Periodic Reporting for period 1 - AntScat (Interactions among Coherent Objects and the Origin of Collectivity in QCD at Colliders)
Reporting period: 2024-04-01 to 2026-03-31
The surprising findings of collective effects in nucleon-nucleus and nucleon-nucleon collisions (and very recently even in electron-positron collisions), previously thought to be exclusive to large systems such as nucleus-nucleus collisions, pose an important challenge to the particle physics community. Both from the heavy-ion and the high-energy communities, intense efforts are devoted to gain a better understanding of the origin of collectivity in QCD. This project revolves around the theoretical study of the interactions among coherent objects and their phenomenological implications. So far, these type of interactions have been absent in the respective state-of-the-art descriptions of the dynamical evolution of QCD at colliders adopted by both communities. The first goal of this project will be to compute the cross-sections for a new set of processes, ranging from the interaction of on-shell particles with colored dipoles to the interactions among dipoles with other dipoles. As a result of these interactions, broadening and stimulated radiation can take place. The features of the dipoles, such as their mass, are imprinted in the scattered object due to the phenomenon of color and quantum interference. These physics translate in the appearance of non-trivial correlations among these objects. A second aspect of the project will consist in the phenomenological modeling that these correlations can have in a) QCD parton showers, the natural scenario from the high-energy front, where interactions among parton and multi-parton objects will be added to the typical splitting processes and b) QCD effective kinetic theory, the natural scenario from the heavy-ion front, where the inclusion of the coherent objects can be modeled by considering them as unstable particles. A detailed study of the observable effects that these new correlations can induce will shed light on the role played by the physics of quantum interference in describing the origin of collectivity in QCD.
The degree to which the quark-gluon plasma formed in heavy-ion collisions can resolve the jet substructure fluctuations that develop within the medium is subject today to intense theoretical and experimental research. From the theoretical side, addressing this question involves understanding the role played by quantum interference in both elastic and inelastic processes between the jet partons and the medium constituents. The main achievement obtained in the course of this project consists in the first computation of the properties of a recoiling medium particle that has interacted with a colored dipole. The novel effects obtained are the following:
-The angular distribution of the recoiling parton is strongly determined by the opening angle of the energetic jet dipole. This is analogous to what was found decades ago for the case of soft gluon emission, and is due to the physics of color coherence.
-If the dipole stays in a color coherent state, successive recoils draw the shape of the antenna (see attached picture).
-The collisional energy loss of a dipole is thus dependent on its substructure due to the limitation of the available phase-space of the recoil, as constrained by quantum interference.
As anticipated in the objectives of the project, considering interactions among coherent objects yields novel effects that link their properties in non-trivial ways, contributing to the measured final-state correlations in the detectors. The calculation performed finds its most natural phenomenological context in the physics of medium response, namely how the medium constituents react to the passage of an energetic jet. My findings reveal that one cannot simply consider two-to-two scatterings when it comes to the description of jet-medium interactions, since the jet is a collection of color correlated dipoles where one cannot really tell which leg of a dipole underwent a scattering.
While still far from having obtained the full set of cross-sections necessary for a complete phenomenological description, these results already validate the points raised in the motivation of the project and encourage further work in this direction.
Although not directly linked to the main objectives of the project, I have performed relevant work in tightly related subjects, such as:
-First semi-analytical description of jet azimuthal anisotropy in heavy-ion collisions, and identification of a coherent angular scale stemming from the jet radius dependence of such azimuthal anisotropy, leading to a universal behaviour which can be measured in experiments both at the RHIC and the LHC.
-First phenomenological study on energy-energy-energy correlators in heavy-ion collisions which offers a way to visualize the shape of the wake induced by the jet in the fluid quark-gluon plasma.
-Phenomenological study of a new type of jet substructure measurement pioneered by ATLAS, in which the properties of large-radius jets composed of skinnier subjets offer new information on both color coherence and medium response effects.
-The angular distribution of the recoiling parton is strongly determined by the opening angle of the energetic jet dipole. This is analogous to what was found decades ago for the case of soft gluon emission, and is due to the physics of color coherence.
-If the dipole stays in a color coherent state, successive recoils draw the shape of the antenna (see attached picture).
-The collisional energy loss of a dipole is thus dependent on its substructure due to the limitation of the available phase-space of the recoil, as constrained by quantum interference.
As anticipated in the objectives of the project, considering interactions among coherent objects yields novel effects that link their properties in non-trivial ways, contributing to the measured final-state correlations in the detectors. The calculation performed finds its most natural phenomenological context in the physics of medium response, namely how the medium constituents react to the passage of an energetic jet. My findings reveal that one cannot simply consider two-to-two scatterings when it comes to the description of jet-medium interactions, since the jet is a collection of color correlated dipoles where one cannot really tell which leg of a dipole underwent a scattering.
While still far from having obtained the full set of cross-sections necessary for a complete phenomenological description, these results already validate the points raised in the motivation of the project and encourage further work in this direction.
Although not directly linked to the main objectives of the project, I have performed relevant work in tightly related subjects, such as:
-First semi-analytical description of jet azimuthal anisotropy in heavy-ion collisions, and identification of a coherent angular scale stemming from the jet radius dependence of such azimuthal anisotropy, leading to a universal behaviour which can be measured in experiments both at the RHIC and the LHC.
-First phenomenological study on energy-energy-energy correlators in heavy-ion collisions which offers a way to visualize the shape of the wake induced by the jet in the fluid quark-gluon plasma.
-Phenomenological study of a new type of jet substructure measurement pioneered by ATLAS, in which the properties of large-radius jets composed of skinnier subjets offer new information on both color coherence and medium response effects.
The first immediate impact of the results obtained in this project involving the computation of interactions among coherent objects will be in the description of elastic scatterings in Monte Carlo models for jet quenching. This is so because the kinematical approximations taken in this first publication are suited to the case of an energetic jet interacting with a soft medium parton. Many models include elastic scatterings and keep track of the recoils, but none could incorporate these effects since they had not been discovered. Given the importance of recoils in jet quenching observables, it is expected that these novel effects will have a sizeable impact in many of those observables, specially jet substructure ones, such as energy correlators.
However, there are a number of questions that still remain unanswered, such as:
-Does the physics change if the recoiling parton is a gluon instead of a quark? If so, why?
-Which is the right way to regulate the collinear divergence when the recoil has very small angle with respect to either leg of the dipole? Which are the right virtual diagrams?
-How does the picture change when one relaxes some kinematical assumptions, such as the one where the recoil energy is much larger than the medium parton rest mass?
-Can we resum an arbitrary number of such scatterings? Do we observe color decoherence because of multiple color rotations, as can be obtained using previous formalisms?
While these questions refer just to the calculation already performed, in order to achieve the longer-term goals of the project (implementation in parton shower and in effective kinetic theory) one will need to extend it in a number of ways:
-Perform dipole-dipole interaction computation
-Relax kinematics to accommodate also soft-soft interaction, in this way allowing for broadening.
-Consider stimulated radiation as a result of the interaction
Once obtained, the implementation of these results will require further non-trivial efforts, concerning mostly modeling assumptions and coding tasks.
In sum, one can say that the consideration of the issues raised and tackled in this project arise naturally if one contemplates the evolution experienced by the field of jet quenching in the last couple of decades, in particular in going from a single parton to a many parton (as produced by an actual jet) scenario. Applying these notions to the description of the interactions among the mini-jets produced in the early times of a heavy-ion collision, or to the non-linear contribution to parton evolution in high-multiplicity jets, offers a great opportunity to gain a better understanding on the striking multi-particle correlations measured across all system sizes.
However, there are a number of questions that still remain unanswered, such as:
-Does the physics change if the recoiling parton is a gluon instead of a quark? If so, why?
-Which is the right way to regulate the collinear divergence when the recoil has very small angle with respect to either leg of the dipole? Which are the right virtual diagrams?
-How does the picture change when one relaxes some kinematical assumptions, such as the one where the recoil energy is much larger than the medium parton rest mass?
-Can we resum an arbitrary number of such scatterings? Do we observe color decoherence because of multiple color rotations, as can be obtained using previous formalisms?
While these questions refer just to the calculation already performed, in order to achieve the longer-term goals of the project (implementation in parton shower and in effective kinetic theory) one will need to extend it in a number of ways:
-Perform dipole-dipole interaction computation
-Relax kinematics to accommodate also soft-soft interaction, in this way allowing for broadening.
-Consider stimulated radiation as a result of the interaction
Once obtained, the implementation of these results will require further non-trivial efforts, concerning mostly modeling assumptions and coding tasks.
In sum, one can say that the consideration of the issues raised and tackled in this project arise naturally if one contemplates the evolution experienced by the field of jet quenching in the last couple of decades, in particular in going from a single parton to a many parton (as produced by an actual jet) scenario. Applying these notions to the description of the interactions among the mini-jets produced in the early times of a heavy-ion collision, or to the non-linear contribution to parton evolution in high-multiplicity jets, offers a great opportunity to gain a better understanding on the striking multi-particle correlations measured across all system sizes.