Azimuthal Angular Correlations Research Articles

Investigating Λ baryon production in p -Pb collisions in jets and the underlying event using angular correlations

First measurements of hadron-Λ (h−Λ) azimuthal angular correlations in p-Pb collisions at sNN =5.02 TeV using the ALICE detector at the Large Hadron Collider are presented. These correlations are used to separate the production of associated Λ baryons into three different kinematic regions, namely those produced in the direction of the trigger particle (near side), those produced in the opposite direction (away side), and those whose production is uncorrelated with the jet axis (underlying event). The per-trigger associated Λ yields in these regions are extracted, along with the near- and away-side azimuthal peak widths, and the results are studied as a function of associated particle pT and event multiplicity. Comparisons with the event generator and previous measurements of the ϕ(1020) meson are also made. The final results indicate that strangeness production in the highest multiplicity p-Pb collisions is enhanced relative to low multiplicity collisions in both the jetlike regions and the underlying event. The production of Λ relative to charged hadrons is also enhanced in the underlying event when compared to the jetlike regions. Additionally, the results hint that strange quark production in the away-side of the jet is modified by soft interactions with the underlying event. ©2025 CERN, for the ALICE Collaboration 2025 CERN

Azimuthal angular correlation of J/ψ plus jet production at the electron-ion collider

By investigating the soft gluon radiation in the J/ψ plus jet photoproduction at the electron-ion collider (EIC), we demonstrate that the azimuthal angular correlations between the leading jet and heavy quarkonium provide a unique probe to the production mechanism of the latter. In particular, a significant cos(ϕ) asymmetry is found for the color-singlet channel, whereas it vanishes or has an opposite sign for color-octet production, depending on the jet transverse momentum. Numerical results of cos(ϕ) and cos(2ϕ) asymmetries employing both the color-singlet model and the nonrelativistic QCD approach are presented for typical kinematics at the future EIC. Published by the American Physical Society 2024

Probing the Quark Orbital Angular Momentum at Electron-Ion Colliders Using Exclusive π^{0} Production.

We propose to detect signals from quark orbital angular momentum (OAM) through exclusive π^{0} production in electron- (longitudinally polarized) proton collisions. Our analysis demonstrates that the sin2ϕ azimuthal angular correlation between the transverse momentum of the scattered electron and the recoil proton serves as a sensitive probe of quark OAM. Additionally, we present a numerical estimate of the asymmetry associated with this correlation for the kinematics accessible at the Electron-Ion Colliders in the U.S. and China. This study aims to pave the way for the first experimental study of quark OAM in relation to the Jaffe-Manohar spin sum rule.

Accessing the gluon GTMD F1,4 in exclusive π0 production in ep collisions

We demonstrate that the longitudinal single target-spin asymmetry in exclusive π0 production in ep collisions can give access to the imaginary part of the gluon generalized transverse momentum distribution (GTMD) F1,4. Such a longitudinal spin asymmetry that results from the Coulomb-nuclear interference effect, leads to a characteristic azimuthal angular correlation of sin2ϕ, where ϕ is the azimuthal angle between the scattered lepton transverse momentum and the recoil proton’s transverse momentum. We also present a numerical estimate of the asymmetry for the kinematics accessible at EIC and EicC. Published by the American Physical Society 2024

Bulk flow and correlation measurements at LHCb

Particle correlations are a powerful tool to study the properties of the bulk nuclear matter produced in relativistic heavy ion collisions. The momentum correlations between identical particles originating from the same particleemitting source, referred to as the Bose-Einstein correlations, measure scales that are related to the geometrical size of the source. The two-particle azimuthal angular correlations measure the spatial anisotropy of produced particles, providing information on collective phenomena arising in the dense nuclear medium. This contribution will discuss new LHCb measurements of Bose- Einstein correlations and, for the first time, the collective flow coefficients in the far forward rapidity region.

Azimuthal Correlations within Exclusive Dijets with Large Momentum Transfer in Photon-Lead Collisions.

The structure of nucleons is multidimensional and depends on the transverse momenta, spatial geometry, and polarization of the constituent partons. Such a structure can be studied using high-energy photons produced in ultraperipheral heavy-ion collisions. The first measurement of the azimuthal angular correlations of exclusively produced events with two jets in photon-lead interactions at large momentum transfer is presented, a process that is considered to be sensitive to the underlying nuclear gluon polarization. This study uses a data sample of ultraperipheral lead-lead collisions at sqrt[s_{NN}]=5.02 TeV, corresponding to an integrated luminosity of 0.38 nb^{-1}, collected with the CMS experiment at the LHC. The measured second harmonic of the correlation between the sum and difference of the two jet transverse momentum vectors is found to be positive, and rising, as the dijet transverse momentum increases. A well-tuned model that has been successful at describing a wide range of proton scattering data from the HERA experiments fails to describe the observed correlations, suggesting the presence of gluon polarization effects.

Measurements of the Elliptic and Triangular Azimuthal Anisotropies in Central ^{3}He+Au, d+Au and p+Au Collisions at sqrt[s_{NN}]=200 GeV.

The elliptic (v_{2}) and triangular (v_{3}) azimuthal anisotropy coefficients in central ^{3}He+Au, d+Au, and p+Au collisions at sqrt[s_{NN}]=200 GeV are measured as a function of transverse momentum (p_{T}) at midrapidity (|η|<0.9), via the azimuthal angular correlation between two particles both at |η|<0.9. While the v_{2}(p_{T}) values depend on the colliding systems, the v_{3}(p_{T}) values are system independent within the uncertainties, suggesting an influence on eccentricity from subnucleonic fluctuations in these small-sized systems. These results also provide stringent constraints for the hydrodynamic modeling of these systems.

Feasibility Studies for the Measurement of Open-Charm Mesons at ALICE-3 Using Decay Channels with Neutral Mesons and Photons in the Final State

ALICE-3 is being designed as a next-generation heavy-ion experiment to be operated at the high-luminosity Large Hadron Collider. With luminosities higher by a factor of fifty, ALICE-3 will be able to study5 properties of quark–gluon matter with probes and precision which were previously unavailable due to small cross sections, high background levels, and insufficient detector sensitivity. In particular, the properties of hot and dense QCD matter will be studied by measuring production cross sections, flow coefficients, azimuthal angular correlations and nuclear modification factors for open-charm hadrons. In this contribution, we present the results of feasibility studies for the measurement of ground and excited states of open-charm mesons in decay channels D0 → K− + π+ + π0, D*(2007)0 → D0 + γ and D*(2010)± → D0 + π± in pp, p-Pb and Pb-Pb collisions at LHC energies using the ALICE-3 experimental setup. We formulate the main requirements for the selection of particles and their combinations to ensure reliable signal extraction in a wide transverse momentum range and estimate the minimum size of the required data samples. The results obtained are also compared to previous findings for the open-charm measurements in different decay channels.

Jet fragmentation via azimuthal angular correlations of electrons from heavy flavor decay in pp, p -Pb, and Pb-Pb collisions using PYTHIA8+Angantyr calculations

Measurements in heavy flavor azimuthal angular correlation provide insight into the production, propagation, and hadronization of heavy flavor jets in ultrarelativistic hadronic and heavy-ion collisions. These measurements across different colliding systems, such as $p\text{\ensuremath{-}}A$ and $A\text{\ensuremath{-}}A$, help us isolate the possible modification in particle production due to cold nuclear matter (CNM) effects and the formation of quark-gluon plasma (QGP), respectively. Jet correlation studies give direct access to the initial parton dynamics produced in these collisions. This article studies the azimuthal angular correlations of electrons from heavy flavor hadron decays in $pp, p$-Pb, and Pb-Pb collisions at $\sqrt{{s}_{NN}}=5.02$ TeV using $\text{PYTHIA8}+\text{Angantyr}$. We study the production of heavy flavor jets with different parton level processes, including multiparton interactions, different color reconnection prescriptions, and initial and final state radiation processes. In addition, we add the hadron-level processes, i.e., Bose-Einstein and rescattering effects, to quantify the effect due to these processes. The heavy flavor electron correlations are calculated in the different trigger and associated ${p}_{\mathrm{T}}$ intervals to characterize the impact of hard and soft scattering in the various colliding systems. The yields and the sigmas associated with the near-side (NS) and away-side (AS) correlation peaks are calculated and studied as a function of associated ${p}_{\mathrm{T}}$ for different trigger ${p}_{\mathrm{T}}$ ranges.

Jet fragmentation via azimuthal angular correlations of heavy flavors in pp collisions at s=7 TeV

Measurements in heavy-flavor azimuthal angular correlation provide insight into the production, propagation, and hadronization of heavy-flavor jets in ultrarelativistic hadronic and heavy-ion collisions. These measurements across different particle species help to isolate the possible modification in particle production and fragmentation due to different mass and quark contents. Jet correlation studies give direct access to the initial parton dynamics produced in these collisions. This article studies the azimuthal angular correlations of heavy-flavor hadrons (charm and beauty mesons and charm baryons) in $pp$ collisions at $\sqrt{s}=7$ TeV using pythia8. We study the production of heavy-flavor jets with different parton-level processes, including multiparton interactions and different color reconnection prescriptions. The heavy-flavor hadrons correlations are calculated in the different triggers and associated ${p}_{\mathrm{T}}$ intervals to characterize the impact of hard and soft scattering. The yields and the widths associated with the near-side (NS) and away-side (AS) correlation peaks are calculated and studied as a function of associated ${p}_{\mathrm{T}}$ for different trigger ${p}_{\mathrm{T}}$ ranges.

Probing non-standard HVV (V = W, Z) couplings in single Higgs production at future electron-proton collider

The couplings of the Higgs boson (H) with massive gauge bosons of weak interaction (V = W, Z), can be probed in single Higgs boson production at the proposed future Large Hadron-Electron Collider (LHeC). In the collision of an electron with a pro- ton, single Higgs production takes place via so-called charged-current (e−p → νeHj) and neutral-current (e−p → e−Hj) processes. We explore the potential of the azimuthal angle correlation between the forward jet and scattered neutrino or electron in probing the non-standard HVV couplings at the collider center-of-mass energy of sqrt{s} ≈ 1.3 TeV. We choose the most general modifications (of CP-even and CP-odd nature) to these couplings due to new physics effects beyond the standard model. We derive exclusion limits on new physics parameters of HV V couplings as a function of integrated luminosity at 95% C.L. using the azimuthal angular correlations in charged- and neutral-current processes. We find that using 1000 fb−1 data, the standard model-like new physics parameters in HWW and HZZ couplings can be constrained with accuracies of 4% and 15%, respectively. The least constrained CP-even parameters of HWW coupling can be as large as 0.04, while those of HZZ coupling can have values around 0.31. Allowed values of CP-odd parameters in HWW and HZZ couplings are found to be around 0.14 and 0.34, respectively. We also study changes in the allowed values of non-trivial new physics parameters in the presence of other parameters.

Azimuthal Angular Correlation as a Boosted Top Jet Substructure.

We propose a novel jet substructure observable of boosted tops that is related to the linear polarization of the W boson in boosted top quark decay, which results in a cos2ϕ angular correlation between the t→bW and W→ff^{'}[over ¯] decay planes. We discuss in detail the origin of such linear polarization by applying Wigner's little group transformation. We show that the unique cos2ϕ angular correlation only exists in the boosted regime but not in the top quark rest frame. We construct an experimental observable for such correlation based on the transverse energy deposition asymmetry in the top jet that does not require the reconstruction of W decay products. The degree of this asymmetry can be used to measure the longitudinal polarization of the top quark, which is an important probe of new physics that couples to the top sector, and can discriminate a boosted top quark jet from its background events, such as QCD jets. A numerical simulation is also performed and found to agree well with the analytic prediction of the standard model.

Coherent energy loss effects in dihadron azimuthal angular correlations in Deep Inelastic Scattering at small x

We perform an exploratory study of the role of coherent, medium-induced energy loss in azimuthal angular correlations in dihadron production in Deep Inelastic Scattering (DIS) at small x where the target proton/nucleus is modeled as a Color Glass Condensate. In this approach coherent radiative energy loss is part of the higher order corrections to the leading order dihadron production cross section. We include the effects of both gluon saturation and coherent radiative energy loss and show that radiative cold-matter energy loss has a significant effect on the so-called coincidence probability for the back to back production of dihadrons in DIS. We also define a double ratio of coincidence probabilities for a nucleus and proton targets and show that it is very robust against higher order radiative corrections.

Polarization as a signature of local parity violation in hot QCD matter

We show that local parity violation due to chirality imbalance in relativistic nuclear collisions can be revealed by measuring the projection of the polarization vector onto the momentum, i.e. the helicity, of final state baryons. The proposed method does not require a coupling to the electromagnetic field, like in the Chiral Magnetic Effect. By using linear response theory, we show that, in the presence of a chiral imbalance, the spin 1/2 baryons and anti-baryons receive an additional contribution to the polarization along their momentum and proportional to the axial chemical potential. The additional, parity-breaking, contribution to helicity can be detected by studying helicity-helicity azimuthal angular correlation.

Study of a background reconstruction method for the measurement of D-meson azimuthal angular correlations

We studied experimental background reconstruction methods for the measurement of the $$D-{\overline{D}}$$ correlation using PYTHIA simulation. The like-sign (LS) and side-band (SB) background methods, which are widely used in the experimental measurements of single D-meson production yields, were deployed for correlation study. It was found that the LS method, which describes the combinatorial background of single $$D^{0}$$ meson yields, fails to reproduce the correlated background in the $$D^{0}-\overline{D^{0}}$$ correlation measurement, while the SB background method yields a good description of the background for both single $$D^0$$ yields and the correlated background of the $$D^{0}-\overline{D^{0}}$$ correlation measurement. We further examined the validity of the correlation methods under different signal-to-background ratios, providing direct references for experimental measurements.

Study on open charm hadron production and angular correlation in high-energy nuclear collisions

We study the production and angular correlation of charm hadrons in hot and dense matter produced in high-energy nuclear–nuclear collisions within a multiphase transport model (AMPT). By triggering additional charm–anticharm quark pair production in the AMPT, the model describes the $$D^0$$ nuclear modification factor in the low and intermediate $$p_{{\rm T}}$$ regions in Au + Au collisions at $$ \sqrt{s_{{\rm NN}}}=200$$ GeV reasonably well. Further exploration of the $$D^0$$ pair azimuthal angular correlation for different centralities shows clear evolution from low-multiplicity to high-multiplicity events, which is associated with the number of charm quark interactions with medium partons during AMPT transport.

The elliptic asymmetry of heavy quarkonia in pA collisions from the initial state

We compute the azimuthal angular correlation between a heavy quarkonium (or an open charm meson) and a charged light hadron produced in pA collisions in the color glass condensate framework. The obtained elliptic asymmetries (v2) of J/ψ and D0 mesons agree with the recent experimental data from the LHC quite well. Our calculation demonstrates that an initial state effect due to the interaction between the partons inside the projectile proton and the dense gluons inside the target nucleus can generate a sizable collectivity which can be measured at RHIC and the LHC. In this framework, we find the mass dependence for the heavy quarkonium v2 is quite weak, while that for the open meson v2 is rather strong. It will be very interesting to test our results regarding other heavy mesons in the future experimental measurements.

Momentum Anisotropy of Leptons from Two-Photon Processes in Heavy-Ion Collisions.

We investigate the azimuthal angular correlation between the lepton transverse momentum P_{⊥} and the impact parameter b_{⊥} in noncentral heavy-ion collisions, where the leptons are produced through two-photon scattering. Among the Fourier harmonic coefficients, a significant v_{4} asymmetry is found for the typical kinematics at RHIC and LHC with a mild dependence on the P_{⊥}, whereas v_{2} is power suppressed by the lepton mass over P_{⊥}. This unique prediction, if confirmed from the experiments, shall provide crucial information on the production mechanism for the dilepton in two-photon processes.

The cos 2\u03d5 azimuthal asymmetry in \u03c10 meson production in ultraperipheral heavy ion collisions

We present a detailed study of vector meson photoproduction in ultraperipheral heavy ion collisions (UPCs). Using the dipole model, we develop a framework for the joint impact parameter and transverse momentum dependent cross sections. We compute the unpolarized cross section and cos 2ϕ azimuthal angular correlation for ρ0 photoproduction with ϕ defined as the angle between the ρ0’s transverse momentum and its decay product pion meson’s transverse momentum. Our result on unpolarized coherent differential cross section gives excellent description to the STAR experimental data. A first compari- son between theoretical calculation and experimental measurement on the cos 2ϕ azimuthal asymmetry, which results from the linearly polarized photons, is performed and reasonable agreement is reached. We find out the characteristic diffractive patterns at both RHIC and LHC energies and predict the impact parameter dependent cos 2ϕ azimuthal asymmetries for ρ0 photoproduction by considering UPCs and peripheral collisions. The future experimental measurements at RHIC and LHC relevant to our calculations will provide a tool to rigorously investigate the coherent and incoherent production of vector meson in UPCs, as well as to probe the nuclear structure in heavy ion collisions.

Collectivity of heavy mesons in proton-nucleus collisions

Using a model based on the Color Glass Condensate framework and the dilute-dense factorization, we systematically study the azimuthal angular correlations between a heavy flavor meson and a light reference particle in proton-nucleus collisions. The obtained second harmonic coefficients (also known as the elliptic flows) for $J/\psi$ and $D^0$ agree with recent experimental data from the LHC. We also provide predictions for the elliptic flows of $\Upsilon$ and $B$ meson, which can be measured in the near future at the LHC. This work can shed light on the physics origin of the collectivity phenomenon in the collisions of small systems.