Transverse imaging of the proton in exclusive diffractive pp scattering

In a recent paper (hep-ph/0608271) we describe a new approach to rapidity gap survival (RGS) in the production of high-mass systems (H = dijet, Higgs, etc.) in exclusive double-gap diffractive pp scattering, pp -> p + H + p. It is based on the idea that hard and soft interactions are approximately independent (QCD factorization), and allows us to calculate the RGS probability in a model-independent way in terms of the gluon generalized parton distributions (GPDs) in the colliding protons and the pp elastic scattering amplitude. Here we focus on the transverse momentum dependence of the cross section. By measuring the "diffraction pattern," one can perform detailed tests of the interplay of hard and soft interactions, and even extract information about the gluon GPD in the proton from the data.Comment: 4 pages, 2 figures, uses ws-procs9x6.cls. Proceedings of XIV International Workshop on Deep Inelastic Scattering (DIS2006), Tsukuba City, Japan, Apr 20 - 24, 200

The Inclusive-Exclusive Connection and the Neutron Negative Central Charge Density

We find an interpretation of the recent finding that the central charge density of the neutron is negative by using models of generalized parton distributions at zero skewness to relate the behavior of deep inelastic scattering quark distributions, evaluated at large values of Bjorken x, to the transverse charge density evaluated at small distances. The key physical input of these models is the Drell-Yan-West relation We find that the d quarks dominate the neutron structure function for large values of Bjorken x, where the large longitudinal momentum of the struck quark has a significant impact on determining the center-of-momentum of the system, and thus the "center" of the nucleon in the transverse position plane.Comment: 17 pages, 8 figures. Text of an invited talk presented by G. A. Miller at the 2008 Division of Nuclear Physics Meeting in Oakland. Prepared for Int. Journ. Mod. Phys.

Izazov eksperimenata virtualnog Comptonovog raspršenja iznad 8 GeV

We discuss the experimental issues confronting measurements of the virtualCompton-scattering (VCS) reaction ep→epγ with electron beams of energy 6 – 30 GeV. We specifically address the kinematics of deeply-virtual-Comptonscattering (deep inelastic scattering, with coincident detection of the exclusive real photon nearly parallel to the virtual photon direction) and large transverse momentum VCS (high energy VCS of arbitrary Q2, and the recoil proton emitted with high-momentum transverse to the virtual photon direction). We discuss the experimental equipment necessary for these measurements. For the deeply virtual Compton scattering, we emphasize the importance of the Bethe-Heitler – Compton interference terms that can be measured with the electron-positron (beam charge) asymmetry, and the electron beam helicity asymmetryRaspravljamo teško´ce s kojima se sučeljavaju mjerenja reakcija virtualnog Comptonovog raspršenja (VCS) ep→epγ pri energijama elektrona od 6 do 30 GeV. Posebno se razmatra kinematika duboko virtualnog Comptonovog raspršenja i VCS s velikim prijenosom impulsa. Raspravljamo mjerne uređaje koji su potrebni za takva mjerenja. Za duboko virtualno Comptonovo raspršenje, naglašavamo važnost Bethe-Heitlerovih interferentnih članova koji se mogu mjeriti asimetrijom elektronpozitron (naboj snopa) i asimetrijom heliciteta elektronskog snopa

Hyperon Photoproduction in the Nucleon Resonance Region

High-statistics cross sections and recoil polarizations for the reactions γ+p → K++ Λ and γ+p→K++ Σ0 have been measured at CLAS for center-of-mass energies between 1.6 and 2.3 GeV. In the K+Λ channel we confirm a resonance-like structure near W = 1.9 GeV at backward kaon angles. Our data show more complex s- and u- channel behavior than previously seen, since structure is also present at forward angles, but not at central angles. The position and width change with angle, indicating that more than one resonance is playing a role. Large positive Λ polarization at backward angles, which is also energy dependent, is consistent with sizable s- or u-channel contributions. Presently available model calculations cannot explain these aspects of the data

η Photoproduction on the Proton for Photon Energies From 0.75 to 1.95 GeV

Differential cross sections for γp→ηp have been measured with tagged real photons for incident photon energies from 0.75 to 1.95 GeV. Mesons were identified by missing mass reconstruction using kinematical information for protons scattered in the production process. The data provide the first extensive angular distribution measurements for the process above W = 1.75  GeV. Comparison with preliminary results from a constituent quark model support the suggestion that a third S11 resonance with mass ∼1.8  GeV couples to the ηN channel

Electroproduction of the Λ(1520) Hyperon

The reaction e→pe′K+Λ(1520) with Λ(1520)→p′K− was studied at electron beam energies of 4.05, 4.25, and 4.46 GeV, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The cos θK+, φK+, Q2, and W dependencies of Λ(1520) electroproduction are presented for the kinematic region 0.9 \u3c Q2 \u3c 2.4 GeV2 and 1.95 \u3c W \u3c 2.65 GeV. Also, the Q2 dependence of the Λ(1520) decay angular distribution is presented for the first time. The cosθK+ angular distributions suggest t-channel diagrams dominate the production process. Fits to the Λ(1520) t-channel helicity frame decay angular distributions indicate the mz = ± ½ parentage accounts for about 60% of the total yield, which suggests this reaction has a significant contribution from t-channel processes with either K+ exchange or longitudinal coupling to an exchanged K*. The Q2 dependence of the Λ(1520) production cross section is the same as that observed for Λ(1116) photo- and electroproduction

Measurement of Beam-Spin Asymmetries for π⁺ Electroproduction Above the Baryon Resonance Region

We report the first evidence for a nonzero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic kinematic region. Data for the reaction e→pe\u27π+X have been obtained using a polarized electron beam of 4.3 GeV with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The amplitude of the sin ᵠ modulation increases with the momentum of the pion relative to the virtual photon, z. In the range z = 0.5-0.8 the average amplitude is 0.038 ± 0.005 ± 0.003 for a missing mass Mx \u3e 1.1 GeV and 0.037 ± 0.007 ± 0.004 for Mx \u3e1.4 GeV

The ep → e\u27p η Reaction at and Above the S₁₁ (1535) Baryon Resonance

New cross sections for the reaction ep → ep η are reported for total center of mass energy W = 1.5-1.86 GeV and invariant momentum transfer Q2 = 0.25-1.5 (GeV/c)2. This large kinematic range allows extraction of important new information about response functions, photocouplings, and etaN coupling strengths of baryon resonances. Newly observed structure at W ~ 1.65 GeV is shown to come from interference between S and P waves and can be interpreted with known resonances. Improved values are derived for the photon coupling amplitude for the S11(1535) resonance