π0 photoproduction on the proton for photon energies from 0.675 to 2.875 GeV

Differential cross sections for the reaction γp→pπ0 have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged photon beam with energies from 0.675 to 2.875 GeV. The results reported here possess greater accuracy in the absolute normalization than previous measurements. They disagree with recent CB-ELSA measurements for the process at forward scattering angles. Agreement with the SAID and MAID fits is found below 1 GeV. The present set of cross sections has been incorporated into the SAID database, and exploratory fits have been extended to 3 GeV. Resonance couplings have been extracted and compared to previous determination

Differential cross sections for the reactions γp→pη and γp→pη′

High-statistics differential cross sections for the reactions γp→pη and γp→pη′ have been measured using the CEBAF large acceptance spectrometer (CLAS) at Jefferson Lab for center-of-mass energies from near threshold up to 2.84 GeV. The η′ results are the most precise to date and provide the largest energy and angular coverage. The η measurements extend the energy range of the world’s large-angle results by approximately 300 MeV. These new data, in particular the η′ measurements, are likely to help constrain the analyses being performed to search for new baryon resonance states

Measurement of ep→e′pπ+π− and Baryon Resonance Analysis

The cross section for the reaction ep ! e′p_+_− was measured in the resonance region for 1.4 The data show resonant structures not visible in previous experiments. The comparison of our data to a phenomenological prediction using available information on N∗ and _ states shows an evident discrepancy. A better description of the data is obtained either by a sizeable change of the properties of the P13(1720) resonance or by introducing a new baryon state, not reported in published analyses

Photoproduction of the ρ0 Meson on the Proton at Large Momentum Transfer

The differential cross section, dσ/dt, for ρ0 meson photoproduction on the proton above the resonance region was measured up to a momentum transfer −t=5GeV2 using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The ρ0 channel was extracted from the measured two charged-pion cross sections by fitting the π+π− and pπ+ invariant masses. The low momentum transfer region shows the typical diffractive pattern expected from Reggeon exchange. The flatter behavior at large −t cannot be explained solely in terms of QCD-inspired two-gluon exchange models. The data indicate that other processes, like quark interchange, are important to fully describe ρ photoproduction

Search for the Photoexcitation of Exotic Mesons in the π+π+π− System

A search for exotic mesons in the π+π+π− system photoproduced by the charge exchange reaction γp→π+π+π−(n) was carried out by the CLAS Collaboration at Jefferson Lab. A tagged-photon beam with energies in the 4.8 to 5.4 GeV range, produced through bremsstrahlung from a 5.744 GeV electron beam, was incident on a liquid-hydrogen target. A partial wave analysis was performed on a sample of 83 000 events, the highest such statistics to date in this reaction at these energies. The main objective of this study was to look for the photoproduction of an exotic JPC=1−+ resonant state in the 1 to 2 GeV mass range. Our partial wave analysis shows production of the a2(1320) and the π2(1670) mesons, but no evidence for the a1(1260), nor the π1(1600) exotic state at the expected levels. An upper limit of 13.5 nb is determined for the exotic π1(1600) cross section, less than 2% of the a2(1320) production

π+ photoproduction on the proton for photon energies from 0.725 to 2.875 GeV

Differential cross sections for the reaction γp→nπ+ have been measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged photon beam with energies from 0.725 to 2.875 GeV. Where available, the results obtained here compare well with previously published results for the reaction. Agreement with the SAID and MAID analyses is found below 1 GeV. The present set of cross sections has been incorporated into the SAID database, and exploratory fits have been made up to 2.7 GeV. Resonance couplings have been extracted and compared to previous determinations. With the addition of these cross sections to the world data set, significant changes have occurred in the high-energy behavior of the SAID cross-section predictions and amplitudes

A kinematically complete measurement of the proton structure function F2 in the resonance region and evaluation of its moments

We measured the inclusive electron-proton cross section in the nucleon resonance region (W\u3c2.5GeV) at momentum transfers Q2 below 4.5(GeV/c)2 with the CLAS detector. The large acceptance of CLAS allowed the measurement of the cross section in a large, contiguous two-dimensional range of Q2 and x, making it possible to perform an integration of the data at fixed Q2 over the significant x interval. From these data we extracted the structure function F2 and, by including other world data, we studied the Q2 evolution of its moments, Mn(Q2), in order to estimate higher twist contributions. The small statistical and systematic uncertainties of the CLAS data allow a precise extraction of the higher twists and will require significant improvements in theoretical predictions if a meaningful comparison with these new experimental results is to be made