Spectroscopy of resonance decays in high-energy heavy-ion collisions

Invariant mass distributions of the hadronic decay products from resonances formed in relativistic heavy ion collision (RHIC) experiments are investigated with a view to disentangle the effects of thermal motion and the phase space of decay products from those of intrinsic changes in the structure of resonances at the freeze-out conditions. Analytic results of peak mass shifts for the cases of both equal and unequal mass decay products are derived. The shift is expressed in terms of the peak mass and width of the vacuum or medium-modified spectral functions and temperature. Examples of expected shifts in meson (e.g., rho, omega, and sigma) and baryon (e.g., Delta) resonances that are helpful to interpret recent RHIC measurements at BNL are provided. Although significant downward mass shifts are caused by widened widths of the $\rho-$meson in medium, a downward shift of at least 50 MeV in its intrinsic mass is required to account for the reported downward shift of 60-70 MeV in the peak of the rho-invariant mass distribution. An observed downward shift from the vacuum peak value of the Delta distinctively signals a significant downward shift in its intrinsic peak mass, since unlike for the rho-meson, phase space functions produce an upward shift for the Delta isobar.Comment: published version with slight change of title and some typos corrected, 12 pages, 5 figure

Density functional study of the adsorption of K on the Ag(111) surface

Full-potential gradient corrected density functional calculations of the adsorption of potassium on the Ag(111) surface have been performed. The considered structures are Ag(111) (root 3 x root 3) R30degree-K and Ag(111) (2 x 2)-K. For the lower coverage, fcc, hcp and bridge site; and for the higher coverage all considered sites are practically degenerate. Substrate rumpling is most important for the top adsorption site. The bond length is found to be nearly identical for the two coverages, in agreement with recent experiments. Results from Mulliken populations, bond lengths, core level shifts and work functions consistently indicate a small charge transfer from the potassium atom to the substrate, which is slightly larger for the lower coverage.Comment: to appear in Phys Rev

The dihadron fragmentation function and its evolution

Dihadron fragmentation functions and their evolution are studied in the process of $e^+e^-$ annihilation. Under the collinear factorization approximation and facilitated by the cut-vertex technique, the two hadron inclusive cross section at leading order (LO) is shown to factorize into a short distance parton cross section and a long distance dihadron fragmentation function. We provide the definition of such a dihadron fragmentation function in terms of parton matrix elements and derive its DGLAP evolution equation at leading log. The evolution equation for the non-singlet quark fragmentation function is solved numerically with a simple ansatz for the initial condition and results are presented for cases of physical interest.Comment: 27 pages, 2 column, Revtex4, 21 figure

Light propagation in non-trivial QED vacua

Within the framework of effective action QED, we derive the light cone condition for homogeneous non-trivial QED vacua in the geometric optics approximation. Our result generalizes the ``unified formula'' suggested by Latorre, Pascual and Tarrach and allows for the calculation of velocity shifts and refractive indices for soft photons travelling through these vacua. Furthermore, we clarify the connection between the light velocity shift and the scale anomaly. This study motivates the introduction of a so-called effective action charge that characterizes the velocity modifying properties of the vacuum. Several applications are given concerning vacuum modifications caused by, e.g., strong fields, Casimir systems and high temperature.Comment: 13 pages, REVTeX, 3 figures, to appear in Phys. Rev.

Two- and Three-Point Functions in the Extended NJL Model

The two-point functions in generalized Nambu--Jona-Lasinio models are calculated to all orders in momenta and quark masses to leading order in $1/N_c$. The use of Ward identities and the heat-kernel expansion allows for a large degree of regularization independence. We also show how this approach works to the same order for three-point functions on the example of the vector-pseudoscalar-pseudoscalar three-point function. The inclusion of the chiral anomaly effects at this level is shown by calculating the pseudoscalar-vector-vector three-point function to the same order. Finally we comment on how (vector-)meson-dominance comes out in the presence of explicit chiral symmetry breaking in both the anomalous and the non-anomalous sectors.Comment: Latex, 42 pages, 3 latex figures, 7 postscript figures included, NORDITA 94/11 N,P. Improvement in the regularization procedure for the PVV three point functio

The effect of spontaneous collapses on neutrino oscillations

We compute the effect of collapse models on neutrino oscillations. The effect of the collapse is to modify the evolution of the `spatial' part of the wave function, which indirectly amounts to a change on the flavor components. In many respects, this phenomenon is similar to neutrino propagation through matter. For the analysis we use the mass proportional CSL model, and perform the calculation to second order perturbation theory. As we will show, the CSL prediction is very small - mainly due to the very small mass of neutrinos - and practically undetectable.Comment: 24 pages, RevTeX. Updated versio

The Ter-Mikayelian Effect on QCD Radiative Energy Loss

The color dielectric modification of the gluon dispersion relation in a dense QCD medium suppresses both the soft and collinear gluon radiation associated with jet production. We compute both the longitudinal and transverse plasmon contributions to the zeroth order in opacity radiative energy loss. This QCD analog of the Ter-Mikayelian effect in QED leads to $\sim 30$ reduction of the energy loss of high transverse momentum charm quarks produced in a QCD plasma with a characteristic Debye mass $\mu\sim 0.5$ GeV.Comment: 18 Pages, 16 Figure

Are direct photons a clean signal of a thermalized quark gluon plasma?

Direct photon production from a quark gluon plasma (QGP) in thermal equilibrium is studied directly in real time. In contrast to the usual S-matrix calculations, the real time approach is valid for a QGP that formed and reached LTE a short time after a collision and of finite lifetime ($\sim 10-20 \mathrm{fm}/c$ as expected at RHIC or LHC). We point out that during such finite QGP lifetime the spectrum of emitted photons carries information on the initial state. There is an inherent ambiguity in separating the virtual from the observable photons during the transient evolution of the QGP. We propose a real time formulation to extract the photon yield which includes the initial stage of formation of the QGP parametrized by an effective time scale of formation $\Gamma^{-1}$. This formulation coincides with the S-matrix approach in the infinite lifetime limit. It allows to separate the virtual cloud as well as the observable photons emitted during the pre- equilibrium stage from the yield during the QGP lifetime. We find that the lowest order contribution $\mathcal{O}(\alpha_{em})$ which does \emph{not} contribute to the S-matrix approach, is of the same order of or larger than the S-matrix contribution during the lifetime of the QGP for a typical formation time $\sim 1 \mathrm{fm}/c$. The yield for momenta $\gtrsim 3 \mathrm{Gev}/c$ features a power law fall-off $\sim T^3 \Gamma^2/k^{5}$ and is larger than that obtained with the S-matrix for momenta $\geq 4 \mathrm{Gev}/c$. We provide a comprehensive numerical comparison between the real time and S-matrix yields and study the dynamics of the build-up of the photon cloud and the different contributions to the radiative energy loss. The reliability of the current estimates on photon emission is discussed.Comment: 31 pages, 12 eps figures, version to appear in PR

Anomalous Pseudoscalar-Photon Vertex In and Out of Equilibrium

The anomalous pseudoscalar-photon vertex is studied in real time in and out of equilibrium in a constituent quark model. The goal is to understand the in-medium modifications of this vertex, exploring the possibility of enhanced isospin breaking by electromagnetic effects as well as the formation of neutral pion condensates in a rapid chiral phase transition in peripheral, ultrarelativistic heavy-ion collisions. In equilibrium the effective vertex is afflicted by infrared and collinear singularities that require hard thermal loop (HTL) and width corrections of the quark propagator. The resummed effective equilibrium vertex vanishes near the chiral transition in the chiral limit. In a strongly out of equilibrium chiral phase transition we find that the chiral condensate drastically modifies the quark propagators and the effective vertex. The ensuing dynamics for the neutral pion results in a potential enhancement of isospin breaking and the formation of $\pi^0$ condensates. While the anomaly equation and the axial Ward identity are not modified by the medium in or out of equilibrium, the effective real-time pseudoscalar-photon vertex is sensitive to low energy physics.Comment: Revised version to appear in Phys. Rev. D. 42 pages, 4 figures, uses Revte

Reflections on the Strong CP Problem

I discuss how anomalies affect classical symmetries and how, in turn, the non-trivial nature of the gauge theory vacuum makes these quantum corrections troublesome. Although no solution seems in sight for the cosmological constant problem, I examine three possible approaches to the strong CP problem involving vacuum dynamics, an additional chiral symmetry, and the possibility of spontaneous CP or P breaking. All of these "solutions" have their own problems and suggest that, at a deep level, we do not understand the nature of CP violation. Nevertheless, it remains extremely important to search for experimental signals predicted by these theoretical "solutions", like invisible axions.Comment: 11 pages, Latex fil