arXiv subjects
Publications and source records attributed to B. Z. Kopeliovich.
This report, prepared for the Community Planning Study - Snowmass 2013 - summarizes the theoretical motivations and the experimental efforts to search for baryon number violation, focussing on nucleon decay and neutron-antineutron oscillations. Present and future nucleon decay search experiments using large underground detectors, as well as planned neutron-antineutron oscillation search experiments with free neutron beams are highlighted.
This paper summarizes discussions of the theoretical developments and the studies performed by the NNbarX collaboration for the 2013 Snowmass Community Summer Study.
Hadrons inclusively produced with large pT in high-energy collisions originate from the jets, whose initial virtuality and energy are of the same order, what leads to an extremely intensive gluon radiation and dissipation of energy at the early stage of hadronization. Besides, these jets have a peculiar structure: the main fraction of the jet energy is carried by a single leading hadron, so such jets are very rare. The constraints imposed by energy conservation enforce an early color neutralization and a cease of gluon radiation. The produced colorless dipole does not dissipate energy anymore and is evolving to form the hadron wave function. The small and medium pT region is dominated by the hydrodynamic mechanisms of hadron production from the created hot medium. The abrupt transition between the hydrodynamic and perturbative QCD mechanisms causes distinct minima in the pT dependence of the suppression factor R_{AA} and of the azimuthal asymmetry v2. Combination of these mechanisms allows to describe the data through the full range of pT at different collision energies and centralities.
Gluons at small x in high-energy nuclei overlap in the longitudinal direction, so the nucleus acts as a single source of gluons, like higher Fock components in a single nucleon, which contribute to inelastic collisions with a high multiplicity of produced hadrons. This similarity helps to make a link between nuclear effects in pA and high-multiplicity pp collisions. Such a relation is well confirmed by data for the J/Psi production rate in high-multiplicity pp events measured recently in the ALICE experiment. Broadening of J/Psi transverse momentum is predicted for high-multiplicity pp collisions.
The analysis of deeply virtual meson production is extended to neutrino-production of the pseudo-Goldstone mesons (pions, kaons, eta-mesons) on nucleons, with the flavor content of the recoil baryon either preserved, or changed to a hyperon from the same SU(3) octet. We rely on the SU(3) relations and express all the cross-sections in terms of the proton generalized parton distributions (GPDs). The corresponding amplitudes are calculated at the leading twist level and in the leading order in α_{s}, using a phenomenological parametrization of GPDs. We also included in the analysis the electromagnetic O(alpha_{em})-corrections to neutrino-induced deeply virtual meson production (\nuDVMP). We found that such electromagnetic corrections decrease with Q^2 in the Bjorken regime less than the standard \nuDVMP handbag contribution, so the electromagnetic mechanism dominates at large Q^2. The electromagnetic corrections give rise to an angular correlation between the lepton and hadron scattering planes with harmonics sensitive to the real and imaginary parts of the DVMP amplitude. These corrections constitute a few percent effect in the kinematics of the forthcoming MINERvA experiment at Fermilab and should be taken into account in precise tests of GPD parametrizations. For virtualities Q^2~100 GeV^2 these corrections become on a par with \nuDVMP handbag contributions.
Starting from the light-cone plasma (LCP) distribution of gluons produced in proton-proton collisions, we derive the invariant charged-particle yield dN/dyd^2pT in heavy-ion collisions. Multiple scattering of partons in the other nucleus leads to pT broadening, which depends on the centrality of the collision. The resulting change of the LCP parameters determines the distributions of charged particles. The charged-particle multiplicity dN/deta at eta=0 normalized to the number of participating nucleons has a universal centrality dependence, common to RHIC and LHC. This universal increase is of geometrical origin. Such a universality can be broken by the energy dependent coherence effect, shadowing and mutual boosting of the saturation scales. However, we found these effects to essentially compensate each other, leaving the centrality dependence of multiplicity approximately universal.
We study the electromagnetic Bethe-Heitler type contribution to neutrino-induced deeply virtual meson production (\nuDVMP). Such O(α_{em})-corrections decrease with Q^{2} in the Bjorken regime less steeply than the standard \nuDVMP handbag contribution. Therefore, they are relatively enhanced at high Q^{2}. The Bethe-Heitler terms give rise to an angular correlation between the lepton and hadron scattering planes with harmonics sensitive to the real and imaginary parts of the DVMP amplitude. These corrections constitute a few percent effect in the kinematics of the forthcoming Minerva experiment at Fermilab and should be taken into account in precision tests of GPD parametrizations. For virtualities Q^{2}~100 GeV^{2} these corrections become on a par with DVMP handbag contributions. A computational code, which can be used for the evaluation of these corrections employing various GPD models is provided.
Predictions for charged hadron, identified light hadron, quarkonium, photon, jet and gauge bosons in p+Pb collisions at sqrt s_NN = 5 TeV are compiled and compared. When test run data are available, they are compared to the model predictions.
The analysis of deeply virtual meson production is extended to neutrino-production of the pseudo-Goldstone mesons (pions, kaons, eta-mesons) on nucleons, with the flavor content of the recoil baryon either remaining intact, or changing to a hyperon from the SU(3) octet. We rely on the SU(3) relations and express all the cross-sections in terms of the proton generalized parton distributions (GPDs). The corresponding amplitudes are calculated at the leading twist level and in the leading order in α_{s}, using a phenomenological parametrization of GPDs. We provide a computational code, which can be used for evaluation of the cross-sections employing various GPD parametrizations. We conclude that these processes can be studied in the experiment Minerva at FERMILAB, which could supplement the measurements at JLAB and help to extract the GPD flavor structure from data.
We study the nonperturbative structure of the axial current and evaluate the two-point light-cone distribution amplitudes (DA) associated with the correlator \int dξexp(-iqξ)<0|\barψ(x)Γψ(y)J_μ^{5}(ξ)|0> within the instanton vacuum model in the leading order in O(1/N_c). Due to the built-in chiral symmetry, four of eight axial DAs are related to that for pions. Knowledge of these nonperturbative objects is important for phenomenological study of high-energy neutrino interactions and semileptonic decays of heavy quarks. We provide a code for evaluation of these DAs and an interpolation formula valid in the region Q^2 < 1 GeV^2.
We present a collection of selected phenomena observed in J/Psi production from proton-nucleus and heavy ion collisions at energies, ranging between the SPS and LHC. The emphasis is placed on the related theoretical ideas or techniques, which are either not widely known, or offer an alternative explanation to the observed nuclear effects.
The Adler relation (AR), which bridges soft interactions of neutrinos and pions, might look as a manifestation of pion dominance. However neutrino cannot fluctuate to a pion because of lepton current conservation, instead it interacts via much heavier hadronic components. This fact leads to nontrivial relations between interaction amplitudes of different hadronic species, in particular, it links diagonal and off-diagonal diffractive interactions of pions. Absorptive corrections break these relations making the AR impossible to hold universally, i.e. for any target and at any energy. We predict a dramatic breakdown of the AR for coherent neutrino-production of pions on nuclei at all energies.
Shadowing is a quantum phenomenon leading to a non-additivity of electroweak cross sections on nucleons bound in a nucleus. It occurs due to destructive interference of amplitudes on different nucleons. Although the current experimental evidence for shadowing is dominated by charged-lepton nucleus scattering, studies of neutrino nucleus scattering have recently begun and revealed unexpected results.
We update the comparison with experiment of the dynamical model of high-energy hadron interactions based on the two scale structure of hadrons. All predictions made over decade ago are confirmed with a high precision by the TOTEM experiment at LHC.
High-pT hadrons produced in hard collisions and detected inclusively bear peculiar features: (i) they originate from jets whose initial virtuality and energy are of the same order; (ii) such jets are rare and have a very biased energy sharing among the particles, namely, the detected hadron carries the main fraction of the jet energy. The former feature leads to an extremely intensive gluon radiation and energy dissipation at the early stage of hadronization, either in vacuum or in a medium. As a result, a leading hadron must be produced on a short length scale. Evaluation within a model of perturbative fragmentation confirms the shortness of the production length. This result is at variance with the unjustified assumption of long production length, made within the popular energy loss scenario. Thus we conclude that the main reason of suppression of high-pT hadrons in heavy ion collisions is the controlled by color transparency attenuation of a high-pT dipole propagating through the hot medium. Adjusting a single parameter, the transport coefficient, we describe quite well the data from LHC and RHIC for the suppression factor R_{AA} as function of pT, collision energy and centrality. We observe that the complementary effect of initial state interaction causes a flattening and even fall of R_{AA} at large pT. The azimuthal anisotropy of hadron production, calculated with no further adjustment, also agrees well with data at different energies and centralities.
Production of leading neutrons in DIS is usually considered as a tool to measure the pion structure function at small x accessible at HERA. The main obstacle is the lack of reliable evaluations of the absorption corrections, which significantly suppress the cross section. We performed a parameter free calculation within the dipole approach and found the absorption corrections to be nearly as strong, as for neutron production in pp collisions. We also included the significant contribution of the iso-vector Reggeons with natural (ρ, a_2) and unnatural (a_1, ρ-π cut) parity with parameters constrained by phenomenology. With a certain modeling for the pion-to-proton ratio of the structure functions we reached good agreement with data from the ZEUS and H1 experiments, successfully reproducing the observed dependence on the fractional neutron momentum z, the photon virtuality Q^2, and the transverse momentum transfer q_T.
Effects of coherence in neutrino-production of pions off nuclei are studied employing the color dipole representation and path integral technique. If the nucleus remains intact, the process is controlled by the interplay of two length scales. One is related to the pion mass and is quite long (at low Q^2), while the other, associated with heavy axial-vector states, is much shorter. The Adler relation is found to be broken at all energies, but especially strongly at ν> 10 GeV, where the cross section is suppressed by a factor ~A^{-1/3}. On the contrary, in a process where the recoil nucleus breaks up into fragments, the Adler relation turns out to be strongly broken at low energies, where the cross section is enhanced by a factor ~A^{1/3}, but has a reasonable accuracy at higher energies, where all the coherence length scales become long.
This report is based on a ten-week program on "Gluons and the quark sea at high-energies", which took place at the Institute for Nuclear Theory in Seattle in Fall 2010. The principal aim of the program was to develop and sharpen the science case for an Electron-Ion Collider (EIC), a facility that will be able to collide electrons and positrons with polarized protons and with light to heavy nuclei at high energies, offering unprecedented possibilities for in-depth studies of quantum chromodynamics. This report is organized around four major themes: i) the spin and flavor structure of the proton, ii) three-dimensional structure of nucleons and nuclei in momentum and configuration space, iii) QCD matter in nuclei, and iv) Electroweak physics and the search for physics beyond the Standard Model. Beginning with an executive summary, the report contains tables of key measurements, chapter overviews for each of the major scientific themes, and detailed individual contributions on various aspects of the scientific opportunities presented by an EIC.