Browsing by Author "Adhikari, K. P."

We report a measurement of a beam–target double-polarisation observable (E) for the γn(p) → K +−(p) reaction. The data were obtained impinging the circularly-polarised energy-tagged photon beam of Hall B at Jefferson Lab on a longitudinally-polarised frozen-spin hydrogen deuteride (HD) nuclear target. The E observable for an effective neutron target was determined for centre-of-mass energies 1.70 ≤ W ≤ 2.30 GeV, with reaction products detected over a wide angular acceptance by the CLAS spectrometer. These new double-polarisation data give unique constraints on the strange decays of excited neutron states. Inclusion of the new data within the Bonn Gatchina theoretical model results in significant changes for the extracted photocouplings of a number of established nucleon resonances. Possible improvements in the PWA description of the experimental data with additional “missing” resonance states, including the N(2120)3/2−resonance, are also quantified.

The CLAS detector was used to obtain the first ever measurement of the electromagnetic decay of the Σ*⁺(1385) from the reaction γp → K⁰Σ*⁺(1385). A real photon beam with a maximum energy of 3.8 GeV was incident on a liquid-hydrogen target, resulting in the photoproduction of the kaon and Σ* hyperon. Kinematic fitting was used to separate the reaction channel from the background processes. The fitting algorithm exploited a new method to kinematically fit neutrons in the CLAS detector, leading to the measured decay widths ratio: Γ(Σ⁺(1385) → Σ⁺γ) / Γ(Σ⁺(1385) → Σ⁺π⁰) = 11.95 ± 2.21 (stat) ⁺⁰.⁵³₋₁.₂₁ (sys) % and a deduced partial width of: Γ(Σ⁺(1385) → Σ⁺γ) = 250.0 ± 56.9 (stat) ⁺³⁴.³₋₄₁.² (sys) keV A U-spin symmetry test using the SU(3) flavor-multiplet representation yields predictions for the Σ*⁺(1385) → Σ⁺γ and Σ*⁰(1385) → Λγ partial widths that agree with the experimental measurements.

Unpolarized and beam-polarized fourfold cross sections ðd4σ=dQ2dxBdtdϕÞ for the ep → e0 p0 γ reaction were measured using the CLAS detector and the 5.75-GeV polarized electron beam of the Jefferson Lab accelerator, for 110 (Q2; xB; t) bins over the widest phase space ever explored in the valencequark region. Several models of generalized parton distributions (GPDs) describe the data well at most of our kinematics. This increases our confidence that we understand the GPD H, expected to be the dominant contributor to these observables. Through a leading-twist extraction of Compton form factors, these results support the model predictions of a larger nucleon size at lower quark-momentum fraction xB.

The beam-spin asymmetry, , for the reaction γ d → pn has been measured using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility (JLab) for six photon-energy bins, between 1.1 and 2.3 GeV, and proton angles in the center-of-mass frame, θc.m., between 25◦ and 160◦. These are the first measurements of beam-spin asymmetries at θc.m. = 90◦ for photon-beam energies above 1.6 GeV, and the first measurements for angles other than θc.m. = 90◦. The angular and energy dependence of is expected to aid in the development of QCD-based models to understand the mechanisms of deuteron photodisintegration in the transition region between hadronic and partonic degrees of freedom, where both effective field theories and perturbative QCD cannot make reliable predictions.

The f0(1500) meson resonance is one of several contenders to have significant mixing with the lightest glueball. This resonance is well established from several previous experiments. Here we present the first photoproduction data for the f0(1500) via decay into the K0 SK 0 S channel using the CLAS detector. The reaction γp → fJp → K0 SK0 Sp, where J = 0,2, was measured with photon energies from 2.7 to 5.1 GeV. A clear peak is seen at 1500 MeV in the background subtracted invariant mass spectra of the two kaons. This is enhanced if the measured 4-momentum transfer to the proton target is restricted to be less than 1.0 GeV2. By comparing data with simulations, it can be concluded that the peak at 1500 MeV is produced primarily at low t, which is consistent with at-channel production mechanism.

We have measured the nuclear transparency of the incoherent diffractive A(e, e ρ0) process in 12C and 56Fe targets relative to 2H using a 5 GeV electron beam. The nuclear transparency, the ratio of the produced ρ0’s on a nucleus relative to deuterium, which is sensitive to ρ A interaction, was studied as function of the coherence length (lc ), a lifetime of the hadronic fluctuation of the virtual photon, and the four-momentum transfer squared (Q 2). While the transparency for both 12C and 56Fe showed no lc dependence, a significant Q 2 dependence was measured, which is consistent with calculations that included the color transparency effects.

Exclusive photoproduction cross sections have been measured for the process γp → pπ0[e+e−(γ )] with the Dalitz decay final state using tagged photon energies in the range of Eγ = 1.275–5.425 GeV. The complete angular distribution of the final state π0, for the entire photon energy range up to large values of t and u, has been measured for the first time. The data obtained show that the cross section dσ/dt, at mid to large angles, decreases with energy as s−6.89±0.26. This is in agreement with the perturbative QCD quark counting rule prediction of s−7. Paradoxically, the size of angular distribution of measured cross sections is greatly underestimated by the QCD-based generalized parton distribution mechanism at highest available invariant energy s = 11 GeV2 . At the same time, the Regge-exchange-based models for π0 photoproduction are more consistent with experimental data.

We report results of Λ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014 GeV electron beam. These results represent the first measurements of the Λ multiplicity ratio and transverse momentum broadening as a function of the energy fraction (𝑧) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high 𝑧 and an enhancement at low 𝑧. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of diquark configurations in the nuclear medium takes place at least part of the time, even at high 𝑧. The trends of these results are qualitatively described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.

First measurements of double-polarization observables in ω photoproduction off the proton are presented using transverse target polarization and data from the CEBAF Large Acceptance Spectrometer (CLAS) FROST experiment at Jefferson Lab. The beam-target asymmetry F has been measured using circularly polarized, tagged photons in the energy range 1200-2700 MeV, and the beam-target asymmetries H and P have been measured using linearly polarized, tagged photons in the energy range 1200-2000 MeV. These measurements significantly increase the database on polarization observables. The results are included in two partial-wave analyses and reveal significant contributions from several nucleon (N∗) resonances. In particular, contributions from new N∗ resonances listed in the Review of Particle Properties are observed, which aid in reaching the goal of mapping out the nucleon resonance spectrum.

We have measured cross sections for the 3He ! pd reaction at photon energies of 0.4–1.4 GeV and a center-of-mass angle of 90 . We observe dimensional scaling above 0.7 GeV at this center-of-mass angle. This is the first observation of dimensional scaling in the photodisintegration of a nucleus heavier than the deuteron

Strange matter is believed to exist in the cores of neutron stars based on simple kinematics. If this is true, then hyperon-nucleon interactions will play a significant part in the neutron star equation of state. Yet, compared to other elastic scattering processes, there is very little data on Λ-N scattering. This experiment utilized the CEBAF Large Acceptance Spectrometer (CLAS) detector to study the Λp → Λp elastic scattering cross section in the incident Λ momentum range 0.9–2.0 GeV=c. These are the first data on this reaction since the 1970s. The new cross sections have significantly better accuracy and precision than the existing world data, and the techniques developed here can also be used in future experiments.

Exclusive 0 electroproduction at a beam energy of 5.75 GeV has been measured with the Jefferson Lab CLAS spectrometer. Differential cross sections were measured at more than 1800 kinematic values in Q2, xB, t, and , in the Q2 range from 1.0 to 4:6 GeV2, t up to 2 GeV2, and xB from 0.1 to 0.58. Structure functions T þ L, TT, and LT were extracted as functions of t for each of 17 combinations of Q2 and xB. The data were compared directly with two handbag-based calculations including both longitudinal and transversity generalized parton distributions (GPDs). Inclusion of only longitudinal GPDs very strongly underestimates T þ L and fails to account for TT and LT, while inclusion of transversity GPDs brings the calculations into substantially better agreement with the data. There is very strong sensitivity to the relative contributions of nucleon helicity-flip and helicity nonflip processes. The results confirm that exclusive 0 electroproduction offers direct experimental access to the transversity GPDs.

The photoproduction of ω mesons off the proton has been studied in the reaction γp → p ω using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry T has been measured in photoproduction from the decay ω → π+π−π0, using a transversely polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant nonzero values are observed for these asymmetries, reaching about 30–40% in the third-resonance region. New measurements for the photon-beam asymmetry are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional ω photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from s-channel resonance production were found in addition to t-channel exchange processes.