Photon Asymmetry Research Articles

Equation of state and initial temperature of quark gluon plasma at RHIC

Abstract In gold-gold collisions of the Relativistic Heavy Ion Collider a perfect fluid of strongly interacting quark gluon plasma (sQGP) is created. The time evolution of this fluid can be described by hydrodynamical models. After an expansion, hadrons are created during the freeze-out period. Their distribution reveals information about the final state. To investigate the time evolution one needs to analyze penetrating probes: e.g. direct photon observations. In this paper we analyze a 1+3 dimensional solution of relativistic hydrodynamics. We calculate momentum distribution, azimuthal asymmetry and momentum correlations of direct photons. Based on earlier fits to hadronic spectra, we compare photon calculations to measurements to determine the equations of state and the initial temperature of sQGP. We find that the initial temperature in the center of the fireball is 507±12 MeV, while for the sound speed we get c s=0.36±0.02. We also estimate a systematic error of these results. We find that the measured azimuthal asymmetry is also compatible with this model. We also predict a photon source that is significantly larger in the out direction than in the side direction.

Erratum: Neutron spin rotation inn⃗-dscattering [Phys. Rev. C78, 014002 (2008)

The neutron spin rotation induced by parity-violating (PV) components in the nucleon-nucleon potential is studied in \vec{n}-d scattering at zero energy. Results are obtained corresponding to the Argonne v_{18} two-nucleon and Urbana-IX three-nucleon strong-interaction potentials in combination with either the DDH or pionless EFT model for the weak-interaction potential. We find that this observable is dominated by the contribution of the long-range part of the PV potential associated with pion exchange. Thus its measurement could provide a further constraint, complementary to that coming from measurements of the photon asymmetry in \vec{n}-p radiative capture, on the strength of this component of the hadronic weak interaction.

First measurement of the circular beam asymmetry in the [formula omitted] reaction

The circular photon asymmetry for π0η photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Δ(1700)D33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D33 amplitude.

The Bjorken sum rule with Monte Carlo and Neural Network techniques

Determinations of structure functions and parton distribution functions have been recently obtained using Monte Carlo methods and neural networks as universal, unbiased interpolants for the unknown functional dependence. In this work the same methods are applied to obtain a parametrization of polarized Deep Inelastic Scattering (DIS) structure functions. The Monte Carlo approach provides a bias-free determination of the probability measure in the space of structure functions, while retaining all the information on experimental errors and correlations. In particular the error on the data is propagated into an error on the structure functions that has a clear statistical meaning. We present the application of this method to the parametrization from polarized DIS data of the photon asymmetries A1p and A1d from which we determine the structure functions g1p(x,Q2) and g1d(x,Q2), and discuss the possibility to extract physical parameters from these parametrizations. This work can be used as a starting point for the determination of polarized parton distributions.

Impact parameter and beam energy dependence for azimuthal asymmetry of direct photons and free protons in intermediate energy heavy-ion collisions

Hard photon emitted from energetic heavy ion collisions is of very interesting since it does not experience the late-stage nuclear interaction, therefore it is useful to explore the early-stage information of matter phase. In this work, we have calculated the impact parameter and beam energy dependence for azimuthal asymmetry, characterized by directed transverse flow parameter F and elliptic asymmetry coefficient v2, of direct photons and the corresponding free protons in intermediate energy heavy-ion collisions. It is further shown the anti-correlated azimuthal asymmetry between direct photons and free protons.

Beam asymmetries in near-thresholdωphotoproduction off the proton

The photoproduction of {omega} mesons off protons has been studied at the Bonn ELSA accelerator from threshold to E{sub {gamma}}=1700 MeV. Linearly polarized beams were produced via coherent bremsstrahlung. Large photon asymmetries in excess of 50% were obtained. For the first time the pion asymmetries associated with the {omega}{yields}{pi}{sup 0}{gamma} decay were measured and found close to zero. The asymmetries indicate s-channel resonance formation on top of t-channel exchange processes.

THE ROLE OF THE QCD ANOMALY IN KAON PHOTOPRODUCTION

We study the kaon photoproduction associated with the production of the ground state Λ. We investigate the role of K and K* exchange in the t -channel, which can be tested by the photon asymmetry in comparison with experimental data. We find a significant amount of contribution from the one-loop diagram induced by the gauged Wess-Zumino-Witten term for QCD anomaly. This enable us to use coupling constants which are determined in a microscopic way, unlike the previous phenomenological studies where the K* coupling strengths were treated as a free parameter.

Handedness of direct photons

The azimuthal asymmetry of direct photons originating from primary hard scatterings between partons is calculated. This can be accounted for by the inclusion of the color dipole orientation, which is sensitive to the rapid variation of the nuclear profile. To this end we introduce the dipole orientation within the saturation model of Golec-Biernat and Wusthoff, while preserving all its features at the cross-section level. We show that the direct photon elliptic anisotropy v2 coming from this mechanism changes sign and becomes negative for peripheral collisions, albeit it is quite small for nuclear collisions at the RHIC energy.

Neutron spin rotation inn→−dscattering

The neutron spin rotation induced by parity-violating (PV) components in the nucleon-nucleon potential is studied in $\stackrel{\ensuremath{\rightarrow}}{n}\text{\ensuremath{-}}d$ scattering at zero energy. Results are obtained corresponding to the Argonne ${v}_{18}$ two-nucleon and Urbana-IX three-nucleon strong-interaction potentials in combination with either the Desplanques-Donoghue-Holstein or pionless effective field theory models for the weak-interaction potential. We find that this observable is dominated by the contribution of the long-range part of the PV potential associated with pion exchange. Thus its measurement could provide a further constraint, complementary to that coming from measurements of the photon asymmetry in $\stackrel{\ensuremath{\rightarrow}}{n}\text{\ensuremath{-}}p$ radiative capture, on the strength of this component of the hadronic weak interaction.

Magnetic interaction induced by the anomaly in kaon-photoproductions

We study the role of magnetic interaction in the photoproduction of the kaon and hyperon. We find that the inclusion of a higher order diagram induced by the Wess–Zumino–Witten term has a significant contribution to the magnetic amplitude, which is compatible to the observed photon asymmetry in the forward angle region. This enables us to use the K∗ coupling constants which have been determined in a microscopic way rather than the phenomenological ones which differ largely from the microscopic ones.

Azimuthal asymmetry of direct photons in intermediate energy heavy-ion collisions

Hard photon emitted from energetic heavy ion collisions is of very interesting since it does not experience the late-stage nuclear interaction, therefore it is useful to explore the early-stage information of matter phase. In this work, we have presented a first calculation of azimuthal asymmetry, characterized by directed transverse flow parameter F and elliptic asymmetry coefficient v2, for proton–neutron bremsstrahlung hard photons in intermediate energy heavy-ion collisions. The positive F and negative v2 of direct photons are illustrated and they seem to be anti-correlated to the corresponding free proton's flow.

Nucleon and Δ resonances inKΣ(1385)photoproduction from nucleons

The reaction mechanisms for $K\ensuremath{\Sigma}(1385)$ photoproduction from the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\Sigma}}^{0}(1385)$ in the resonance energy region are investigated in a hadronic model. Both contributions from $N$ and \ensuremath{\Delta} resonances of masses around 2 GeV as given in the Review of Particle Data Group and by the quark model predictions are included. The Lagrangians for describing the decays of these resonances into $K\ensuremath{\Sigma}(1385)$ are constructed with the coupling constants determined from the decay amplitudes predicted by a quark model. Comparing the resulting total cross section for the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\Sigma}}^{0}(1385)$ with the preliminary data from the Thomas Jefferson National Accelerator Facility, we find that the most important contributions are from the two-star-rated resonances $\ensuremath{\Delta}(2000){F}_{35},\ensuremath{\Delta}(1940){D}_{33}$, and $N(2080){D}_{13}$, as well as the missing resonance $N{\frac{3}{2}}^{\ensuremath{-}}(2095)$ predicted in the quark model. Predictions on the differential cross section and photon asymmetry in this reaction are also given.

Azimuthal anisotropy of direct photons

The electromagnetic bremsstrahlung produced by a quark interacting with nucleons or nuclei is azimuthally asymmetric. In the light-cone dipole approach this effect is related to the orientation dependent dipole cross section. Such a radiation anisotropy is expected to contribute to the azimuthal asymmetry of direct photons in $pA$ and $AA$ collisions, as well as in deep-inelastic scattering and in the production of dileptons.

Regge approach to charged pion photoproduction at invariant energies above 2 GeV

A Regge model with absorptive corrections is employed in a global analysis of the world data on positive and negative pion photoproduction for photon energies from 3 to 8GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross-sections and single polarization observables at - t ⩽ 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3GeV. Differences between our predictions and data in this energy region are systematically examined as possible signals for the presence of excited baryons. We find that the data available for the polarized photon asymmetry show promising resonance signatures at invariant energies around 2GeV. With regard to differential cross-sections the analysis of negative pion photoproduction data, obtained recently at JLab, indicates likewise the presence of resonance structures around 2GeV.

Hadronic parity violation in [formula omitted] with effective field theory

The parity-violating nucleon–nucleon (NN) potential is considered up to next-to-next-to leading order in heavy-baryon chiral perturbation theory. We include one-pion exchange at leading order and the two-pion exchange and two-nucleon contact terms at next-to-next-to-leading order. The effects of intermediate (two-pion exchange) and short-range (two-nucleon contact) terms are probed by calculating the photon asymmetry Aγ in n→p→dγ employing Siegert's theorem and an accurate phenomenological potential for the parity-conserving NN interaction. We explore in detail the uncertainties due to the parameters that control the contribution of the short-range interaction. We obtain about 20% uncertainty in the value of Aγ up to the next-to-next-to leading order. We discuss the implication of this uncertainty for the determination of the weak pion–nucleon coupling constant and how the uncertainty can be reduced.

Single transverse-spin asymmetry in very forward and very backward neutral particle production for polarized proton collisions at [formula omitted

In the 2001–2002 running period of the Relativistic Heavy Ion Collider (RHIC), transversely polarized protons were accelerated to 100 GeV for the first time, with collisions at s=200 GeV. We present results from this run for single transverse-spin asymmetries for inclusive production of neutral pions, photons and neutrons of the energy region 20–100 GeV for forward and backward production for angles between 0.3 mrad and 2.2 mrad with respect to the polarized proton direction. An asymmetry of AN=(−0.090±0.006±0.009)×(1.0−0.24+0.47) was observed for forward neutron production, where the errors are statistical and systematic, and the scale error is from the beam polarization uncertainty. The forward photon and π0, and backward neutron, photon, and π0 asymmetries were consistent with zero. The large neutron asymmetry indicates a strong interference between a spin–flip amplitude, such as one pion exchange which dominates lower energy neutron production, and remaining spin–non-flip amplitudes such as reggeon exchange.

Dependence of the C12(γ→,pd) reaction on photon linear polarisation

The sensitivity of the C 12 ( γ → , pd ) reaction to photon linear polarisation has been determined at MAMI, giving the first measurement of the reaction for a nucleus heavier than 3 He. Photon asymmetries and cross sections were measured for E γ = 170 to 350 MeV. For E γ below the Δ resonance, reactions leaving the residual 9 Be near its ground state show a positive asymmetry of up to 0.3, similar to that observed for 3 He suggesting a similar reaction mechanism for the two nuclei.

Azimuthal flow of decay photons in relativistic nuclear collisions

An overwhelming fraction of photons from relativistic heavy-ion collisions has its origin in the decay of ${\ensuremath{\pi}}^{0}$ and \ensuremath{\eta} mesons. We calculate the azimuthal asymmetry of the decay photons for several azimuthally asymmetric pion distributions. We find that the ${k}_{T}$ dependence of the elliptic flow parameter ${v}_{2}$ for the decay photons closely follows the elliptic flow parameter ${v}_{2}^{{\ensuremath{\pi}}^{0}}$ evaluated at ${p}_{T}\ensuremath{\approx}{k}_{T}+\ensuremath{\delta}$, where $\ensuremath{\delta}\ensuremath{\approx}0.1\text{\ensuremath{-}}0.2$ GeV, for typical pion distributions measured in nucleus-nucleus collisions at relativistic energies. Similar results are obtained for photons from the $2\text{\ensuremath{-}}\ensuremath{\gamma}$ decay of \ensuremath{\eta} mesons. Assuming that the flow of ${\ensuremath{\pi}}^{0}$ is similar to those for ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ for which independent measurements would be generally available, this ansatz can help in identifying additional sources for photons. Taken along with quark number scaling suggested by the recombination model, it may help to estimate ${v}_{2}$ of the parton distributions in terms of azimuthal asymmetry of the decay photons at large ${k}_{T}$.

Measurement of the x- and Q2-dependence of the asymmetry A1 on the nucleon

We report results for the virtual photon asymmetry $A_1$ on the nucleon from new Jefferson Lab measurements. The experiment, which used the CEBAF Large Acceptance Spectrometer and longitudinally polarized proton ($^{15}$NH$_3$) and deuteron ($^{15}$ND$_3$) targets, collected data with a longitudinally polarized electron beam at energies between 1.6 GeV and 5.7 GeV. In the present paper, we concentrate on our results for $A_1(x,Q^2)$ and the related ratio $g_1/F_1(x,Q^2)$ in the resonance and the deep inelastic regions for our lowest and highest beam energies, covering a range in momentum transfer $Q^2$ from 0.05 to 5.0 GeV$^2$ and in final-state invariant mass $W$ up to about 3 GeV. Our data show detailed structure in the resonance region, which leads to a strong $Q^2$--dependence of $A_1(x,Q^2)$ for $W$ below 2 GeV. At higher $W$, a smooth approach to the scaling limit, established by earlier experiments, can be seen, but $A_1(x,Q^2)$ is not strictly $Q^2$--independent. We add significantly to the world data set at high $x$, up to $x = 0.6$. Our data exceed the SU(6)-symmetric quark model expectation for both the proton and the deuteron while being consistent with a negative $d$-quark polarization up to our highest $x$. This data setshould improve next-to-leading order (NLO) pQCD fits of the parton polarization distributions.

Incoherent pion photoproduction on the deuteron in the first resonance region

Incoherent pion photoproduction on the deuteron is studied in the first resonance region. The unpolarized cross section, the beam asymmetry, and the vector and tensor target asymmetries are calculated in the framework of a diagrammatic approach. Pole diagrams and one-loop diagrams with $\mathit{NN}$ scattering in the final state are taken into account. An elementary operator for pion photoproduction on the nucleon is taken in various on-shell forms and calculated using the SAID and MAID multipole analyses. Model dependence of the obtained results is discussed in some detail. A comparison with predictions of other works is given. Although a reasonable description of many available experimental data on the unpolarized total and differential cross sections and photon asymmetry has been achieved, in some cases a significant disagreement between the theory and experiment has been found. Invoking known information on the reactions $\ensuremath{\gamma}d\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}d$ and $\ensuremath{\gamma}d\ensuremath{\rightarrow}\mathit{np}$ we predict the total photoabsorption cross section for deuterium. We find that our values strongly overestimate experimental data in the vicinity of the \ensuremath{\Delta} peak.