Coherent J/ψ Electroproduction on ^{4}He and ^{3}He at the Electron-Ion Collider: Probing Nuclear Shadowing One Nucleon at a Time.

While the phenomenon of gluon nuclear shadowing at small x has been getting confirmation in QCD analyses of various LHC measurements involving heavy nuclei, it has not been possible so far to establish experimentally the number of target nucleons responsible for nuclear shadowing in a given process. To address this issue, we study coherent J/ψ electroproduction on ^{4}He and ^{3}He in the kinematics of a future electron-ion collider and show that this process has the power to disentangle the contributions of the interaction with a specific number of nucleons k, in particular, with two nucleons at the momentum transfer t≠0. We predict a dramatic shift of the t dependence of the differential cross section toward smaller values of |t| due to a nontrivial correlation between x and k.

Selected topics in diffraction with protons and nuclei: past, present, and future.

We review a broad range of phenomena in diffraction in the context of hadron-hadron, hadron-nucleus collisions and deep inelastic lepton-proton/nucleus scattering focusing on the interplay between the perturbative QCD and non-perturbative models. We discuss inclusive diffraction in DIS, phenomenology of dipole models, resummation and parton saturation at low, hard diffractive production of vector mesons, inelastic diffraction in hadron-hadron scattering, formalism of color fluctuations, inclusive coherent and incoherent diffraction as well as soft and hard diffraction phenomena in hadron-hadron/nucleus and photon-nucleus collisions. For each topic we review key results from the past and present experiments including HERA and the LHC.

Neutron Valence Structure from Nuclear Deep Inelastic Scattering.

Mechanisms of spin-flavor SU(6) symmetry breaking in quantum chromodynamics (QCD) are studied via an extraction of the free neutron structure function from a global analysis of deep inelastic scattering (DIS) data on the proton and on nuclei from A=2 (deuterium) to 208 (lead). Modification of the structure function of nucleons bound in atomic nuclei (known as the EMC effect) are consistently accounted for within the framework of a universal modification of nucleons in short-range correlated (SRC) pairs. Our extracted neutron-to-proton structure function ratio F_{2}^{n}/F_{2}^{p} becomes constant for x_{B}≥0.

Probing the core of the strong nuclear interaction.

The strong nuclear interaction between nucleons (protons and neutrons) is the effective force that holds the atomic nucleus together. This force stems from fundamental interactions between quarks and gluons (the constituents of nucleons) that are described by the equations of quantum chromodynamics. However, as these equations cannot be solved directly, nuclear interactions are described using simplified models, which are well constrained at typical inter-nucleon distances but not at shorter distances.

Tracking fast small color dipoles through strong gluon fields at the LHC.

We argue that the process gamma+A-->J/psi+"gap"+X at large momentum transfer q(2) provides a quick and effective way to test the onset of a novel perturbative QCD regime of strong absorption for the interaction of small dipoles at the collider energies. We find that already the first heavy-ion run at the LHC will allow one to study this reaction with sufficient statistics via ultraperipheral collisions, hence probing the interaction of qq dipoles of sizes approximately 0.2 fm with nuclear media down to x approximately 10(-5).

Color fluctuations in the nucleon in high-energy scattering.

We study quantum fluctuations of the nucleon's parton densities by combining QCD factorization for hard processes with the notion of cross section fluctuations in soft diffraction. The fluctuations of the small-x gluon density are related to the ratio of inelastic and elastic vector meson production in ep scattering. A simple dynamical model explains the HERA data and predicts the x and Q2 dependence of the ratio.

How to probe high gluon densities in pp collisions at the Large Hadron Collider.

We present a model for hadron production in the proton fragmentation region in pp collisions at the CERN Large Hadron Collider which accounts for the first time for effects of very strong small x gluon fields. Average transverse momenta acquired by the valence quarks exceed 1 GeV/c for central collisions and result in the suppression of leading baryon production and an additional energy flow to smaller rapidities. A strong dependence on the impact parameter will allow one to investigate the propagation of leading partons through gluon fields of a strength comparable to the ones encountered in heavy ion collisions at the LHC and in cosmic-ray-air interactions at highest energies.

Measurement of nuclear transparency for the A(e,e'pi+) reaction.

We have measured the nuclear transparency of the A(e,e'pi+) process in 2H, 12C, 27Al, 63Cu, and 197Au targets. These measurements were performed at the Jefferson Laboratory over a four momentum transfer squared range Q2=1.1 to 4.

Evidence for strong dominance of proton-neutron correlations in nuclei.

We analyze recent data from high-momentum-transfer (p, pp) and (p, ppn) reactions on carbon. For this analysis, the two-nucleon short-range correlation (NN-SRC) model for backward nucleon emission is extended to include the motion of the NN pair in the mean field. The model is found to describe major characteristics of the data.

Measurement of two- and three-nucleon short-range correlation probabilities in nuclei.

The ratios of inclusive electron scattering cross sections of 4He, 12C, and 56Fe to 3He have been measured at 1 < xB <. At Q2 > 1.4 GeV2, the ratios exhibit two separate plateaus, at 1.

Probing small parton densities in ultraperipheral A A and pA collisions at the CERN large Hadron Collider.

We calculate photoproduction rates for several hard processes in ultraperipheral proton-lead and lead-lead collisions at the CERN Large Hadron Collider (LHC) with square root of sNN = 8.8 and 5.5 TeV, respectively, which could be triggered in the large LHC detectors.

High-density QCD and cosmic-ray air showers.

We discuss particle production in the high-energy, small-x limit of QCD where the gluon density of hadrons is expected to become nonperturbatively large. Strong modifications of the phase-space distribution of produced particles as compared to leading-twist models are predicted, which reflect in the properties of cosmic-ray induced air showers in the atmosphere. Assuming hadronic primaries, our results suggest a light composition near Greisen-Zatsepin-Kuzmin cutoff energies.

Nuclear shadowing and extraction of Fp2-Fn2 at small x from deuteron collider data.

We demonstrate that leading twist nuclear shadowing leads to large corrections for the extraction of the neutron structure function Fn2 from the future deuteron collider data both in the inclusive and in the tagged structure function modes. We suggest several strategies to address the extraction of Fn2 and to measure at the same time the effect of nuclear shadowing via the measurement of the distortion of the proton spectator spectrum in the semi-inclusive eD-->e'pX process.

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.

Proton breakup in high-energy pA collisions from perturbative QCD.

We argue that the distribution of hadrons near the longitudinal light cone in central high-energy pA collisions is computable in weak coupling QCD. This is because the density of gluons per unit transverse area in the dense target at saturation provides an intrinsic semihard momentum scale, Q(s). We predict that the longitudinal distribution of (anti)baryons and mesons steepens with increasing energy and atomic number of the target and that the transverse momentum distribution broadens.

Measuring double-parton distributions in nucleons at proton-nucleus colliders.

We predict a strong enhancement of multijet production in proton-nucleus collisions at collider energies, as compared to a naive expectation of a cross section similar to A. The study of the process would allow one to measure, for the first time, the double-parton distribution functions in a nucleon in a model-independent way and hence to study both the longitudinal and the transverse correlations of partons.

Revealing the black-body regime of small-x deep-inelastic scattering through final-state signals.

We derive the major characteristics of inclusive and diffractive final states in deep-inelastic scattering off heavy nuclei for the high-energy (small-x) kinematics in which the limit of complete absorption is reached for the dominant hadronic fluctuations in the virtual photon (the black-body limit of the process). Both the longitudinal and transverse distributions of the leading hadrons are found to be strikingly different from the corresponding ones within the leading-twist approximation, and hence provide unambiguous signals for the onset of the black-body limit.

QCD rescattering and high energy two-body photodisintegration of the deuteron.

Photon absorption by a quark in one nucleon followed by its high-momentum transfer interaction with a quark in the other may produce two final-state nucleons with high relative momentum. We sum the relevant quark rescattering diagrams to show that the scattering amplitude depends on a convolution between the large angle pn scattering amplitude, the hard photon-quark interaction vertex, and the low-momentum deuteron wave function. The computed absolute values of the cross section are in reasonable agreement with the data.

Hard exclusive electroproduction of decuplet baryons in the large N(c) limit.

The cross sections and transverse spin asymmetries in the hard exclusive electroproduction of decuplet baryons are calculated in the large N(c) limit and found to be comparable to that of octet baryons. Large N(c) selection rules for the production amplitudes are derived, leading to new sensitive tests of the spin aspects of the QCD chiral dynamics both in the nonstrange and strange sectors. Importance of such studies for the reliable extraction of the pion form factor from pion electroproduction is explained.