Long-range Angular Correlations Research Articles

Emergence of Long-Range Angular Correlations in Low-Multiplicity Proton-Proton Collisions.

This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at sqrt[s]=13 TeV. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of 1.4<|Δη|<1.8 and a transverse momentum of 1<p_{T}<2 GeV/c, as a function of the charged-particle multiplicity measured at midrapidity. This Letter extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly interacting medium is unlikely to be formed. The precision of the new low multiplicity results allows for the first direct quantitative comparison with the results obtained in e^{+}e^{-} collisions at sqrt[s]=91 GeV and sqrt[s]=183-209 GeV, where initial-state effects such as preequilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range 8≲⟨N_{ch}⟩≲24 where the e^{+}e^{-} results have good precision, the measured ridge yields in pp collisions are substantially larger than the limits set in e^{+}e^{-} annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in e^{+}e^{-} annihilations do not contribute significantly to the emergence of long-range correlations in pp collisions.

Investigating hidden sectors at future e+e− colliders through two-particle angular correlations

Exploring long-range angular correlations among emitted particles in high-energy collisions provides an opportunity to uncover physics beyond the Standard Model like Hidden Valley (HV) models. We focus on a hidden QCD-like sector, where the interplay between HV matter and QCD partonic cascades could enhance azimuthal correlations between final-state particles. Our investigation, performed at detector level, specifically targets the detectability of these phenomena at future e+e− colliders, yielding a cleaner experimental signature as compared to the Large Hadron Collider (LHC). Remarkably, the observation of ridge structures in the two-particle correlation function may suggest the existence of New physics.

Cosmological analogies in the search for new physics in high-energy collisions

In this paper, analogies between multiparticle production in high-energy collisions and the time evolution of the early universe are discussed. A common explanation is put forward under the assumption of an unconventional early state: a rapidly expanding universe before recombination (last scattering surface), followed by the CMB, later evolving up to present days, versus the formation of hidden/dark states in hadronic collisions followed by a conventional QCD parton shower yielding final-state particles. In particular, long-range angular correlations are considered pointing out deep connections between the two physical cases potentially useful for the discovery of new physics.

Measurement of long-range two-particle azimuthal correlations in Z-boson tagged pp collisions at sqrt{s}=8 and 13\xa0TeV

Results are presented from the measurement by ATLAS of long-range (|Delta eta |>2) dihadron angular correlations in sqrt{s}=8 and 13 TeV pp collisions containing a Z boson. The analysis is performed using 19.4 mathrm {fb}^{-1} of sqrt{s}=8 TeV data recorded during Run 1 of the LHC and 36.1 mathrm {fb}^{-1} of sqrt{s}=13 TeV data recorded during Run 2. Two-particle correlation functions are measured as a function of relative azimuthal angle over the relative pseudorapidity range 2<|Delta eta |<5 for different intervals of charged-particle multiplicity and transverse momentum. The measurements are corrected for the presence of background charged particles generated by collisions that occur during one passage of two colliding proton bunches in the LHC. Contributions to the two-particle correlation functions from hard processes are removed using a template-fitting procedure. Sinusoidal modulation in the correlation functions is observed and quantified by the second Fourier coefficient of the correlation function, v_{2,2}, which in turn is used to obtain the single-particle anisotropy coefficient v_{2}. The v_{2} values in the Z-tagged events, integrated over 0.5<p_{mathrm {T}}<5 GeV, are found to be independent of multiplicity and sqrt{s}, and consistent within uncertainties with previous measurements in inclusive pp collisions. As a function of charged-particle p_{mathrm {T}}, the Z-tagged and inclusive v_{2} values are consistent within uncertainties for p_{mathrm {T}}<3 GeV.

Centrality dependent long-range angular correlations of intermediate-pT D-mesons and charged particles in pPb collisions at the LHC energy

The high-multiplicity events of pPb collisions at at the LHC exhibit unforeseen collective behavior. One of the possible explanations to the collectivity could be the formation of thermalized partonic matter, like the one formed in relativistic nucleus–nucleus collisions and is described by the hydrodynamic models. This article presents a study on the centrality dependent long-range two-particle azimuthal correlations of D-mesons and charged particles in pPb collisions at . The study has been conducted on the events, generated with the EPOS3 hydrodynamic code that reproduces most of the features of the pPb data at the LHC energy. There appears a ridge-like structure in the long-range two-particle angular correlations of D-mesons, in the intermediate pT-range, and charged particles in the simulated high-multiplicity pPb events.

Measurement of emission-angle anisotropy via long-range angular correlations with high- pT hadrons in d+Au and p+p collisions at sNN=200 GeV

We present measurements of two-particle angular correlations between high-transverse-momentum ($2<p_T<11$ GeV/$c$) $\pi^0$ observed at midrapidity ($|\eta|<0.35$) and particles produced either at forward ($3.1<\eta<3.9$) or backward ($-3.7<\eta<-3.1$) rapidity in $d$$+$Au and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The azimuthal angle correlations for particle pairs with this large rapidity gap in the Au-going direction exhibit a ridge-like structure that persists up to $p_T{\approx}6$ GeV/$c$ and which strongly depends on collision centrality, which is a similar characteristic to the hydrodynamical particle flow in A+A collisions. The ridge-like structure is absent in the $d$-going direction as well as in $p$$+$$p$ collisions, in the transverse-momentum range studied. The results indicate that the ridge-like structure is shifted in the Au-going direction toward more central collisions, similar to the charged-particle pseudorapidity distributions.

Measurement of long-range angular correlations and azimuthal anisotropies in high-multiplicity p+Au collisions at sNN=200 GeV

We present the first measurements of long-range angular correlations and the transverse momentum dependence of elliptic flow $v_2$ in high-multiplicity $p$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. A comparison of these results with previous measurements in high-multiplicity $d$$+$Au and $^3{\rm He}$$+$Au collisions demonstrates a relation between $v_2$ and the initial collision eccentricity $\varepsilon_2$, suggesting that the observed momentum-space azimuthal anisotropies in these small systems have a collective origin and reflect the initial geometry. Good agreement is observed between the measured $v_2$ and hydrodynamic calculations for all systems, and an argument disfavoring theoretical explanations based on momentum-space domain correlations is presented. The set of measurements presented here allows us to leverage the distinct intrinsic geometry of each of these systems to distinguish between different theoretical descriptions of the long-range correlations observed in small collision systems.

Measurements of long-range angular correlation and anisotropy in d+Au collisions at sNN=200 GeV from PHENIX

We present azimuthal angular correlations measured in central d+Au collisions at sNN=200GeV between charged hadrons and energy deposited in calorimeter towers. The charged hadrons are measured at mid-rapidity |η|<0.35, and the energy deposits are measured at forward and backward rapidities (−3.7<η<−3.1, Au-going direction and 3.1<η<3.9, d-going direction). An enhanced near-side angular correlation across |Δη|> 2.75 is observed in d+Au collisions. We also present the correlation between energy deposited in the d-going and Au-going direction. Similar to previous case, an enhancement is also observed in the near-side pairs correlations in most central d+Au collisions which across |Δη|>6.2. Using the event-plane method applied to the Au-going energy distribution, we extract the anisotropy strength v2 for inclusive charged hadrons at mid-rapidity up to pT=4.5GeV/c. We also present the measurement of v2 for identified π± and (anti)protons in central d+Au collisions, and observe a mass-ordering pattern similar to that seen in A+A heavy ion collisions. These results are compared with measurements from p+Pb at sNN=5.02TeV.

Long-range angular correlations at the LHC with ALICE

The observation of long-range correlations on the near and away side -the double ridge- in high-multiplicity p-Pb collisions at sqrt(s_NN) = 5.02 TeV and its similarity to Pb-Pb collisions remains one of the open questions from the p-Pb run at the Large Hadron Collider (LHC). It has been attributed to mechanisms that involve initial-state effects, such as gluon saturation and colour connections forming along the longitudinal direction, and final-state effects, such as parton-induced interactions and collective effects developing in a high-density system possibly formed in these collisions. In order to understand the nature of this double- ridge structure the two-particle correlation analysis has been extended to identified particles. The observed mass dependence in p-Pb resembles the pattern observed in Pb-Pb collisions and is compatible with expectations from hydrodynamics. The subtraction of the long-range structures allows the study of the hadron production belonging to the fragmentation of jets originating from low momentum-transfer scatterings (minijets).

Long-range angular correlations of π, K and p in p–Pb collisions at [formula omitted

Angular correlations between unidentified charged trigger particles and various species of charged associated particles (unidentified particles, pions, kaons, protons and antiprotons) are measured by the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV in the transverse-momentum range 0.3<pT<4 GeV/c. The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range |ηlab|<0.8. Fourier coefficients are extracted from the long-range correlations projected onto the azimuthal angle difference and studied as a function of pT and in intervals of event multiplicity. In high-multiplicity events, the second-order coefficient for protons, v2p, is observed to be smaller than that for pions, v2π, up to about pT=2 GeV/c. To reduce correlations due to jets, the per-trigger yield measured in low-multiplicity events is subtracted from that in high-multiplicity events. A two-ridge structure is obtained for all particle species. The Fourier decomposition of this structure shows that the second-order coefficients for pions and kaons are similar. The v2p is found to be smaller at low pT and larger at higher pT than v2π, with a crossing occurring at about 2 GeV/c. This is qualitatively similar to the elliptic-flow pattern observed in heavy-ion collisions. A mass ordering effect at low transverse momenta is consistent with expectations from hydrodynamic model calculations assuming a collectively expanding system.

Communication: Long-range angular correlations in liquid water

At ambient conditions the intermolecular correlation in liquid water is generally believed to be short ranged as shown in the atomic pair distribution functions (PDFs) obtained from scattering experiments or from theoretical predictions. However, atom-atom PDFs provide only a partial description of the higher dimensional intermolecular correlation function that depends on both the positions and orientations of water molecules. Here we study the atomic PDFs of liquid water as well as the angular correlation function (ACF) using a classical density functional theory. We demonstrate that, different from the PDFs, the ACF exhibits long-range oscillatory decay extending up to tens of molecular diameters. The theoretical predictions are in good agreement with molecular simulations and corroborate recent experimental results from the second harmonic light scattering experiments.

Long-range angular correlations on the near and away side in p–Pb collisions at [formula omitted

Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5<pT,assoc<pT,trig<4 GeV/c. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield obtained in low-multiplicity events is subtracted from the one in high-multiplicity events. The excess on the near-side is qualitatively similar to that recently reported by the CMS Collaboration, while the excess on the away-side is reported for the first time. The two-ridge structure projected onto azimuthal angle is quantified with the second and third Fourier coefficients as well as by near-side and away-side yields and widths. The yields on the near side and on the away side are equal within the uncertainties for all studied event multiplicity and pT bins, and the widths show no significant evolution with event multiplicity or pT. These findings suggest that the near-side ridge is accompanied by an essentially identical away-side ridge.

Strong Signal of Dynamical Long-Range Correlation among Target Fragments in Relativistic and Ultra-Relativistic Nuclear Interactions

This paper reports an investigation on the two-particle long-range angular correlation among the target fragments produced in 28Si−AgBr interactions at 14.5 AGeV, 16O−AgBr interactions at 60 AGeV and 32S−AgBr interactions at 200 AGeV. The experimental data have been compared with Monte Carlo simulated events to extract dynamical correlation. The data exhibit two-particle long-range correlation in emission angle space at all energies.

Orientational order in dense molecular hydrogen: A first-principles path-integral Monte Carlo calculation.

A quantum-mechanical simulation of orientational order in solid molecular hydrogen is presented, based on path-integral Monte Carlo calculations. The effective potential between nearest-neighbor hydrogen molecules is derived from first-principles total energy calculations, taking full account of the symmetries of the Hamiltonian within a hexagonal close-packed arrangement of the molecule centers. A first-order orientational order transition is found by studying short- and long-range angular correlations and the behavior of the corresponding order parameters. A pressure-temperature phase diagram is constructed and compared to relevant experimental and theoretical results.

Long-range angular correlations of waves in a tube geometry.

On met en evidence l'existence des correlations angulaires inhabituelles pour les ondes transmises et reflechies, pour un milieu aleatoire ayant la forme d'un long tube. La brillance du spot du «speckle» optique s'etend bien au-dessus de sa taille angulaire normale Δθ=λ/w (ou w est la largeur du tube) et chute seulement comme une loi en puissance avec la force oscillatoire. On montre une disparition graduelle de l'effet de memoire lorsque la longueur du tube excede sa largeur

New evaluation of the angular correlation of positron annihilation spectra for noble and d-transition metals

A new fit to the long-range angular correlation of annihilation radiation data of the sum of a parabola, so-called Ferrell function, and a Gaussian for noble and d-transition metals is presented. The three functions are considered to describe the positron annihilation with three different electron groups: nearly free, d-like and rare-gas core electrons, respectively.

A theoretical study of simple liquid crystal models

The reference hypernetted-chain (RHNC) theory is solved for simple liquid crystal models and the results obtained are compared with Monte Carlo calculations. It is shown that the RHNC approximation gives an accurate estimate of the stability limit of the isotropic phase with respect to fluctuations of nematic symmetry. Furthermore, it provides a good description of the growth of long-range angular correlation in the pretransitional region. The temperature dependence of the inverse Kerr constant given by the RHNC theory is examined and qualitatively compared with mean field theory and experimental results. Finally, the reference linearized hypernetted-chain (RLHNC) theory and the mean spherical approximation (MSA) are also solved and are shown to be less accurate than the RHNC theory for the model considered.