We argue that the pattern of the deviation from the Glauber approximation prediction for the centrality dependence of the rate of forward jet production observed in pA collisions at the LHC provides the first experimental evidence that parton configu
rations in the projectile proton containing a parton with large $x$ interact with a nuclear target with a significantly smaller than average cross section and have smaller than average size. We implement the effects of fluctuations of the interaction strength and, using the ATLAS analysis of how hadron production at backward rapidities depends on the number of wounded nucleons, make quantitative predictions for the centrality dependence of the jet production rate as a function of the $x$-dependent interaction strength $sigma(x)$. We find that sigma(x)sim 0.6 ~sigma_{tot}(pp) gives a good description of the x=0.6 data and may shed a light on the origin of the EMC effect.
We review the evolution of the studies of diffractive processes in the strong interaction over the last 60 years. First, we briefly outline the early developments of the theory based on analyticity and unitarity of the S-matrix, including the derivat
ion and exploration of the Regge trajectories and related moving cuts. Special attention is paid to the concept of the Pomeron trajectory introduced for description of total, elastic and diffractive cross sections at high energies and to the emergence of the dynamics of multi-Pomeron interactions.The role of large longitudinal distances and color coherent phenomena for the understanding of inelastic diffraction in hadron-hadron scattering and deep inelastic scattering is emphasized. The connection of these phenomena to the cancellation of the contribution of the Glauber approximation in hadron-nucleus collisions and to the understanding of the Gribov-Glauber approximation is explained. The presence of different scales in perturbative QCD due to masses of heavy quarks has led to the emergence of numerous new phenomena including non-universality of the slopes of Regge trajectories made of light and heavy quarks and non-universal energy dependence of elastic cross sections. The application of the perturbative QCD techniques allowed us to calculate from the first principles the interaction of small transverse size color singlets with hadrons leading to the development of the quantitative theory of hard exclusive reactions and to the successful prediction of many regularities in hard large mass diffraction. It also led to the prediction of the phenomenon of complete transparency of nuclear matter in QCD in special processes. The conflict of perturbative QCD with probability conservation for high energy processes of virtual photon-nucleon scattering is explained. Some properties of the new QCD regime are outlined.
Applying exact QCD sum rules for the baryon charge and energy-momentum we demonstrate that if nucleons are the only degrees of freedom of nuclear wave function, the structure function of a nucleus would be the additive sum of the nucleon distribution
s at the same Bjorken x = AQ^2/2(p_Aq)< 0.5 up to very small Fermi motion corrections if x>0.05. Thus the difference of the EMC ratio from one reveals the presence of non-nucleonic degrees of freedom in nuclei. Using exact QCD sum rules we show that the ratio R_A(x_p,Q^2) used in experimental studies, where x_p = Q^2/2q_0 m_p deviates from one even if a nucleus consists of nucleons with small momenta only. Use of the Bjorken x leads to additional decrease of R_A(x,Q^2) as compared to the x_p plots. Coherent contribution of equivalent photons into photon component of parton wave function of a nucleus unambiguously follows from Lorentz transformation of the rest frame nucleus Coulomb field. For A~200 photons carry ~0.0065 fraction of the light momentum of nucleus almost compensates the difference between data analysis in terms of Bjorken x and x_p. Different role of higher twist effects for Q^2 probed at electron and muon beams is emphasized. Direct observations of large and predominantly nucleonic short-range correlations in nuclei pose a serious challenge for most of the models of the EMC effect for x>0.6. The data are consistent with a scenario in which the hadronic EMC effect reflects fluctuations of inter nucleon interaction due to fluctuations of color distribution in the interacting nucleons. The dynamic realization of this scenario is the model in which the 3q (3qg) configurations with x > 0.5 parton have a weaker interaction with nearby nucleons, leading to suppression of such configurations giving a right magnitude of the EMC effect. The directions for the future studies and challenging questions are outlined.
Photons as well as quarks and gluons are constituents of the infinite momentum frame (IMF) wave function of an energetic particle. They are mostly equivalent photons whose amplitude follows from the Lorentz transformation of the particle rest frame C
oulomb field into the IMF and from the conservation of the electromagnetic current. We evaluate in a model independent way the dominant photon contribution propto alpha_{em}(Z^2/A^{4/3})ln(1/R_{A}m_{N}x) to the nuclear structure functions as well as the term propto alpha_{em}Z/A. In addition we show that the definition of x consistent with the exact kinematics of eA scattering (with exact sum rules) works in the same direction as the nucleus field of equivalent photons. Combined, these effects account for the bulk of the EMC effect for xle 0.5 where Fermi motion effects are small. In particular for these x the hadronic mechanism contribution to the EMC effect does not exceed sim 3% for all nuclei. Also the A-dependence of the hadronic mechanism of the EMC effect for x > 0.5 is significantly modified.
We summarize recent progress in the studies of the short-rang correlations (SRC) in nuclei in high energy electron and hadron nucleus scattering and suggest directions for the future high energy studies aimed at establishing detailed structure of two
-nucleon SRCs, revealing structure of three nucleon SRC correlations and discovering non-nucleonic degrees of freedom in nuclei.
Novel processes probing the decay of nucleus after removal of a nucleon with momentum larger than Fermi momentum by hard probes finally proved unambiguously the evidence for long sought presence of short-range correlations (SRCs) in nuclei. In combin
ation with the analysis of large $Q^2$, A(e,e)X processes at $x>1$ they allow us to conclude that (i) practically all nucleons with momenta $ge$ 300 MeV/c belong to SRCs, consisting mostly of two nucleons, ii) probability of such SRCs in medium and heavy nuclei is $sim 25%$, iii) a fast removal of such nucleon practically always leads to emission of correlated nucleon with approximately opposite momentum, iv) proton removal from two-nucleon SRCs in 90% of cases is accompanied by a removal of a neutron and only in 10% by a removal of another proton. We explain that observed absolute probabilities and the isospin structure of two nucleon SRCs confirm the important role that tensor forces play in internucleon interactions. We find also that the presence of SRCs requires modifications of the Landau Fermi liquid approach to highly asymmetric nuclear matter and leads to a significantly faster cooling of cold neutron stars with neutrino cooling operational even for $N_p/N_n le 0.1$. The effect is even stronger for the hyperon stars. Theoretical challenges raised by the discovered dominance of nucleon degrees of freedom in SRCs and important role of the spontaneously broken chiral symmetry in quantum chromodynamics (QCD) in resolving them are considered. We also outline directions for future theoretical and experimental studies of the physics relevant for SRCs.
We evaluate the large momentum transfer $J/psi$ photoproduction with rapidity gaps in ultraperipheral proton-ion collisions at the LHC which provides an effective method of probing dynamics of large t elastic hard QCD Pomeron interactions. It is show
n that the experimental studies of this process would allow to investigate the energy dependence of cross section of elastic scattering of a small $cbar c$ dipole off the gluon over a wide range of invariant energies 10^3 < s_{cbar c - gluon} < 10^6 GeV^2. The accessible energy range exceeds the one reached in gamma p at HERA by a factor of 10 and allows the kinematic cuts which improve greatly sensitivity to the Pomeron dynamics as compared to the HERA measurements. The cross section is expected to change by a factor ge 20 throughout this interval and our estimates predict quite reasonable counting rates for this process with the several of the LHC detectors.
