Beam Spin Asymmetries in DVCS with CLAS at 4 .8 GeV

We report measurements of the beam spin asymmetry in Deeply Virtual Compton Scattering (DVCS) at an electron beam energy of 4.8 GeV using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The DVCS beam spin asymmetry has been measured in a wide range of kinematics, 1(GeV/c)$^2$ $<Q^2<2.8$(GeV/c)$^2$, $0.12<x_B<0.48$, and 0.1 (GeV/c)$^2$ $<-t<0.8$(GeV/c)$^2$, using the reaction \pEpX. The number of H$(e,e^{\prime}\gamma p)$ and H$(e,e^{\prime}\pi^0 p)$ events are separated in each $(Q^2,x_B,t)$ bin by a fit to the line shape of the H$(e,e^{\prime}p)X$ $M_x^2$ distribution. The validity of the method was studied in detail using experimental and simulated data. It was shown, that with the achieved missing mass squared resolution and the available statistics, the separation of DVCS-BH and $\pi^0$ events can reliably be done with less than 5% uncertainty. The $Q^2$- and $t$-dependences of the $\sin\phi$ moments of the asymmetry are extracted and compared with theoretical calculations

First observation of the Λ(1405) line shape in electroproduction

We report the first observation of the line shape of the Λ(1405) from electroproduction, and show that it is not a simple Breit-Wigner resonance. Electroproduction of K+Λ(1405) off the proton was studied by using data from CLAS at Jefferson Lab in the range 1.0&#60;Q2&#60;3.0 (GeV/c)2. The analysis utilized the decay channels Σ+π− of the Λ(1405) and pπ0 of the Σ+. Neither the standard Particle Data Group resonance parameters, nor free parameters fitting to a single Breit-Wigner resonance represent the line shape. In our fits, the line shape corresponds approximately to predictions of a two-pole meson-baryon picture of the Λ(1405), with a lower mass pole near 1368 MeV/c2 and a higher mass pole near 1423 MeV/c2. Furthermore, with increasing photon virtuality the mass distribution shifts toward the higher mass pole

First Observation of Large Missing-Momentum (e,e'p) Cross-Section Scaling and the onset of Correlated-Pair Dominance in Nuclei

International audienceWe report the first measurement of $x_B$-scaling in $(e,e'p)$ cross-section ratios off nuclei relative to deuterium at large missing-momentum of $350 \leq p_{miss} \leq 600$ MeV/c. The observed scaling extends over a kinematic range of $0.7 \leq x_B \leq 1.8$, which is significantly wider than $1.4 \leq x_B \leq 1.8$ previously observed for inclusive $(e,e')$ cross-section ratios. The $x_B$-integrated cross-section ratios become constant (i.e., scale) beginning at $p_{miss}\approx k_F$, the nuclear Fermi momentum. Comparing with theoretical calculations we find good agreement with Generalized Contact Formalism calculations for high missing-momentum ($> 375$ MeV/c), suggesting the observed scaling results from interacting with nucleons in short-range correlated (SRC) pairs. For low missing-momenta, mean-field calculations show good agreement with the data for $p_{miss}\le k_F$, and suggest that contributions to the measured cross-section ratios from scattering off single, un-correlated, nucleons are non-negligible up to $p_{miss}\approx 350$ MeV/c. Therefore, SRCs become dominant in nuclei at $p_{miss}\approx 350$ MeV/c, well above the nuclear Fermi Surface of $k_F \approx 250$ MeV/c

Design of the ECCE Detector for the Electron Ion Collider

International audienceThe EIC Comprehensive Chromodynamics Experiment (ECCE) detector has been designed to address the full scope of the proposed Electron Ion Collider (EIC) physics program as presented by the National Academy of Science and provide a deeper understanding of the quark-gluon structure of matter. To accomplish this, the ECCE detector offers nearly acceptance and energy coverage along with excellent tracking and particle identification. The ECCE detector was designed to be built within the budget envelope set out by the EIC project while simultaneously managing cost and schedule risks. This detector concept has been selected to be the basis for the EIC project detector