High-density QCD and saturation of nuclear partons

We review the recent finding of the two-plateau momentum distribution of sea quarks in deep inelastic scattering off nuclei in the saturation regime. The diffractive plateau which dominates for small p measures precisely the momentum distribution of quarks in the beam photon, the rôle of the nucleus is simply to provide an opacity. The plateau for truly inelastic DIS exhibits a substantial nuclear broadening of the momentum distribution. Despite this nuclear broadening, the observed final-state and initial-state sea quark densities do coincide exactly. We emphasize how the saturated sea is generated from the nuclear-diluted Weizsäcker-Williams because of the anti-collinear splitting of gluons into sea quarks.Received: 30 September 2002, Published online: 22 October 2003PACS: 11.80.La Multiple scattering - 13.87.-a Jets in large-Q ² scattering - 24.85. + p Quarks, gluons, and QCD in nuclei and nuclear processe     

Saturation of nuclear partons: Fermi statistics or nuclear opacity?

We derive the two-plateau momentum distribution of final state (FS) quarks produced in deep inelastic scattering (DIS) off nuclei in the saturation regime. The diffractive plateau, which dominates for small p, measures precisely the momentum distribution of quarks in the beam photon; the role of the nucleus is simply to provide an opacity. The plateau for truly inelastic DIS exhibits a substantial nuclear broadening of the FS momentum distribution. We discuss the relationship between the FS quark densities and the properly defined initial state (IS) nuclear quark densities. The Weizsäcker-Williams glue of a nucleus exhibits a substantial nuclear dilution, still soft IS nuclear sea saturates because of the anti-collinear splitting of gluons into sea quarks.     

Charm and bottom production at RHIC

The current status of charm and bottom production measurements at RHIC is summarized. Heavy-flavor data from p+p collisions at provide a crucial testing ground for perturbative QCD calculations and serve as a baseline for measurements in nucleus-nucleus collisions. Due to their large mass heavy quarks are produced in hard parton scattering processes in the earliest phase of a nucleus-nucleus collisions such that they can be used to probe the hot and dense medium that is formed in the course of Au+Au collisions at RHIC.     

Measurement of open beauty production at HERA in the D*μ final state

The production of beauty quarks with a and a muon in the final state has been measured with the ZEUS detector at HERA using an integrated luminosity of 114 pb^-1. Low transverse-momentum thresholds for the muon and D^* meson allow for a measurement of beauty production closer to the production threshold than previous measurements. The beauty signal was extracted using the charge correlations and angular distributions of the muon with respect to the D^* meson. Cross sections for photoproduction and deep inelastic scattering are somewhat higher than, but compatible with, next-to-leading-order QCD predictions, and compatible with other measurements.     

Nuclear shadowing and heavy-ion collisions at high energies

Nuclear shadowing effects for quarks and gluons are calculated using information on diffractive deep inelastic scattering on a nucleon. Role of these effects in interactions of hadrons and nuclei with nuclei at high energies is investigated. A decrease in particle densities for heavy-ion collisions in comparison with the Glauber model is predicted and nuclear modification factors are calculated. Distributions of gluons in nuclei are used to predict suppression of heavy quarkonia. The parameter-free calculation of J/ψ in DAu and AuAu collisions is in a good agreement with recent RHIC data. Predictions for heavy quarkonia suppression in heavy-ion collisions at LHC are formulated.     

57Fe Mössbauer study of FeNx (x=0.25≈0.91) alloys

⁵⁷Fe Mössbauer measurements have been perforned at the first time on iron nitrides, FeN_x(x>0.5), films prepared by rf sputtering and the measurements for γ-FeN_0.09, γ'-Fe₄N, ε-Fe_2–3N and ζ-Fe₂N have been also performed. Two new phases for x>0.5 have been identified: One has a ZnS-type structure denoted by γ" Another phase is denoted by γ''' in which iron atoms constitute a fcc lattice but the details of occupation sites of nitrogen atoms are not yet clear. The isomer shift of γ" relative to α-Fe at 300 K is rather small in comparison with that of other well known nitrides. The roam temperature Mössbauer spectra of γ''' consist of a sum of two doublets. Mössbauer spectra of γ" at 10 K do not indicate magnetically split pattern, but those of γ''' show magnetically split patterns at 10 K with the hyperfine fields of 30 T and 48 T, respectively. The component of 48 T is due to oxides, not due to nitrides.     

Raman scattering study of anomalous spin, charge, and lattice dynamics in the charge-ordered phase of Bi1-xCaxMnO3 (x > 0.5)

We report an inelastic light scattering study of the effects of charge ordering on the spin, charge, and lattice dynamics of Bi1-xCaxMnO3 (x>0.5). We find that charge ordering results in anomalous phonon behavior, such as the appearance of "activated" modes. More significantly, however, the transition to the charge-ordered phase results in the appearance of a quasielastic scattering response with the symmetry of the spin-chirality operator ( T(1g)); this scattering response is thus indicative of magnetic or chiral spin fluctuations in the antiferromagnetic charge-ordered phase.     

On the suppression of the gluon radiation for quark jets penetrating a dense quark gas

Suppression of the gluon-emission probability due to secondary scattering (the Landau-Pomeranchuk effect) is considered for an ultra-relativistic quark penetrating a dense gas of thermal quarks. With some appeal to QED, a simple expression for the suppression factor is obtained which reveals strong damping throughout the gluon spectrum for gas densities typical of nuclear matter. The suppression, which is due to multiple rather than single scattering, is in particular pronounced for soft gluon emission. The obtained reduction factor is believed to describe the damping of the Abelian radiation component to within a factor of \(\sqrt 2 \) . For the non-Abelian component comparison of emission and scattering angles hints at a suppression comparable to that pertaining to the Abelian component.     

Evidence for two back-to-back high-p T jets inK + p interactions at 70 GeV/c

Nucleon structure functions obtained from neutrino and anti-neutrino scattering on iron nuclei at high energies (E _v =30 to 250 GeV) are presented. These results are compared with the results of other lepton-nucleon scattering experiments. The structure functions are used to test the validity of the Gross-Llewellyn-smith sum rule, which measures the number of valence quarks in the nucleons, and to obtain leading and second order QCD fits.     

Role of Nd/Ba disorder on the penetration depth of Nd(1+x)Ba(2-x)Cu(3)O(7-delta) thin films

We report on a study on the effect of Nd/Ba disorder on the ab-plane penetration depth of epitaxial Nd(1+x)Ba(2-x)Cu(3)O(7-delta) thin films. While in stoichiometric samples lambda(T) at low temperature is linear, Nd-rich films exhibit a quadratic law. For low Nd excess (x<0.04), a satisfying fit is obtained using the "dirty" d-wave model assuming that Nd ions at Ba sites act as strong scattering centers. At high x (x>0.15) the data are explained if Nd/Ba disorder becomes less effective as a source of scattering. The effect of localization has been discussed to account for the experimental results.     

Nature of e(g) electron order in La(1-x)Sr(1+x)MnO(4)

X-ray scattering measurements of the low-temperature structure of La(1-x)Sr(1+x)MnO(4) ( 0.33< or =x< or =0.67) indicate the existence of three distinct regions: a disordered phase (x<0.4), a charge-ordered phase (x> or =0.5), and a mixed phase (0.4< or =x<0.5). For x>0.5, the modulation vector associated with the charge order is incommensurate with the lattice and depends linearly on the concentration of e(g) electrons. The primary superlattice reflections are strongly suppressed along the modulation direction and the higher harmonics are weak, implying the existence of a largely transverse and nearly sinusoidal structural distortion, consistent with a charge-density wave of the e(g) electrons.     

Disappearance of Mott Oscillations in Sub-barrier Elastic Scattering of Identical Nuclei and Atomic Ions

The scattering of identical nuclei at low energies exhibits conspicuous Mott oscillations which can be used to investigate the presence of components in the predominantly Coulomb interaction arising from several physical effects. It is found that at a certain critical value of the Sommerfeld parameter the Mott oscillations disappear and the cross section becomes quite flat. We call this effect Transverse Isotropy (TI). The critical value of the Sommerfeld parameter at which TI sets in is found to be \({\eta_{c} = \sqrt{3s + 2}}\), where s is the spin of the nuclei participating in the scattering. No TI is found in the Mott scattering of identical Fermionic nuclei. The critical center of mass energy corresponding to \({\eta_c}\) is found to be \({E_c = 0.40}\) MeV for \({\alpha + \alpha}\) (s = 0), 1.2 MeV for \({^{6}}\)Li + \({^{6}}\)LI (s = 1) and 7.1 MeV for \({^{10}}\)B + \({^{10}}\)B (s = 3). We further found that the inclusion of the nuclear interaction induces a significant modification in the TI. We suggest measurements at these sub-barrier energies for the purpose of extracting useful information about the nuclear interaction between light heavy ions. We also suggest extending the study of the TI to the scattering of identical atomic ions.     

Distribution of charmed constituent quarks in hadrons

By using a statistical approach within noncovariant perturbation theory, the distributions of light and charmed quarks in hadrons are derived with allowance for the charmed-quark mass. The parameters of the model are extracted from a comparison with NA3 data on the hadroproduction of J/ψ particles. A reanalysis of EMC data on charm production in muon-nucleon scattering is performed. In relation to the conventional source of charmed quarks from photon-gluon fusion, the EMC data are found to suggest the presence of an additional contribution from deep-inelastic scattering on charmed quarks at large x. The resulting admixture of Fock states that contain charmed quarks in the decomposition of the proton wave function is about 1%. The approach presented for the excitation of Fock states involving charmed quarks can also be applied to states featuring beauty quarks, as well as to the hadronic component of the virtual photon (resolved photon component).     

On the break-down of completeness of wave operators in potential scattering

We study the Schrödinger operator with a potential that vanishes at infinity but the rate of falloff of the potential depends on the direction. It turns out that for such potentials scattering theory becomes in general multichannel.     

PHENIX experiment: Recent results

An overview of the latest physics results obtained in the PHENIX experiment by studying pp, dAu, and AuAu interactions at √sNN = 200 GeV is given. Results of measurements of nuclear-modification factors, direct-photon production, and properties of light vector mesons and particles containing heavy c and b quarks are presented. These results are used to deduce information about the properties of dense parton matter produced in collisions of relativistic heavy nuclei at the Relativistic Heavy Ion Collider (RHIC).     

Color-conductivity in nuclei and the EMC-effect

We advocate that gluons and quarks of sufficiently short wavelengths are delocalized in nuclei. This hypothesis leads us to structure functions measured in μ-nucleus scattering, which depend at fixedx only on the ratio of the resolving power 1/Q and the radius of the nucleusR _A, whereA denotes the mass number of the nucleus. Thus we suggest that the structure functionF ₂(x, Q ²,A) per nucleon of an isoscalar nucleusA scales essentially as \(F_2 (x,Q^2 ,A) = \tilde F_2 (x,R_A^2 \cdot Q^2 )\) with a universal function \(\tilde F_2\) . The ratio of the so obtained structure functions of iron to deuterium agrees rather well with the one measured recently by the European Muon Collaboration. This observation implies that nuclei are “color-insulators” at lowQ ², but “color conductors” at largeQ ².     

Parity-violating electron scattering – an experimental overview

We discuss the current status and future plans of world-wide efforts of parity-violating asymmetry measurements in the scattering of longitudinally polarized electrons off unpolarized fixed targets. One thrust is the measurements of nucleon neutral weak form factors at intermediate four-momentum transfer (0.1 < Q ² < 1) (GeV/c)² which provides information about the role of virtual strange quarks on the charge and current distributions inside nucleons. A new topic is the elastic neutral weak amplitude at very low Q ² from scattering off a heavy spinless nucleus, which is sensitive to the presence of a neutron skin. Finally, we discuss the neutral current elastic amplitude at very low Q ² off protons and electrons and in the DIS regime off deuterium, which allows precision measurements of the weak mixing angle at low energy and is thus sensitive to new physics at the TeV scale. The physics implications of recent results, potential measurements from experiments under construction as well as new ideas at future facilities are discussed.     

Contribution of the hadronic component of a virtual photon to the structure function for charm leptoproduction at high <Emphasis Type="Italic">x</Emphasis> and <Emphasis Type="Italic">Q</Emphasis><Superscript>2</Superscript>

The contribution of the hadronic component of a virtual photon to the structure function for charm leptoproduction is calculated. This contribution comes from the scattering of c quarks of a virtual photon on proton quarks and gluons. A comparison of the results of our calculations for this structure function with relevant data obtained by the European Muon Collaboration (EMC) for μ⁺p scattering gives reasons to believe that the contribution of the resolved photon may be responsible for the excess in these EMC data over the predictions of the model of photon-gluon fusion at high momentum transfers. Therefore, it may become possible to describe the EMC data without resort to the hypothesis of a nonperturbative charmed-quark admixture in the proton wave function (intrinsic-charm hypothesis).     

Ion-induced quark-gluon implosion

We investigate nuclear fragmentation in the central proton-nucleus and nucleus-nucleus collisions at the energies of CERN LHC. Within the semiclassical approximation we argue that because of the fast increase with energy of the cross sections of soft and hard interactions each nucleon is stripped in the average process off "soft" partons and fragments into a collection of leading quarks and gluons with large p(t). Valence quarks and gluons are streaming in the opposite directions when viewed in the c.m. of the produced system. The resulting pattern of the fragmentation of the colliding nuclei leads to an implosion of the quark and gluon constituents of the nuclei. The nonequilibrium state produced at the initial stage in the nucleus fragmentation region is estimated to have densities >/=50 GeV/fm(3) at the LHC energies and probably >/=10 GeV/fm(3) at BNL RHIC.