arXiv subjects
Publications and source records attributed to Mark Strikman.
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.
More than 100 people participated in a discussion session at the DIS08 workshop on the topic What HERA may provide. A summary of the discussion with a structured outlook and list of desirable measurements and theory calculations is given.
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 combination 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 argue that significant probability of protons with momenta above their Fermi surface leads for proton concentrations p/n > 1/8 to the enhancement of termally excited direct and modified URCA processes within a cold neutron star, and to a nonzero probability of direct URCA processes for small proton concentrations (p/n < 1/8). We evaluate high momentum tails of neutron, proton and electrons distributions within a neutron star. We expect also significantly faster neutrino cooling of hyperon stars.
We evaluate the large momentum transfer $J/ψ$ 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 shown that the experimental studies of this process would allow to investigate the energy dependence of cross section of elastic scattering of a small $c\bar c$ dipole off the gluon over a wide range of invariant energies 10^3 < s_{c\bar c - gluon} < 10^6 GeV^2. The accessible energy range exceeds the one reached in γp 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.
I review history of the color transparency (CT) which started with discovery of the $J/ψ$ meson, discovery of high energy CT phenomena and the recent progress in the investigations of CT at intermediate energies.
We outline several directions for future investigations of the three-dimensional structure of nucleon, including multiparton correlations, color transparency, and branching processes at hadron colliders and at hadron factories. We also find evidence that pQCD regime for non-vacuum Regge trajectories sets in for $-t\ge 1 {GeV}^2$ leading to nearly t-independent trajectories.
We argue that in the small x processes, in the black disc QCD regime (BDR) a very forward parton propagating through the nuclear matter should loose a significant and increasing with energy and atomic number fraction of its initial energy as a result of dominance of inelastic interactions, causality and energy-momentum conservation. We evaluate these energy losses and find them to lead to the significant suppression of the forward jet production in the central NA collisions at collider energies with a moderate suppression of recoiling jet at central rapidities. We confront our expectations with the recent RHIC data of the STAR collaboration on the probability, $P$, for emission of at least one fast hadron at a central rapidity in association with production of a very forward high $p_t$ neutral pion in $pp$ and $dAu$ collisions. We calculate the A-dependence of $P$, and find that the data imply a strong suppression of leading pion production at central impact parameters. We also conclude that production of recoil jets in the hard subprocess is not suppressed providing further evidence for the dominance of peripheral collisions. Both features of the data are consistent with the onset of BDR. We suggest new phenomena and new observables to investigate BDR at RHIC and LHC.
At the LHC, the leading partons in the nuclei are expected to interact with the maximal possible strength - black disk limit - up to transverse momenta of the order of few GeV. We demonstrate that in this limit the densities of the quark - gluon systems produced in the central AA collisions in the nucleus fragmentation regions should exceed 300 GeV/fm^3 which is at least as high as the densities discussed for the central region. Experimental signatures of such a regime are also discussed.
We present a model independent procedure for extracting deep-inelastic structure function of "free'' neutron from the electron - deuteron scattering with protons produced in the target fragmentation region of the reaction. This procedure is based on the extrapolation of t, which describes the invariant momentum transfered to the proton, to the unphysical region corresponding to the mass of the struck neutron. We demonstrate that the impulse approximation diagram of the reaction has a pole at this limit with a residue being proportional to the "free" neutron structure function. The method is analogous to that of Chew and Low for extraction of the ``free'' pion-pion and neutron-neutron cross sections from p(pion,p)X and d(n,n)pn reactions respectively. We demonstrate that in the extrapolation the final state interaction amplitudes are smooth functions of $t$ and have negligible contribution in the extracted ``free'' nucleon structure function. We also estimate the range of the recoil nucleon momenta which could be used for successful extrapolation procedure.
We calculate production rates for several hard processes in ultraperipheral proton-nucleus and nucleus-nucleus collisions at the LHC. The resulting high rates demonstrate that some key directions in small $x$ research proposed for HERA will be accessible at the LHC through these ultraperipheral processes. Indeed, these measurements can extend the HERA $x$ range by roughly a factor of 10 for similar virtualities. Nonlinear effects on the parton densities will thus be significantly more important in these collisions than at HERA.
A non-relativistic potential-model version of the factorization assumption, used in perturbative QCD calculations of hadronic form factors, is used, along with the Born approximation valid at high energies, to derive a remarkably simple relationship between the impulse approximation contribution to the deuteron form factor at high momentum transfer and the high energy neutron-proton scattering amplitude. The relation states that the form factor at a given value of $Q^2$ is proportional to the scattering amplitude at a specific energy and scattering angle. This suggests that an accurate computation of the form factors at large $Q^2$ requires a simultaneous description of the phase-shifts at a related energy, a statement that seems reasonable regardless of any derivation. Our form factor-scattering amplitude relation is shown to be accurate for some examples. However, if the potential consists of a strong short distance repulsive term and a strong longer ranged attractive term, as typically occurs in many realistic potentials, the relation is found to be accurate only for ridiculously large values of $Q$. More general arguments, using only the Schroedinger equation, suggest a strong, but complicated, relationship between the form factor and scattering amplitude. Furthermore, the use of recently obtained soft potentials, along with an appropriate current operator, may allow calculations of form factors that are consistent with the necessary phase shifts.
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 semi-hard momentum scale, Qs. 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. We show that the evolution of high moments of the longitudinal net baryon distribution with Qs is determined by the anomalous dimension gamma_qq.
We argue that the t-dependence of the two-gluon form factor of the nucleon should be given by $Γ(t)=(1-t/m_{2g}^2)^{-2}$ with $m_{2g}^2\approx 1 GeV^2$. We demonstrate that this form provides a good description of the t-dependence of the cross section of the elastic photoproduction of $J/ψ$-mesons between the threshold region of $E_γ=11 GeV$ (Cornell), $E_γ=19 GeV$ (SLAC) and $E_γ=100 GeV$ (FNAL) including the strong energy dependence of the t-slope. It is also well matched with the recent HERA data. The same assumption explains also the t-dependence of $ϕ$-meson electroproduction near threshold at $W=2.3 GeV, Q^2=1.0 GeV^2$.
We analyze the light cone (Ioffe) time structure of the gluon distribution function in the double logarithmic approximation. We show that due to QCD evolution Ioffe equation is modified. The characteristic light cone time of the gluons does not increase as fast with increasing energy (decreasing Bjorken x) as predicted by the parton distributions exhibiting Bjorken scaling due to the increase of the transverse momenta of the gluons in the DGLAP ladder.
The interaction of spatially small dipoles with nucleons, nuclei is calculated in the DGLAP approximation at the top of HERA energies and found to be close to the $S$-channel unitarity limit in the case of the color octet dipoles. The DGLAP analyses of the current diffractive data appear to support this conclusion as they indicate a $\sim 30-40%$ probability of the gluon induced diffraction for $Q^2\sim 4 $GeV$^2$. The need for the high-precision measurements of the $t$-dependence of inclusive and exclusive diffraction for pinpointing higher twist effects in the gluon sector is emphasized.The $eA$ collisions at HERA would provide a strong amplification of the gluon densities allowing to reach deep into the regime of nonlinear QCD evolution. Connection between the leading twist nuclear shadowing and leading twist diffraction in $ep$ scattering is explained. The presented model independent results for the nuclear shadowing for light nuclei indicate much larger shadowing for the gluon sector than for the sea quark sector.It is argued that HERA in $eA$ mode would be able to discover a number of new phenomena including large gluon shadowing, large nonlinearities in parton evolution, small $x$ color transparency in the vector meson production followed by color opacity at $x\le .005$, large probability of inclusive diffraction. Implications for the nucleus-nucleus collisions at LHC are discussed as well.
We outline major ideas involved in discussion of color coherence phenomena (CCP) at intermediate energies. We point out that the recent advances in calculating cross sections of hard exclusive processes off light nuclei allow to use the lightest nuclei for sensitive tests of CCP. Consistency of the results of the measurements of color transparency in quasielastic A(p,2p) and A(e,e$'$p) processes is emphasized. Evidence for presence of significant color fluctuations in nucleons and pions emerging from the study of diffractive processes is summarized. A new class of hard processes leading to three particle final state is suggested for electron and hadron projectiles. A number of new experiments are suggested to probe color fluctuations in hadrons.
Properties of excited baryonic states are investigated in the context of electroproduction of baryon resonances off the deuteron. In particular, the hadronic radii and the compositeness of baryon resonances are studied for kinematic situations in which their hadronic reinteraction is the dominant contribution. Specifically, we study the reaction $d(e,e'S_{11})N$ at $Q^2\ge 1 GeV^2$ for kinematics in which the produced hadronic state reinteracts predominantly with the spectator nucleon. A comparison of constituent quark model and effective chiral Lagrangian calculations of the $S_{11}$ shows substantial sensitivity to the structure of the produced resonance.