Azimuthal Angular Correlations Research Articles

Charm hadron azimuthal angular correlations in Au + Au collisions at $$\sqrt{s_{\mathrm{NN}}} = 200$$ GeV from parton scatterings

The dynamic evolution of the charm hadron in hot quark matter was studied in the framework of a multiphase transport (AMPT) model. We first reproduced the open charm hadron $$D_0 p_{\mathrm{T}}$$ spectrum in Au + Au collisions at $$\sqrt{s_{\mathrm{NN}}} = 200$$ GeV by triggering the $${\mathrm{c}} {\bar{\mathrm{c}}}$$ production in AMPT, and then the elliptic flow of charm hadrons was described with different parton cascade cross sections. Charm hadron azimuthal angular correlations were proposed, and they are affected by the different parton cross-section parameter applied in the model, which can facilitate our understanding of the loss of collision energy of charm quarks in hot quark medium and can stimulate further experimental studies.

Dijet azimuthal correlations and conditional yields in pp and p+Pb collisions at sNN=5.02TeV with the ATLAS detector

This paper presents a measurement of forward-forward and forward-central dijet azimuthal angular correlations and conditional yields in proton-proton ($pp$) and proton-lead ($p$+Pb) collisions as a probe of the nuclear gluon density in regions where the fraction of the average momentum per nucleon carried by the parton entering the hard scattering is low. In these regions, gluon saturation can modify the rapidly increasing parton distribution function of the gluon. The analysis utilizes 25 pb$^{-1}$ of $pp$ data and 360 $\mu \mathrm{b}^{-1}$ of $p$+Pb data, both at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, collected in 2015 and 2016, respectively, with the ATLAS detector at the LHC. The measurement is performed in the center-of-mass frame of the nucleon-nucleon system in the rapidity range between $-$4.0 and 4.0 using the two highest transverse momentum jets in each event, with the highest transverse momentum jet restricted to the forward rapidity range. No significant broadening of azimuthal angular correlations is observed for forward-forward or forward-central dijets in $p$+Pb compared to $pp$ collisions. For forward-forward jet pairs in the proton-going direction, the ratio of conditional yields in $p$+Pb collisions to those in $pp$ collisions is suppressed by approximately 20%, with no significant dependence on the transverse momentum of the dijet system. No modification of conditional yields is observed for forward-central dijets.

Lepton-Jet Correlations in Deep Inelastic Scattering at the Electron-Ion Collider.

We propose the lepton-jet correlation in deep inelastic scattering as a unique tool for the tomography of nucleons and nuclei at the electron-ion collider (EIC). The azimuthal angular correlation between the final state lepton and jet depends on the transverse momentum dependent quark distributions. We take the example of single transverse spin asymmetries to show the sensitivity to the quark Sivers function. When the correlation is studied in lepton-nucleus collisions, transverse momentum broadening effects can be used to explore cold nuclear matter effects. These features make lepton-jet correlations an important new hard probe at the EIC.

Elliptic Flow of Heavy Quarkonia in pA Collisions.

Using the dilute-dense factorization in the color glass condensate framework, we investigate the azimuthal angular correlation between a heavy quarkonium and a charged light hadron in proton-nucleus collisions. We extract the second harmonic v_{2}, commonly known as the elliptic flow, with the light hadron as the reference. This particular azimuthal angular correlation between a heavy meson and a light hadron was first measured at the LHC recently. The experimental results indicate that the elliptic flows for heavy flavor mesons (J/ψ and D^{0}) are almost as large as those for light hadrons. Our calculation demonstrates that this result can be naturally interpreted as an initial state effect due to the interaction between the incoming partons from the proton and the dense gluons inside the target nucleus. Since the heavy quarkonium v_{2} exhibits a weak mass dependence according to our calculation, we predict that the heavy quarkonium ϒ should have a similar elliptic flow as compared to that of the J/ψ, which can be tested in future measurements.

Multiparticle azimuthal angular correlations in pA collisions

In the Color Glass Condensate formalism, we evaluate the 3-dipole correlator up to the $\frac{1}{N_c^4}$ order with $N_c$ being the number of colors, and compute the azimuthal cumulant $c_{123}$ for 3-particle productions. In addition, we discuss the patterns appearing in the $n$-dipole formula in terms of $\frac{1}{N_c}$ expansions. This allows us to conjecture the $N_c$ scaling of $c_n\{m\}$, which is crosschecked by our calculation of $c_2\{4\}$ in the dilute limit.

Pseudorapidity and transverse momentum dependence of flow harmonics in pPb and PbPb collisions

Measurements of azimuthal angular correlations are presented for high-multiplicity pPb collisions at $\sqrt{s_\mathrm{NN}}=$ 5.02 TeV and peripheral PbPb collisions at $\sqrt{s_\mathrm{NN}}=$ 2.76 TeV. The data used in this work were collected with the CMS detector at the CERN LHC. Fourier coefficients as functions of transverse momentum and pseudorapidity are studied using the scalar product method, 4-, 6-, and 8-particle cumulants, and the Lee-Yang zeros technique. The influence of event plane decorrelation is evaluated using the scalar product method and found to account for most of the observed pseudorapidity dependence.

Study of isolated-photon and jet momentum imbalance in pp and PbPb collisions

In this paper, we study the production of isolated-photon plus a jet in pp and PbPb collisions, which can be used as an important probe to the jet transport property in quark gluon plasma created in heavy ion collisions. Normally, there are two types of observables associated with the production of isolated-photon plus a jet, namely, the azimuthal angular correlation and the transverse momentum imbalance. To understand both observables in the full kinematical region, we need to employ the perturbative QCD calculation, which takes into account the hard splitting of partons, together with the Sudakov resummation formalism, which resums soft gluon splittings. Furthermore, by introducing energy-loss into the system, we calculate the enhancement of the momentum imbalance distribution for AA as compared to pp collisions and make predictions for future unfolded experimental data. In addition, in order to extract the jet transport coefficient more precisely in our numerical calculation, we also distinguish quark jets from gluon jets, since they interact with quark gluon plasma with different strengths. This work provides a reliable theoretical tool for the calculation of the gamma-jet correlation, which can lead us to a more precise extraction of the jet transport coefficient in relativistic heavy-ion collisions.

Azimuthal Angular correlations in high energy processes in QCD

Angular (azimuthal or polar) correlations between produced particles in high energy hadronic/nuclear collisions carry a wealth of information on the dynamics of the strong interactions in the novel regime where parton densities of the target hadron or nucleus are high. As such they probe a relatively unexplored region of QCD kinematic phase space. Here we consider azimuthal angular correlations between three jets produced in Deep Inelastic Scattering in the kinematics region corresponding to low xBjorken of the target and show that high gluon density effects in the target lead to a disappearance of the away-side peaks.

3 parton production at DIS at small x

We use the spinor helicity formalism to calculate the cross section for production of three partons of a given polarization in Deep Inelastic Scattering (DIS) off proton and nucleus targets at small Bjorken x. The target proton or nucleus is treated as a classical color field (shock wave) from which the produced partons scatter multiple times. The resulting expressions are used to study azimuthal angular correlations between produced partons in order to probe the gluon structure of the target hadron or nucleus as well as to study energy loss in DIS reactions.

Jet Production at RHIC and LHC

Recent results on jet production in heavy ion collisions at RHIC and the LHC are discussed, with emphasis on inclusive jet yields and semi-inclusive hadron-triggered and vector boson-triggered recoil jet yields as well as their azimuthal angular correlations. I will also discuss the constraints that these observables impose on the opacity of the medium, the flavour dependence of energy loss, the interplay of perturbative and non perturbative effects and the change of the degrees of freedom of the medium with the resolution of the probe.

Jet-hadron correlations relative to the event plane at the LHC with ALICE

In ultra relativistic heavy-ion collisions at the Large Hadron Collider (LHC), conditions are met to produce a hot, dense and strongly interacting medium known as the Quark Gluon Plasma (QGP). Quarks and gluons from incoming nuclei collide to produce partons at high momenta early in the collisions. By fragmenting into collimated sprays of hadrons, these partons form 'jets'. The outgoing partons scatter and interact with the medium, leading to a manifestation of medium modifications of jets in the final state, known as jet quenching. Within the framework of perturbative QCD, jet production is well understood in pp collisions. We use jets measured in pp interactions as a baseline reference for comparing to heavy-ion collision systems to detect and study jet quenching. The jet quenching mechanism can be studied through the angular correlations of jets with charged hadrons and is examined in transverse momentum (pT) bins of the jets, pT bins of the associated hadrons, and as a function of collision centrality. A robust and precise background subtraction method is used in this analysis to remove the complex, flow dominated, heavy-ion background. The analysis of angular correlations for different orientations of the jet relative to the event plane allows for the study of the path-length dependence of medium modifications to jets. The event plane dependence of azimuthal angular correlations of charged hadrons with respect to the axis of an R=0.2 reconstructed full (charged + neutral) jet in Pb–Pb collisions at sNN=2.76 TeV in ALICE is presented. Results are compared for three angular bins of the jet relative to the event plane in mid-peripheral events. The yields relative to the event plane are presented and then quantified through yield ratio calculations. The results show no significant path-length dependence on the medium modifications.

A CGC/saturation approach for angular correlations in proton\u2013proton scattering

We generalized our model for the description of hard processes, and calculate the value of the azimuthal angular correlations (Fourier harmonics v_n), for proton–proton scattering. The energy and multiplicity independence, as well as the value of v_n, turns out to be in accord with the experimental data, or slightly larger. Therefore, before making extreme assumptions on proton–proton collisions, such as the production of a quark–gluon plasma in large multiplicity events, we need to understand how these affect the Bose–Einstein correlations, which have to be taken into account since the Bose–Einstein correlations are able to describe the angular correlations in proton–proton collisions, without including final state interactions.

Gluon tomography from deeply virtual Compton scattering at small x

We present a full evaluation of the deeply virtual Compton scattering (DVCS) cross section in the dipole framework in the small-x region. The result features the $\cos \phi$ and $\cos 2\phi$ azimuthal angular correlations which have been missing in previous studies based on the dipole model. In particular, the $\cos 2\phi$ term is generated by the elliptic gluon Wigner distribution whose measurement at the planned electron-ion collider (EIC) provides an important information about the gluon tomography at small-x. We also show the consistency with the standard collinear factorization approach based on the quark and gluon generalized parton distributions (GPDs).

Hunting the Gluon Orbital Angular Momentum at the Electron-Ion Collider.

Applying the connection between the parton Wigner distribution and orbital angular momentum (OAM), we investigate the probe of the gluon OAM in hard scattering processes at the planned electron-ion collider. We show that the single longitudinal target-spin asymmetry in the hard diffractive dijet production is very sensitive to the gluon OAM distribution. The associated spin asymmetry leads to a characteristic azimuthal angular correlation of sin(ϕ_{q}-ϕ_{Δ}), where ϕ_{Δ} and ϕ_{q} are the azimuthal angles of the proton momentum transfer and the relative transverse momentum between the quark-antiquark pair. This study may motivate a first measurement of the gluon OAM in the proton spin sum rule.

Spinor helicity methods in high-energy factorization: Efficient momentum-space calculations in the Color Glass Condensate formalism

We use the spinor helicity formalism to calculate the cross section for production of three partons of a given polarization in Deep Inelastic Scattering (DIS) off proton and nucleus targets at small Bjorken x. The target proton or nucleus is treated as a classical color field (shock wave) from which the produced partons scatter multiple times. We reported our result for the final expression for the production cross section and studied the azimuthal angular correlations of the produced partons in [1]. Here we provide the full details of the calculation of the production cross section using the spinor helicity methods.

Azimuthal angular correlations of D mesons with charged particles in pp collisions at √s = 7 TeV and p-Pb collisions at √s = 7 TeV and p-Pb collisions at √SNN = 5.02 TeV with the ALICE experiment at the LHC

Heavy flavour studies played an important role in the understanding of energy loss in the formed medium in heavy-ion collisions through different observables (the nuclear modification factor RAA , the elliptic flow v2 ). In this paper, results on heavy-flavour correlations will be presented. This observable is sensitive to the fragmentation processes of heavy flavours as well as the system of the collisions. First, the reason why studies on correlations are interesting will be discussed. The ALICE experimental step will be presented. Then the analysis strategy is explained. Finally, the results are presented.

JIMWLK evolution of the odderon

We study the effects of a parity-odd "odderon" correlation in JIMWLK renormalization group evolution at high energy. Firstly we show that in the eikonal picture where the scattering is described by Wilson lines, one obtains a strict mathematical upper limit for the magnitude of the odderon amplitude compared to the parity even pomeron one. This limit increases with N_c, approaching infinity in the infinite N_c limit. We use a systematic extension of the Gaussian approximation including both 2- and 3-point correlations which enables us to close the system of equations even at finite N_c. In the large-N_c limit we recover an evolution equation derived earlier. By solving this equation numerically we confirm that the odderon amplitude decreases faster in the nonlinear case than in the linear BFKL limit. We also point out that, in the 3-point truncation at finite N_c, the presence of an odderon component introduces azimuthal angular correlations ~ cos(n phi) at all n.

Polarized 3 parton production in inclusive DIS at small x

Azimuthal angular correlations between produced hadrons/jets in high energy collisions are a sensitive probe of the dynamics of QCD at small x. Here we derive the triple differential cross section for inclusive production of 3 polarized partons in DIS at small x using the spinor helicity formalism. The target proton or nucleus is described using the Color Glass Condensate (CGC) formalism. The resulting expressions are used to study azimuthal angular correlations between produced partons in order to probe the gluon structure of the target hadron or nucleus. Our analytic expressions can also be used to calculate the real part of the Next to Leading Order (NLO) corrections to di-hadron production in DIS by integrating out one of the three final state partons.

Exploring Jet-Hadron correlations at the LHC with ALICE

In relativistic heavy ion collisions at the Large Hadron Collider (LHC), the conditions are met to produce the hot and dense, strongly interacting medium known as the Quark Gluon Plasma (QGP). The QGP, a state of matter created shortly after the Big Bang, is a phase where the deconfinement of quarks and gluons is hypothesized. Jets, the collimated sprays of hadrons from fragmenting partons, are a key probe of the medium. The experimental methods used for jet measurements at ALICE to remove, reduce, and correct for the underlying background event will be presented. In pp collisions, jet production is well understood within the framework of perturbative QCD and acts as a rigorous baseline measurement for jet quenching measurements. By comparing to heavy ion collision systems, we can study the suppression of the number of jets seen and study the modification of the pT or angular distributions of jet fragments. Azimuthal angular correlations of charged hadrons with respect to the axis of a full (charged + neutral) reconstructed (trigger) jet in Pb-Pb and pp collisions at √sNN = 2.76 TeV in ALICE will be presented here. Newly developed combinatoric background subtraction methods and their improvement compared to prior techniques will be discussed.

Di-jet production and angular correlations in DIS at NLO

Angular correlations are a sensitive probe of the dynamics of QCD at high energy. In particular azimuthal angular correlations between two hadrons produced in Deeply Inelastic Scattering (DIS) of a virtual photon on a hadron or nucleus offer the best environment in which to investigate high gluon density (gluon saturation) effects expected to arise at small x . Here we give a progress report on our derivation of Next to Leading Order (NLO) corrections to di-jet (di-hadron) production in DIS.