Azimuthal Angular Correlations Research Articles

Effects of triangular flow on di-hadron azimuthal correlations in relativistic heavy ion collisions

Using the AMPT model for relativistic heavy ion collisions, we have studied the di-hadron azimuthal angular correlations triggered by emitted jets in Au+Au collisions at center of mass energy $\sqrt{s_{NN}}=200$ GeV and impact parameter $b=8$ fm. A double-peak structure for the associated particles at the away side of trigger particles is obtained after subtracting background correlations due to the elliptic flow. Both the near-side peak and the away-side double peaks in the azimuthal angular correlations are, however, significantly suppressed (enhanced) in events of small (large) triangular flow, which are present as a result of fluctuations in the initial collision geometry. After subtraction of background correlations due to the triangular flow, the away-side double peaks change into a single peak with broad shoulders on both sides. The away side of the di-hadron correlations becomes essentially a single peak after further subtraction of higher-order flows.

Charm and beauty searches using electron-D0 azimuthal correlations and microvertexing techniques in STAR experiment at RHIC

The energy loss of heavy quarks in the hot and dense matter created at RHIC, can be used to probe the properties of the medium. Both charm and beauty quarks contribute to the non-photonic electrons through their semi-leptonic decays. It is essential to determine experimentally the relative contribution of charm and beauty quarks to understand the suppression of heavy flavors at high PT in central Au+Au collisions. The azimuthal angular correlations of non-photonic electrons with the reconstructed D0 allow to disentangle the contribution of charm and beauty and to reduce the background below the D0 invariant mass as well. We discuss the STAR measurement of non-photonic electron and D0 → Kπ azimuthal correlations in p+p collisions at 200 GeV. Furthermore, we show results from the application of microvertexing techniques for charm and beauty searches in Cu+Cu and Au+Au collisions at 200 GeV using the information of the Silicon tracker of STAR.

Reaction plane angle dependence of dihadron azimuthal correlations from a multiphase transport model calculation

Dihadron azimuthal angle correlations relative to the reaction plane have been investigated in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV using a multiphase transport model (AMPT). Such reaction plane azimuthal-angle-dependent correlations can shed light on the path-length effect of energy loss of high-transverse-momentum particles propagating through a hot dense medium. The correlations vary with the trigger particle azimuthal angle with respect to the reaction plane direction, ${\ensuremath{\phi}}_{s}={\ensuremath{\phi}}_{T}\ensuremath{-}{\ensuremath{\Psi}}_{\mathrm{EP}}$, which is consistent with the experimental observation by the STAR Collaboration. The dihadron azimuthal angle correlation functions on the away side of the trigger particle present a distinct evolution from a single-peak to a broad, possibly double-peak structure when the trigger particle direction goes from in-plane to out-of-plane with the reaction plane. The away-side angular correlation functions are asymmetric with respect to the back-to-back direction in some regions of ${\ensuremath{\phi}}_{s}$, which could provide insight into the testing ${v}_{1}$ method for reconstructing the reaction plane. In addition, both the root-mean-square width (${W}_{\mathrm{rms}}$) of the away-side correlation distribution and the splitting parameter $(D)$ between the away-side double peaks increase slightly with ${\ensuremath{\phi}}_{s}$, and the average transverse momentum of away-side-associated hadrons shows a strong ${\ensuremath{\phi}}_{s}$ dependence. Our results indicate that a strong parton cascade and resultant energy loss could play an important role in the appearance of a double-peak structure in the dihadron azimuthal angular correlation function on the away side of the trigger particle.

Partonic effects on dijet correlations in relativistic heavy ion collisions

Within a parton transport model, we show that diffusion wakes and Mach cone wakes produced from energy loss by a fast parton propagating in the quark-gluon plasma lead to the observed azimuthal angular correlations of hadrons in central $\mathrm{Au}+\mathrm{Au}$ collisions at RHIC. The magnitude and shape of the wakes are shown to be sensitive measures of the initial parton density and the parton scattering cross section. The shear viscosity to entropy density ratio, $\ensuremath{\eta}/s$, of the plasma is found to be remarkably similar to the lower bound $\ensuremath{\eta}/s=1/4\ensuremath{\pi}$ obtained in the AdS/CFT conjecture.

Properties of Dihadron Correlations for p+p Collisions at √sNN = 200 GeV

The dihadron azimuthal angular correlations for p+p collisions at √sNN = 200 GeV are simulated by a multiphase transport model. The dispersions of near-side and away-side peaks, indicated by the width of Gaussian fit functions and the rms width, decrease with the transverse momentum of associated particles. This trend is consistent with the experimental results. Conditional-yields are also calculated to obtain distributions of the associated particle transverse momentum for both away-side and near-side. Furthermore, the hadronic rescattering effects make the conditional-yield distributions softer.

Study of b quark contributions to non-photonic electron yields by azimuthal angular correlations between non-photonic electrons and hadrons

Azimuthal angular correlations between non-photonic electrons and charged hadrons from p + p collisions at = 200 GeV have been observed in RHIC-STAR. The shapes of the correlation function at each pT are fit with PYTHIA calculations and the relative contribution of B meson decay to the non-photonic electron yields has been extracted as a function of electron pT up to 9.5 GeV/c. Our experimental analysis indicates that bottom quark contribution is about 50% of the non-photonic electron yields for pT greater than 5 GeV/c and is consistent with a fixed-order-next-to-leading-log perturbative QCD calculation. Implications on the bottom quark energy loss in the hot and dense medium will also be discussed combining the results with the previously reported large suppression of non-photonic electron yields.

Heavy quark results from STAR

I report the most recent measurements on open heavy flavor production at RHIC on behalf of the STAR collaboration. The total charm production cross section in midrapidity at RHIC energy is found to approximately scale by number of binary collisions in d + Au, Cu + Cu and Au + Au collisions. The nuclear modification factor of non- photonic electrons is strongly suppressed in central Au + Au collisions, suggesting substantial heavy quark energy loss at RHIC. The bottom decay contribution to non-photonic elec- trons was studied via the e-h and e-D 0 azimuthal angular correlations. The bottom contribution is found to be impor- tant at pT > 5G eV /c, and is consistent with the FONLL calculation within uncertainties. Charm production through gluon jet splitting was measured by studying the D ∗± con- tents in the fully reconstructed jets in p + p collisions. This rate is consistent with pQCD evaluation of gluon splitting into a pair of charm quarks and subsequent hadronization.

Studying the relative bottom contribution via heavy-quark decay electron measurements in STAR

We report measurements of the azimuthal angular correlation distribution of heavy-quark decay electrons and open charmed mesons in pp collisions at \(\sqrt{s}=200\) GeV in the STAR experiment at RHIC. This measurement in combination with current theoretical model calculations allows to extract the relative bottom contribution to the heavy-quark decay electrons, which is important for the interpretation of the observed strong suppression of the high-p T electron yield in central Au+Au collisions.

Parton transverse momenta and Drell-Yan dilepton production

The differential cross section for the dilepton production is calculated, including Fermi motion of hadron constituents as well as emission from the ladders in the formalism of unintegrated parton distributions. We use unintegrated parton distributions which fulfill Kwieci\ifmmode \acute{n}\else \'{n}\fi{}ski evolution equations. Both zeroth- and first-order (for matrix element) contributions are included. We calculate azimuthal angular correlations between charged leptons and deviations from the ${p}_{t}({l}^{+})={p}_{t}({l}^{\ensuremath{-}})$ relation. We concentrate on the distribution in the dilepton-pair transverse momentum. We find incident energy and virtuality dependence of the distribution in the transverse momentum of the lepton pair. We study also azimuthal correlations between jet and dilepton pairs and correlation in the $({p}_{1t}(\mathrm{\text{jet}}),{p}_{2t}({l}^{+}{l}^{\ensuremath{-}}))$ space. The results are compared with experimental data of the R209 and UA1 collaborations.

Azimuthal correlations and alignment of particles in gamma families

Azimuthal angular correlations and the alignment of photons are studied in gamma families recorded by the Pamir Collaboration in a carbon x-ray emulsion chamber. The present interpretation of these experimental data is based on a model of semihard parton scattering in nucleon-nucleus collisions and on arguments favoring the production of exotic beam strings and heavy leading resonances undergoing quasicoplanar decays.

Transverse momentum and pseudorapidity dependences of Mach-like correlations for central Au+Au collisions atsNN=200GeV

The transverse-momentum and pseudorapidity dependences of partonic Machlike shock wave have been studied by using a multiphase transport model with both partonic and hadronic interactions. The splitting parameter $D$, i.e., half distance between two splitting peaks on away side in di-hadron azimuthal angular ($\ensuremath{\Delta}\ensuremath{\phi}$) correlations, slightly increases with the transverse momentum of associated hadrons (${p}_{T}^{\mathrm{assoc}}$), which is consistent with preliminary experimental trend, owing to different interaction-lengths/numbers between wave partons and medium in strong parton cascade. However, the splitting parameter $D$ as a function of pseudorapidity of associated hadrons (${\ensuremath{\eta}}^{\mathrm{assoc}}$), stays flat in the mid-pseudorapidity region and rapidly drops in the high-pseudorapidity region, which is a result of different violent degrees of jet-medium interactions in the medium that has different energy densities in the longitudinal direction. It is proposed that the research on the properties of Machlike correlation can shed light on the knowledge of both partonic and hadronic interactions at RHIC.

Jet Properties from Di-hadron Correlations in [formula omitted] Collisions at [formula omitted

An analysis of high p T hadron spectra associated with high p T π 0 particles in p + p collisions at s = 200 GeV is presented. The shape of the azimuthal angular correlation is used to determine the value of partonic intrinsic momentum 〈 k T 2 〉 = 2.68 ± 0.07 (stat) ± 0.34 (sys) GeV / c . The effect of k T -smearing of inclusive π 0 cross section is discussed.

Two Particle Azimuthal Correlations at High Transverse Momentum in Pb-Au at 158 AGeV/c

The analysis of two-particle azimuthal angular correlations at high transverse momentum from Pb+Au collisions at 158 AGeV/c at SPS reveals substantial modifications of the away-side peak as compared to the distributions from p+p reactions. The data recorded with the CERES Time-Projection Chamber, which provides excellent tracking efficiency, suggest that the observed modification of the back-to-back structure implies significant re-interactions of the scattered partons within the medium. These findings at top SPS energy show features qualitatively similar to the results obtained at RHIC from Au+Au collisions at s = 200 GeV / c and Cu+Cu collisions at s = 62 GeV / c [S.S. Adler et al., PHENIX Collaboration, arXiv:nucl-ex/0507004, M. McCumber and J. Frantz, arXiv:nucl-ex/0511048]. We present the centrality and charge dependent conditional di-jet yields for similar transverse momentum windows as investigated by the RHIC experiments [S.S. Adler et al., PHENIX Collaboration, arXiv:nucl-ex/0507004, M. McCumber and J. Frantz, arXiv:nucl-ex/0511048, J. Adams et al., STAR Collaboration, Phys. Rev. Lett. 95 (2005) 152301, arXiv:nucl-ex/0501016].

Measurements of bottom-antibottom azimuthal production correlations in proton-antiproton collisions ats=1.8 TeV

We have measured the azimuthal angular correlation of bottom anti-bottom production, using 86.5 pb-1 of data collected by Collider Detector at Fermilab (CDF) in proton anti-proton collisions at sqrt{s}=1.8 TeV during 1994-1995. In high-energy p-pbar collisions, such as at the Tevatron, bottom anti-bottom production can be schematically categorized into three mechanisms. The leading-order (LO) process is ``flavor creation,'' where both the bottom and anti-bottom quarks substantially participate in the hard scattering and result in a distinct back-to-back signal in final state. The ``flavor excitation'' and the ``gluon splitting'' processes, which appear at next-leading-order (NLO), are known to make a comparable contribution to total bottom anti-bottom cross section, while providing very different opening angle distributions from the LO process. An azimuthal opening angle between bottom and anti-bottom, Delta phi, has been used for the correlation measurement to probe the interaction creating bottom anti-bottom pairs. The Delta phi distribution has been obtained from two different methods. One method measures the Delta phi between bottom hadrons using events with two reconstructed secondary vertex tags. The other method uses b-bbar --> (J/psi X)(l X') events, where the charged lepton (l) is an electron or a muon, to measure Delta phi between bottom quarks. The bottom anti-bottom purity is determined as a function of Delta phi by fitting the decay length of the J/psi and the impact parameter of the lepton. Both methods quantify the contribution from higher-order production mechanisms by the fraction of the b-bbar pairs produced in the same azimuthal hemisphere, f_toward. The measured f_toward values are consistent with both parton shower Monte Carlo and NLO QCD predictions.

Supergravity in (2 + 1) dimensionsfrom (3 + 1)-dimensional supergravity

The double-diffractive production of chi_c and chi_b mesons, with a rapidity gap on either side, is studied, using both the Regge formalism and the perturbative QCD approach. Due to the rather low scale, the exclusive double-diffractive process pp \to p + chi + p is predicted to dominate, whereas the probability that the incoming protons dissociate is expected to be relatively small. We evaluate the corresponding chi production cross sections at the Tevatron and LHC energies. For the double-diffractive process with proton dissociation, it is possible to select events with large transverse momenta transferred through the rapidity gaps, by measuring the transverse energy, E_\perp, flows in the proton fragmentation regions. Then the large E_\perp provides a scale to justify the use of perturbative QCD, and to allow a spin-parity analysis of the centrally produced system to be performed, by studying the azimuthal angular correlations between the directions of the forward and backward E_\perp flows. The central production of the new X(3872) charmonium state is considered.

Double-diffractive $\chi$ meson production at the hadron colliders

The double-diffractive production of $\chi_c$ and $\chi_b$ mesons, with a rapidity gap on either side, is studied, using both the Regge formalism and the perturbative QCD approach. Due to the rather low scale, the exclusive double-diffractive process $pp\to p + \chi + p$ is predicted to dominate, whereas the probability that the incoming protons dissociate is expected to be relatively small. We evaluate the corresponding $\chi$ production cross sections at the Tevatron and LHC energies. For the double-diffractive process with proton dissociation, it is possible to select events with large transverse momenta transferred through the rapidity gaps, by measuring the transverse energy, $E_\perp$ , flows in the proton fragmentation regions. Then the large $E_\perp$ provides a scale to justify the use of perturbative QCD, and to allow a spin-parity analysis of the centrally produced system to be performed, by studying the azimuthal angular correlations between the directions of the forward and backward $E_\perp$ flows. The central production of the new X(3872) charmonium state is considered.

On azimuthal spin correlations in Higgs plus jet events at LHC

We consider the recent proposal that the distribution of the difference between azimuthal angles of the two accompanying jets in gluon-fusion induced Higgs-plus-two-jet events at LHC reflects the CP of the Higgs boson produced. We point out that the hierarchy between the Higgs boson mass and the jet transverse energy makes this observable vulnerable to logarithmically enhanced higher-order perturbative corrections. We present an evolution equation that describes the scale variation of the azimuthal angular correlation for the two jets. The emission of extra partons leads to a significant suppression of the correlation. Using the HERWIG Monte Carlo event generator, we carry out a parton-shower analysis to confirm the findings.

Elliptic emission of K+ and π+ in 158 A·GeV Pb + Pb collisions

An event-by-event analysis of the azimuthal angular correlation with respect to the reaction plane has been carried out for K + and π + emission near mid-rapidity in 158 A·GeV Pb + Pb collisions. In semi-central collisions, K + mesons are found to be preferentially emitted out of the reaction plane, while π + mesons are emitted in the reaction plane. The results suggest that the kaon emission is influenced by in-medium potential effects in addition to collective flow effects.

Li-Li azimuthal angular correlations: A test for emission from a rotating source versus instantaneous multifragmentation.

Azimuthal angle correlations have been measured for Li-Li pairs from $^{40}\mathrm{Ar}$ + $^{197}\mathrm{Au}$, $^{\mathrm{nat}}\mathrm{Ag}$, $^{\mathrm{nat}}\mathrm{Cu}$, $^{27}\mathrm{Al}$ (17A, 27A, and 34A MeV). Many of these correlations exhibit enhancements at \ensuremath{\Delta}cphi of 0\ifmmode^\circ\else\textdegree\fi{} and 180\ifmmode^\circ\else\textdegree\fi{}, the classical pattern for evaporation from a hot, high-spin source. A very different pattern is predicted by a simple multifragmentation model, i.e., a peak at \ensuremath{\Delta}cphi\ensuremath{\approxeq}60\ifmmode^\circ\else\textdegree\fi{}. This peak is driven by the rapid Coulomb explosion of a nonrotating nucleus. The latter pattern is not observed experimentally, however, if collective rotation is included in the multifragmentation model, its predictions are more consistent with the observations. Such comparisons can give a promising test for sequential emission from a rotating source versus instantaneous explosive multifragmentation, but one needs a very good selection of collision centrality to reduce the role of the collective rotation. For most of these data the dominant driving forces seem to be rotational motion perturbed by final-state Coulomb repulsions for time delays of the order of ${10}^{\mathrm{\ensuremath{-}}22}$ s between successive emissions of Li fragments.

Dynamical aspects of intermediate mass fragment emission in the reaction of 32S + Ag at 3 A-MeV

The emission of intermediate mass fragments (IMFs) has been studied using the 4π array, AMPHORA. The energy spectra, the angular distributions, the multiplicities, and the charge distributions are studied inclusively as well as in coincidence with projectile-like fragments (PLFs) and other fragments. The low-energy component of the inclusive fragment spectra can be reproduced by a statistical binary decay code GEMINI, although the calculated absolute cross sections are an order of magnitude smaller than the experimental IMF cross sections. At intermediate angles the fragments are dominated by IMFs with Z ≤ 10 both for inclusive and coincidence events and the energy spectra of the fragments show hard components which cannot be explained as statistical emission. For this non-equilibrium component strong azimuthal angular correlations are observed in IMF-PLF and IMF-IMF coincidence events. Both the energy spectra and the azimuthal angular correlations of the non-equilibrium component are well reproduced by an extended classical dynamical model.