30 research outputs found
Pentaquarks: review of the experimental evidence
Pentaquarks, namely baryons made by 4 quarks and one antiquark have been
predicted and searched for since several decades without success. Theoretical
and experimental advances in the last 2 years led to the observation of a
number of pentaquark candidates. We review the experimental evidence for
pentaquarks as well as their non-observations by some experiments, and discuss
to which extend these sometimes contradicting informations may lead to a
consistent picture.Comment: Contribution to the International Conference on 'Strangeness in Quark
Matter', 15-21 Sept. 2004, Cape Tawn, South Afric
Measurement of the Deuteron Structure Function Fā in the Resonance Region and Evaluation of its Moments
Inclusive electron scattering off the deuteron has been measured to extract the deuteron structure function F2 with the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. The measurement covers the entire resonance region from the quasielastic peak up to the invariant mass of the final-state hadronic system W similar or equal to 2.7 GeV with four-momentum transfers Q2 from 0.4 to 6 (GeV/c)2. These data are complementary to previous measurements of the proton structure function F2 and cover a similar two-dimensional region of Q2 and Bjorken variable x. Determination of the deuteron F2 over a large x interval including the quasielastic peak as a function of Q2, together with the other world data, permit a direct evaluation of the structure function moments for the first time. By fitting the Q2 evolution of these moments with an OPE-based twist expansion we have obtained a separation of the leading twist and higher twist terms. The observed Q2 behavior of the higher twist contribution suggests a partial cancelation of different higher twists entering into the expansion with opposite signs. This cancelation, found also in the proton moments, is a manifestation of the duality phenomenon in the F2 structure function
Differential Cross Sections for + p ā Kāŗ + Y for Ī and Ī£ā° Hyperons
High-statistics cross sections for the reactions + p ā Kāŗ + Ī and + p ā Kāŗ + Ī£ā° have been measured using CLAS at Jefferson Lab for center-of-mass energies W between 1.6 and 2.53 GeV, and for -0.85 \u3c cos Īø Kc.m. \u3c +0.95. In the Kāŗ + Ī channel we confirm a resonance-like structure near W=1.9 GeV at backward kaon angles. The position and width of this structure change with angle, indicating that more than one resonance is likely playing a role. The Kāŗ + Ī channel at forward angles and all energies is well described by a t-channel scaling characteristic of Regge exchange, whereas the same scaling applied to the Kāŗ + Ī£ā° channel is less successful. Several existing theoretical models are compared to the data, but none provide a good representation of the results
Measurement of Deeply Virtual Compton Scattering Beam-Spin Asymmetries
The beam-spin asymmetries in the hard exclusive electroproduction of photons on the proton (āepāepĪ³ ) were measured over a wide kinematic range and with high statistical accuracy. These asymmetries result from the interference of the Bethe-Heitler process and of deeply virtual Compton scattering. Over the whole kinematic range (xB from 0.11 to 0.58, Q2 from 1 to 4.8āāGeV2, āt from 0.09 to 1.8āāGeV2), the azimuthal dependence of the asymmetries is compatible with expectations from leading-twist dominance, A ā asinĻ/(1+ccosĻ). This extensive set of data can thus be used to constrain significantly the generalized parton distributions of the nucleon in the valence quark sector
Magnetic moment of the pentaquark with light-cone QCD sum rules
In this article, we study the magnetic moment of the pentaquark state as diquark-diquark-antiquark () state in the
framework of the light-cone QCD sum rules approach. The numerical results
indicate the magnetic moment of the pentaquark state is about
.Comment: 10 pages, 1 figure. The main contents of this article is included in
hep-ph/0503007, this article will not be submitted to a journal for
publicatio
Beam-Helicity Asymmetries in Double-Charged-Pion Photoproduction on the Proton
Beam-helicity asymmetries for the two-pion-photoproduction reaction āp ā p Ļ+Ļ- have been studied for the first time in the resonance region for center-of-mass energies between 1.35 and 2.30 GeV. The experiment was performed at Jefferson Lab with the CEBAF Large Acceptance Spectrometer using circularly polarized tagged photons incident on an unpolarized hydrogen target. Beam-helicity-dependent angular distributions of the final-state particles were measured. The large cross-section asymmetries exhibit strong sensitivity to the kinematics and dynamics of the reaction. The data are compared with the results of various phenomenological model calculations, and show that these models currently do not provide an adequate description for the behavior of this new observable
Ī·ā Photoproduction on the Proton for Photon Energies from 1.527 to 2.227 GeV
Differential cross sections for the reaction Ī³pāĪ·ā²p have been measured with the CLAS spectrometer and a tagged photon beam with energies from 1.527 to 2.227 GeV. The results reported here possess much greater accuracy than previous measurements. Analyses of these data suggest for the first time the coupling of the Ī·ā²N channel to both the S11(1535) and P11(1710) resonances, known to couple strongly to the Ī·N channel in photoproduction on the proton, and the importance of J=3/2 resonances in the process
Light Vector Mesons in the Nuclear Medium
The light vector mesons (Ļ,Ļ, and Ļ) were produced in deuterium, carbon, titanium, and iron targets in a search for possible in-medium modifications to the properties of the Ļ meson at normal nuclear densities and zero temperature. The vector mesons were detected with the CEBAF Large Acceptance Spectrometer (CLAS) via their decays to e+eā. The rare leptonic decay was chosen to reduce final-state interactions. A combinatorial background was subtracted from the invariant mass spectra using a well-established event-mixing technique. The Ļ-meson mass spectrum was extracted after the Ļ and Ļ signals were removed in a nearly model-independent way. Comparisons were made between the Ļmass spectra from the heavy targets (A\u3e2) with the mass spectrum extracted from the deuterium target. With respect to the Ļ-meson mass, we obtain a small shift compatible with zero. Also, we measure widths consistent with standard nuclear many-body effects such as collisional broadening and Fermi motion
Moments of the spin structure functions g(1)(p) and g(1)(d) for 0.05 \u3c Q\u3csup\u3e2\u3c/sup\u3e \u3c 3.0 GeV\u3csup\u3e2\u3c/sup\u3e
The spin structure functions g1 for the proton and the deuteron have been measured over a wide kinematic range in x and Q2 using 1.6 and 5.7 GeV longitudinally polarized electrons incident upon polarized NH3 and ND3 targets at Jefferson Lab. Scattered electrons were detected in the CEBAF Large Acceptance Spectrometer, for 0.05 \u3c Q2 \u3c 5GeV2 and W \u3c 3 GeV. The first moments of g1 for the proton and deuteron are presented ā both have a negative slope at low Q2, as predicted by the extended GerasimovāDrellāHearn sum rule. The first extraction of the generalized forward spin polarizability of the proton Ī³ p 0 the Q2 evolution of the first moment of g1 shows agreement in leading order with Heavy Baryon Chiral Perturbation Theory. However, a significant discrepancy is observed between the Ī³ p 0 data and Chiral Perturbation calculations for Ī³ p 0, even at the lowest Q2
Beam Spin Asymmetries in Deeply Virtual Compton Scattering (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, 1.02\u3c2.8 (GeV/c)2, 0.12\u3cxB\u3c0.48, and 0.1\u3cāt\u3c0.8 (GeV/c)2, using the reaction āepāeā²pX. The number of H(e,eā²Ī³p) and H(e,eā²Ļ0p) events are separated in each (Q2,xB,t) bin by a fit to the line shape of the H(e,eā²p)X M2x 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 DVCSāBethe-Heitler and Ļ0 events can reliably be done with less than 5% uncertainty. Also, the Q2 and t dependences of the sinĻ moments of the asymmetry are extracted and compared with theoretical calculations