Search

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., Comment: 3 pages. Uses aipproc.cls. Contributed to proceedings of PANIC05, Santa Fe, NM, Oct. 24-28, 2005

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., Comment: 15 pages, 4 eps figures

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., Comment: Submitted to Phys. Rev. Lett., 4 pages, 5 figures

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., Comment: 5 pages, 2 figures, RevTeX; slightly revised version to appear in PRL

We argue that the t-dependence of the two-gluon form factor of the nucleon should be given by $\Gamma(t)=(1-t/m_{2g}^2)^{-2}$ 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 $J/\psi$-mesons between the threshold region of $E_{\gamma}=11 GeV$ (Cornell), $E_{\gamma}=19 GeV$ (SLAC) and $E_{\gamma}=100 GeV$ (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 $\phi$-meson electroproduction near threshold at $W=2.3 GeV, Q^2=1.0 GeV^2$., Comment: 10 pages, 4 figures

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., Comment: 13 pages, 1 figure

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., Comment: 14 pages, 4 figures, The discussion has been extended by including numerical examples and general arguments

Photon absorption by a quark in one nucleon followed by its high momentum transfer interaction with a quark in the other may produce two 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 cross sections are in reasonable agreement with the data., Comment: Four pages Latex, uses espcrc1.sty. Presented at 15th International Conference on Particle and Nuclei (PANIC 99), Uppsala, Sweden, 10-16 Jun 1999

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., Comment: 17 pp, 12 fig. Plenary talk to be published in the proceedings of 7th International Workshop on Deep Inelastic Scattering and QCD (DIS 99), Zeuthen, Germany, 19-23 Apr 1999. Submitted to Nucl.Phys.Proc.Suppl

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., Comment: 14 pages,6 figures,Dedicated to Koichi Yazaki on the occasion of his 60th birthday. Invited talk at KEK-Tanashi International Symposium on Physics of Hadrons and Nuclei, Tokyo, Japan, 14-17 Dec 1998

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., Comment: 4 pages, revised version to be published in Phys. Rev. Lett

The hard exclusive electroproduction of $\pi^0,\eta,\eta'$ mesons by the longitudinal photons is considered. We calculate the ratios of the $\pi^0 : \eta : \eta'$ yields for the electroproduction processes off nucleons and deuterons based on the QCD factorization theorem, the low energy QCD chiral dynamics, and the QCD anomaly. Thus investigation of these ratios gives a new way to probe the physics of the spontaneous chiral symmetry breaking., Comment: 14 pages, 2 figures, RevTeX. Discussion of the factorization is expanded, discussion of the two-photon production of the pseudoscalar mesons, references and acknowledgements are added, typos are corrected. The results of the paper are unchanged

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., Comment: 24 pages, 5 figures

We study nuclear physics in the chiral limit ($m_u$, $m_d=0$) in which the pion mass vanishes. We find that the deuteron mass is changed little, but that P--wave nucleon--nucleon scattering volumes are infinite. This motivates an investigation of the possibilities that there could be a two--nucleon $^3P_0$ bound state, and that the nuclear matter ground state is likely to be a condensed state of nucleons paired to those quantum numbers. However, the short distance repulsion in the nucleon--nucleon potential is not affected by the chiral limit and prevents such new chiral possibilities. Thus the chiral limit physics of nuclei is very similar to that of nature. Using the chiral limit to simplify QCD sum rule calculations of nuclear matter properties seems to be a reasonable approximation., Comment: 9 pages, 1 figure, submitted to Phys. Rev C

Predicted two-photon Higgs production with heavy ions at LHC is shown to be reduced due to the large Coulomb dissociation cross section. Incorporating the effect of dissociation reduces the production of a 100 GeV Higgs by about a factor of three compared to rates in the literature calculated without this effect., Comment: 5 pages, latex, revtex source, two postscript figures

We formulate and prove a QCD factorization theorem for hard exclusive electroproduction of mesons in QCD. The proof is valid for the leading power in Q and all logarithms. This generalizes previous work on vector meson production in the diffractive region of small x. The amplitude is expressed in terms of off-diagonal generalizations of the usual parton densities. The full theorem applies to all kinds of meson and not just to vector mesons. The parton densities used include not only the ordinary parton density, but also the helicity density (g_1 or $\Delta q$) and the transversity density ($h_1$ or $\delta q$), and these can be probed by measuring the polarization of the produced mesons with unpolarized protons., Comment: 50 pages, revtex, figures included, uses epsf.sty. Correction of citations, and of predictions for meson production

We argue that the probability of gap survival in dijet production in $\gamma p$ scattering as measured by ZEUS may be due to the color transparency phenomenon and suggest ways to test this hypothesis in the future $\gamma p$ and $\gamma A$ processes., Comment: 4 pages, LaTeX, one Postscript figure included using epsfig.sty. To appear in the conference proceedings of the Workshop "Future Physics at HERA"

We argue that current data on nuclear shadowing confirm expectations of the color singlet models of nuclear shadowing. We demonstrate that unitarity constrains require nuclear shadowing for gluons to be very significant already for $x \le 10^{-3}$ . Physics of coherent diffraction production of vector mesons off light nuclei is also discussed., Comment: Talk at the Workshop on Deep Inelastic Scattering and QCD, Paris, April 1995, 5 pages LATEX, uses "qcdparis.sty" (appended at end), no figures

Coherence phenomena, and the non-universality of parton structure of the effective Pomeron are explained. New hard phenomena directly calculable in QCD such as diffractive electroproduction of states with $M^2\ll Q^2$ as well as new options to measure the light-cone wave functions of various hadrons are considered. An analogue of Bjorken scaling is predicted for the diffractive electroproduction of $\rho$ mesons at large momentum transfers and for the production of large rapidity gap events, as observed at HERA. A phenomenological QCD evolution equation is suggested to calculate the basic characteristics of the large rapidity gap events. The increase of parton densities at small $x$ as well as new means to disentangle experimentally soft and hard physics are considered. We discuss constraints on the increase of deep inelastic amplitudes with $Q^2$ derived from unitarity of the S matrix for collisions of wave packets. New ways to probe QCD physics of hard processes at large longitudinal distances and to answer the long standing problems on the origin of the Pomeron are suggested. Unresolved problems and perspectives of small $x$ physics are also outlined., Comment: Talk at the Workshop on Deep Inelastic Scattering and QCD, Paris, April 1995, 17 pages LATEX, uses "qcdparis.sty" needed (appended at end), one ps figure in separeted uuncode file

Coherence phenomena, the increase with energy of coherence length and the non-universality of parton structure of the effective Pomeron are explained. New hard phenomena directly calculable in QCD such as diffractive electroproduction of states with $M^2\ll Q^2$ and the color transparency phenomenon as well as new options to measure the light-cone wave functions of various hadrons are considered. An analogue of Bjorken scaling is predicted for the diffractive electroproduction of $\rho$ mesons at large momentum transfers and for the production of large rapidity gap events, as observed at HERA. A phenomenological QCD evolution equation is suggested to calculate the basic characteristics of the large rapidity gap events. The increase of parton densities at small $x$ as well as new means to disentangle experimentally soft and hard physics are considered. We discuss constraints on the increase of deep inelastic amplitudes with $Q^2$ derived from the inconsistency of QCD predictions for inclusive and exclusive processes and from unitarity of the S matrix for collisions of wave packets. New ways to probe QCD physics of hard processes at large longitudinal distances and to answer the long standing problems on the origin of the Pomeron are suggested. Unresolved problems and perspectives of small $x$ physics are also outlined., Comment: 1 figure updated, comments added, some numerical coefficients are corrected (to appear in SLAC summer school, 1994)

Diffractive scattering involves exchange of a Pomeron to make a rapidity gap. It is normally assumed that to get a hard scattering in diffraction, one may treat the Pomeron as an ordinary particle, which has distributions of gluons and quarks. We show that this is not so: When we use perturbative QCD, there is a breakdown of the factorization theorem. The whole Pomeron can initiate the hard scattering, even though it is not point-like. Qualitatively, but not quantitatively, this gives the same effect as a delta function term in the gluon density in a Pomeron., Comment: 13 pages (+ 5 figures, not included), LaTeX, PSU/TH/116