Multi-particle correlation techniques are frequently used to study jet shapes and yields in hadronic and nuclear collisions. To date, a standard assumption applied in such analyses is that the observed correlations arise from either jets and associated hard scattering phenomena, or from a background component due to combinatorial pairs connected only through whole even correlations. Within this assumption of two essentially independent sources, a fundamental problem centers around determining the relative contributions of each component. We discuss the methods commonly used to establish the background yield in jet correlation analyses, with a full explanation of the absolute background normalization technique which establishes the background yield without assumptions about the shape of jet correlations. This is especially important in relativistic heavy ion collisions where the jet shapes are significantly distorted from the well separated back-to-back di-jets observed in proton-proton collisions.
Recent theoretical studies have indicated that the topological features of away-side jet fragments can be significantly altered by medium-induced modifications. The leading candidates resulting from such modifications are Mach Cones and deflected jets. We show that three particle correlations are able to distinguish between these different modification scenarios. Initial results from an application of the method to Au+Au collisions at RHIC ($\sqrt{s_{NN}}=200$ GeV) are presented.
Using theoretical tools developed by the JET Collaboration in which one employes 2+1D or 3+1D hydrodynamic models for the bulk medium evolution and jet quenching models, the combined data on suppression of single inclusive hadron spectra at both RHIC and LHC are systematically analyzed with five different approaches to the parton energy loss. The jet transport parameter is extracted from the best fits to the data with values of $\hat q \approx 1.2\pm 0.3$ and $1.9\pm 0.7$ GeV$^2$/fm in the center of the most central Au+Au collisions at $\sqrt{s}=200$ GeV and Pb+Pb collisions at $\sqrt{s}=2.67$…
Dihadron azimuthal correlations in Au+Au collisions at $\sqrt{S_{NN}}$=200 GeV have been explored by using a multi-phase transport (AMPT) model. In order to obtain the contributions from jet-medium interactions, the combined harmonic flow background is subtracted from the raw dihadron correlation functions. The signals are compared in three associated transverse momentum ($p_{T}^{assoc}$) bins: 0.2-0.8 GeV/c, 0.8-1.4 GeV/c and 1.4-2.0 GeV/c from central to semi-peripheral geometries. The medium modifications are observed from changes in the signal shape and the relative jet contribution has be…
Based on the factorization in perturbative QCD, a jet cross sections in heavy-ion collisions can be expressed as a convolution of the jet cross section in $p+p$ collisions and a jet energy loss distribution. Using this simple expression and the Markov Chain Monte Carlo method, we carry out Bayesian analyses of experimental data on jet spectra to extract energy loss distributions for both single inclusive and $γ$-triggered jets in $Pb+Pb$ collisions with different centralities at two colliding energies at the Large Hadron Collider. The average jet energy loss has a dependence on the initial jet…
Di-hadron and jet-hadron correlations are commonly used in relativistic heavy ion collisions to study the soft component of jets in a quark gluon plasma. There is a large correlated background which is described by the Fourier decomposition of the azimuthal anisotropy where $v_n$ is the $n$th order coefficient. The path length dependence of partonic energy loss can be studied by varying the angle of the high momentum trigger particle or jet relative to a reconstructed event plane. This modifies the shape of the background correlated with that event plane. The original derivation of the shape o…
A modification of the internal structure of jets is expected due to the production of a dense QCD medium, the Quark Gluon Plasma, in heavy-ion collisions. We discuss some aspects of jet reconstruction in p+p and A+A collisions and emphasize the dramatically increased contribution of the underlying event in nucleus-nucleus collisions as compared with the vacuum case. We conclude with its consequences on the full jet spectrum and fragmentation function extraction at LHC.
Within the framework of the generalized QCD factorization formalism, we perform the first global analysis of the jet transport coefficient ($\hat q$) for cold nuclear matter. The analysis takes into account the world data on transverse momentum broadening in semi-inclusive electron-nucleus deep inelastic scattering, Drell-Yan dilepton and heavy quarkonium production in proton-nucleus collisions, as well as the nuclear modification of the structure functions in deep inelastic scattering, comprising a total of 215 data points from 8 data sets. For the first time, we clarify quantitatively the un…
The STAR Collaboration at RHIC presents a systematic study of high transverse momentum charged di-hadron correlations at small azimuthal pair separation \dphino, in d+Au and central Au+Au collisions at $\rts = 200$ GeV. Significant correlated yield for pairs with large longitudinal separation \deta is observed in central Au+Au, in contrast to d+Au collisions. The associated yield distribution in \detano$\times$\dphi can be decomposed into a narrow jet-like peak at small angular separation which has a similar shape to that found in d+Au collisions, and a component which is narrow in \dphi and \…
A simple model to investigate jet quenching and correlated errors for centrality-dependent nuclear-modification factors in relativistic heavy-ion collisions
The PHENIX collaboration has developed novel methodologies for reliable extraction of jet functions from two and three particle azimuthal correlation functions measured at mid-rapidity in Au+Au collisions at $\sqrt s=200$ GeV. The extracted jet shape and the yield of jet-associated partner hadrons (per trigger hadron) are found to vary with particle type and collision centrality, indicating a significant effect of the nuclear collision medium on the (di)jet fragmentation process.
These proceedings review recent results from two and three particle correlations in heavy ion collisions. In particular we discuss the modified structure of the away side jet correlations. Under the assumption that the away side can be decomposed into a punch through component at $Δϕ=π$ and a shoulder component with a peak displaced from $π$ many similar properties are observed between the ridge and shoulder. The particle ratios, yields and $p_T$ spectra are in near agreement. We also highlight important future measurements, including investigating if the decomposition of the away side jet cor…
Jets correlated with isolated photons are a promising channel to study jet quenching in heavy-ion collisions, as photons do not interact strongly and therefore constrain the $Q^2$ of the initial hard scattering. We present the isolated photon-jet correlations measured in Pb--Pb collisions at $\sqrt{s_{\text{NN}}}$ = 5.02 TeV by the ALICE collaboration. We study correlations of isolated photons above 20 GeV/$c$ with charged-particle jets above 10 GeV/$c$, reconstructed with the anti-$k_\text{T}$ algorithm. The correlations probe the lowest jet $p_\text{T}$ range ever measured at LHC energies, a…
Systematic measurements of pseudorapidity ($Δη$) and azimuthal ($Δϕ$) correlations between high-$p_t$ charged hadrons in $\sqrt{s_{NN}}$=200 GeV Au+Au collisions are presented. An enhancement of correlated yield at large $Δη$ on the near-side is observed. This effect persists up to trigger $p_{t}^{trig}$ $\sim$ 9 GeV/c, indicating that it is associated with jet production. More detailed analysis suggests distinct short-range and long-range components in the correlation.
The large statistics accumulated during the LHC Run-1 and Run-2 have provided the unique opportunity to study the properties of the Quark-Gluon Plasma using rare hard probes. The ALICE Collaboration has presented new results on the jet shape observables in p--Pb and Pb--Pb collision. Using data from Run-2, the elliptic flow of open and hidden charm mesons have been measured with much better precision, confirming that charm quarks participate in the collective dynamic expansion with the medium. In p--Pb collisions the absence of jet quenching correlated with high event activity has been confirm…
Early measurements at the Relativistic Heavy Ion Collider (RHIC) demonstrated jet quenching through the suppression of pairs of high momentum hadrons. These dihadron correlations have a large correlated background. As understanding of the background improved, it was recognized in the field that a significant term was omitted from the background and several dihadron correlation results were quantitatively and qualitatively incorrect. The original measurements demonstrating jet quenching have not been revisited. These measurements are repeated in this paper in a kinematic range similar to the or…
We summarize the latest achievements about the extraction of the transversity parton distribution and proton tensor charge based on an analysis of pion-pair production in deep-inelastic scattering off transversely polarized targets. Recently released data for proton and deuteron targets by HERMES and COMPASS allow for a flavor separation of the valence components of transversity. At variance with the Collins effect, this extraction is performed in the framework of collinear factorization and relies on di-hadron fragmentation functions. The latter have been taken from the first recent analysis …
This Letter presents measurements of correlated production of nearby jets in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV using the ATLAS detector at the Large Hadron Collider. The measurement was performed using 0.14 ${\rm nb}^{-1}$ of data recorded in 2011. The production of correlated jet pairs was quantified using the rate, $R_{ΔR}$, of "neighbouring" jets that accompany "test" jets within a given range of angular distance, $ΔR$, in the pseudorapidity--azimuthal angle plane. The jets were measured in the ATLAS calorimeter and were reconstructed using the anti-$k_{t}$ algorithm w…
For a quantitative interpretation of reconstructed jet properties in heavy-ion collisions it is paramount to characterize the contribution from the underlying event and the influence of background fluctuations on the jet signal. In addition to the pure number fluctuations, region-to-region correlated background within one event can enhance or deplete locally the level of background and modify the jet energy. We show a first detailed assessment of background effects using different probes embedded into heavy-ion data and quantify their influence on the reconstructed jet spectrum.
Two-particle jet-like azimuthal correlations have revealed intriguing modifications to the away-side of high pt trigger particles in relativistic heavy-ion collisions. Three-particle jet-like azimuthal correlation and three-particle azimuthal cumulant have been analyzed in experiments in attempt to distinguish conical emission of jet-correlated particles from other physics mechanisms. We investigate the difference between three-particle jet-like correlation and three-particle cumulant in azimuth. We show, under the circumstance where the away-side two-particle correlation is relatively flat in…
We review a recently proposed phenomenological framework to establish the notions of QCD factorization and universality of jet cross sections in the heavy-ion environment. First results of a global analysis of the nuclear modification factor of inclusive jets are presented where we extract medium modified jet functions using a Monte Carlo sampling approach. We observe that gluon jets are significantly more suppressed than quark jets. In addition, we study the jet radius dependence of the inclusive jet cross section in heavy-ion collisions and comment on a recent measurement from CMS. By consid…
We propose a new phenomenological approach to establish QCD factorization of jet cross sections in the heavy-ion environment. Starting from a factorization formalism in proton-proton collisions, we introduce medium modified jet functions to capture the leading interaction of jets with the hot and dense QCD medium. A global analysis using a Monte Carlo sampling approach is performed in order to reliably determine the new jet functions from the nuclear modification factor of inclusive jets at the LHC. We find that gluon jets are significantly more suppressed due to the presence of the medium tha…
We study the medium modification of jets correlated with large transverse momentum photons at the LHC via a transport and perturbative QCD hybrid model which incorporates the contributions from both elastic collisions and radiative energy loss experienced by the parton showers. Calculations are performed for the modification of the photon-tagged jet yield, the photon-jet energy imbalance, and the azimuthal distribution of away-side jets. The modifications of photon-tagged jets with different $x_T = p_{T, J}/p_{T, γ}$ values are studied and they exhibit different centrality and jet cone size de…
We consider the one-parameter family of jet substructure observables known as angularities using the specific case of inclusive jets arising from photoproduction events at an Electron-Ion Collider (EIC). We perform numerical calculations at next-to-leading logarithmic accuracy within perturbative QCD and compare our results to PYTHIA 6 predictions. Overall, we find good agreement and conclude that jet substructure observables are feasible at the EIC despite the relatively low jet transverse momentum and particle multiplicities. We investigate the size of subleading power corrections relevant a…
