Experimental results on the $\Sigma^+(1189)$ hyperon transverse polarization
in photoproduction on a hydrogen target using the CLAS detector at Jefferson
laboratory are presented. The ...$\Sigma^+(1189)$ was reconstructed in the
exclusive reaction $\gamma+p\rightarrow K^{0}_{S} + \Sigma^+(1189)$ via the
$\Sigma^{+} \to p \pi^{0}$ decay mode. The $K^{0}_S$ was reconstructed in the
invariant mass of two oppositely charged pions with the $\pi^0$ identified in
the missing mass of the detected $p\pi^+\pi^-$ final state. Experimental data
were collected in the photon energy range $E_{\gamma}$ = 1.0-3.5 GeV
($\sqrt{s}$ range 1.66-2.73 GeV). We observe a large negative polarization of
up to 95%. As the mechanism of transverse polarization of hyperons produced in
unpolarized photoproduction experiments is still not well understood, these
results will help to distinguish between different theoretical models on
hyperon production and provide valuable information for the searches of missing
baryon resonances.
We report on parity-violating asymmetries in the nucleon resonance region measured using \(5 - 6\) GeV longitudinally polarized electrons scattering off an unpolarized deuterium target. These results ...are the first parity-violating asymmetry data in the resonance region beyond the \(\Delta(1232)\), and provide a verification of quark-hadron duality in the nucleon electroweak \(\gamma Z\) interference structure functions at the (10-15)% level. The results are of particular interest to models relevant for calculating the \(\gamma Z\) box-diagram corrections to elastic parity-violating electron scattering measurements.
We report the measurement of near threshold neutral pion electroproduction cross sections and the extraction of the associated structure functions on the proton in the kinematic range \(Q^2\) from 2 ...to 4.5 GeV\(^2\) and \(W\) from 1.08 to 1.16 GeV. These measurements allow us to access the dominant pion-nucleon s-wave multipoles \(E_{0+}\) and \(S_{0+}\) in the near-threshold region. In the light-cone sum-rule framework (LCSR), these multipoles are related to the generalized form factors \(G_1^{\pi^0 p}(Q^2)\) and \(G_2^{\pi^0 p}(Q^2)\). The data are compared to these generalized form factors and the results for \(G_1^{\pi^0 p}(Q^2)\) are found to be in good agreement with the LCSR predictions, but the level of agreement with \(G_2^{\pi^0 p}(Q^2)\) is poor.
We have measured cross sections for the gamma+3He->p+d reaction at photon energies of 0.4 - 1.4 GeV and a center-of-mass angle of 90 deg. 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.
PRL 109, 112001 (2012) Exclusive $\pi^0$ 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 $Q^2$, $x_B$, $t$, and
$\phi_\pi$, in the $Q^2$ range from 1.0 to 4.6 GeV$^2$,\ $-t$ up to 2 GeV$^2$,
and $x_B$ from 0.1 to 0.58. Structure functions $\sigma_T +\epsilon \sigma_L,
\sigma_{TT}$ and $\sigma_{LT}$ were extracted as functions of $t$ for each of
17 combinations of $Q^2$ and $x_B$. The data were compared directly with two
handbag-based calculations including both longitudinal and transversity GPDs.
Inclusion of only longitudinal GPDs very strongly underestimates $\sigma_T
+\epsilon \sigma_L$ and fails to account for $\sigma_{TT}$ and $\sigma_{LT}$,
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 non-flip
processes. The results confirm that exclusive $\pi^0$ electroproduction offers
direct experimental access to the transversity GPDs.
We propose to measure the photo-production cross section of \(J/{\psi}\) near threshold, in search of the recently observed LHCb hidden-charm resonances \(P_c\)(4380) and \(P_c\)(4450) consistent ...with 'pentaquarks'. The observation of these resonances in photo-production will provide strong evidence of the true resonance nature of the LHCb states, distinguishing them from kinematic enhancements. A bremsstrahlung photon beam produced with an 11 GeV electron beam at CEBAF covers the energy range of \(J/{\psi}\) production from the threshold photo-production energy of 8.2 GeV, to an energy beyond the presumed \(P_c\)(4450) resonance. The experiment will be carried out in Hall C at Jefferson Lab using a 50{\mu}A electron beam incident on a 9% copper radiator. The resulting photon beam passes through a 15 cm liquid hydrogen target, producing \(J/{\psi}\) mesons through a diffractive process in the \(t\)-channel, or through a resonant process in the \(s\)- and \(u\)-channel. The decay \(e^+e^-\) pair of the \(J/{\psi}\) will be detected in coincidence using the two high-momentum spectrometers of Hall C. The spectrometer settings have been optimized to distinguish the resonant \(s\)- and \(u\)-channel production from the diffractive \(t\)-channel \(J/{\psi}\) production. The \(s\)- and \(u\)-channel production of the charmed 5-quark resonance dominates the \(t\)-distribution at large \(t\). The momentum and angular resolution of the spectrometers is sufficient to observe a clear resonance enhancement in the total cross section and \(t\)-distribution. We request a total of 11 days of beam time with 9 days to carry the main experiment and 2 days to acquire the needed \(t\)-channel elastic \(J/{\psi}\) production data for a calibration measurement. This calibration measurement in itself will greatly enhance our knowledge of \(t\)-channel elastic \(J/{\psi}\) production near threshold.
The CLAS Collaboration provides a comment on the physics interpretation of the results presented in a paper published by M. Amaryan et al. regarding the possible observation of a narrow structure in ...the mass spectrum of a photoproduction experiment.
Eur. Phys. J. A (2013) 49:1-16 The exclusive electroproduction of $\pi^+$ above the resonance region was
studied using the $\rm{CEBAF}$ Large Acceptance Spectrometer ($\rm{CLAS}$) at
Jefferson ...Laboratory by scattering a 6 GeV continuous electron beam off a
hydrogen target. The large acceptance and good resolution of $\rm{CLAS}$,
together with the high luminosity, allowed us to measure the cross section for
the $\gamma^* p \to n \pi^+$ process in 140 ($Q^2$, $x_B$, $t$) bins:
$0.16<x_B<0.58$, 1.6 GeV$^2<$$Q^2$$<4.5$ GeV$^2$ and 0.1 GeV$^2<$$-t$$<5.3$
GeV$^2$. For most bins, the statistical accuracy is on the order of a few
percent. Differential cross sections are compared to two theoretical models,
based either on hadronic (Regge phenomenology) or on partonic (handbag diagram)
degrees of freedom. Both can describe the gross features of the data reasonably
well, but differ strongly in their ingredients. If the handbag approach can be
validated in this kinematical region, our data contain the interesting
potential to experimentally access transversity Generalized Parton
Distributions.
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