Glimpsing Colour in a World of Black and White

The past 40 years have taught us that nucleons are built of constituents that carry colour charges with interactions governed by Quantum Chromodynamics (QCD). How experiments (past, present and future) at Jefferson Lab probe colourless nuclei to map out these internal colour degrees of freedom is presented. When combined with theoretical calculations, these will paint a picture of how the confinement of quarks and gluons, and the structure of the QCD vacuum, determine the properties of all (light) strongly interacting states.Comment: 8 pages, 9 figures. Invited talk at the Rutherford Centennial Conference on Nuclear Physics, University of Manchester, 8-12 August 2011. To appear in the Proceeding

Double hadron leptoproduction in the nuclear medium

First measurement of double-hadron production in deep-inelastic scattering has been measured with the HERMES spectrometer at HERA using a 27.6 GeV positron beam with deuterium, nitrogen, krypton and xenon targets. The influence of the nuclear medium on the ratio of double-hadron to single-hadron yields has been investigated. Nuclear effects are clearly observed but with substantially smaller magnitude and reduced $A$-dependence compared to previously measured single-hadron multiplicity ratios. The data are in fair agreement with models based on partonic or pre-hadronic energy loss, while they seem to rule out a pure absorptive treatment of the final state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter

Quark helicity distributions in the nucleon for up, down, and strange quarks from semi--inclusive deep--inelastic scattering

Polarized deep--inelastic scattering data on longitudinally polarized hydrogen and deuterium targets have been used to determine double spin asymmetries of cross sections. Inclusive and semi--inclusive asymmetries for the production of positive and negative pions from hydrogen were obtained in a re--analysis of previously published data. Inclusive and semi--inclusive asymmetries for the production of negative and positive pions and kaons were measured on a polarized deuterium target. The separate helicity densities for the up and down quarks and the anti--up, anti--down, and strange sea quarks were computed from these asymmetries in a ``leading order'' QCD analysis. The polarization of the up--quark is positive and that of the down--quark is negative. All extracted sea quark polarizations are consistent with zero, and the light quark sea helicity densities are flavor symmetric within the experimental uncertainties. First and second moments of the extracted quark helicity densities in the measured range are consistent with fits of inclusive data

Exclusive Leptoproduction of rho^0 Mesons from Hydrogen at Intermediate Virtual Photon Energies

Measurements of the cross section for exclusive virtual-photoproduction of rho^0 mesons from hydrogen are reported. The data were collected by the HERMES experiment using 27.5 GeV positrons incident on a hydrogen gas target in the HERA storage ring. The invariant mass W of the photon-nucleon system ranges from 4.0 to 6.0 GeV, while the negative squared four-momentum Q^2 of the virtual photon varies from 0.7 to 5.0 GeV^2. The present data together with most of the previous data at W > 4 GeV are well described by a model that infers the W-dependence of the cross section from the dependence on the Bjorken scaling variable x of the unpolarized structure function for deep-inelastic scattering. In addition, a model calculation based on Off-Forward Parton Distributions gives a fairly good account of the longitudinal component of the rho^0 production cross section for Q^2 > 2 GeV^2.Comment: 10 pages, 6 embedded figures, LaTeX for SVJour(epj) document class. Revisions: curves added to Fig. 1, several clarifications added to tex

Evidence for a narrow |S|=1 baryon state at a mass of 1528 MeV in quasi-real photoproduction

Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel p K^0_S --> p pi^+ pi^-. A peak is observed in the p K^0_S invariant mass spectrum at 1528 +/- 2.6 (stat) +/-2.1 (syst) MeV. Depending on the background model,the naive statistical significance of the peak is 4--6 standard deviations and its width may be somewhat larger than the experimental resolution of sigma=4.3 -- 6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Theta^{+}(uuddbar(s)) pentaquark baryon. No signal for an hypothetical Theta^{++} baryon was observed in the pK^+ invariant mass distribution. The absence of such a signal indicates that an isotensor Theta is excluded and an isovector Theta is unlikely.Comment: 8 pages, 4 figure

Publisher Correction: The asymmetry of antimatter in the proton.

In Fig. 1c of this Article, the x-axis scale was inadvertently duplicated from Fig. 1b. The original Article has been corrected online

The asymmetry of antimatter in the proton.

The fundamental building blocks of the proton-quarks and gluons-have been known for decades. However, we still have an incomplete theoretical and experimental understanding of how these particles and their dynamics give rise to the quantum bound state of the proton and its physical properties, such as its spin1. The two up quarks and the single down quark that comprise the proton in the simplest picture account only for a few per cent of the proton mass, the bulk of which is in the form of quark kinetic and potential energy and gluon energy from the strong force2. An essential feature of this force, as described by quantum chromodynamics, is its ability to create matter-antimatter quark pairs inside the proton that exist only for a very short time. Their fleeting existence makes the antimatter quarks within protons difficult to study, but their existence is discernible in reactions in which a matter-antimatter quark pair annihilates. In this picture of quark-antiquark creation by the strong force, the probability distributions as a function of momentum for the presence of up and down antimatter quarks should be nearly identical, given that their masses are very similar and small compared to the mass of the proton3. Here we provide evidence from muon pair production measurements that these distributions are considerably different, with more abundant down antimatter quarks than up antimatter quarks over a wide range of momenta. These results are expected to revive interest in several proposed mechanisms for the origin of this antimatter asymmetry in the proton that had been disfavoured by previous results4, and point to future measurements that can distinguish between these mechanisms

Measurement of flavor asymmetry of the light-quark sea in the proton with Drell-Yan dimuon production in p+p and p+d collisions at 120 GeV

Evidence for a flavor asymmetry between the ū and d¯ quark distributions in the proton has been found in deep-inelastic scattering and Drell-Yan experiments. The pronounced dependence of this flavor asymmetry on x (fraction of nucleon momentum carried by partons) observed in the Fermilab E866 Drell-Yan experiment suggested a drop of the d¯(x)/ū(x) ratio in the x>0.15 region. We report results from the SeaQuest Fermilab E906 experiment with improved statistical precision for d¯(x)/ū(x) in the large x region up to x=0.45 using the 120 GeV proton beam. Two different methods for extracting the Drell-Yan cross section ratios, σpd/2σpp, from the SeaQuest data give consistent results. The d¯(x)/ū(x) ratios and the d¯(x)-ū(x) differences are deduced from these cross section ratios for 0.1

The SeaQuest spectrometer at Fermilab

The SeaQuest spectrometer at Fermilab was designed to detect oppositely-charged pairs of muons (dimuons) produced by interactions between a 120 GeV proton beam and liquid hydrogen, liquid deuterium and solid nuclear targets. The primary physics program uses the Drell–Yan process to probe antiquark distributions in the target nucleon. The spectrometer consists of a target system, two dipole magnets and four detector stations. The upstream magnet is a closed-aperture solid iron magnet which also serves as the beam dump, while the second magnet is an open aperture magnet. Each of the detector stations consists of scintillator hodoscopes and a high-resolution tracking device. The FPGA-based trigger compares the hodoscope signals to a set of pre-programmed roads to determine if the event contains oppositely-signed, high-mass muon pairs