Forward Neutral-Pion Transverse Single-Spin Asymmetries in p plus p Collisions at s=200 GeV

We report precision measurements of the Feynman x (x(F)) dependence, and first measurements of the transverse momentum (p(T)) dependence, of transverse single-spin asymmetries for the production of pi(0) mesons from polarized proton collisions at s=200 GeV. The x(F) dependence of the results is in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the p(T) dependence at fixed x(F) are not consistent with these same perturbative QCD-based calculations.

Determination of the parity of the neutral pion via its four-electron decay

We present a new determination of the parity of the neutral pion via the double Dalitz decay pi(0) -> e(+)e(-)e(+)e(-). Our sample, which consists of 30511 candidate decays, was collected from K(L) -> pi(0)pi(0)pi(0) decays in flight at the KTeV-E799 experiment at Fermi National Accelerator Laboratory. We confirm the negative pi(0) parity and place a limit on scalar contributions to the pi(0) -> e(+)e(-)e(+)e(-) decay amplitude of less than 3.3% assuming CPT conservation. The pi(0)gamma(*)gamma(*) form factor is well described by a momentum-dependent model with a slope parameter fit to the final state phase-space distribution. Additionally, we have measured the branching ratio of this mode to be B(pi(0) -> e(+)e(-)e(+)e(-)) = (3.26 +/- 0.18) x 10(-5).

Quantitative constraints on the transport properties of hot partonic matter from semi-inclusive single high transverse momentum pion suppression in Au plus Au collisions at root S(NN)=200 GeV

The PHENIX experiment has measured the suppression of semi-inclusive single high-transverse-momentum pi(0)'s in Au+Au collisions at root s(NN) = 200 GeV. The present understanding of this suppression is in terms of energy loss of the parent (fragmenting) parton in a dense color-charge medium. We have performed a quantitative comparison between various parton energy-loss models and our experimental data. The statistical point-to-point uncorrelated as well as correlated systematic uncertainties are taken into account in the comparison. We detail this methodology and the resulting constraint on the model parameters, such as the initial color-charge density dN(g)/dy, the medium transport coefficient <(q) over cap >, or the initial energy-loss parameter epsilon(0). We find that high-transverse-momentum pi(0) suppression in Au+Au collisions has sufficient precision to constrain these model-dependent parameters at the +/- 20-25% (one standard deviation) level. These constraints include only the experimental uncertainties, and further studies are needed to compute the corresponding theoretical uncertainties.

Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at s=200 GeV

We report a measurement of the longitudinal double-spin asymmetry A(LL) and the differential cross section for inclusive pi(0) production at midrapidity in polarized proton collisions at s=200 GeV. The cross section was measured over a transverse momentum range of 1 < p(T)< 17 GeV/c and found to be in good agreement with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry was measured in the range of 3.7 < p(T)< 11 GeV/c and excludes a maximal positive gluon polarization in the proton. The mean transverse momentum fraction of pi(0)'s in their parent jets was found to be around 0.7 for electromagnetically triggered events.

Neutral pion production in Au plus Au collisions at root s(NN)=200 GeV

The results of midrapidity (0 < y < 0.8) neutral pion spectra over an extended transverse momentum range (1 < p(T) < 12 GeV/c) in root s(NN) = 200 GeV Au + Au collisions, measured by the STAR experiment, are presented. The neutral pions are reconstructed from photons measured either by the STAR Barrel Electro-Magnetic Calorimeter or by the Time Projection Chamber via tracking of conversion electron-positron pairs. Our measurements are compared to previously published pi(+/-) and pi(0) results. The nuclear modification factors R(CP) and R(AA) of pi(0) are also presented as a function of p(T). In the most central Au + Au collisions, the binary collision scaled pi(0) yield at high p(T) is suppressed by a factor of about 5 compared to the expectation from the yield of p + p collisions. Such a large suppression is in agreement with previous observations for light quark mesons and is consistent with the scenario that partons suffer considerable energy loss in the dense medium formed in central nucleus-nucleus collisions at the Relativistic Heavy Ion Collider.

Pion interferometry in Au plus Au and Cu plus Cu collisions at s(NN)=62.4 and 200 GeV

We present a systematic analysis of two-pion interferometry in Au+Au collisions at s(NN)=62.4 GeV and Cu+Cu collisions at s(NN)=62.4 and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The multiplicity and transverse momentum dependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.

Pion femtoscopy in p plus p collisions at root s=200 GeV

The STAR Collaboration at the BNL Relativistic Heavy Ion Collider has measured two-pion correlation functions from p + p collisions at root s = 200 GeV. Spatial scales are extracted via a femtoscopic analysis of the correlations, though this analysis is complicated by the presence of strong nonfemtoscopic effects. Our results are put into the context of the world data set of femtoscopy in hadron-hadron collisions. We present the first direct comparison of femtoscopy in p + p and heavy ion collisions, under identical analysis and detector conditions.

Pion mass effects on axion emission from neutron stars through NN bremsstrahlung processes

The rates of axion emission by nucleon-nucleon bremsstrahlung are calculated with the inclusion of the full momentum contribution from a nuclear one pion exchange (OPE) potential. The contributions of the neutron-neutron (nn), proton-proton (pp) and neutron-proton (np) processes in both the non-degenerate and degenerate limits are explicitly given. We find that the finite-momentum corrections to the emissivities are quantitatively significant for the non-degenerate regime and temperature-dependent, and should affect the existing axion mass hounds. The trend of these nuclear effects is to diminish the emissivities. (C) 2009 Elsevier B.V. All rights reserved.

Pion: space-like structure

The scalar form factor describes modifications induced by the pion over the quark condensate. Assuming that representations produced by chiral perturbation theory can be pushed to high values of negative-t, a region in configuration space is reached (r < R similar to 0.5 fm) where the form factor changes sign, indicating that the condensate has turned into empty space. A simple model for the pion incorporates this feature into density functions. When supplemented by scalar-meson excitations, it yields predictions close to empirical values for the mean square radius (< r(2)>(pi)(S) = 0.59 fm(2)) and for one of the low energy constants ((l) over bar (4) = 4.3), with no adjusted parameters.

Accuracy of three electronic apex locators in the presence of different irrigating solutions

The present study compared the accuracy of three electronic apex locators (EALs) - Elements Diagnostic®, Root ZX® and Apex DSP® - in the presence of different irrigating solutions (0.9% saline solution and 1% sodium hypochlorite). The electronic measurements were carried out by three examiners, using twenty extracted human permanent maxillary central incisors. A size 10 K file was introduced into the root canals until reaching the 0.0 mark, and was subsequently retracted to the 1.0 mark. The gold standard (GS) measurement was obtained by combining visual and radiographic methods, and was set 1 mm short of the apical foramen. Electronic length values closer to the GS (± 0.5 mm) were considered as accurate measures. Intraclass correlation coefficients (ICCs) were used to verify inter-examiner agreement. The comparison among the EALs was performed using the McNemar and Kruskal-Wallis tests (p < 0.05). The ICCs were generally high, ranging from 0.8859 to 0.9657. Similar results were observed for the percentage of electronic measurements closer to the GS obtained with the Elements Diagnostic® and the Root ZX® EALs (p > 0.05), independent of the irrigating solutions used. The measurements taken with these two EALs were more accurate than those taken with Apex DSP®, regardless of the irrigating solution used (p < 0.05). It was concluded that Elements Diagnostic® and Root ZX® apex locators are able to locate the cementum-dentine junction more precisely than Apex DSP®. The presence of irrigating solutions does not interfere with the performance of the EALs.

Detailed study of the K(L)->pi(0)pi(0)pi(0) Dalitz plot

Using a sample of 68.3x10(6) K(L)->pi(0)pi(0)pi(0) decays collected in 1996-1999 by the KTeV (E832) experiment at Fermilab, we present a detailed study of the K(L)->pi(0)pi(0)pi(0) Dalitz plot density. We report the first observation of interference from K(L)->pi(+)pi(-)pi(0) decays in which pi(+)pi(-) rescatters to pi(0)pi(0) in a final-state interaction. This rescattering effect is described by the Cabibbo-Isidori model, and it depends on the difference in pion scattering lengths between the isospin I=0 and I=2 states, a(0)-a(2). Using the Cabibbo-Isidori model, and fixing (a(0)-a(2))m(pi)(+)=0.268 +/- 0.017 as measured by the CERN-NA48 collaboration, we present the first measurement of the K(L)->pi(0)pi(0)pi(0) quadratic slope parameter that accounts for the rescattering effect: h(000)=(+0.59 +/- 0.20(stat)+/- 0.48(syst)+/- 1.06(ext))x10(-3), where the uncertainties are from data statistics, KTeV systematic errors, and external systematic errors. Fitting for both h(000) and a(0)-a(2), we find h(000)=(-2.09 +/- 0.62(stat)+/- 0.72(syst)+/- 0.28(ext))x10(-3), and m(pi)(+)(a(0)-a(2))=0.215 +/- 0.014(stat)+/- 0.025(syst)+/- 0.006(ext); our value for a(0)-a(2) is consistent with that from NA48.

Measurement of transverse single-spin asymmetries for J/psi production in polarized p plus p collisions at root s=200 GeV

We report the first measurement of transverse single-spin asymmetries in J/psi production from transversely polarized p + p collisions at root s = 200 GeV with data taken by the PHENIX experiment in 2006 and 2008. The measurement was performed over the rapidity ranges 1.2 < vertical bar y vertical bar < 2.2 and vertical bar y vertical bar < 0.35 for transverse momenta up to 6 GeV/c. J/psi production at the Relativistic Heavy Ion Collider is dominated by processes involving initial-state gluons, and transverse single-spin asymmetries of the J/psi can provide access to gluon dynamics within the nucleon. Such asymmetries may also shed light on the long-standing question in QCD of the J/psi production mechanism. Asymmetries were obtained as a function of J/psi transverse momentum and Feynman-x, with a value of -0.086 +/- 0.026(stat) +/- 0.003(syst) in the forward region. This result suggests possible nonzero trigluon correlation functions in transversely polarized protons and, if well defined in this reaction, a nonzero gluon Sivers distribution function.

Suppression of away-side jet fragments with respect to the reaction plane in Au plus Au collisions at root s(NN)=200 GeV

Pair correlations between large transverse momentum neutral pion triggers (p(T) = 4-7 GeV/c) and charged hadron partners (p(T) = 3-7 GeV/c) in central (0%-20%) and midcentral (20%-60%) Au + Au collisions at root s(NN) = 200 GeV are presented as a function of trigger orientation with respect to the reaction plane. The particles are at larger momentum than where jet shape modifications have been observed, and the correlations are sensitive to the energy loss of partons traveling through hot densematter. An out-of-plane trigger particle produces only 26 +/- 20% of the away-side pairs that are observed opposite of an in-plane trigger particle for midcentral (20%-60%) collisions. In contrast, near-side jet fragments are consistent with no suppression or dependence on trigger orientation with respect to the reaction plane. These observations are qualitatively consistent with a picture of little near-side parton energy loss either due to surface bias or fluctuations and increased away-side parton energy loss due to a long path through the medium. The away-side suppression as a function of reaction-plane angle is shown to be sensitive to both the energy loss mechanism and the space-time evolution of heavy-ion collisions.

High-p(T) pi(0) production with respect to the reaction plane in Au plus Au collisions at s(NN)=200 GeV

Measurements of the azimuthal anisotropy of high-p(T) neutral pion (pi(0)) production in Au+Au collisions at s(NN)=200 GeV by the PHENIX experiment are presented. The data included in this article were collected during the 2004 Relativistic Heavy Ion Collider running period and represent approximately an order of magnitude increase in the number of analyzed events relative to previously published results. Azimuthal angle distributions of pi(0) mesons detected in the PHENIX electromagnetic calorimeters are measured relative to the reaction plane determined event-by-event using the forward and backward beam-beam counters. Amplitudes of the second Fourier component (v(2)) of the angular distributions are presented as a function of pi(0) transverse momentum (p(T)) for different bins in collision centrality. Measured reaction plane dependent pi(0) yields are used to determine the azimuthal dependence of the pi(0) suppression as a function of p(T), R(AA)(Delta phi,p(T)). A jet-quenching motivated geometric analysis is presented that attempts to simultaneously describe the centrality dependence and reaction plane angle dependence of the pi(0) suppression in terms of the path lengths of hypothetical parent partons in the medium. This set of results allows for a detailed examination of the influence of geometry in the collision region and of the interplay between collective flow and jet-quenching effects along the azimuthal axis.

Charged Kaon Interferometric Probes of Space-Time Evolution in Au plus Au Collisions at s(NN)=200 GeV

Bose-Einstein correlations of charged kaons are used to probe Au+Au collisions at s(NN)=200 GeV and are compared to charged pion probes, which have a larger hadronic scattering cross section. Three-dimensional Gaussian source radii are extracted, along with a one-dimensional kaon emission source function. The centrality dependences of the three Gaussian radii are well described by a single linear function of N(part)(1/3) with a zero intercept. Imaging analysis shows a deviation from a Gaussian tail at r greater than or similar to 10 fm, although the bulk emission at lower radius is well described by a Gaussian. The presence of a non-Gaussian tail in the kaon source reaffirms that the particle emission region in a heavy-ion collision is extended, and that similar measurements with pions are not solely due to the decay of long-lived resonances.