Baryon resonances and their properties from the CLAS data

The study of the baryon spectrum is a fundamental part of the scientific program in Hall B at Jefferson Lab. By using the CLAS detector, reactions involving electromagnetic production of exclusive hadronic final states in the kinematic region of baryon resonances have been extensively measured, with the purpose of extracting information on non-strange baryon excited states. Some selected topics in the N ^* CLAS experimental program are outlined and corresponding new experimental results are reported and briefly discussed.     

Search for Theta++ pentaquarks in the exclusive reaction gammap-->K+K-p

The reaction gammap --> pK+K- was studied at Jefferson Lab with photon energies from 1.8 to 3.8 GeV using a tagged photon beam. The goal was to search for a Theta++ pentaquark, a narrow, doubly charged baryon state having strangeness S=+1 and isospin I=1, in the pK+ invariant mass spectrum. No statistically significant evidence of a Theta++ was found. Upper limits on the total and differential cross section for the reaction gammap --> K-Theta++ were obtained in the mass range from 1.5 to 2.0 GeV/c2, with an upper limit for a narrow resonance with a mass M(Theta++) = 1.54 GeV/c2 of about 0.15 nb, 95% C.L.. This result places a stringent upper limit on the Theta++ width Gamma(Theta++) <0.1 MeV/c2.     

Neutron transversity measurement at Jefferson Lab with a polarized 3He target

A first measurement of single target spin asymmetry (SSA) from semi-inclusive electroproduction of charged pions from a transversely polarized ³He target in deep-inelastic-scattering kinematics will take data in Hall A at Jefferson Lab in 2008. Such SSA will allow for an extraction of the much desired information on the Collins and Sivers asymmetry from the “neutron” in order to probe the quark transversity distributions. The experiment will be a coincidence measurement with the BigBite spectrometer detecting the scattered electrons and the left-arm HRS spectrometer detecting the charged pions. The prospect of future 11 GeV measurements at Jefferson Lab in Hall A using a solenoid magnetic detector system is also discussed in this talk. Correspondence: H. Gao, Department of Physics and the Triangle Universities Nuclear Laboratory, Duke University, Durham, NC 27708, USA     

Proton knock-out in Hall A

Proton knock-out is studied in a broad program in Hall A at Jefferson Lab. The first experiment performed in Hall A studied the ¹⁶O(e,e'p) reaction. Since then proton knock-out experiments have studied a variety of aspects of that reaction, from single-nucleon properties to its mechanism, such as final-state interactions and two-body currents, in nuclei from ²H to ¹⁶O. In this review the accomplishments of this program will be summarized and an outlook given of expected future results.Received: 1 November 2002, Published online: 15 July 2003PACS: 25.30.Rw Electroproduction reactions - 24.70.+s Polarization phenomena in reactionsFor the Jefferson Lab Hall A Collaboration     

Precise polarization measurements via detection of compton scattered electrons

The Qweak experiment at Jefferson Lab aims to make a 4% measurement of the parity-violating asymmetry in elastic scattering at very low Q ² of a longitudinally polarized electron beam off a proton target. One of the dominant experimental systematic uncertainties in Qweak will result from determining the beam polarization. A new Compton polarimeter was installed in the fall of 2010 to provide a non-invasive and continuous monitoring of the electron beam polarization in Hall C at Jefferson Lab. The Compton-scattered electrons are detected in four planes of diamond micro-strip detectors. We have achieved the design goals of <1% statistical uncertainty per hour and expect to achieve <1% systematic uncertainty.     

CLAS+FROST: new generation of photoproduction experiments at Jefferson Lab.

A large part of the experimental program in Hall B of the Jefferson Lab is dedicated to baryon spectroscopy. Photoproduction experiments are essential part of this program. CEBAF Large Acceptance Spectrometer (CLAS) and availability of circularly and linearly polarized tagged photon beams provide unique conditions for this type of experiments. Recent addition of the Frozen Spin Target (FROST) gives a remarkable opportunity to measure double and triple polarization observables for different pseudo-scalar meson photopro-duction processes. For the first time, a complete or nearly complete experiment becomes possible and will allow model independent extraction of the reaction amplitude. An overview of the experiment and its current status is presented.     

The generalized Gerasimov-Drell-Hearn sum of the neutron at low Q<Superscript>2</Superscript> using a polarized <Superscript>3</Superscript>He target

A recently finished experiment to evaluate the generalized Gerasimov-Drell-Hearn sum on ³He and the neutron in a Q² range between 0.03 GeV²/c² and 1.2 GeV²/c² will be discussed. The experiment was conducted in Hall A at Jefferson Lab using a polarized ³He target in combination with a highly polarized electron beam.     

Observation of an exotic baryon with S=+1 in photoproduction from the proton

The reaction gamma p-->pi(+)K(-)K(+)n was studied at Jefferson Laboratory using a tagged photon beam with an energy range of 3-5.47 GeV. A narrow baryon state with strangeness S=+1 and mass M=1555+/-10 MeV/c(2) was observed in the nK(+) invariant mass spectrum. The peak's width is consistent with the CLAS resolution (FWHM=26 MeV/c(2)), and its statistical significance is (7.8+/-1.0)sigma. A baryon with positive strangeness has exotic structure and cannot be described in the framework of the naive constituent quark model. The mass of the observed state is consistent with the mass predicted by the chiral soliton model for the Theta(+) baryon. In addition, the pK(+) invariant mass distribution was analyzed in the reaction gamma p-->K(-)K(+)p with high statistics in search of doubly charged exotic baryon states. No resonance structures were found in this spectrum.     

Observation of an exotic S = +1 baryon in exclusive photoproduction from the deuteron

In an exclusive measurement of the reaction gammad-->K(+)K(-)pn, a narrow peak that can be attributed to an exotic baryon with strangeness S=+1 is seen in the K(+)n invariant mass spectrum. The peak is at 1.542+/-0.005 GeV/c(2) with a measured width of 0.021 GeV/c(2) FWHM, which is largely determined by experimental mass resolution. The statistical significance of the peak is (5.2+/-0.6)sigma. The mass and width of the observed peak are consistent with recent reports of a narrow S=+1 baryon by other experimental groups.     

Few-body form factor measurements in Hall A of JLab

There is an active program of few-body form factor measurements in Hall A of Thomas Jefferson National Accelerator Facility (Jefferson Laboratory). Recent measurements of the deuteron elastic electric, A(Q²), and magnetic, B(Q²), structure functions will be presented. Planned measurements of the form factors of ³He and ⁴He will also be discussed.     

Dynamically polarized target for the g 2 p and G E p experiments at Jefferson Lab

Recently, two experiments were concluded in Hall A at Jefferson Lab which utilized a newly assembled, solid, polarized hydrogen target. The primary components of the target are a new, high cooling power ⁴He evaporation refrigerator, and a re-purposed, superconducting split-coil magnet. It has been used to polarize protons in irradiated NH₃ at a temperature of 1 K and at fields of 2.5 and 5.0 tesla. Maximum polarizations of 55% and 95% were obtained at those fields, respectively. To satisfy the requirements of both experiments, the magnet had to be routinely rotated between angles of 0°, 6°, and 90° with respect to the incident electron beam.     

Searching for gluonic excitations —The Hall D project at Jlab

Hybrid mesons are produced when the gluonic degrees of freedom are excited within normal mesons. A large fraction of these gluonic excitations can be identified using unique combinations of spin, parity and charge conjugation (J ^PC ) quantum numbers which are not allowed for ordinary $q\bar q$ bound states. Photon beams are expected to be particularly favorable for the production of such states, which are required by the quark confining mechanism of QCD. Mapping out the spectrum and decay modes of these hybrid mesons is the necessary first step in understanding the nature of confinement. Plans are underway at Jefferson Lab to upgrade the energy of the electron accelerator to 12 GeV. With 12 GeV electrons, a 9 GeV linearly polarized photon beam will be produced using the coherent bremsstrahlung technique. Along with this energy upgrade, a hermetic detector housed in new experimental hall (Hall D) will be used to collect data on photoproduced mesons with unprecedented statistics.     

Search for Theta+ (1540) Pentaquark in High-Statistics Measurement of gammap-->K0K+n at CLAS

The exclusive reaction gammap-->K0K+n was studied in the photon energy range between 1.6 and 3.8 GeV searching for evidence of the exotic baryon Theta+ (1540)-->nK+. The decay to nK+requires the assignment of strangeness S=+1 to any observed resonance. Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility corresponding to an integrated luminosity of 70 pb-1. No evidence for the Theta+ pentaquark was found. Upper limits were set on the production cross section as function of center-of-mass angle and nK+ mass. The 95% C.L. upper limit on the total cross section for a narrow resonance at 1540 MeV was found to be 0.8 nb.     

φ-meson photoproduction off the protons by using linearly polarized photons in the mid- to higher-t regimes at threshold energies

Observables from vector meson photoproduction by linearly-polarized photons can be expressed in term of bllinear combinations of helicity amplitudes parameterized by the Spin Density Matrix Elements (SDMEs). These SDMEs give straightforward relations for understanding the nature of the parity exchange at threshold energies, as well as for extracting signatures of the Okubo-Zweig-Iizuka violation. This paper will show preliminary measurements of SDMEs for (γ)p→φp in the photon energy range of 1.7 to 1.9 GeV (momentum transfer squared t range of -1.2 to -0.25 GeV~2) and 1.9 to 2.1 GeV (t range of -1.4 to -0.25 GeV~2) from the g8b experimental data collected in the summer of 2005 in the Hall B of Jefferson Lab.     

Kaon electro-production on protons at JLab in Hall A

The elementary reaction of kaon exclusive electro-production on protons has been studied in a broad kinematical range at Jefferson Lab in Hall A. Data have been taken at different values of the invariant center-of-mass energy W in the range W=1.8-2.2 GeV, for two values of the transferred 4-momentum Q² =1.9 and 2.4 (GeV/c)². Each kinematics was measured at different electron beam energies so as to separate the longitudinal (L) and transverse (T) contributions to the cross-section. The LT interference term has also been measured for a limited number of kinematics. The preliminary data are compared to results of different models developed in the framework of hadronic field and Regge theories.     

Isobar channels and nucleon resonances in π + π − electroproduction on protons

Acomprehensive set of differential cross sections for the reaction γ _ν p → π ^? π ⁺ p at the square of the photon 4-momentum in the range 0.2 < Q ² < 0.6 GeV² and the invariant mass of final-state hadrons in the range 1.3 < W < 1.6 GeV was first obtained with the CLAS detector at the Thomas Jefferson Laboratory. An analysis of these data on the basis of the phenomenological model developed by physicists from the Institute of Nuclear Physics at Moscow State University (INP MSU) and Thomas Jefferson Laboratory (INP MSU-Hall B at Jefferson Lab Collaboration) made it possible to determine, for the first time, the contributions of all isobar channels to the differential cross sections in question. The possibility of extracting the Q ² dependences of the electromagnetic form factors for the P ₁₁(1440) and D ₁₃(1520) resonances in a kinematical region that is the most sensitive to the contribution of the meson-baryon cloud to the structure of nucleon resonances is demonstrated.     

The Third Generation (e,e′K~+) A Hypernuclear Spectroscopy at Jlab

We are now preparing for the third generation (e,e′K~+) A hypernuclear spectroscopic experiment at Hall C,Jefferson Lab (USA).The goal of the experiment is the precise spectroscopy of hypernuclei in wide mass region.We have constructed a new high resolution electron spectrometer "HES" dedicated to (e,e′K~+) hypernuclear study in Japan and it was shipped to JLab in February,2008.We will discuss about the physics of the (e,e′K~+) hypernuclear study at JLab and report the current preparation status of the third generatrion experiment.     

Search for the Θ<Superscript>+</Superscript> pentaquark: experimental statuspentaquark: experimental status

The possible existence of a baryon with strangeness S=+1, the Θ⁺(1540), is presently one of the most exciting topics in hadron physics. Such a state must minimally posses 4 quarks together with one valence\(\bar s\) antiquark. The current experimental status of searches for the Θ⁺(1540) is reviewed, open questions and future prospects are discussed.