Direct photons measured by the PHENIX experiment at RHIC

Results from the PHENIX experiment at RHIC on direct photon production in p+p, d+Au, and Au+Au collisions at sqrt(s_NN) = 200 GeV are presented. In p+p collisions, direct photon production at high p_T behaves as expected from perturbative QCD calculations. The p+p measurement serves as a baseline for direct photon production in Au+Au collisions. In d+Au collisions, no effects of cold nuclear matter are found within the large uncertainty of the measurement. In Au+Au collisions, the production of high p_T direct photons scales as expected for particle production in hard scatterings. This supports jet quenching models, which attribute the suppression of high p_T hadrons to the energy loss of fast partons in the medium produced in the collision. Low p_T direct photons, measured via e+e- pairs with small invariant mass, are possibly related to the production of thermal direct photons.Comment: 5 pages, 5 figures, Proceedings of the Hot Quarks 2006 Workshop for young scientists on the physics of ultra-relativistic nucleus-nucleus collisions, Villasimius, Sardinia, Italy, May 15--20, 200

First-Order System Least Squares and the Energetic Variational Approach for Two-Phase Flow

This paper develops a first-order system least-squares (FOSLS) formulation for equations of two-phase flow. The main goal is to show that this discretization, along with numerical techniques such as nested iteration, algebraic multigrid, and adaptive local refinement, can be used to solve these types of complex fluid flow problems. In addition, from an energetic variational approach, it can be shown that an important quantity to preserve in a given simulation is the energy law. We discuss the energy law and inherent structure for two-phase flow using the Allen-Cahn interface model and indicate how it is related to other complex fluid models, such as magnetohydrodynamics. Finally, we show that, using the FOSLS framework, one can still satisfy the appropriate energy law globally while using well-known numerical techniques.Comment: 22 pages, 8 figures submitted to Journal of Computational Physic

Di-hadron azimuthal correlation and Mach-like cone structure in parton/hadron transport model

In the framework of a multi-phase transport model (AMPT) with both partonic and hadronic interactions, azimuthal correlations between trigger particles and associated scattering particles have been studied by the mixing-event technique. The momentum ranges of these particles are $3< p^{trig}_T< 6$ GeV/$c$ and $0.15< p_{T}^{assoc} < 3$ GeV/$c$ (soft), or $2.5<p^{trig}_T<$ 4 GeV/$c$ and $1< p_{T}^{assoc} < 2.5$ GeV/$c$ (hard) in Au + Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. A Mach-like structure has been observed in correlation functions for central collisions. By comparing scenarios with and without parton cascade and hadronic rescattering, we show that both partonic and hadronic dynamical mechanisms contribute to the Mach-like structure of the associated particle azimuthal correlations. The contribution of hadronic dynamical process can not be ignored in the emergence of Mach-like correlations of the soft scattered associated hadrons. However, hadronic rescattering alone cannot reproduce experimental amplitude of Mach-like cone on away-side, and the parton cascade process is essential to describe experimental amplitude of Mach-like cone on away-side. In addition, both the associated multiplicity and the sum of $p_{T}$ decrease, whileas the $$ increases, with the impact parameter in the AMPT model including partonic dynamics from string melting scenario.Comment: 9 pages, 5 figures; Physics Letters B 641, 362-367 (2006

Robust block preconditioners for biot’s model

In this paper, we design robust and efficient block preconditioners for the two-field formulation of Biot’s consolidation model, where stabilized finite-element discretizations are used. The proposed block preconditioners are based on the well-posedness of the discrete linear systems. Block diagonal (norm-equivalent) and block triangular preconditioners are developed, and we prove that these methods are robust with respect to both physical and discretization parameters. Numerical results are presented to support the theoretical results

Robust preconditioners for a new stabilized discretization of the poroelastic equations

In this paper, we present block preconditioners for a stabilized discretization of the poroelastic equations developed in [C. Rodrigo, X. Hu, P. Ohm, J. Adler, F. Gaspar, and L. Zikatanov, Comput. Methods Appl. Mech. Engrg., 341 (2018), pp. 467-484]. The discretization is proved to be well-posed with respect to the physical and discretization parameters and thus provides a framework to develop preconditioners that are robust with respect to such parameters as well. We construct both norm-equivalent (diagonal) and field-of-value-equivalent (triangular) preconditioners for both the stabilized discretization and a perturbation of the stabilized discretization, which leads to a smaller overall problem after static condensation. Numerical tests for both two-and three-dimensional problems confirm the robustness of the block preconditioners with respect to the physical and discretization parameters

System-size scan of dihadron azimuthal correlations in ultra-relativistic heavy ion collisions

System-size dependence of dihadron azimuthal correlations in ultra-relativistic heavy ion collision is simulated by a multi-phase transport model. The structure of correlation functions and yields of associated particles show clear participant path-length dependences in collision systems with a partonic phase. The splitting parameter and root-mean-square width of away-side correlation functions increase with collision system size from $^{14}$N+$^{14}$N to $^{197}$Au+$^{197}$Au collisions. The double-peak structure of away-side correlation functions can only be formed in sufficient "large" collision systems under partonic phase. The contrast between the results with partonic phase and with hadron gas could suggest some hints to study onset of deconfinment.Comment: 8 pages, 4 figures, 1 table; Nucl. Phys. A (accepted

Adler Function, Sum Rules and Crewther Relation of Order O(alpha_s^4): the Singlet Case

The analytic result for the singlet part of the Adler function of the vector current in a general gauge theory is presented in five-loop approximation. Comparing this result with the corresponding singlet part of the Gross-Llewellyn Smith sum rule [1], we successfully demonstrate the validity of the generalized Crewther relation for the singlet part. This provides a non-trivial test of both our calculations and the generalized Crewther relation. Combining the result with the already available non-singlet part of the Adler function [2,3] we arrive at the complete ${\cal O}(\alpha_s^4)$ expression for the Adler function and, as a direct consequence, at the complete ${\cal O}(\alpha_s^4)$ correction to the $e^+ e^-$ annihilation into hadrons in a general gauge theory.Comment: 4 pages, 1 figure. Final published versio

One-Loop QCD Mass Effects in the Production of Polarized Bottom and Top Quarks

The analytic expressions for the production cross sections of polarized bottom and top quarks in $e^+e^-$ annihilation are explicitly derived at the one-loop order of strong interactions. Chirality-violating mass effects will reduce the longitudinal spin polarization for the light quark pairs by an amount of $3\%$, when one properly considers the massless limit for the final quarks. Numerical estimates of longitudinal spin polarization effects in the processes $e^+e^-\to b\bar{b}(g)$ and $e^+e^- \to t\bar{t}(g)$ are presented.Comment: 17 p. (5 figs available upon request), LaTeX, MZ-TH/93-30, RAL/93-81, FTUV/93-4

Adler Function, DIS sum rules and Crewther Relations

The current status of the Adler function and two closely related Deep Inelastic Scattering (DIS) sum rules, namely, the Bjorken sum rule for polarized DIS and the Gross-Llewellyn Smith sum rule are briefly reviewed. A new result is presented: an analytical calculation of the coefficient function of the latter sum rule in a generic gauge theory in order O(alpha_s^4). It is demonstrated that the corresponding Crewther relation allows to fix two of three colour structures in the O(alpha_s^4) contribution to the singlet part of the Adler function.Comment: Talk presented at 10-th DESY Workshop on Elementary Particle Theory: Loops and Legs in Quantum Field Theory, W\"orlitz, Germany, 25-30 April 201

Constraining the parameters of binary systems through time-dependent light deflection

A theory is derived relating the configuration of the cores of active galaxies, specifically candidates for presumed super-massive black hole binaries (SMBHBs), to time-dependent changes in images of those galaxies. Three deflection quantities, resulting from the monopole term, mass quadrupole term, and spin dipole term of the core, are examined. The resulting observational technique is applied to the galaxy 3C66B. This technique is found to under idealized circumstances surpass the technique proposed by Jenet et al. in accuracy for constraining the mass of SMBHB candidates, but is exceeded in accuracy and precision by Jenet's technique under currently-understood likely conditions. The technique can also under favorable circumstances produce results measurable by currently-available astronomical interferometry such as very-long baseline-interferometry (VLBI).Comment: 15 pages, 2 figures, accepted in General Relativity & Gravitatio