created in heavy ion collisions. In anticipation of the commissioning and first data-taking of the detector in 2023, a RIKEN-BNL Research Center (RBRC) workshop was organized to collect theoretical input and identify compelling aspects of the physics program. This paper compiles theoretical predictions from the workshop participants for jet quenching, heavy flavor and quarkonia, cold QCD, and bulk physics measurements at sPHENIX.

nucl-ex↗

arXiv2023

Quantum to classical parton evolution in the QGP

We study the time evolution of the density matrix of a high energy quark in the presence of a dense QCD background that is modeled as a stochastic Gaussian color field. At late times, we find that only the color singlet component of the quark's reduced density matrix survives the in-medium evolution and that the density matrix becomes asymptotically diagonal in both transverse position and momentum spaces. In addition, we observe an accelerated entropy growth due to the larger phase space being explored by the quark and that the quantum and classical quark entropies converge at late times. We further observe that the quark state loses all memory of the initial condition. Combined with the fact that the reduced density matrix satisfies Boltzmann-diffusion transport, we conclude that the quark reduced density matrix can be interpreted as a classical phase space distribution.

hep-ph↗

arXiv2023

Jet suppression and azimuthal anisotropy at RHIC and LHC

Jets are multi-partonic systems that develop before interactions with the quark-gluon plasma set in and lead to energy loss and modifications of their substructure. Jet modification depends on the degree to which the medium can resolve the internal jet structure that is dictated by the physics of coherence governed by a critical angle $θ_c$. Using resummed quenching weights that incorporate the IOE framework for medium-induced radiation and embedding the system into a realistic heavy-ion environment we compute the dependence of jet suppression on the cone angle $R$ of the jet, both at RHIC and the LHC. At RHIC kinematics we see a very mild cone angle dependence for the range of $R$ studied, similar to what was found at the LHC. We also present results for the jet azimuthal anisotropy $v_2$ as a function of $R$. We observe that as centrality is decreased, $v_2$ for moderate $R$ jets sequentially collapse towards the result for small $R = 0.1$. The reason of this sequential grouping is the evolution of $θ_c$ with centrality due to its strong dependence on the in-medium traversed length. For jets with $R > θ_c$, traversing shorter lengths within the medium will make a larger difference than for jets with $R < θ_c$, since the size of the resolved phase-space over which quenching weights are resummed will be reduced. For this reason, $v_2(R)$ is quite sensitive to the typical value of $θ_c$ at a given centrality.

hep-ph↗

arXiv2023

Low and moderate $x$ gluon contribution to exclusive Compton scattering processes

We revisit the high energy semi-classical description of the exclusive processes DVCS, TCS, and Double DVCS by explicitly keeping track of the Feynman $x$ dependence in both the hard and the hadronic matrix elements. This is achieved by a modification of the standard shock wave approximation to derive the effective Feynman rules, which leads to a generic expression on which we then perform a partial twist expansion to get rid of quantities suppressed by the proper physical scales. We obtain a compact factorized master formula that can be used to investigate the Bjorken limit at leading twist. In particular, we recover the full one-loop result in the collinear limit for pure gluon exchange with the target. Finally, we discuss the subtleties in taking the simultaneous collinear and small $x$ limit.

hep-ph↗

arXiv2023

Energy loss effects in EECs at LO

In recent years, there has been an effort towards establishing a more complete picture for jet substructure in the presence of the quark gluon plasma. Such a program requires not only a more detailed description of medium induced effects, but also the design of novel substructure observables. Very recently, it has been noticed that Energy Energy correlators (EECs) might provide one type of such observables. Although the full extent of their sensitivity to the medium has not been completely explored, they are capable to resolve the transverse structure of the jet. In particular, they are sensitive to the critical angle separating coherent and decoherent jet evolution in the medium. In this talk, we show for the first time the effects of medium induced radiative energy loss in EECs at leading order in the number of vacuum-like emissions. The calculation takes into account all order soft gluon emissions, in the large $N_c$ limit and neglecting subdominant interfering contributions.

hep-ph↗

arXiv2023

The case for an EIC Theory Alliance: Theoretical Challenges of the EIC

We outline the physics opportunities provided by the Electron Ion Collider (EIC). These include the study of the parton structure of the nucleon and nuclei, the onset of gluon saturation, the production of jets and heavy flavor, hadron spectroscopy and tests of fundamental symmetries. We review the present status and future challenges in EIC theory that have to be addressed in order to realize this ambitious and impactful physics program, including how to engage a diverse and inclusive workforce. In order to address these many-fold challenges, we propose a coordinated effort involving theory groups with differing expertise is needed. We discuss the scientific goals and scope of such an EIC Theory Alliance.

hep-ph↗