The forward geometry of the LHCb detector provides unprecedented access to both the very high and very low regions of Bjorken $x$ inside the nucleon. With full particle ID and a fast DAQ, LHCb is able to fully reconstruct plentiful charged particles and $\pi^{0}$ mesons, as well as relatively rare probes such as $Z$ bosons and heavy quarks, providing a unique set of constraints on nucleon...
Heavy quarks are produced in the early stage of heavy ion collisions due to their large mass, and experience the entire evolution of the QCD medium. The baryon-to-meson ratio, in particular, the $\Lambda_c^+$/$D^0$ ratio, provides valuable information on charm hadronization mechanisms, testing the role of coalescence in the Quark-Gluon Plasma created in PbPb collisions. In $p$Pb collisions,...
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 $\rm T_RENTo$ and the state-of-the-art (2+1)D v-USPhydro, for the simulation of jet quenching phenomena in a more realistic...
The ALICE experiment at the Large Hadron Collider (LHC) is designed to investigate the properties of the quark-gluon plasma created in high-energy heavy-ion collisions. During the successful data-taking campaigns of LHC Run 1 and Run 2 (2009 - 2018), it recorded data for a variety of collision systems and different center-of-mass energies. As particle production at the LHC is driven by a...
In the early stages of heavy-ion collisions, at the highest energies, the system begins in a highly anisotropic state which is far from equilibrium. At later times, the dynamic evolution of the system is well described in the framework of relativistic hydrodynamics which requires local thermodynamic equilibrium. The KoMPoST framework has had some success in bridging the gap between these...
High energy nuclear collisions produce far-from-equilibrium matter with a high density of gluons at early times. We identify for the first time two local order parameters for condensation, which can occur as a consequence of the large density of gluons. We demonstrate that an initial over-occupation of gluons can lead to the formation of a macroscopic zero mode towards low momenta that scales...
A nonperturbative charm production contribution, known as intrinsic charm, has long been speculated. While it has yet to be satisfactorily proven, there have been recent tantalizing hints. Several experiments, either taking data or
planned, could proivde definitive evidence in the next few years. Experiments that have taken $J/\psi$ and $D$ meson data include SeaQuest at Fermilab and SMOG...
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 $\sqrt{s_\textrm{NN}}~=~2.76$~TeV that jet quenching leads to significant...
Measurements of two-particle correlations in $pp$ collisions show the presence of long-range correlations along $\Delta\eta$ that are strikingly similar to those seen in heavy-ion collisions. The similarity between the $pp$ and heavy-ion measurements raises the possibility that a tiny droplet of the QGP is produced even in $pp$ collisions. However, models that attribute the correlation in $pp$...
Within a microscopic kinetic description based on the Boltzmann equation, we evaluate the importance of the pre-equilibrium stage in high-energy heavy-ion collisions for final state observables over a large range of viscosity and system size. We use our results to determine the range of applicability of an effective description in relativistic viscous hydrodynamics. We find that hydrodynamics...
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...
Measurements of quarkonia production in peripheral and ultra-peripheral heavy-ion collisions are sensitive to photon-photon and photon-nucleus interactions, the partonic structure of nuclei, and to the mechanisms of vector-meson production. LHCb has studied production of the $J/\psi$ and $\psi(2S)$ charmonium states in peripheral and ultra-peripheral collisions using PbPb data at forward...
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 $k_{\mathrm{T,g}}$ 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...
Minijets, created by perturbative hard QCD collisions at moderate energies, can represent a significant portion of the total multiplicity of a heavy-ion collision event. Since their transverse momenta are larger than the typical saturation scale describing the bulk of the equilibrating QGP, they do not in general hydrodynamize at the same pace than the bulk of the collision. In this work we...
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...
In relativistic heavy ion collisions, the charged ions produce an intense flux of equivalent photons. Thus, photon-induced processes are the dominant interaction mechanism when the colliding nuclei have a transverse separation larger than the nuclear diameter. In these ultra-peripheral collisions (UPCs), the photon provides a clean, energetic probe of the partonic structure of the nucleus,...
The second-order Fourier coefficients ($v_{2}$) of $\Upsilon$(1S) and J/$\psi$ mesons in high-multiplicity pPb collisions is studied using data collected by the CMS experiment at a nucleon-nucleon center-of-mass energy 8.16 TeV. The dimuons used to reconstruct the heavy quarkonium states are correlated with charged hadrons using long-range two-particle correlation techniques. The measurement...
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...
The second- and third-order Fourier coefficients of charmonium states are measured in PbPb collisions with CMS. With this new analysis, extending to a higher $p_\mathrm{T}$ region, we investigate further the high-$p_\mathrm{T}$ J/$\psi$ $v_{2}$ in heavy ion collisions. The nonprompt J/$\psi$ $v_{2}$ probes the different behavior of charm and bottom quarks induced by interactions with the QGP...
We construct an improved 3+1D version of the Trento initial state model, which includes rapidity-dependent fluctuations. The correlation between the fluctuations at different rapidities is controlled by a new parameter. We then use this improved model to study rapidity-dependent observables for ultracentral collisions. It it known that ultracentral flow at midrapidity is sensitive to...
Suppression of open heavy flavors and quarkonia in heavy-ion collisions is among the most informative probes of the quark-gluon plasma. Interpreting the full wealth of data obtained from the collision events requires a precise understanding of the evolution of heavy quarks and quarkonia as they propagate through the nearly thermal and strongly coupled plasma. Only in the past few years,...
We study the impact of the Glasma fields, used to describe the very-early stage of heavy-ion collisions, on the transport of hard probes, namely heavy quarks and jets. We perform numerical simulations of the strong classical fields using techniques from real-time lattice gauge theory. The resulting fields are used as background for the classical transport of ensembles of particles, described...
Measurements of bottomonium states in heavy-ion collisions provide a powerful tool to study both initial-state effects on heavy-quark production and final-state interactions between heavy quarks and the quark-gluon plasma (QGP). The ATLAS experiment at LHC has measured the production of bottomonium states $\Upsilon$(1S), $\Upsilon$(2S), and $\Upsilon$(3S), in Pb+Pb and $pp$ collisions at a...
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...
The intriguing phenomena emerging in the high-density QCD matter are being widely studied in the heavy ion program at the LHC and will be understood more deeply during the high luminosity LHC (HL-LHC) era. The CMS experiment is under the Phase II upgrade towards the HL-LHC era. A new timing detector is proposed with timing resolution for minimum ionization particles (MIP) to be 30ps. The MIP...
Hadronic collisions produce prompt photons that are characterized by a large transverse momentum and absence of event activity in their vicinity. Photons are a robust probe of cold nuclear matter effects in small and large collision systems because they do not interact strongly and are thus insensitive to medium-induced final-state effects. Prompt photon production is dominated by the Compton...
Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This talk presents a series of measurements of dilepton production from photon fusion performed by the ATLAS Collaboration, which provide strong constraints on the nuclear photon flux, its dependence on the impact parameter and photon energy, and can also...
We present the first study of coherent exclusive quarkonium (J/psi, Upsilon) photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs) at the LHC in the framework of collinear factorization and next-to-leading order (NLO) perturbative QCD. We make predictions for the J/psi and Upsilon rapidity distributions for the cases of lead (Pb) and oxygen (O) beams and quantify their...
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...
An important sign of the creation of the Quark-Gluon Plasma in heavy-ion collisions is the observation of jet energy-loss. Energetic, high transverse momentum ($p_{T}$) partons produced at the moment of initial hard scattering are influenced by the evolution history of the medium and lose energy via interactions. In this work we compare two models of low virtuality radiative energy loss:...
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...
In Spring 2023, the sPHENIX detector at BNL’s Relativistic Heavy Ion Collider (RHIC) will begin measuring a suite of unique heavy flavor and quarkonia observables with unprecedented statistics and kinematic reach at the RHIC energies using combined EM and hadronic calorimeters and high precision tracking. A MAPS-based vertex detector upgrade to sPHENIX, the MVTX, will provide a precise...
The high-intensity beams provided by the CERN SPS in a wide energy interval offer a unique opportunity to investigate the region of the QCD phase diagram at high baryochemical potential. The NA60+ experiment, proposed for taking data with heavy-ion collisions at the SPS in the next years, has a strong potential for investigating the QCD phase diagram via measurements of rare probes in a...
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 LHCb spectrometer has the unique capability to function as a fixed-target experiment by injecting gas into the LHC beampipe while proton or ion beams are circulating. The resulting beam+gas collisions cover an unexplored energy range that is above previous fixed-target experiments, but below the top RHIC energy for AA collisions. Here we present new results on open charm, $J/\psi$, and...
Heavy quarks (i.e. charm and beauty) are powerful tools to characterize the quark-gluon plasma (QGP) produced in heavy-ion collisions. Although they are initially produced out of kinetic equilibrium via hard partonic scattering processes, recent measurements of anisotropic flow of charmed hadrons [1] pose the question regarding the possible thermalization of heavy quarks in the medium. Our...
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...
We present a consistent photon production calculation from hadronic cross sections, including bremsstrahlung and 2-to-2 reactions, matching the usually employed thermal rates [1,2]. Using the hadronic transport approach SMASH as the afterburner for the hadronic stage at RHIC and LHC energies, we find a significant increase in the calculated momentum anisotropies of these photons due to...
The addition of a Forward Calorimeter (FoCal) to the ALICE experiment is proposed for LHC Run 4 to provide unique constraints on the low-x gluon structure of protons and nuclei via forward measurements of direct photons. A new high-resolution electromagnetic Si-W calorimeter using both Si-pad and Si-pixel layers is being developed to discriminate single photons from pairs of photons...
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...
Photon-induced reactions in ultra-peripheral collisions (UPCs) of heavy nuclei at the LHC have been studied using the ALICE detector for several years. The ALICE detector can measure the photoproduction cross section for vector mesons at various rapidities, centre-of-mass energies and collision systems. Beyond the recent ALICE studies of the rapidity and momentum transfer dependence of...
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...
PHENIX presents the simultaneous measurement of high pt (8-18 GeV/c) direct γ and π0 production in d+Au collisions at 200 GeV. The analysis is performed for different events samples selected by event activity. The direct γ-to-π0 ratio is independent of event activity, except for events with the highest activity where the ratio is slightly enhanced. Final state effects are expected to be small...
Relativistic heavy-ion beams at the LHC are accompanied by a large flux of equivalent photons, leading to multiple photon-induced processes. This talk presents searches for physics beyond the standard model enabled by photon-photon processes in both di-tau and di-photon final states. The tau-pair production measurements can constrain the tau lepton’s anomalous magnetic dipole moment (g-2), and...
In studies of the QGP, it has been observed that in high-energy heavy-ion (A+A) collisions, high-momentum hadrons and heavy flavor electrons are suppressed, indicating that QGP is strongly coupled, and that quarks lose energy as they traverse the medium. In order to quantitatively discuss this important energy loss mechanism, for charged hadrons consisting of light quarks, in addition to 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...
The microscopic production mechanism of light (anti)nuclei in high-energy hadronic collisions is still mysterious and is a highly debated topic in the scientific community. Two different phenomenological models are typically used to describe the experimental data: the statistical hadronization model and baryon coalescence. In the former, light nuclei are emitted from a source in local thermal...
The total rate of heavy quark production can be calculated with perturbative QCD techniques. However, the fraction of heavy quarks that pair with a light quark (forming mesons) versus the fractions combine with two other quarks (baryons) baryons or 3 or more other quarks (exotic states) is sensitive to the nonperturbative hadronization process. LHCb is uniquely well suited to study such...
Photons are emitted at all stages of relativistic heavy-ion collisions and do not interact with the medium strongly. With access to the versatility of RHIC, measurements of low momentum direct photons are made possible across different system size and beam energies. An excess of direct photons, above prompt photon production from hard scattering processes, is observed for a system size...
Electroweak bosons produced in hard-scattering processes at the early stage of the collision, are efficient probes of the initial state of the collision. While the W measurements in pp collisions are a stringent test of perturbative QCD-based calculations and production mechanisms, they can constrain the nuclear parton distribution functions in p-Pb and Pb-Pb collisions.
Electroweak bosons...
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, $\Delta R_{\rm axis}$, in central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV with ALICE. We compare this...
Understanding the modification of jets and high-$p_T$ probes in small systems requires the integration of soft and hard physics. We present recent developments in extending the JETSCAPE framework to build an event generator, which includes correlations between soft and hard partons, to study jet observables in small systems. The multi-scale physics of the collision is separated into different...
Thermal photons are vital tool to study Quark-Gluon Plasma The photon production rate from the plasma at some temperature T is proportional to the transverse spectral function $\rho_T(\omega=|\vec k|,\vec k)$. One can calculate the photon production rate also from the difference between $\rho_T(\omega,\vec k)$ (transverse) and $\rho_L(\omega,\vec k)$ (longitudinal) correlator as $\rho_L$...
Photonuclear interactions are studied in ultra-peripheral p-Pb collisions with the ALICE experiment, where the photon radiated by a Pb nucleus probes the gluon density of the proton at low Bjorken-x. The exclusive $\rm{J/\psi}$ photoproduction cross section $\rm{\sigma(\gamma + p \rightarrow J/\psi + p)}$ is expected to follow a power law trend as $x$ decreases, but it should deviate from this...
The classical field approximation to Color Glass Condensate for two colliding nuclei has been solved in the literature using numerical methods and recursive analytic solution. In the weak field limit, analytic solutions in transverse momentum space have also been known for some time. Based on the latter, we derive expressions for the space-time dependence of classical gluon 2-point functions...
We developed a formalism to study momentum anisotropy, in particular, the collective flow $v_2$, in the ultra-relativistic onium-onium scattering. We derived the impact-parameter dependent cross section up to the next-to-leading order in the eikonal approximation. With this formalism, we are able to interpret the origin and behavior of $v_2$ in the dilute limit, by investigating the elementary...
During the last years, there has been an increasing interest in how the quark mass affects the jet quenching phenomena and dynamics of heavy flavor in HIC. Here, we will present a new effect, which consists in anisotropic broadening and gluon radiation sourced by the background flow, transverse to the parton momentum, and sensitive to the quark mass. This effect appears due to a modification...
Correlation functions of energy flow operators have been recently proposed as a tool to identify the onset of colour coherence within the jets. As a promising exploration avenue to unveil the scales of the Quark-Gluon Plasma, it has yet to be demonstrated how the medium back-reaction to the jet propagation will blur such identification. In this work, by using a perturbative prescription to...
Recent measurements by the CMS and ATLAS experiments reveal a deficit of charged particles in pp collisions with excited Υ(nS) states compared to the Υ(1S) ground state. This observation is suggested to be a manifestation of excited bottomonia suppression in pp interactions. The analysis presented in this talk is an independent approach, complementary to the CMS and ATLAS analyses, based on...
This project aims the application of non-extensive statistics, more specifically that proposed by C. Tsallis, in the study of the transverse momentum distribution of mesons composed of charm quarks produced in collisions between heavy ions at relativistic energies. Non-extensive statistics has been very successful in the description of transverse momentum spectra of particles produced in...
Keywords : Heavy-ion Collisions, Event generator, EPOS, PHSD, EPOSi+PHSDe, Quark-Gluon
Plasma
Ultrarelativistic heavy-ion collisions at RHIC and the LHC provide a hot and ultra-dense form of matter composed of deconfined quarks and gluons, named QGP. Different models like EPOS and PHSD allow to study the space-time evolution of such heavy-ion collisions. Their dynamics is complicated;...
The hotspot model has proven to be an efficient tool to study coherent and incoherent diffraction HERA data by modelling the initial state fluctuations of the gluon density of the proton. The hotspot model in its original form is a non-perturbative model applicable for low momentum transfer and underestimates the incoherent cross section in orders of magnitude when extended for large momentum...
In this work, we assess the impact of the expansion of the medium on angular distribution of gluons at different kinematical scales in a medium-induced cascade . Firstly, we study the scaling of the gluon spectra at low$-x$ between expanding and static medium profiles and apply them to obtain the transverse momentum broadened spectra. The numerical solutions for the in-medium cascades are...
In the context of jet-medium interaction, we consider the response of QCD-like plasma to energy/momentum disturbance as a function of the gradient. For both N=4 super-Yang Mills theory in strong coupling limit and kinetic theory under relaxation time approximation, we find that hydrodynamic modes continue dominating medium's response even in the region where Knudsen number is large. However,...
Heavy quarks, like charm quarks, are produced early in the relativistic heavy-ion collisions and probe all stages of the evolution of the created medium – the Quark Gluons Plasma. Two-particle correlations at low relative momentum (the femtoscopic correlations) are sensitive to the interactions in the final state and the extent of the region from which correlated particles are emitted...
Constraining the initial condition of the QGP using experimental observables is one of the most important challenges in our field. Recent studies show that the Pearson Correlation Coefficient (PCC) between $v_n$ and event-wise mean transverse momentum $[p_\mathrm{T}]$, $\rho_n(v_n, [p_\mathrm{T}])$, and $[p_\mathrm{T}]$ fluctuations can probe several ingredients of the initial state. This talk...
Measurements of jet substructure in ultra-relativistic heavy ion collisions suggest that the jet showering process is modified by the interaction with quark gluon plasma. Modifications of the hard substructure of jets can be explored with modern data-driven techniques. In this study, a machine learning approach to the identification of quenched jets is designed. Jet showering processes are...
We have studied the influence of realistic modeling of the medium formed in
Relativistic Heavy-Ion collisions on Jet Quenching phenomena. We used JEWEL to
simulate the medium modified parton shower and coupled it with vUSP-hydro$+\rm
T_RENTo$ models. We have studied the influence of these combination of models on jet observables
such as $R_{AA}$, jet mass, $x_J$ and subjet...
Heavy flavor jets are powerful tools to gain insight into the in-medium partonic energy loss mechanisms and the transport properties of the quark-gluon plasma (QGP) in high-energy nuclear collisions. In this work, we present the first theoretical study of the longitudinal momentum fraction $z_{||}$ carried by $\rm{D^0}$ meson in jets in Pb+Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The...
The ALICE experiment at the LHC investigates the properties of the hot and dense nuclear matter created in heavy-ion collisions. By comparing the particle production in pp and p--Pb collisions, possible nuclear initial state effects can be isolated. Measurements of the $\omega$ meson $p_\text{T}$-spectra in pp and p--Pb collisions not only allow for a determination of the nuclear modification...
Recent results of charmed baryon production in $pp$ collisions showed a significant enhancement of the baryon-to-meson ratio compared with the expectation based on $e^{+}e^{-}$ collisions. This indicates that the charm fragmentation into hadrons is not an universal process among different collision systems, and different mechanisms may play a role in the hadronic collisions with respect to...
Jets can be copiously produced in heavy-ion collisions at the LHC energies. Their calibration is crucial for precise measurements of various processes, such as top-quark pair production. The poster presents the measurement of jet energy scale and resolution in proton-lead collisions collected at 8.16 TeV in 2016. The balance between Z boson and jet transverse momenta is explored for jet...
The precise measurement of the neutral meson production in pp collisions can be used to constrain fragmentation functions and parton density functions needed by pQCD calculations. Additionally, those measurements serve as input for direct photon analyses.
Moreover, the dependence of the neutral meson cross section on the event charged-particle multiplicity could give further insight into...
Measurements of the production of hadrons containing heavy quarks (charm and beauty) allow a study of cold nuclear matter (CNM) effects such as gluon saturation, shadowing and energy loss in p-Pb collisions. Understanding these effects is important for the proper interpretation of results in Pb-Pb collisions. In addition, the measurements provide the possibility to investigate the...
We present a new high-statistics measurement of inclusive $J/\psi$ production versus event multiplicity in $p$+$p$ collisions at $\sqrt{s} = 500$ GeV with the STAR experiment at RHIC. At mid-rapidity, calorimeter-triggered events are selected for candidate $J/\psi$ detection in the dielectron decay channel. Existing measurements at both $\sqrt{s} = 200$ GeV from STAR and $\sqrt{s} = 7$ TeV...
Jet fragmentation allows us to explore the evolution process of the QCD jets. It can be studied using the transverse momentum ($j_\mathrm{T}$) and longitudinal momentum fraction ($z$) of constituent particles. The $j_\mathrm{T}$ distributions of jet fragments have been measured in pp and p—Pb collisions at $\sqrt{s}$, $\sqrt{s_{NN}}$ = 5.02 TeV with ALICE, and various parton-shower models...
The differences in hadron chemistry observed at $e^{+}e^{-}$ machines versus hadron colliders may indicate that the mechanisms by which partons evolve into visible matter are not universal. In particular, the influence of the underlying event in hadron collisions requires further study. With full particle ID, precision vertexing, and a high rate DAQ, the LHCb detector is uniquely well suited...
Measurements of jet properties in small systems provide insights into perturbative and non-perturbative QCD aspects of jet structure and cold nuclear matter effects. Additionally, recent studies of high-multiplicity final states in small collision systems exhibit signatures of collective effects that are conventionally associated with the formation of hot and dense, color-deconfined QCD...
Partonic scatterings with high momentum transfer occur before the formation of the quark-gluon plasma (QGP) in heavy-ion (A+A) collisions and result in collimated collections of hadrons called jets. The modification of the parton shower in the QGP compared to that in proton-proton (p+p) collisions offers insight into the nature of the medium's interaction with colored probes. Typically, this...
Nuclear parton distribution functions (nPDFs) are an essential part in predictions of heavy-ion collisions. nPDFs have been determined via "global QCD analysis" in which nPDF-dependent prediction of a given process compares with its actual measurement. The challenging part of nPDF extraction is the uncertainty estimation. The most common approach for this purpose is Hessian method which has...
Production measurements of heavy quark pairs in pp collisions are a known tool to test perturbative quantum chromodynamics calculations. In addition, they provide a reference for the corresponding studies in nuclear collisions. Indeed, in Pb-Pb collisions, the heavy quarks are produced at the early stages of the collision and can then experience full medium evolution. Open heavy flavor hadrons...
We present unquenched correlator data and corresponding reconstructed spectral functions for quarkonium in both pseudoscalar and vector channels. Correlators are obtained using clover-improved Wilson fermions on $N_f=2+1$ HISQ lattices. To be on the constant line of physics bare quark masses are tuned to reproduce the mass spectrum of quarkonium by comparing the mass spectrum obtained from the...
We have studied the shear ($\eta$) and bulk ($\zeta$) viscosities of hot QCD medium in a weak magnetic field. These viscosities are calculated by using the relaxation time approximation of kinetic theory in weak magnetic field limit, where temperature scale dominates over the energy scale related to magnetic field. It is found that the weak magnetic field reduces both $\eta$ and $\zeta$,...
Direct computations of QCD real-time observables like transport coefficients are very difficult due to the infamous sign problem. The complex Langevin (CL) method is a promising approach to overcome it by using a real-time formulation of QCD on a complex time contour. Studying SU($N_c$) gauge theories with CL, we find that current stabilization techniques are insufficient to obtain correct...
Measurements of the jet substructure in Pb+Pb collisions provide information on the jet quenching in the quark-gluon plasma (QGP) created in these collisions, over a wide range of energy scales. This poster presents ATLAS measurement of the suppression of yields of large-radius jets and its dependence on the jet substructure, characterized by the presence of sub-jets. This measurement is...
Measurements of the lightest open-charm baryon, $\Lambda_{\rm{c}}^{+}$, production can provide important information about the quark coalescence process of hadronization in the quark-gluon plasma (QGP). With strange quark yields being enhanced in the presence of the QGP medium, the production of $\rm{D}^{+}_{\rm{s}}$ is expected to be enhanced in heavy ion collisions if recombination plays an...
The production of quarkonia in high-energy heavy-ion collisions has been studied extensively to understand their production mechanisms and properties of Quark-Gluon Plasma (QGP). PHENIX has the capability to measure $J/\psi$ with muon arms at forward and backward rapidity ($1.2<|y|<2.2$) and charged particle multiplicity with silicon trackers at various acceptance ranges. The recent PHENIX...
Groomed jet substructure measurements, the momentum splitting fraction $z_g$ and the groomed jet radius $R_g$, for inclusive, D$^0$-tagged and B$^0$-tagged jets in $pp$ and central Pb+Pb collisions at $\sqrt{s}=5.02$ TeV are investigated. Theoretical results for light-quark initiated and gluon initiated jets are provided as references though experimentally indistinguishable for now. Charged...
We study the suppression of leading two hadrons within jets and the modifications of their flavor correlations in heavy ion collisions. The di-hadron system is robust against the underlying event background therefore allows its precision measurements. Their suppression is sensitive to any partonic energy loss mechanism and can be used to cleanly test the onset of jet quenching in the...
In early stage of high energy nuclear collisions, the Lorentz contracted colliding nuclei nuclei fly near the light cone, with large-x partons acting as static sources of small-x modes that constitute the Color-Glass Condensate (CGC) fields. By interacting CGC fields, the chromoelectric and chromomagnetic fields are formed, they are the glasma fields. The glasma fields serve as initial...
We present a full set of the Boltzmann Equation in Diffusion Approximation (BEDA) for studying thermal equilibration of quarks and gluons. Using BEDA, we first analyse thermalization and quark production of spatially homogeneous systems initially made of pure gluons. We observe that soft partons, dominantly produced via medium-induced radiation, rapidly fill a thermal distribution with an...
Dielectrons are an exceptional tool to study the evolution of the medium created in heavy-ion collisions. In central collisions, the energy densities are sufficient to create a quark-gluon plasma (QGP). Thermal e$^{+}$e$^{-}$ pairs with invariant mass ($m_{ee}$) around 1.5 GeV/$c^{2}$ can be used to estimate the temperature of the QGP.
At LHC energies, the cross section of heavy-flavour (HF)...
We consider some corrections relevant to energy loss in small collision systems, for which the temperature times the size of the system isn't large, $T\times L\sim1$.
First, we present the derivation of the explicit small path length correction to the DGLV opacity expansion. We then show first results from an energy loss model including these corrections, demonstrating the additional...
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...
In this contribution, new results for beauty measurements with ALICE are presented. The production of beauty hadrons can be accessed with measurements of leptons from beauty- and charm-hadron decays as well as the reconstruction of non-prompt charmed hadrons.
We show the nuclear modification factor of electrons from beauty hadron decays in Pb-Pb collisions at $\sqrt{s_{\rm \scriptscriptstyle...
Photonuclear reaction is induced by the strong electromagnetic field generated by ultrarelativistic heavy ion collisions. This process has been extensively studied in ultra-peripheral collisions (UPC). Photoproduced quarkonia are used to probe the nuclear gluon distributions at low Bjorken-$x$. In recent years, the coherent photoproduction of the J/$\psi$ vector meson has also been observed in...
While perturbative QCD is sufficient for understanding parton energy loss at large transverse momentum ($p_\mathrm{T}$) in heavy-ion collisions, a simultaneous description of the heavy flavor nuclear modification factor ($R_\mathrm{AA}$) and elliptic flow coefficient ($v_2$) at low to intermediate $p_\mathrm{T}$ still remains challenging due to non-perturbative interactions between heavy...
We present a unique signal of jet-induced medium excitations: the enhancement of baryon-to-meson ratios around the quenched jets. To illustrate this, we study jet-particle correlations and the distributions of jet-induced identified particles with respect to the jet direction in Pb+Pb collisions at the LHC via a multi-phase transport model. We find a strong enhancement of baryon-to-meson...
We discuss the evolution of initial momentum anisotropy in the early-stage quark-gluon plasma. We use kinetic theory to study the far-from-equilibrium evolution of an expanding plasma with an anisotropic momentum-space distribution. We identify slow and fast degrees of freedom in the far-from-equilibrium plasma from the evolution of moments of this distribution. At late times, the slow modes...
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...
We present a novel approach to nonperturbatively estimate the heavy quark momentum diffusion coefficient, which is a key input for the theoretical description of heavy quarkonium production in heavy ion collisions, and is important for the understanding of the elliptic flow and nuclear suppression factor of heavy flavor hadrons. In the heavy quark limit, this coefficient is encoded in the...
Small systems display large anisotropic flow coefficients that can potentially be interpreted as a hydrodynamic signal. At these moderate multiplicities anisotropic flow is however relatively sensitive to subtle effects. These include the precise experimental procedure, rapidity coverage and gaps as well as effects due to resonance decays. In this talk we quantify these effects for pPb, OO and...
The proper treatment of hadronic resonances plays an important role for many aspects of heavy ion collisions. We expect this to be the case also for hadronization, due to the large degeneracies of excited states, and the abundant production of hadrons from their decays. We show how a comprehensive treatment of excited meson states can be incorporated into quark recombination, and in extension,...
The understanding of the charm-quark hadronisation in high-energy hadronic collisions has recently improved. The production of D mesons relative to each other is compatible with those measured in $\mathrm{e^{+}e^{-}}$ collisions. The charm baryon-to-meson yield ratios show an enhancement from those measured at lepton colliders. This modification depends on the charged-particle multiplicity....
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$\rightarrow$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 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),...
High-multiplicity measurements in pp and p-Pb collisions have revealed the presence of phenomena typically attributed to the creation of a quark-gluon plasma (QGP). Events with multiple parton-parton interactions (MPIs) have been proposed as one possible explanation of this observation. MPIs play a significant role in describing the soft component of the hadronic interactions, and at LHC...
Viscous hydrodynamics serves as a successful mesoscopic description of the Quark-Gluon Plasma (QGP) on large time and distance scales. Since highly energetic Jets deposit part of their energy into the QGP in a very localized fashion, it is important to understand to what extent the propagation of the deposited energy can be described within hydrodynamics. We investigate this problem by...
We investigate the early time dynamics of heavy ion collisions studying the time evolution of the energy-momentum tensor as well as energy-momentum correlations within a uniformly thermalizing holographic QGP. From these quantities, we suggest a far-from equilibrium definition of shear viscosity, which is a crucial property of QCD matter as it significantly determines the generation of...
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...
Quarkonia production in hadronic collisions is an important experimental observable that sheds light on the heavy quark interaction with the nuclear medium. While the bound quarkonium states undergo dissociation and recombination in PbPb collisions, in $p$Pb collisions they can suffer from a combination of initial and final state effects such as shadowing and comover breakup. The exotic...
We demonstrate the unique opportunities for small systems studies offered by complementing the future run of $^{16}$O+$^{16}$O collisions at the CERN LHC with collisions of bowling-pin-shaped $^{20}$Ne isotopes.
1. Origin of collectivity: A comprehensive campaign of hydrodynamic calculations (~20 million simulated events) demonstrates that the impact of the extreme shape of neon on...
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...
We present a new 3+1D resolved model for the initial state of ultrarelativistic Heavy-Ion collisions, based on the $k_\perp$-factorized Color Glass Condensate hybrid approach [1-4]. This new model responds to the need for a rapidity-resolved initial-state Monte Carlo event generator which can deposit the relevant conserved charges (energy, charge and baryon densities) both in the midrapidity...
The interaction of heavy quarks with the quark-gluon plasma (QGP) affects their azimuthal distribution and $p_{\rm{T}}$ spectrum, hence measurement of azimuthal anisotropy coefficients ($v_n$) and nuclear modification factors ($R_{\rm{AA}}$) of heavy flavor hadrons turns out to be an important probe of the QGP. However, simultaneous modeling of $v_n$ and $R_{\rm{AA}}$ is still challenging....
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...
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...
We compute the heavy quark momentum diffusion coefficient using effective kinetic theory for a system going through bottom-up isotropization until approximate hydrodynamization. We find that when matching the nonthermal diffusion coefficient to the thermal one for the same energy density, the observed deviations throughout the whole evolution are withing 30% from the thermal value. When...
Heavy-ion collisions at $\sqrt{s_{\rm NN}} \sim 10$ GeV probes the QCD phase diagram at large baryon densities. However, because the longitudinal Lorentz contraction is small at these collision energies, understanding the dynamics during the early phase of the collision is essential for the subsequent modeling of the system evolution, and for constraining the QGP transport properties at finite...
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 $\rm{e}^{+}\rm{e}^{-}$ and $\rm{ep}$ collisions. However, recent measurements...
One of the most prominent features of the quark gluon plasma is its near-perfect fluid behavior. An important outstanding question is establishing the degree to which heavy flavor particles flow with the bulk system. Measurements of the Fourier coefficient v2 of light and heavy flavor hadrons and quarkonia can provide insight into the properties of the medium. At low transverse momentum (pT)...
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...
We present predictions for the suppression of D and B-mesons at $\sqrt{s}=5.5$ TeV at the LHC in Pb+Pb collisions. We assume that the QGP is strongly coupled, and show the centrality and momentum dependence of the nuclear modification factor at midrapidity. We quantify the systematic theoretical uncertainties associated with the mapping of parameters in $N=4$ SYM theory to QCD, as well as the...
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...
Shower development dynamics for a jet traveling through the quark-gluon plasma (QGP) is a multi-scale process, where the heavy flavor mass is an important scale. During the high virtuality portion of the jet evolution in the QGP, emission of gluons from a heavy flavor is modified owing to heavy quark mass. In-medium stimulated radiation of heavy flavor is sensitive to microscopic processes...
Since 2011 a wide variety of measurements suggest the existence of strong collectivity in collisions of small systems such as proton-proton (pp) and proton-nucleus (pPb) with hydrodynamic models and gluon saturation in the initial state as two theory alternatives showing consistency with the observations. These results raise the question as to whether such phenomena may be present in even...
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...
Analyzing data from nuclear lepton Deep-Inelastic Scattering, Drell- Yan processes, and W and Z boson production, we show that factorizing nuclear structure into quasi-free nucleons and universally modified close-proximity Short Range Correlated (SRC) nucleon pairs allows us to fully describe the quark-gluon structure of nuclei down to very-low momentum fractions. This is the first combined...
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...
Polarization measurements represent an important tool for the understanding of particle production mechanisms occurring in proton–proton collisions. When considering heavy-ion collisions, quarkonium polarization could also be used to investigate the characteristics of the hot and dense medium, the quark-gluon plasma (QGP) created at LHC energies. It has been hypothesized that quarkonium states...
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...
We show for the first time that heavy flavor quenching and flow can be utilized to probe the equation of state (EoS) of quark-gluon plasma (QGP) produced in relativistic heavy-ion collisions. Based on our quasi-particle linear Boltzmann transport (QLBT) model that is coupled to a (3+1)-dimensional viscous hydrodynamic simulation of the QGP and a hybrid fragmentation-coalescence approach for...
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 recent results on the in-medium spectral function of the rho(770) vector meson and the a1(1260) axial-vector meson in nuclear matter, as well as on the resulting thermal dilepton rate. As an effective description of the thermodynamics and the phase structure of nuclear matter we use a chiral baryon-meson model, taking into account the effects of fluctuations from scalar mesons,...
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...
J/$\psi$ is an important probe to study the properties of the quark-gluon plasma (QGP) created in heavy-ion collisions. Measurements of J/$\psi$ yield suppression in Au+Au collisions at $\sqrt{s_\mathrm{NN}}$ = 200 GeV suggest that J/$\psi$ production in heavy-ion collisions is affected by the interplay of several effects, such as dissociation, regeneration in the QGP, and the cold nuclear...
Electromagnetic probes ($\gamma, \gamma^{\ast}$) offer a unique opportunity to study the conditions during heavy-ion collisions. They are produced throughout the whole evolution of the colliding system and can penetrate the strongly interacting medium to bring direct information from their origins to the detector. In this manner, it is possible to not only probe freeze-out, but also earlier...
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...