E895 Collaboration Research Articles

Parametrization of the nuclear structure function

In this paper, the parametrization of the nuclear structure function which is directly constrained by the dynamics of QCD in its high-energy limit is considered. This simple parameterization of the nuclear structure function is obtained from the proton experimental data by relying on a Froissart-bounded parametrization of the proton structure function. This phenomenological model describes high-energy QCD in the presence of saturation effects. Numerical calculations and comparison with available data from NMC, EMC and E665 Collaborations demonstrate that the suggested method by N.Armesto, C.A.Salgado and U.A. Wiedemann (ASW model) provides reliable ratio of nuclear structure functions $F_{2}^{A}/AF^{p}_{2}$ at low $x$ for light and heavy nuclei. The magnitude of nuclear shadowing is predicted for various kinematic regions and can be applied as well in analysis of ultra-high energy processes by future experiments at electron-ion colliders.

Considering the saturation effect in nuclei based on the Kharzeev-Levin-Nardi model

We consider the total and longitudinal cross sections by using the color dipole model according to the known Kharzeev-Levin-Nardi model in nuclei. Our calculations can predict the low-$x$ electron-nucleus cross sections, the saturation effect, and the ratio between the structure functions for different nuclei and energies of colliding particles. The results indicate the differences between two perturbative and nonperturbative regions at low $x$, which are compared with the H1, ZEUS, New Muon, and E665 Collaborations.

Anisotropic collective flow of Λ-hyperons produced in [formula omitted] collisions at [formula omitted

Features of anisotropic collective flow and spectral temperatures have been determined for Λ hyperons emitted from C + C collisions, at incident momentum of 4.2 A GeV / c , measured using the Propane Bubble Chamber of JINR at Dubna. Moreover, characteristics of protons and of negative pions, emitted from those collisions, have been determined and provided for comparison. The directed and elliptic flows of Λs both agree in sign with the corresponding flows of protons. Parameters of the directed and elliptic flows for Λs agree further, within errors, with the corresponding parameters for the co-produced protons. This contrasts an earlier finding by the E895 Collaboration of the directed flow being significantly weaker for Λs than protons, in the much heavier Au + Au system, at comparable incident momentum. Particle spectral temperatures in the C + C collisions have been determined focusing independently on either center-of-mass energy, transverse energy or transverse momentum distributions. For either protons or negative pions, the temperatures were found to be approximately the same, no matter whether the emission of those particles was associated with Λ production or not. Results of the measurements have been compared to the results of simulations within the Quark–Gluon String Model.

Transverse momentum and transverse mass distributions of charged hadrons produced in Au–Au collisions at high energies

The transverse momentum distribution and the transverse mass distribution of charged hadrons produced in nucleus–nucleus collisions at high energies are described by using a two-cylinder model. The results calculated by the model are compared and found to be in agreement with the experimental data of the STAR and E895 Collaborations, measured in Au–Au collisions at the relativistic heavy ion collider (RHIC) and alternating-gradient synchrotron (AGS) energies, respectively. In the energy range concerned, the excitation degree of emission source close to the central axis of cylinders increases obviously with the collision centrality and incident energy increasing, but it does not show any obvious change with the increase of the (pseudo)rapidity in central collisions. The excitation degree of emission source close to the side-surface of cylinders does not show any obvious change with the collision centrality, the (pseudo)rapidity, and the incident energy increasing.

Analysis of Low-Energy Pion Spectra

The transverse mass spectra and the rapidity distributions of $\pi^+$ and $\pi^-$ in Au-Au collisions at 2, 4, 6, and 8 GeV$\cdot$A by E895 collaboration are fitted using an elliptically expanding fireball model with the contribution from the resonance decays and the final state Coulomb interaction. The ratio of the total number of produced $\pi^-$ and $\pi^+$ is used to fit the data. The resulting freeze-out temperature is rather low($T_f < 60$ MeV) with large transverse flow and thus resonance contribution is very small. The difference in the shape of $m_t$ spectra of the oppositely charged pions are found to be due to the Coulomb interaction of the pions.

Small-xnuclear shadowing from diffractive scattering

We calculate nuclear shadowing ratios at small Bjorken-$x$ for nuclei in the mass range $3lAl239$ in the framework of a generalized form of the Gribov theory. The diffractive dissociation cross sections needed as inputs are taken from the FNAL (Fermi National Laboratory) and HERA (Hadron-Electron Ring Accelerator) experiments. Our calculations cover a wide range of energies between 12.8 and 231 GeV. The effects of the inclusion of subleading Reggeons on nuclear shadowing are investigated. At lower energies our results are compared to available data from the New Muon Collaboration (NMC) and E665 Collaboration (E665) experiments at $x\ensuremath{\simeq}{10}^{\ensuremath{-}4}$. At higher energies subleading Reggeons are found to contribute significantly to nuclear shadowing.

Mass dependence of nuclear shadowing at small Bjorken-xfrom diffractive scattering

We calculate the nuclear shadowing ratio for a wide range of nuclei at small Bjorken-x in the framework of Gribov theory. The coherent contribution to the (virtual) photon-nucleon cross section is obtained in terms of the diffractive dissociation cross section. Information on diffraction from FNAL and HERA is used. Our results are compared to available data from the New Muon Collaboration and E665 Collaboration experiments at x{approx_equal}10{sup -4}.

Differential flow of protons in [formula omitted] collisions at AGS energies

We study the proton sideward and elliptic differential flow for Au+ Au collisions at AGS energies (2–8 A GeV) in a microscopic relativistic transport model that includes all baryon resonances up to a mass of 2 GeV as well as string degrees of freedom for the higher hadronic excitations. In order to explore the sensitivity of the various differential flows to the nuclear equation of state (EoS) we use three different parameterizations of the scalar and vector mean-fields, i.e., NL2 (soft), NL23 (medium) and NL3 (hard), with their momentum dependence fitted to the experimental Schrödinger equivalent potential (at normal nuclear matter density ρ 0 ) up to kinetic energies of 1 GeV. We calculate the excitation function of sideward and elliptic flow within these parameter sets for Au+ Au collisions and compare with the recent data from the E895 Collaboration as a function of rapidity, impact parameter and transverse momentum, respectively. We find that the best description of the differential data is provided by a rather ‘stiff’ EoS at 2 A GeV (NL3) while at higher bombarding energies (4–8 A GeV) a ‘medium’ EoS leads to the lowest χ 2 with respect to the data. However, the differences in the transverse and elliptic flows (from the different parameter sets) become of minor significance at 4–8 A GeV. We attribute this insensitivity to a similar reduction of the vector potential in all models and to the dominance of string degrees of freedom at these bombarding energies.

Neutral strange particle dynamics in Au + Au collisions at AGS energies

The E895 Collaboration has investigated a number of mixed-particle correlations involving neutral strange particles (K0s and Λ) in 2, 4, 6 and 8 A GeV Au + Au collisions at the AGS. The first measurements for pΛ correlations indicate a correlation magnitude of about 50% with a 26 MeV width. Detailed comparisons between data and theory suggest an emitting source with an effective Gaussian radius close to 4.5 fm. This radius is much larger than the value of 2.8 fm predicted by the relativistic transport code ART. Pairs of pK0, π−Λ and π−K0 are found to display essentially zero correlation at 6 A GeV. The latter observations are not unexpected considering that the proton–K0 interaction has a very small scattering length and there are no very well known interactions between pions and Λs or K0s.

Longitudinal Flow of Protons from(2–8)AGeVCentralAu+AuCollisions

Rapidity distributions of protons from central 197Au+197Au collisions measured by the E895 Collaboration in the energy range from (2-8)A GeV at the Brookhaven AGS are presented. Longitudinal flow parameters derived using a thermal model including collective longitudinal expansion are extracted from these distributions. The results show an approximately linear increase in the longitudinal flow velocity, <betagamma>(L), as a function of the logarithm of beam energy.

Laying the groundwork at the AGS: Recent results from experiment E895

The E895 Collaboration at the Brookhaven AGS has performed a systematic investigation of Au+Au collisions at 2-8 AGeV, using a large-acceptance Time Projection Chamber. In addition to extensive measurements of particle flow, spectra, two-particle interferometry, and strangeness production, we have performed novel hybrid analyses, including azimuthally-sensitive pion HBT, extraction of the six-dimensional pion phasespace density, and a first measurement of the Lambda-proton correlation function.

Energy dependence of the contribution of pion exchange to large-rapidity-gap events in deep inelastic scattering

We study the energy dependence of the contribution of pion exchange to large-rapidity-gap events in deep inelastic scattering. The results show that this contribution can be quite significant at low energy and that the LRG events observed by the E665 Collaboration in $\ensuremath{\mu}\mathrm{Xe}$ and $\ensuremath{\mu}\mathrm{D}$ interactions at 490 GeV can be reasonably well described in terms of meson exchange. We also show that the distribution of the maximum rapidity for all hadrons is quite different from that for charged hadrons only and that the former exhibits also shoulderlike structure for events at 490 GeV similar to that at DESY HERA.

The quark-antiquark asymmetry of the nucleon sea from Λ and [formula omitted] fragmentation

We present a general analysis of the spin transfer for Λ and Λ production in deep-inelastic scattering of polarized charged leptons on the nucleon, and find that the pattern of different behaviors of Λ and Λ production observed by the E665 Collaboration suggests the possibility of quark-antiquark asymmetries either in the quark to Λ fragmentation functions and/or in the quark and antiquark distributions of the target proton. We also point out that the strange-antistrange asymmetry of the nucleon sea may produce an observable contribution to the different behaviors of Λ and Λ production. We find that a softer s(x) than s( x) as predicted by the light-cone baryon-meson fluctuation model of intrinsic quark-antiquark pairs of the nucleon sea might lead to a reasonable picture. However, the magnitude is still too small to explain the E665 data and the conclusion has also strong model-dependence. This may suggest the importance of quark-antiquark asymmetry in the quark to Λ fragmentation functions, provided that the E665 data are confirmed.

The quark–antiquark asymmetry of the nucleon sea from Λ and [formula omitted] fragmentation

We present a general analysis of the spin transfer for Λ and Λ production in deep-inelastic scattering of polarized charged leptons on the nucleon, and find that the pattern of different behaviors of Λ and Λ production observed by the E665 Collaboration suggests the possibility of quark–antiquark asymmetries either in the quark to Λ fragmentation functions and/or in the quark and antiquark distributions of the target proton. We also point out that the strange-antistrange asymmetry of the nucleon sea may produce an observable contribution to the different behaviors of Λ and Λ production. We find that a softer s(x) than s( x) as predicted by the light-cone baryon-meson fluctuation model of intrinsic quark–antiquark pairs of the nucleon sea might lead to a reasonable picture. However, the magnitude is still too small to explain the E665 data and the conclusion has also strong model-dependence. This may suggest the importance of quark–antiquark asymmetry in the quark to Λ fragmentation functions, provided that the E665 data are confirmed.

Vector-meson electroproduction from generalized vector dominance

Including destructively interfering off-diagonal transitions of diffraction-dissociation type, we arrive at a formulation of GVD for exclusive vector-meson production in terms of a continuous spectral representation of dipole form. The transverse cross-section, σ T,γ ∗ p→V p , behaves asymptotically as 1/ Q 4, while R V≡σ L,γ ∗ p→V p /σ T,γ ∗ p→V p becomes asymptotically constant. Contributions violating s-channel helicity conservation stay at the level established in low-energy photoproduction and diffractive hadron–hadron interactions. The data on ρ 0-meson production from the Fermilab E665 Collaboration and on φ- and ρ 0-meson production from HERA are found to be in agreement with these predictions.

Neutral strange particle production and flow at AGS energies

In recent experiments at the AGS, the E895 Collaboration has made detailed measurements of neutral strange particles (K0s and Λ) over a broad range of incident energies and impact parameters for the Au + Au system. Preliminary results are presented for the production yields and the sidewards flow of these particles. The production yields show relatively good agreement with existing systematics. The Λ particles show flow results which are similar to those reported for protons, albeit with a smaller magnitude. On the other hand, the K0s seem to follow an anti-flow pattern suggesting that the kaon vector potential plays an important role in high-density nuclear matter.

K−/K+ratios in relativistic heavy-ion collisions

We study ${K}^{\ensuremath{-}}{/K}^{+}$ ratios as a function of centrality (participant nucleon number), transverse mass ${(m}_{t}),$ and rapidity, in heavy-ion collisions at beam energies between 1 and $2 A\mathrm{GeV}.$ We use the relativistic transport model that includes explicitly the strangeness degrees of freedom and consider two scenarios for kaon properties in dense matter, one with and one without medium modifications of their properties. In both scenarios, the ${K}^{\ensuremath{-}}{/K}^{+}$ ratio does not change very much with the centrality, while the $K/\ensuremath{\pi}$ and $\overline{K}/\ensuremath{\pi}$ ratios increase with increasing centrality. Significant differences are predicted, both in magnitudes and shapes, for the ${m}_{t}$ spectra and rapidity distributions of ${K}^{\ensuremath{-}}{/K}^{+}$ ratio. Experimental measurement of these ratios, currently under investigation by the FOPI, KaoS, E866, and E895 Collaborations, will be useful in revealing the kaon in-medium properties.

Collective flow in Au+Au collisions between 2–8 A GeV at AGS

The E895 collaboration has undertaken a systematic and exclusive study of Au+Au collisions between 2–8 A GeV. We present preliminary results on collective flow observables as a function of beam energy. Data are compared with theoretical predictions.

Diffractive phenomena and shadowing in deep-inelastic scattering

Shadowing effects in deep-inelastic lepton-nucleus scattering probe the mass spectrum of diffractive leptoproduction from individual nucleons. We explore this relationship using current experimental information on both processes. In recent data from the NMC and E665 collaboration, taken at small x << 0.1 and Q^2 < 1 GeV^2, shadowing is dominated by the diffractive excitation and coherent interaction of low mass vector mesons. If shadowing is explored at small x << 0.1 but large Q^2 >> 1 GeV^2 as discussed at HERA, the situation is different. Here dominant contributions come from the coherent interaction of diffractively produced heavy mass states. Furthermore we observe that the energy dependence of shadowing is directly related to the mass dependence of the diffractive production cross section for free nucleon targets.

Unusual effects of diffraction dissociation for multiproduction in deep inelastic scattering on nuclei

In hadron-nucleus interactions, the stronger is nuclear shadowing in the total cross section the higher is the multiplicity of secondary hadrons. In deep inelastic scattering nuclear shadowing at smallx is associated with the hadronlike behaviour of photons as contrasted to the pointlike behaviour in the non-shadowing region of largex. In this paper we predict dilution of multiproduction of secondary hadrons, and weaker fragmentation of the target nucleus, in deep inelastic leptoptoproduction on nuclei in the shadowing region of smallx as compared to the non-shadowing region of largex. This paradoxial conclusion has its origin in nuclear enhancement of the coherent diffraction dissociation of photons which affects multiproduction in the shadowing region and does not contribute in the non-shadowing region. We present numerical predictions for multiproduction in theμXe interactions studied by the Fermilab E665 collaboration.