Exclusive pi^+ production at HERMES

Hard exclusive production in deep inelastic lepton scattering provides access to the unknown Generalized Parton Distributions (GPDs) of the nucleon. At HERMES, different observables for hard exclusive pi^+ production have been measured with a 27.6 GeV positron beam on an internal hydrogen gas target. First preliminary results for the unpolarized ep->enpi^+ total cross section for 1.5<Q^2<10.5 GeV^2 and for 0.02<x<0.8 are presented and compared to GPD calculations. The final result for the single-spin asymmetry using a longitudinal polarized target is also reported.Comment: 5 pages, 3 figures, contribution to the XII International Workshop on Deep Inelastic Scattering, April 14-18, 2004, Strbske Pleso, Slovaki

DVCS and hard exclusive meson production

Hard exclusive processes have come to the frontier of nucleon structure physics as they offer experimental access to the Generalised Parton Distributions (GPDs). GPDs have quickly risen in importance as they provide a basis for novel representations of the nucleon as an extended object in space (“nucleon tomography”). A lot of excitement about GPDs is connected to the unique possibility for providing access to the total angular momentum carried by quarks and gluons in the nucleon, a question of fundamental importance in QCD spin physics. This contribution aims at giving an overview about existing measurements of hard exclusive processes, which span over a wide kinematic range, as provided by the HERA collider experiments till the JLab fixed target experiments. A brief summary about the status of GPD extractions in global analyses of these data is given as well. More detailed discussions of the various measurements as well as of model calculations and phenomenology of GPDs can be found at http://ecsac.ictp.it/transversity2011/programma.php

The Spin Structure of the Nucleon

This article reviews our present understanding of QCD spin physics: the proton spin puzzle and new developments aimed at understanding the transverse structure of the nucleon. We discuss present experimental investigations of the nucleon's internal spin structure, the theoretical interpretation of the different measurements and the open questions and challenges for future investigation.Comment: Review, 43 pages, 23 figures, to appear in Reviews of Modern Physic

Azimuthal Asymmetries in Meson Electroproduction at HERMES

The measurement of single‐spin azimuthal asymmetries for pseudoscalar meson production in semi‐inclusive deep‐inelastic scattering of 27.6 GeV electrons off a longitudinally polarised hydrogen and deuterium target is reported by the HERMES experiment. A significant target‐spin asymmetry amplitude in the azimuthal distribution of charged and neutral pions and positively charged kaons relative to the lepton scattering plane has been observed. The dependence on the relevant kinematic variables which are the Bjorken variable x, the meson fractional energy z and the meson transverse momentum P⊥ has been investigated as well. The results are compared to predictions of model calculations which are base on a fragmentation function that varies with the transverse polarisation of the struck quark. In addition, data from the measurement of a single beam‐spin azimuthal asymmetry in the electroproduction of positive pions in semi‐inclusive and semi‐exclusive deep‐inelastic scattering will be presented

The HERMES Back Drift Chambers

The tracking system of the HERMES spectrometer behind the bending magnet consists of two pairs of large planar 6-plane drift chambers. The design and performance of these chambers is described. This description comprises details on the mechanical and electronical design, information about the gas mixture used and its properties, results on alignment, calibration, resolution, and efficiencies, and a discussion of the experience gained through the first three years of operation.Comment: 21 pages, LaTex, 16 figures include

Investigation of single spin asymmetries in pi^{+} electroproduction

The azimuthal single target-spin asymmetries for pi^{+} production in semi-inclusive deep inelastic scattering of leptons off longitudinally polarized protons are evaluated using two main approaches available in the literature. It is shown that the approximation where the twist-2 transverse quark spin distribution in the longitudinally polarized nucleon is small enough to be neglected leads to a consistent description of all existing asymmetries observed by the HERMES experiment.Comment: 11 pages with 4 figures, LaTe

Dark matter search in a Beam-Dump eXperiment (BDX) at Jefferson Lab

MeV-GeV dark matter (DM) is theoretically well motivated but remarkably unexplored. This Letter of Intent presents the MeV-GeV DM discovery potential for a 1 m$^3$ segmented plastic scintillator detector placed downstream of the beam-dump at one of the high intensity JLab experimental Halls, receiving up to 10$^{22}$ electrons-on-target (EOT) in a one-year period. This experiment (Beam-Dump eXperiment or BDX) is sensitive to DM-nucleon elastic scattering at the level of a thousand counts per year, with very low threshold recoil energies ($\sim$1 MeV), and limited only by reducible cosmogenic backgrounds. Sensitivity to DM-electron elastic scattering and/or inelastic DM would be below 10 counts per year after requiring all electromagnetic showers in the detector to exceed a few-hundred MeV, which dramatically reduces or altogether eliminates all backgrounds. Detailed Monte Carlo simulations are in progress to finalize the detector design and experimental set up. An existing 0.036 m$^3$ prototype based on the same technology will be used to validate simulations with background rate estimates, driving the necessary R$\&$D towards an optimized detector. The final detector design and experimental set up will be presented in a full proposal to be submitted to the next JLab PAC. A fully realized experiment would be sensitive to large regions of DM parameter space, exceeding the discovery potential of existing and planned experiments by two orders of magnitude in the MeV-GeV DM mass range.Comment: 28 pages, 17 figures, submitted to JLab PAC 4

Precise Measurements of Beam Spin Asymmetries in Semi-inclusive π\u3csup\u3e0\u3c/sup\u3e Production

We present studies of single-spin asymmetries for neutral pion electroproduction in semi-inclusive deep-inelastic scattering of 5.776 GeV polarized electrons from an unpolarized hydrogen target, using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. A substantial sin phi(h) amplitude has been measured in the distribution of the cross section asymmetry as a function of the azimuthal angle φh of the produced neutral pion. The dependence of this amplitude on Bjorken x and on the pion transverse momentum is extracted with significantly higher precision than previous data and is compared to model calculations. (C) 2011 Elsevier B.V