Event: Workshop on Light-Ion Collisions at the LHC - 2025

Venue: CERN, Switzerland

Time: December 2, 2025

When viewed at length scales of order the inverse of its temperature, quark-gluon plasma (QGP) behaves as a strongly coupled liquid. However, when it is probed at sufficiently short length scales, or equivalently sufficiently high momentum-exchange, asymptotic freedom mandates the presence of quark- and gluon-like quasiparticles. High-energy partons within jets produced in a light- or heavy-ion collision can trigger such high momentum-exchanges via Molière scatterings, making them valuable probes with which to study the microscopic, particulate structure of the droplet of QGP formed in the same collision. To study the effects of Molière scatterings in light- and heavy-ion collisions, we begin by describing how Molière scatterings are implemented in the Hybrid Strong/Weak Coupling Model of jet quenching. We then reproduce CMS measurements of charged-hadron $R_{AA}$ in OO collisions using Hybrid Model calculations. We show that agreement with experimental data is obtained only when Molière scatterings between jet-partons and medium-quasiparticles are included in the model.

Studies of jet quenching in PbPb collisions often suffer from the effects of selection bias due to jet energy-loss – namely that jets which have lost the least amount of energy to the plasma are the ones selected for study. Since high-$p_{T}$ particles within jets in OO collisions lose significantly less energy to the plasma than in PbPb collisions, jet substructure observables in OO collisions suffer far less from such jet-selection bias effects. As a result, compared to PbPb collisions, jet substructure observables in OO collisions encode the underlying jet-medium interactions in a more accessible manner. With this in mind, we show how medium-induced modifications to the Soft Drop angle $R_{g}$ of anti-$k_{t}$ $R = 0.2$ and $R = 0.4$ jets in PbPb collisions are primarily driven by the effects of selection bias due to jet-energy loss. Due to the significantly lower amount of jet energy-loss in OO collisions, we then show that the dominant cause of modifications to $R_{g}$ distributions of anti-$k_{t}$ $R = 0.2$ and $R = 0.4$ jets in OO collisions is the presence of Molière scatterings. In particular, we show that Molière scatterings in OO collisions result in a higher number of jets with $R_g > 0.2$ compared to pp collisions, whereas significant jet suppression in PbPb collisions results in a lower number of jets with $R_g > 0.2$ compared to pp collisions. Thus, light-ion collisions, and in particular OO collisions, provide a uniquely powerful environment in which to reveal the presence of Molière scatterings and to study microscopic structure of QGP.

Presentation Info: https://indico.cern.ch/event/1597414/contributions/6780503/

Event: 2025 Fall Meeting of the APS Division of Nuclear Physics

Venue: Chicago, Illinois

Time: October 20, 2025

We first report Hybrid Model calculations that reproduce experimental results published by ATLAS in 2023 on R_AA for R = 1 jets in PbPb collisions. These jets are identified by first reconstructing anti-kt R = 0.2 subjets and then re-clustering them. Following ATLAS, we investigate how R_AA for these large-radius jets depends on the “Hard Group angle” between the two R = 0.2 skinny subjets involved in the final clustering step of the R = 1 jet. We also study the dependence of R_AA for these jets on the resolution length of quark-gluon plasma (QGP), defined such that the medium can only resolve partons in a jet shower that are separated by more than this length. We demonstrate that this measurement pioneered by ATLAS rules out any picture in which an entire parton shower loses energy coherently as a single entity.

We further use this setup of R = 1 jets reclustered from R = 0.2 skinny subjets to evaluate the Soft Drop angle, notated as dR_12, using all charged-particle tracks that are associated with each R = 1 jet. Following another ATLAS measurement published in 2025, we use Hybrid Model calculations to investigate the dependence of R = 1 jet R_AA on the Soft Drop angle dR_12. We demonstrate that this new measurement from ATLAS rules out any picture in which all partons in a shower lose energy fully incoherently. Therefore, our analysis demonstrates, for the first time, that the experimental data favors a nonzero QGP resolution length.

Presentation Info: https://schedule.aps.org/dnp/2025/events/S07/7

Event: 21st International Conference on QCD in Extreme Conditions (XQCD 2025)

Venue: Wrocław, Poland

Time: July 4, 2025

We begin by using Hybrid Model calculations to reproduce experimental results published by ATLAS in 2023 on R_AA for R = 1 jets in Pb+Pb collisions. These jets are identified by first reconstructing anti-kt R = 0.2 subjets and then re-clustering them. Following ATLAS, we investigate how R_AA for these jets depends on the angle between the two subjets involved in the final clustering step of the R = 1 jet. We also study the dependence of R_AA for these jets on the resolution length of quark-gluon plasma (QGP), defined such that the QGP-medium can only resolve partons in a jet shower that are separated by more than this length. We demonstrate that this measurement pioneered by ATLAS rules out any picture in which an entire parton shower loses energy coherently as a single entity.

We further use this setup of R = 1 jets reclustered from R = 0.2 skinny subjets to evaluate the soft drop angle, notated as dR_12, using all charged-particle tracks that are associated with each R = 1 jet. Following another ATLAS measurement published in 2025, we use Hybrid Model calculations to investigate the dependence of R = 1 jet R_AA on the soft drop angle dR_12. We demonstrate that this new measurement from ATLAS rules out any picture in which all partons in a shower lose energy fully incoherently. Therefore, our analysis demonstrates, for the first time, that the QGP has a finite nonzero resolution length.

Presentation Slides: https://indico.cern.ch/event/1334113/contributions/6289816/

Event: XXXI International Conference on Ultra-relativistic Nucleus-Nucleus Collisions (Quark Matter 2025)

Venue: Franfurt, Germany

Time: April 7, 2025

In heavy-ion collisions, jets formed from hard-scattered partons experience an overall energy loss and have a modified internal structure compared to vacuum jets. These modifications are a result of the interactions between the energetic partons in a jet shower and the strongly coupled quark-gluon plasma (QGP). As the jet traverses the QGP, it loses momentum to the medium, which in turn responds to the presence of the jet. This “medium response” modifies the momentum distribution of the (soft) hadrons produced when the QGP freezes out. Since these hadrons carry the momentum lost by the parton shower, they and the modified shower both contribute to the energy flow in jets. The quantitative description of the medium response is an open question under active investigation. Recently, the projected N-point energy correlators (ENCs) have seen a resurgence of interest to probe vacuum QCD. For the first time [1], we will present a computation of the full three point energy-energy-energy correlation function in heavy-ion collisions and demonstrate its use for studying the shape of the energy flow originating from medium response. For this study, we utilize the Hybrid Model that implements a hydrodynamical medium response via the wake. We will show that measuring three-point correlation functions offer a promising experimental avenue for imaging the wake of the jet as when the three angles are well-separated the three-point correlator is dominated by the medium response.

[1] arXiv:2407.13818v2

Presentation Slides: https://indico.cern.ch/event/1334113/contributions/6289816/

Event: Workshop on High Energy Probes of the Initial Stages

Venue: CERN Theoretical Physics Department, Switzerland

Time: April 2, 2025

Quark gluon plasma (QGP), when viewed at length scales of order the inverse of its temperature, behaves as a strongly coupled liquid. However, when it is probed at shorter length scales or with sufficiently high momentum transfer, asymptotic freedom mandates the presence of quark-like and gluon-like quasi-particles. High energy partons within jets can trigger these high-momentum exchanges, making jets valuable probes for revealing the presence of such quasi-particles. In this talk, we describe an implementation of such elastic scatterings within the hybrid strong/weak coupling model. High-energy partons in jets undergo elastic Molière scatterings with quasi-particles in the medium. A jet parton that scatters is deflected, kicking a medium parton, which recoils. Subsequently, as both of these partons propagate further through the medium they each lose energy and momentum to the medium, producing hydrodynamic wakes in the droplet of QGP. That is, elastic scattering results in modifications to both the parton shower and to the wake that the shower excites in the droplet of QGP.

Energy-energy-correlators (EECs) characterize the substructure of the energy flow within jets. Using two-point and three-point EEC observables we are able to reveal the relevant angular regions at which (modified) parton showers and wakes in the QGP each dominate, offering a new way with which to visualize and constrain the corresponding dynamics. We compare our calculations to recent CMS measurements of two-point EECs of charged-particle tracks in anti-kt R = 0.4 jets. We show that our calculations agree with the CMS measurements only when elastic scattering is included and when the elastically scattered recoil-partons produce their own wakes. We also reveal our predictions for the first ALICE measurements of two-point EECs of R = 0.2 jets in PbPb collisions.

Presentation Slides: https://indico.cern.ch/event/1487879/contributions/6413104/attachments/3042895/5377440/ArjunKudinoor_CERN_High_Energy_Probes_2025.pdf

Event: Hot Jets 2025: Advancing the Understanding of High Temperature QCD with Jets

Venue: University of Illinois, Urbana-Champaign

Time: January 8, 2025

Quark gluon plasma (QGP), when viewed at length scales of order the inverse of its temperature, behaves as a strongly coupled liquid. However, when it is probed at shorter length scales or with sufficiently high momentum transfer, asymptotic freedom mandates the presence of quark-like and gluon-like quasi-particles. High energy partons within jets can trigger these high-momentum exchanges, making jets valuable probes for revealing the presence of such quasi-particles. In this talk, we describe an implementation of such elastic scatterings within the hybrid strong/weak coupling model. High-energy partons in jets undergo elastic Molière scatterings with quasi-particles in the medium. A jet parton that scatters is deflected, kicking a medium parton, which recoils. Subsequently, as both of these partons propagate further through the medium they each lose energy and momentum to the medium, producing hydrodynamic wakes in the droplet of QGP. That is, elastic scattering results in modifications to both the parton shower and to the wake that the shower excites in the droplet of QGP.

Energy-energy-correlators (EECs) characterize the substructure of the energy flow within jets. Using two-point and three-point EEC observables we are able to reveal the relevant angular regions at which (modified) parton showers and wakes in the QGP each dominate, offering a new way with which to visualize and constrain the corresponding dynamics. We compare our calculations to recent CMS measurements of two-point EECs of charged-particle tracks in anti-kt R = 0.4 jets. We show that our calculations agree with the CMS measurements only when elastic scattering is included and when the elastically scattered recoil-partons produce their own wakes.

Presentation Slides: https://indico.global/event/1374/contributions/27643/

Event: 2024 Fall Meeting of the APS Division of Nuclear Physics

Venue: Hilton Park Plaza, Boston, USA

Time: Tuesday, October 8 at 2:24-2:36pm ET

We begin by using Hybrid Model calculations to reproduce experimental results published by ATLAS in 2023 on R_AA for R = 1.0 jets in Pb+Pb collisions. These jets are identified via first reconstructing anti-kt R = 0.2 subjets and then reclustering them. Following ATLAS, we investigate how R_AA for these large-radius jets depends on the angle between the two subjets involved in the final clustering step of the R = 1.0 jet. We also study the dependence of R_AA for these jets on the resolution length of QGP, defined such that the medium can only resolve partons in the jet shower that are separated by more than this length scale. Our investigation suggests that measurements like those pioneered by ATLAS can constrain the resolution length of QGP.

We make further use of this setup by analyzing the response of the medium to the passage of large-radius R = 2.0 jets containing two R = 0.2 subjets, produced in gamma-jet events, and identified as above. We introduce novel jet-shape observables that allow us to visualize the angular shape of the soft hadrons originating from the wakes that wide jets with two skinny subjets excite in the droplet of QGP, as a function of the angular separation between the subjets. We find that even when the two hard subjets are 0.8 to 1.0 radians apart, a single broad wake is produced. When the two subjets are even farther apart the presence of two sub-wakes is revealed. In our Monte Carlo study, the new observables that we introduce allow us to visualize how jet structure shapes jet wakes unambiguously. We close by showing that similar clarity may be achieved experimentally by measuring the jet shape observables we have introduced using only those hadrons with low transverse momenta.

*This work was supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics grant DE-SC0011090. The work of ASK was supported by a Euretta J. Kellett Fellowship, awarded by Columbia University.

Presentation Info: https://meetings.aps.org/Meeting/DNP24/Session/F04.3

Event: Jet Modification and Hard-Soft Correlations [Soft Jet 2024]

Venue: University of Tokyo, Tokyo, Japan

Time: Saturday, September 28 at 2:30-2:50pm JT

We begin by using Hybrid Model calculations to reproduce experimental results published by ATLAS in 2023 on R_AA for R = 1.0 jets in Pb+Pb collisions. These jets are identified via first reconstructing anti-kt R = 0.2 subjets and then reclustering them. Following ATLAS, we investigate how R_AA for these large-radius jets depends on the angle between the two subjets involved in the final clustering step of the R = 1.0 jet. We also study the dependence of R_AA for these jets on the resolution length of QGP, which suggests that measurements like those pioneered by ATLAS can constrain this property of QGP.

We also use this setup to analyze the response of the medium to the passage of large-radius R = 1.0 jets containing two R = 0.2 subjets, produced in gamma-jet events, and identified as above. We introduce novel jet-shape observables that allow us to visualize the angular shape of the soft hadrons originating from wakes that wide jets with two skinny subjets excite in the droplet of QGP, as a function of the angular separation between the subjets. We find that even when two hard subjets are 0.8 to 1.0 radians apart, a single broad wake is produced. When the two subjets are even farther apart, the presence of two sub-wakes is revealed. We show that the way in which the structure of hard jets shapes their soft wakes can be visualized with similar clarity in experiments by measuring the observables we have introduced using only soft hadrons with low transverse momenta.

Finally, we study the effects of Rutherford-like scattering (aka elastic or Moliere scattering) on two-point energy correlators within the context of the Hybrid Model. We compare Hyrbid Model calculations to recent CMS measurements of the two-point energy correlator in Pb+Pb collisions. We demonstrate that our Hybrid Model calculations agree with the CMS measurements only when elastically scattered particles in the medium produce their own wakes.

Slides: https://indico.cern.ch/event/1403965/contributions/6057581/

Event: 12th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions [Hard Probes 2024]

Venue: Dejima Messe, Nagasaki, Japan

Time: Wednesday September 25, at 9:40-10:00am JT

We begin by using Hybrid Model calculations to reproduce experimental results published by ATLAS in 2023 on R_AA for R = 1.0 jets in Pb+Pb collisions. These jets are identified via first reconstructing anti-kt R = 0.2 subjets and then reclustering them. Following ATLAS, we investigate how R_AA for these large-radius jets depends on the angle between the two subjets involved in the final clustering step of the R = 1.0 jet. We also study the dependence of R_AA for these jets on the resolution length of QGP, which suggests that measurements like those pioneered by ATLAS can constrain this property of QGP.

We make further use of this setup by analyzing the response of the medium to the passage of large-radius R = 2.0 jets containing two R = 0.2 subjets, produced in gamma-jet events, and identified as above. We introduce novel jet-shape observables that allow us to visualize the angular shape of the soft hadrons originating from the wakes that wide jets with two skinny subjets excite in the droplet of QGP, as a function of the angular separation between the subjets. We find that even when the two hard subjets are 0.8 to 1.0 radians apart, a single broad wake is produced. When the two subjets are even farther apart the presence of two sub-wakes is revealed. We show that the way in which jet structure shapes jet wakes can be visualized with similar clarity in experiments by measuring the observables we have introduced using only those hadrons with low transverse momenta.

Slides: https://indico.cern.ch/event/1339555/contributions/6040802/

Event: CTP Graduate Student Seminar

Venue: Cosman Room, Center for Theoretical Physics, MIT

Time: Friday, September 13 at 12:30-1:30pm ET

Microseconds after the Big Bang, our universe was filled with a hot, strongly coupled, liquid phase of QCD-matter called Quark Gluon Plasma (QGP). Physicists are able to recreate droplets of this primordial liquid in high energy collisions of large nuclei. Such “heavy ion collisions” also produce highly energetic sprays of particles called “jets” that traverse and interact with the expanding droplet of QGP. Since these interactions with the plasma modify the jets produced in heavy ion collisions, such jets allow us to probe the properties of these tiny droplets of Big Bang matter. One such modification includes a loss in the total energy of a jet as it traverses the QGP produced in a collision. This lost energy is deposited into the plasma, sourcing wakes. The wakes left by jets that plow through droplets of QGP are yet to be unambiguously observed in experiments.

In this talk, I describe my recent efforts to image and characterize the structure of such jets and the wakes they produce. My work employs a Monte Carlo model of heavy ion collisions that incorporates holographic calculations of energy loss experienced by light quarks and gluons as they traverse the strongly coupled plasma. I show that the wakes produced by jets in this model can be imaged using two observables of interest: correlation functions of the energies of final-state hadrons produced in heavy ion collisions (energy correlators) and the angular distributions of the energies of those final-state hadrons with low-momenta (jet shapes). Finally, I introduce a novel modification to the latter observable that enables us to characterize how the structure of jets shapes the structure of their wakes. The theoretical calculations I perform may be realized in experiments, thereby illuminating an avenue to image and characterize the structure of jet-induced wakes in nature.

Slides: https://columbia.edu/~ask2262/Talks/2024/kudinoor-mit-ctp-seminar-2024-09-13.pdf

Event: Energy Correlators at the Collider Frontier [Workshop]

Venue: Mainz Institute for Theoretical Physics, Johannes Gutenberg-Universität Mainz

Time: Wednesday, July 10 at 12:00-12:30 PM CET

This talk, which is the fourth in a coordinated sequence of four talks, will be an analysis of coordinate choices and the effects of jet wakes on three-point energy correlators. Using energy-energy-energy correlators to “image” the wakes that jets excite in the hydrodynamic fluid produced in a heavy ion collision is the goal of the work that is described in this four-talk series, and the Hybrid strong/weak coupling model of heavy ion collisions is a tool that will be employed in obtaining the new results presented in each talk. This is the abstract for the whole sequence of four talks.

Recently, the projected N-point energy correlators (ENCs) have seen a resurgence of interest for hadronic collisions at RHIC and the LHC to probe vacuum QCD.  Here, we will show that the full three-point energy-energy-energy correlation (EEEC) function can be useful for studying the shape of energy flow within jets. In vacuum, it has been shown that these correlators elucidate the collinear singularity of vacuum QCD. For the first time, we will show how EEEC can uniquely characterize the energy flow originating from the jet-induced medium response in heavy-ion collisions. In heavy-ion collisions, jets formed from hard-scattered partons experience an overall energy loss and have a modified internal structure compared to vacuum jets. This is due to interactions between the energetic partons in a jet shower and the strongly coupled quark-gluon plasma (QGP). As the jet traverses the QGP, it loses momentum to the medium, momentum which becomes a wake in the droplet of QGP and subsequently becomes soft particles carrying momentum in the jet direction. A quantitative description of this “medium response” is an area of active investigation. For this study, we utilize the Hybrid Model that implements a hydrodynamical medium response via the wake. We will show that measuring three-point correlation functions offer promising experimental avenues for imaging this wake of the jet as when the three angles are well-separated the three-point correlator is dominated by the medium response. Along the way, we will introduce new coordinates for specifying the three angles that are well-suited to the case when these angles are far from collinear, as in the regime where the EEEC is dominated by particles coming from jet wakes.

Slides: https://indico.mitp.uni-mainz.de/event/358/contributions/4990/

Event: Conference on Everything

Venue: Wolfson Hall, Churchill College, Cambridge University

Time: Saturday, April 27 at 5:30-5:45 PM BST

Microseconds after the Big Bang, our universe was filled with a hot, dense, nearly-frictionless liquid phase of matter called Quark Gluon Plasma (QGP). This soup of quark and gluons is the origin of matter — including the protons and neutrons — we see around us today. In the hopes of unraveling our cosmic origin story, physicists recreate tiny droplets of QGP in nuclear collisions at incredibly high energies. In this talk, I introduce the physics that governs QGP, explain how it is studied in heavy ion (nuclear) collision experiments, and describe the theoretical techniques that I use to probe the earliest, hottest, and most liquid liquid of our universe.

Event: Part III Student Seminars in Mathematics

Venue: MR 13, Center for Mathematical Sciences, Cambridge University

Time: Saturday, March 14 at 1:00-1:50 PM BST

During relativistic heavy ion collisions a deconfined, strongly coupled, hot-QCD phase of matter called quark gluon plasma (QGP) is formed. Over the last decade, formal advancements like the AdS/CFT duality have helped shed light on the otherwise intractable strongly coupled regime of QCD phenomenology. In this talk, I describe the basics of the AdS/CFT duality and its applications to my research on the substructure of jets and their wakes in heavy ion collisions. I also demonstrate how wake-substructure depends nontrivially on the substructure of the jets from which wakes originate. Finally, I explain how this nontrivial dependence between jet and wake-substructures reveals signatures for observing jet-wakes in experiments.

Slides: https://columbia.edu/~ask2262/Talks/2024/kudinoor-cambridge-cms-2024-03-14.pdf

Event: Part III Student Seminars in Mathematics

Venue: Potter Room (MR 19), Center for Mathematical Sciences, Cambridge University

Time: Thursday, November 30 at 1:00-1:40 PM BST

During relativistic heavy ion collisions a deconfined, strongly coupled, hot-QCD phase of matter called quark gluon plasma (QGP) is formed. Over the last decade, formal advancements like the AdS/CFT duality have helped shed light on the otherwise intractable strongly coupled regime of QCD phenomenology. In this introductory talk, I describe the basics of the AdS/CFT duality and its applications to heavy ion collisions in the context of my research on the hybrid strong/weak coupling model of QGP physics.

Event: 2-Minute Thesis Conference

Venue: Wolfson Hall, Churchill College, Cambridge University

Time: Tuesday, November 21 at 5:30-6:30 PM BST

Microseconds after the Big Bang, before atoms even formed, our universe was filled with a frictionless liquid of subatomic particles called quarks and gluons. Today we study this liquid, called a quark gluon plasma, in nuclear collisions at incredibly high energies.

This talk is accessible to the general public.