Measurement of dijet pT correlations in Pb+Pb and pp collisions at √sNN = 2.76 TeV with the ATLAS detector

Measurements of dijets in both Pb+Pb and pp collisions at a nucleon--nucleon centre-of-mass energy of √sNN=2.76 TeV are presented. The measurements were performed with the ATLAS detector at the LHC using data samples with integrated luminosities of 0.14 nb−1 and 4.0 pb−1 for the Pb+Pb and pp data samples, respectively. Jets were reconstructed using the anti-ktkt algorithm with R=0.4. A background subtraction procedure was applied to correct the jet kinematics for the large underlying event present in Pb+Pb collisions. Measurements are reported of the normalized yields 1NdNdxJ, where xJ=pT2/pT1 and pT1 and pT2 are the leading and subleading jet transverse momenta, respectively. The results are presented as a function of pT1 and collision centrality. The results were obtained by measuring the two-dimensional pT1−pT2 distributions and applying an unfolding procedure to account for experimental resolution in the measurement of both jets' transverse momenta simultaneously. The distributions are found to be similar in peripheral Pb+Pb collisions and pp collisions, but highly modified in central Pb+Pb collisions. The results are consistent with expectations from partonic energy loss

Recent results on hard processes in p+Pb, Pb+Pb, and pp collisions from the ATLAS Experiment at the LHC

Measurements of recent results on hard probes in p+Pb, Pb+Pb, and pp collision systems are presented. This includes measurements of electro-weak bosons, jets, and quarkonia. Electro-weak boson results serve as a reference for other measurements, while jets and quarkonia investigate properties of the hot dense medium produced in heavy ion collisions

Recent results on hard processes in p+Pb, Pb+Pb, and pp collisions from the ATLAS Experiment at the LHC

In relativistic heavy-ion collisions, a hot medium with a high density of unscreened colour charges is produced. Hard processes that are produced by parton-parton scatterings in the early stages of the collision can be used as probes of the medium since the initial state is mostly understood such that any differences from pp collisions in the final state can be attributed to interactions with the medium. The initial and final state effects are separated by comparing p+Pb and pp collisions. Recent ATLAS results on electro-weak bosons, jets, and quarkonia are discussed in both p+Pb and Pb+Pb collisions, compared to pp. Recent results on ultra–peripheral collisions by ATLAS are also shown

Measurement of the primary Lund jet plane density in pp collisions at s=13\sqrt{s} = \rm{13} TeV with ALICE

Precision measurements of jet substructure are used as a probe of fundamental QCD processes. The primary Lund jet plane density is a two-dimensional visual representation of the radiation off the primary emitter within the jet that can be used to isolate different regions of the QCD phase space. A new measurement of the primary Lund plane density for inclusive charged-particle jets in the transverse momentum range of 20 and 120 GeV/$c$ in pp collisions at $\sqrt{s} =$ 13 TeV with the ALICE detector will be presented. This is the first measurement of the Lund plane density in an intermediate jet $p_{\rm T}$ range where hadronization and underlying event effects play a dominant role. The projections of the Lund plane density onto the splitting scale $k_{\rm T}$ and splitting angle $\Delta{R}$ axis are shown, highlighting the perturbative/non-perturbative and wide/narrow angle regions of the splitting phase space. Through a 3D unfolding procedure, the Lund plane density is corrected for detector effects which allows for quantitative comparisons to MC generators to provide insight into how well generators describe different features of the parton shower and hadronization.Precision measurements of jet substructure are used as a probe of fundamental QCD processes. The primary Lund jet plane density is a two-dimensional visual representation of the radiation off the primary emitter within the jet that can be used to isolate different regions of the QCD phase space. A new measurement of the primary Lund plane density for inclusive charged-particle jets in the transverse momentum range of 20 and 120 GeV/$c$ in pp collisions at $\sqrt{s} =$ 13 TeV with the ALICE detector will be presented. This is the first measurement of the Lund plane density in an intermediate jet $p_{\rm T}$ range where hadronization and underlying event effects play a dominant role. The projections of the Lund plane density onto the splitting scale $k_{\rm T}$ and splitting angle $\Delta{R}$ axis are shown, highlighting the perturbative/non-perturbative and wide/narrow angle regions of the splitting phase space. Through a 3D unfolding procedure, the Lund plane density is corrected for detector effects which allows for quantitative comparisons to MC generators to provide insight into how well generators describe different features of the parton shower and hadronization

Recent results from the ATLAS heavy ion program

The heavy-ion program in the ATLAS experiment at the LHC originated as an extensive program to probe and characterize the hot, dense matter created in relativistic lead-lead collisions. In recent years, the program has also broadened to a detailed study of collective behavior in smaller systems. In particular, the techniques used to study larger systems are also applied to proton-proton and proton-lead collisions over a wide range of particle multiplicities, to try and understand the early-time dynamics which lead to similar flow-like features in all of the systems. Another recent development is a program studying ultra-peripheral collisions, which provide gamma-gamma and photonuclear processes over a wide range of CM energy, to probe the nuclear wavefunction. This talk presents a subset of the the most recent results from the ATLAS experiment based on Run 1 and Run 2 data, including measurements of collectivity over a wide range of collision systems, potential nPDF modifications — using electroweak bosons, inclusive jets, and quarkonia — and photonuclear dijet production

Dijet asymmetries in Pb+Pb and pp collisions with the ATLAS detector

The phenomenon of events containing highly asymmetric dijet pairs is one of the most striking results in heavy ion physics. It has provided the first direct observation of in-medium jet energy loss at the LHC. Detailed measurements of centrality-dependent dijet imbalance in $\sqrt{s_{NN}}=$2.76 TeV Pb+Pb collisions using data collected in the 2011 LHC heavy ion run are presented. The new analysis fully corrects to the particle level. The results show a centrality-dependent modification of the dijet asymmetry distribution accompanied by an unmodified angular correlation between two jets in the dijet system. Detailed studies of the dijet asymmetry as a function of the leading jet transverse momentum and jet radius are presented. The reference measurement of the dijet asymmetry in the $\it{pp}$ collisions at the same center of mass energy is also shown

Jet measurements in heavy-ion collisions with the ATLAS detector

In relativistic heavy-ion collisions, a hot medium with a high density of unscreened colour charges is produced. Jets are produced by parton-parton scatterings in the early stages of the collision, and are observed to be attenuated as they propagate through the hot matter. One manifestation of this energy loss is a lower yield of jets emerging from the medium than expected in the absence of medium effects. Another manifestation of energy loss is the modification of both dijet transverse energy balance, and a similar modification of photon-jet correlations. Finally, the internal structure of jets is also observed to be modified, from a careful study of fragmentation functions. In this talk, the latest ATLAS results on single jet suppression, dijet suppression, photon-jet correlations, and modification of the jet internal structure in both p+Pb and Pb+Pb collisions, compared to pp, will be presented