11th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions

Jets are excellent probes for studying the deconfined matter formed in heavy ion collisions. Measurements of jet yield and substructure as a function of jet resolution parameter $R$ over a wide range in jet $p_{\mathrm{T}}$ probe the mechanisms underlying the interaction between jets and the QGP, notably the role of opening angle of the hardest jet shower components, and of the angular...

The $R$-dependence of jet observables provides a new tool in understanding the interplay between the jet energy-loss mechanism and medium response in heavy-ion collisions. We have coupled the Monte Carlo event generators JEWEL and PYTHIA, with initial conditions from the ${\mathrm{T}}_{\mathrm{R}}\mathrm{E}\mathrm{N}\mathrm{T}\mathrm{o}$ and the state-of-the-art (2+1)D v-USPhydro, for the simulation of jet quenching phenomena in a more realistic...

High-energy partons are well established to lose energy when traversing the hot and dense medium produced in heavy-ion collisions. This results in a modification to the transverse momentum distributions of jets, producing a phenomenon known as jet quenching. It has been previously established in Pb+Pb collisions at $\sqrt{s_{\text{NN}}}=2.76$~TeV that jet quenching leads to significant...

Jet energy loss and transverse momentum broadening are twin consequences of a jet interacting with a hot and dense QCD matter in heavy-ion collisions. The underlying interaction can be represented by jet transport coefficient $\hat{q}$ distributed in the whole phase space of the bulk medium. The gradient of $\hat{q}$ perpendicular to the momentum direction of an energetic parton leads to an...

Jet-medium interactions receive large non-perturbative contributions from classical gluons, i.e. infrared gluons with high occupation numbers. These contributions affect transverse jet momentum broadening and medium-induced radiation. Both depend significantly on the in-medium dispersion of hard partons, encoded in their so-called asymptotic mass.
In this talk, I shall show how the...

Following the formalism developed in our preceding works [1], a non-perturbative light-front Hamiltonian approach, we investigated the momentum broadening of a quark jet inside a SU(3) colored medium. We performed the numerical simulation of the real-time jet evolution in the Fock space of |q> + |qg>, at an extensive range of $p^{+}$, and various medium densities. With the obtained light-front...

Jet substructure observables provide unique probes of the properties of the quark-gluon plasma (QGP). In this talk, we report new measurements of groomed jet substructure in central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. We present the first application of Dynamical Grooming in heavy-ion collisions to search for excess $k_{\mathrm{T},\mathrm{g}}$ emissions as a signature of point-like...

Jets produced from hard scatterings of partons early in heavy-ion collisions traverse through the quark-gluon plasma (QGP) medium and get modified relative to vacuum ($p$+$p$ collision) baseline. These modifications can change the distributions of jet shape observables, which are related to jet fragmentation and its internal structure, and calculated based on the intra-jet angular energy...

In a series of recent publications [1,2,3], we have proposed a factorized approach, based on perturbative QCD, for the evolution of a jet in a dense quark-gluon plasma, together with its implementation as a Monte-Carlo parton shower and successful applications to the phenomenology of jet quenching.
In the original formulation of the parton shower, the collisions between the jet constituents...

We will discuss the use of recoil-free jet observables to systematically benchmark jet modification studies with precision and sensitivity, starting from the hardest components of jets. Here we focus on the recoil-free jet axis in defining di-jet and photon-jet angular decorrelation. This observable is not affected by the huge underlying event background and can be calculated and measured...

The transverse momentum broadening coefficient serves as a key ingredient in characterising the quenching of a jet as it propagates through the QGP. While it has recently been understood to receive quantum, radiative corrections featuring potentially large logarithmic enhancements at relative order $g^2$ [1, 2], it is still not clear how these corrections compare quantitatively with their...

Measuring the jet substructure in heavy-ion collisions provides exciting new opportunities to study detailed aspects of the dynamics of jet quenching in the hot and dense QCD medium created in these collisions. In this talk, we present new ATLAS measurements of jet substructure performed using various jet (de)clustering and grooming techniques. Measurements of inclusive jet suppression...

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...

The measurement of jets recoiling from a trigger hadron provides unique probes of medium-induced modification of jet production. Jet deflection via multiple soft scatterings with the medium constituents or single-hard Molière scatterings off quasi-particles in the medium are expected to modify the azimuthal correlation between the trigger hadron and recoiling jet. The $R$-dependence of recoil...

The leading jet transport coefficients $\hat{q}$ or ${\hat{e}}_2$ encode transverse or longitudinal momentum broadening of a hard parton traversing a hot medium. Computing their normalization and temperature dependence from first principles is key to appreciating the observed suppression of high-transverse momentum probes at RHIC or LHC collision energies. We present a first continuum...

We study the interaction of leading jet partons in a strongly interacting quark-gluon plasma (sQGP) medium based on the effective dynamical quasi-particle model (DQPM). The DQPM describes the non-perturbative nature of the sQGP at finite temperature $T$ and baryon chemical potential ${\mu }_B$ based on a propagator representation of massive off-shell partons whose properties are adjusted to...

In this work, by exploiting jet substructure techniques, we identify the transition from early-time perturbative splittings to late-time non-perturbative emissions and its associated timescale at both RHIC and LHC energies. We introduce three experimentally robust splittings along the jet clustering tree, each related to the perturbative, non-perturbative-like regions and the transition...

The heavy-ion collisions produce a hot, dense medium, and high-momentum partons from the collision traverse this medium while losing energy to it. This talk presents new measurements of the azimuthal dependence with respect to the event plane of single jet yields and high momentum charged particles yields in Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV. As the distance traversed by the...

The injection of energy and momentum from a jet into the QGP generates a wake, which leads to soft and semi-hard particle creation correlated with the jet direction after the QGP hadronizes. As several jet quenching studies have shown, this medium response phenomenon plays a crucial role in our understanding of many jet structure and substructure observables. Nevertheless, a detailed account...

Measurements at RHIC and the LHC show strongly enhanced baryon-to-meson yield ratios at intermediate transverse momenta ($p_T$) in high-energy nuclear collisions compared to $p$+$p$ baseline. This enhancement is attributed to the following QGP effects: strong hydrodynamic flow and parton recombination. Jet probes have been used extensively to gain insights into QGP properties, with...

We employ machine learning techniques to identify important features that distinguish jets produced in heavy-ion collisions from jets produced in proton-proton collisions [1]. We formulate the problem using binary classification and focus on leveraging machine learning in ways that inform theoretical calculations of jet modification: (i) we quantify the information content in terms of Infrared...

In recent years jet substructure observables have been used at the LHC as instruments to search for new physics and test perturbative and non-perturbative processes in QCD. In heavy-ion collisions, these jet substructure observables can additionally elucidate the production and evolution of the QCD medium. The jet mass is one such observable that probes the momentum transfer scale of the...

We present a new model for jet quenching from coherent radiation in a brick medium. The jet energy loss is simulated as a perturbative final-state vacuum parton shower followed by a medium-induced shower originating from elastic and radiative collisions with the medium constituents. Coherency is achieved by starting with trial gluons that acts as field dressing of the initial jet parton. These...

Modifications of the internal structure of jets through interactions with the QGP produced in heavy-ion collisions, referred to as jet quenching, are used to study the properties of the QGP. In this talk, we present the first measurement of the angle between pairs of jet axes, $\mathrm{\Delta }{R}_{\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{s}}$, in central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV with ALICE. We compare this...

As high-energy light quarks and gluons traverse the Quark-Gluon Plasma, they are expected to lose energy mainly via medium-induced gluon bremsstrahlung. Thus, a basic assumption in pQCD-based frameworks of radiative energy loss is that it depends on the QCD color factor of the initiating parton. In this talk, ATLAS presents two measurements in Pb+Pb collisions aimed at constraining the...

The interaction between the jet and QGP fluid will deflect particles associated with the jet from their initial direction. Such deflection will depend on the energy of the jet constitutes and the velocity of the flow. The soft particles suffering stronger deflection will drift towards the direction of the flowing medium, away from the center of the jet cone where the hard particles are...

Interactions of high-energy partons with the strongly-coupled quark-gluon plasma lead to parton energy loss, as well as broadening of the partons' transverse and longitudinal momentum distributions. Energy loss and momentum broadening resulting from soft parton-plasma interactions can be quantified with transport coefficients, factorizing their effect from hard (perturbative) parton-plasma...

Over the last decades, the theoretical picture of how hadronic jets interact with nuclear matter has been extended to account for the medium’s finite longitudinal length and expansion. However, only recently a first-principle approach has been developed that allows to couple the jet evolution to the medium flow and anisotropic structure in the dilute limit. In this talk, we will show how to...

The suppression of jets in heavy-ion collisions can provide detailed information about the hot, dense plasma formed in these collisions at the LHC. The energy loss mechanism can be studied by measuring differences in the suppression of $b$-tagged and inclusive jets in $pp$ and Pb+Pb collisions. Besides the $b$-tagged jet measurements, an alternative method for probing the interactions of heavy...

In recent years, a lot of effort has been put into expanding established jet-quenching formalisms to account for higher-order or energy-suppressed medium-induced effects. Understanding how such contributions emerge is important to have a more complete picture of jet evolution in the medium and to extract more detailed properties of the underlying matter. However, such efforts are in general...

The properties of partonic fragmentation in QCD are dependent on the flavours of the partons involved in the 1$\to$2 splitting processes underpinning parton showers. These flavour dependencies arise due both to the different Casimir factors of quarks and gluons, as well as the mass of heavy quarks. Heavy-flavour jets provide a unique experimental tool to probe these flavour...

In this talk, we introduce a novel approach to minimise selection biases associated to the modification of the quark- vs. gluon-initiated jet fraction in order to assess the presence of other medium-induced effects, namely color decoherence. More concretely, we propose to explore the rapidity dependence of jet substructure observables. So far, all jet substructure measurements at mid-rapidity...

In this talk, we will discuss the possibility of using the dead cone of heavy quarks as a region of the Lund plane where medium-induced gluon radiation can be isolated and characterised. We propose to use jet grooming techniques to identify a particular splitting in the jet tree that is both perturbative and sensitive to the dead-cone effect. In particular, we introduce a new jet substructure...

Jet substructure is a powerful tool to probe the time evolution of a parton shower. However, many of the analysis methods used to extract splitting formation times from jet substructure, such as Soft Drop grooming and the Lund plane, focus on the hardest radiation of the jet. A complementary observable with growing theoretical and experimental interest, the 2-point Energy Correlator (EEC),...

The dynamics of jet formation in heavy ion collisions (HICs) is influenced by the presence of a quark-gluon plasma (QGP) and is imprinted into a jet's multi-scale substructure. In recent work, we demonstrated that the two-point energy correlator, measured on a massless in-medium jet, provides a sensitive probe of this dynamics and can robustly identify the scales defined by the properties of...

High-energy partons are capable of triggering high-momentum exchanges with quark-like and gluon-like QGP quasi-particles that can be observed at sufficiently short length scales. In this work we present an implementation of a central aspect of this physics within the hybrid strong/weak coupling model. Interaction with the quasi-particles results in elastic, Moliere scatterings, leading to...

When an energetic parton traverses the hot QCD medium, it may suffer multiple scattering and energy losses. The medium-induced gluon radiation for a massive quark will be suppressed relative to that of a light quark due to the dead-cone effect. The development of new declustering techniques of jet evolution makes a direct study of the dead-cone effect in the QCD medium possible for the first...

Jet quenching, one of the signatures of the quark-gluon plasma, is a well established experimental phenomenon at RHIC and LHC. However, a detailed characterization of the expected dependence of jet-medium interactions on the flavor of the parton initiating the shower is yet to be settled. This talk presents the first b jet shapes measurements in 5.02 TeV PbPb and pp collisions collected by the...

Bin migration effects hinder a direct connection between the nuclear modification factor $R_{AA}$ and the energy lost by jets. $R_{AA}$ compares yields of jets, in pp and AA collisions, that are reconstructed with the same $p_T$ and is thus biased by the steeply falling jet spectrum. To mitigate these effects, Brewer et al. [1] introduced a novel observable to directly quantify average jet...

Given recent works showing jet quenching’s sensitivity to the dynamics of the pre-hydrodynamic phase of heavy-ion collisions, addressing medium-induced radiation in the initial stages becomes crucial. In this talk, we derive the BDMPS-Z emission spectrum off a hard parton accounting for additional medium-induced emissions arising from its vacuum propagation in the pre-hydrodynamics phase. By...

Reference measurements in p+p collisions are crucial for understanding jet quenching. PHENIX has a suite of new jets measurements in p+p collisions: fragmentation function, transverse momentum, jt, distribution, the radial profile, and splitting functions.

Jet quenching effects can also be studied with high momentum hadrons and two-particle correlations. The distribution of hadrons...

Fragmentation functions are one of the key components of the factorisation theorem used to calculate heavy-flavour hadron production cross sections. The non-perturbative nature of fragmentation functions necessitates their constraint through experimental measurements, commonly performed in the clean environments of ${\mathrm{e}}^{+}{\mathrm{e}}^{-}$ and $\mathrm{e}\mathrm{p}$ collisions. However, recent measurements...

We study jet fragmetation via final-state parton splittings in the medium. These processes are usually calculated theoretically by invoking one or two approximations: the large-Nc and the eikonal approximations. We want to develop methods to do the calculations without using these approximations, and to quantify the error that is introduced by employing them.
As partons go through the medium...

Jets are multi-partonic systems that develop before interactions with the QGP 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 ${\theta }_c$. Using resummed quenching weights that incorporate the IOE...

Partonic scatterings with large momentum transfer occur before the formation of the quark-gluon plasma (QGP) in heavy-ion collisions, resulting in collimated collections of hadrons known as jets. As a jet traverses and interacts with the QGP medium, it loses energy via collisional and radiative processes, known as jet quenching. The magnitude of the energy loss can be quantified by the ratio...

We present a new approach to jet substructure in heavy-ion collisions based on the study of correlation functions of energy flow operators (energy correlators). This approach is based on the insight that the dynamics of the QGP is imprinted at specific time scales in the jet, which will be reflected as changes in the shape of the correlator. We analyze the two-point correlator of an in-medium...

We study the energy deposition of a high-momentum parton traveling through a Quark-Gluon Plasma using QCD kinetic theory. We show that the energy is first transported to the soft sector by collinear cascade and then isotropised by elastic scatterings. Remarkably, we find that the jet wake can be well described by a thermal distribution function with angle-dependent temperature. This could be...

We investigate how the scale-dependent jet-medium interactions affect the jet substructure observables in heavy-ion collisions via event-by-event Monte Carlo simulation using the JETSCAPE framework. Jets are dynamic probes with varying virtualities and energies of partonic constituents in their shower evolution. The various internal medium structures involved in interactions at different...

We revisit the picture of jets propagating in the quark-gluon plasma. In addition to vacuum radiation, related to the high initial virtuality, jet particles scatter on the medium constituents resulting in induced emissions. Analytical approaches to resumming these interactions have traditionally dealt separately with multiple, soft, or rare, hard scatterings. A full resummation has so far only...

Jets are multi-scale objects that connect asymptotically free partons to confined hadrons. Jet substructure measurements in vacuum provide essential insight into the parton evolution and the ensuing non-perturbative processes.

In this study, we use the SoftDrop grooming technique, based on the angular-ordered Cambridge/Aachen reclustering algorithm, to probe correlations between jet...

The fast evolution of the QGP makes its interaction with jets an inherently time-dependent process. However, this crucial dimension is missing from current jet quenching measurements, which hence provide a mere average quantification of the medium properties. In this talk, we propose that jet substructure observables allow access to the QGP time structure. By identifying the recursive steps of...

In this talk we review jet production in a large variety of collision systems using the JETSCAPE event generator and Hybrid Hadronization. Hybrid Hadronization combines quark recombination, applicable when distances between partons in phase space are small, and string fragmentation appropriate for dilute parton systems. It can therefore smoothly describe the transition from very dilute parton...

We present measurements of the semi-inclusive distribution of charged-particle jets recoiling from $\gamma$ and ${\pi }^0$ triggers in $p$+$p$ and central Au+Au collisions, and from charged hadron triggers in smaller collision systems (Ru+Ru and Zr+Zr) at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV by STAR. The large uncorrelated background in heavy-ion collisions is removed using the event mixing...

While experimental studies on jet quenching have achieved a large sophistication, the theoretical description of this phenomenon still misses some important points. One of them is the interplay of vacuum-like emissions, usually formulated in momentum space, with the medium induced ones that demand an interplay with a space-time picture of the medium and thus must be formulated in position...

By scattering with the medium, partons produced by an in-medium shower can change their flavor. At these intermediate energy scales, the rate of gluon conversion to quarks and antiquarks can be more than double the reverse process of quark conversion. Consequently, interactions with the medium lead to a ring of fermions around the hard parton [1]. We present estimates of the range of angles...

Jet shape is studied with a linear Boltzmann transport model for event-by-event simulations of photon-tagged jets in heavy-ion collisions. The transverse momentum asymmetry $A_{\perp }$ is shown to increase with the initial transverse position when the gradient of jet transport coefficient $\hat{q}$ increases until at the edge of the nonuniform medium. On one hand,the shape of the photon-tagged...

The path-length dependence of jet quenching can help to constrain different jet quenching mechanisms in heavy-ion collisions. However, measuring an explicit value for this dependence has proven challenging. Traditional approaches, which consider anisotropic jet suppression arising from geometric asymmetries, have successfully measured a non-zero azimuthal dependence of jet modification with...

Small systems such as pp or p+Pb collisions exhibit evidence of collective behavior strikingly similar to that in Pb+Pb collisions. However, while jet quenching is readily observed in Pb+Pb collisions, no evidence has been found in small systems to date, raising fundamental questions about the nature of the system created in these collisions. This talk reports a measurement by the ATLAS...

Jet quenching parameter $\hat{q}$ is essential for characterizing the interaction strength between jet partons and nuclear matter. Based on the quark-meson (QM) model, we develop a new framework for calculating $\hat{q}$ at finite chemical potentials, in which $\hat{q}$ is related to the spectral function of the chiral order parameter. A perturbative calculation up to the one-loop order...