Azimuthal anisotropy of electrons from heavy flavor decays in \sqrt{s_{NN}} =200 GeV Au-Au collisions at PHENIX

The PHENIX experiment has measured the azimuthal anisotropy parameter v₂, the second harmonic of the azimuthal distribution, for electrons at mid-rapidity (|η|<0.35) as a function of transverse momentum (0.5<p_T (GeV/c)<5.0) in Au+Au collisions at  =200 GeV. From the result we have calculated the non-photonic electron v₂, which is expected to reflect charm quark azimuthal anisotropy, by subtracting the v₂ of electrons from other sources such as photon conversions and Dalitz decays.     

Strange particle production at RHIC

We report STAR measurements of mid-rapidity yields for the Λ , , K _S ⁰ , Ξ ⁻, , Ω ⁻, particles in Cu + Cu and Au + Au  GeV collisions. We show that at a given number of participating nucleons, bulk strangeness production is higher in Cu + Cu collisions compared to Au + Au collisions at the same center of mass energy, counter to predictions from the Canonical formalism. We compare both the Cu + Cu and Au + Au yields to AMPT and EPOS predictions, and find they reproduce key qualitative aspects of the data. Finally, we investigate other scaling parameters and find bulk strangeness production for both the measured data and theoretical predictions, scales better with the number participants that undergo more than one collision.     

Longitudinal broadening of near-side jets due to parton cascade

Longitudinal broadening along the Δη direction on the near side in the two-dimensional (Δφ×Δη) di-hadron correlation distribution has been studied for central Au+Au collisions at  GeV, within a dynamical multi-phase transport model. It was found that longitudinal broadening is generated by a longitudinal flow induced by a strong parton cascade in central Au+Au collisions, to be compared with p+p collisions at  GeV. The longitudinal broadening may shed light on the strongly interacting partonic matter at RHIC.     

Recent results from PHOBOS on particle production at high p T

A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity (Δη) and azimuthal angle (Δφ) correlations between high transverse momentum charged hadrons (p _T >2.5 GeV/c) and all associated charged particles is presented at both short-range (small Δη) and long-range (large Δη) over a continuous detector acceptance covering −4<Δη<2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au + Au collisions at  GeV. The results provide new information about the longitudinal (Δη) extent of the near-side ‘ridge’ structure, first observed by the STAR Collaboration over a narrower η range. In central Au + Au collisions the ridge structure extends to at least Δη=4, and its strength completely diminishes as collisions become more peripheral. Presenter of the paper on the 3rd International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions, 8–14 June 2008, Illa da Toxa (Galicia-Spain).     

Identified two-hadron correlations at STAR using Λ, Λ̄, and K0Short triggers with charged hadrons in Au + Au collisions at $\sqrt{s_{NN}}$ =200 GeV

Here we present initial studies of two-dimensional angular correlations of Λ, Λ̄, and K⁰ _Short triggers with unidentified charged hadrons in Au+Au collisions at  =200 GeV measured by STAR. Distributions of pseudo-rapidity difference, Δη, and azimuthal separation, Δφ, are constructed containing structures observed in unidentified hadron correlations, including a jet peak at small Δη-Δφ accompanied by a flow-like ridge extended over Δη. These features are studied as a function of centrality via integrated yields and fitting to projections onto Δη and Δφ axes. Yields are found to be consistent with unidentified correlation analyses, and no clear distinction is observed between the three species. PACS  25.75.Gz     

Investigation of the high-p<Subscript>T</Subscript> strange baryon anomalies at RHIC

Results from RHIC have shown that there is an enhanced baryon/meson ratio in the intermediate transverse momentum range (2<p_T<6 GeV/c) in Au+Au collisions at both  =130 and 200 GeV. This was initially demonstrated by measurements of the p̄/π^- ratio which was then extended in p_T by the Λ/K⁰ _S ratio. The data were successfully described by models utilising different hadronization mechanisms: those having recombination of quarks and others having an interplay between flow, jet quenching and incorporating baryon junction loops. The strange particle data from the first Au+Au run at  =200 GeV gave tantalising hints that the observed enhancement of baryons compared to mesons was diminished by a p_T of 6 GeV/c, but a lack of statistics in this range made a definitive statement impossible. Here we present an extended analysis of identified strange baryons and mesons in Au+Au collisions at  =200 GeV using data obtained by the STAR experiment from the 2004 running period. The increase in statistics extends the measurement of Λ hyperons out to at least 7 GeV/c and K⁰ _S mesons out to 9 GeV/c. This data allows us to place limits on the range where in-vacuum fragmentation functions are applicable and the effect of baryon dominance is reduced. We also discuss the prospects for making these measurements using multiply-strange baryons and mesons (Ω and ϕ).     

Centrality dependence of v 2 in Au + Au at \sqrt{s_{NN}}=200 GeV

One of the most striking results is the large elliptic flow (v ₂) at RHIC. Detailed mass and transverse momentum dependence of elliptic flow are well described by ideal hydrodynamic calculations for p _T<1 GeV/c, and by parton coalescence/recombination picture for p _T=2–6 GeV/c. The systematic error on v ₂ is dominated by so-called “non-flow effects”, which are correlations other than flow, such as resonance decays and jets. It is crucial to understand and reduce the systematic error from non-flow effects in order to understand the underlying collision dynamics. In this paper, we present the centrality dependence of v ₂ with respect to the first harmonic event plane at ZDC-SMD (v ₂{ZDC-SMD}) in Au + Au collisions at  GeV. A large rapidity gap (|Δη|>6) between midrapidity and the ZDC-SMD could enable us to minimize possible non-flow contributions. We compare the results of v ₂{ZDC-SMD} with v ₂{BBC}, which is measured by event plane determined at |η|=3.1–3.9. Possible non-flow contributions in those results will be discussed.     

Test of quantum mechanics by neutron interferometry

Measurements of the π⁰ and direct photon nuclear modification factors in p+p and A+A collisions in the PHENIX experiment at RHIC will be reviewed and recent results at  GeV will be presented. Using for the first time the p+p reference measured in the same experiment instead of averaging world data the π⁰ suppression turns out to be almost as large at  GeV as at  GeV, implying gluon densities dN^g/dy>800. Possible origins of photon suppression at high p_T in  GeV are discussed in light of the new results on photon R_AA at  GeV.     

On the Rate of Explosion for Infinite Energy Solutions of the Spatially Homogeneous Boltzmann Equation

Let μ ₀ be a probability measure on ℝ³ representing an initial velocity distribution for the spatially homogeneous Boltzmann equation for pseudo Maxwellian molecules. As long as the initial energy is finite, the solution μ _t will tend to a Maxwellian limit. We show here that if , then instead, all of the mass “explodes to infinity” at a rate governed by the tail behavior of μ ₀. Specifically, for L0, define

Neutral meson production at high p<Subscript>T</Subscript> with the PHENIX experiment at RHIC

Over the first five years of operation the PHENIX experiment at RHIC has collected a wealth of data for various systems and collision energies that is providing valuable information for the understanding of the suppression pattern observed in central Au+Au collisions at . An overview on transverse-momentum (p_T) spectra of π⁰ and η in different collision energies and systems is presented.     

<Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> suppression and <Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> spectra in RHIC and LHC energy collisions

In a hydrodynamic model, we have studied J/ψ production in Au+Au/Cu+Cu collisions at RHIC energy, GeV. At the initial time, J/ψ’s are randomly distributed in the fluid. As the fluid evolves in time, the free streaming J/ψ’s are dissolved if the local fluid temperature exceeds a threshold temperature T _J/ψ . Sequential melting of charmonium states (χ _c , ψ ^′ and J/ψ), with melting temperatures , T _J/ψ ≈2T _c and feed-down fraction F≈0.3, explains the PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions. J/ψ p _T spectra and the nuclear modification factor in Au+Au collisions are also well explained in the model. The model however overpredicts the centrality dependence of J/ψ suppression in Cu+Cu collisions by 20–30%. The J/ψ p _T spectra are underpredicted by 20–30%. The model predicts that in central Pb+Pb collisions at LHC energy,  GeV, J/ψ’s are suppressed by a factor of ∼10. The model predicted a J/ψ p _T distribution in Pb+Pb collisions at LHC is similar to that in Au+Au collisions at RHIC.     

First-Order Intertwining Operators with Position Dependent Mass and <Emphasis Type="Italic">η</Emphasis>-Weak-Pseudo-Hermiticity Generators

A Hermitian and an anti-Hermitian first-order intertwining operators are introduced and a class of η-weak-pseudo-Hermitian position-dependent mass (PDM) Hamiltonians are constructed. A corresponding reference-target η-weak-pseudo-Hermitian PDM—Hamiltonians’ map is suggested. Some η-weak-pseudo-Hermitian -symmetric Scarf II and periodic-type models are used as illustrative examples. Energy-levels crossing and flown-away states phenomena are reported for the resulting Scarf II spectrum. Some of the corresponding η-weak-pseudo-Hermitian Scarf II- and periodic-type-isospectral models ( -symmetric and non- -symmetric) are given as products of the reference-target map.     

Study of B c → J / ψ π , η c π decays with?perturbative?QCD?approach

The B _c →J/ψ π, η _c π decays are studied with the perturbative QCD approach. It is found that the form factors and for the B _c →J/ψ, η _c transitions and the branching ratios are sensitive to the parameters ω, v, f _J/ψ and , where ω and v are the parameters of the charmonium wave functions for a Coulomb potential and the harmonic-oscillator potential, respectively, and f _J/ψ and are the decay constants of the J/ψ and η _c mesons, respectively. The large branching ratios and the clear signals of the final states make the B _c →J/ψ π, η _c π decays the prospective channels for measurements at the hadron colliders.     

Recent high-<Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> results from STAR

We present selected recent results of multi-hadron correlation measurements in azimuth and pseudorapidity at intermediate and high p _T in Au+Au collisions at , from the STAR experiment at RHIC. At intermediate p _T , measurements are presented that attempt to determine the origin of the associated near-side (small Δφ) yield at large pseudo-rapidity difference Δη that is found to be present in heavy ion collisions. In addition, results are reported on new multi-hadron correlation measures at high-p _T that use di-hadron triggers and multi-hadron cluster triggers with the goal to constrain the underlying jet kinematics better than in the existing measurements of inclusive spectra and di-hadron correlations.     

Study of W-boson polarisations and triple gauge boson couplings in the reaction e+e-→W+W- at LEP 2

A determination of the single W spin density matrix (SDM) elements in the reaction e⁺e^-→W⁺W^-→lνqq̄(l=e/μ) is reported at centre-of-mass energies between 189 and 209 GeV. The data sample used corresponds to an integrated luminosity of 520 pb^-1 taken by DELPHI between 1998 and 2000. The single W SDM elements, ρ_ττ’ ^W± (τ,τ’=± 1 or 0), are determined as a function of the W^- production angle with respect to the e^- beam direction and are obtained from measurements of the W decay products by the application of suitable projection operators, Λ_ττ’, which assume the V-A coupling of the W-boson to fermions. The measured SDM elements are used to obtain the fraction of longitudinally polarised Ws, with the result: at a mean energy of 198 GeV. The SDM elements are also used to determine the triple gauge couplings Δg₁ ^Z,Δκ_γ,λ_γ and g₄ ^Z, and . For the CP-violating couplings the results of single parameter fits are: The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model.     

Determination of the η and η<Superscript>′</Superscript> mixing angle from the pseudoscalar transition form factors

The possible range of the η– mixing angle is determined from the transition form factors F_ηγ(Q²) and with the help of up-to-date experimental data. For this purpose, the quark-flavor mixing scheme is adopted and the pseudoscalar transition form factors are calculated in the framework of light-cone pQCD, in which the transverse-momentum corrections and the contributions beyond the leading Fock state have been carefully taken into consideration. We construct a phenomenological expression to estimate the contributions to the form factors beyond the leading Fock state, based on their asymptotic behavior at Q²→0 and . By taking the quark-flavor mixing scheme, our results lead to , where the first error comes from the experimental uncertainty and the second error from the uncertainties of the parameters of the wavefunction. The possible intrinsic charm component in η and is discussed, and our present analysis also disfavors a large intrinsic charm component in η and , e.g. . PACS 13.40.Gp; 12.38.Bx; 14.40.Aq     

What can we learn from high-p<Subscript>T</Subscript> correlations of neutral strange baryons and mesons at RHIC?

We present results on two-particle azimuthal correlations of neutral strange baryons (Λ, Λ̄) and mesons (K_S ⁰) for p_T=2–6 GeV/c associated with non-identified charged particles in d+Au and Au+Au collisions at  =200 GeV measured by the STAR experiment. We investigate in detail the associated yield of charged particles as a function centrality of the collision and transverse momentum of trigger and associated particles to look for possible flavor, baryon/meson and particle/anti-particle differences. We compare our results to the proton and pion triggered correlations as well as to a fragmentation and recombination model. PACS 25.75.-q; 25.75.Gz     

Suppression of high-pT non-photonic electrons in Au+Au collisions at $\sqrt{s_{NN}}$ =200 GeV

The strong suppression of high p_T hadrons observed at RHIC has led to the interpretation that energetic partons lose their energy via induced gluon radiation in the hot and dense matter before fragmenting into hadrons. The study of heavy quark production can extend our understanding of this scenario. Due to the dead cone effect, the suppression of heavy quark mesons at high p_T is expected to be smaller than that observed for charged hadrons at the same energy. The measurement of non-photonic single electrons up to high p_T provides information on charm and beauty production. The semi-leptonic decays of D and B mesons are the dominant contribution to the non-photonic electron spectra. The preliminary spectra from p+p, d+Au and Au+Au collisions at  =200 GeV have been extracted for mid-rapidity non-photonic electrons in the range 1.5<p_T (GeV/c)<10. The corresponding nuclear modification factors (R_AA) are presented and show a large suppression in central Au+Au collisions, indicating an unexpectedly large energy loss for heavy quarks in the hot and dense matter created at RHIC. This observed suppression is compared to recent theoretical models. PACS  13.85.Qk; 13.20.Fc; 13.20.He; 25.75.Dw     

Measurement of single muon yields from charm semileptonic decay at $\sqrt{s_{NN}}$ =200 GeV Au + Au collisions at STAR

We report on the first measurement of single muon from charm semileptonic decays at low transverse momentum (p_T) in  =200 GeV Au+Au collisions. Muon identification was obtained using the STAR time projection chamber in conjunction with a time-of-flight detector. The p_T spectra of electron and muon from charm semileptonic decays are presented. The measured D→μ+X at p_T<0.25 GeV/c greatly constrains the charm total cross section. The charm differential cross section dσ_cc/dy is found to be consistent with the number of binary collision scaling. PACS  25.75.Dw; 25.75.-q     

Five-Body Calculation of Resonance and Scattering States of the uudd\bar{s} System

The scattering problem of the system, in the standard non-relativistic quark model of Isgur-Karl, is solved for the first time, by treating the large five-body model space, including the NK scattering channel, accurately with the Gaussian expansion method and the Kohn-type coupled-channel variational method. The calculated NK scattering phase shift shows no resonance in the energy region of the reported pentaquark Θ⁺(1540) that is, at 0–500 MeV above the NK threshold (1.4–1.9 GeV in mass). The phase shift does show two resonances just above 500 MeV: a broad ⁺ resonance with a width of Γ ∼ 110 MeV located at ∼ 520 MeV (∼ 2.0 GeV in mass) and a sharp ⁻ resonance with Γ = 0.12 MeV at 540 MeV.