104 research outputs found
Photoproduction of Ï(1020) mesons on the proton at large momentum transfer
The cross section for Ï meson photoproduction on the proton has been measured for the first time up to a four-momentum transfer ât=4GeV2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. At low four-momentum transfer, the differential cross section is well described by Pomeron exchange. At large four-momentum transfer, above ât=1.8GeV2, the data support a model where the Pomeron is resolved into its simplest component, two gluons, which may couple to any quark in the proton and in the Ï
Theoretical study of incoherent phi photoproduction on a deuteron target
We study the photoproduction of phi mesons in deuteron, paying attention to
the modification of the cross section from bound protons to the free ones with
the aim of comparing with recent results at LEPS. For this purpose we take into
account Fermi motion in single scattering and rescattering of the phi to
account for phi absorption on a second nucleon as well as the rescattering of
the proton. We find that the contribution of the double scattering is much
smaller than the typical cross section of gamma p to phi p in free space, which
implies a very small screening of the phi production in deuteron. The
contribution from the proton rescattering, on the other hand, is found to be
not negligible compared to the cross section of gamma p to phi p in free space,
and leads to a moderate reduction of the phi photoproduction cross section on a
deuteron at forward angles if LEPS set up is taken into account. The Fermi
motion allows contribution of the single scattering in regions forbidden by
phase space in the free case. In particular, we find that for momentum
transferred squared close to the maximum value, the Fermi motion changes
drastically the shape of d sigma / dt, to the point that the ratio of this
cross section to the free one becomes very sensitive to the precise value of t
chosen, or the size of the bin used in an experimental analysis. Hence, this
particular region of t does not seem the most indicated to find effects of a
possible phi absorption in the deuteron. This reaction is studied theoretically
as a function of t and the effect of the experimental angular cuts at LEPS is
also discussed, providing guidelines for future experimental analyses of the
reaction.Comment: 17 pages, 16 figure
Systematic Regge theory analysis of omega photoproduction
Systematic analysis of available data for -meson photoproduction is
given in frame of Regge theory. At photon energies above 20 GeV the
reaction is entirely dominated by Pomeron exchange.
However, it was found that Pomeron exchange model can not reproduce the
and data at high energies
simultaneously with the same set of parameters. The comparison between
and data indicates a large room for meson exchange contribution to
-meson photoproduction at low energies. It was found that at low
energies the dominant contribution comes from and -meson exchanges.
There is smooth transition between the meson exchange model at low energies and
Regge theory at high energies.Comment: 7 pages, 8 figures, revtex
Exclusive Ï0 Meson Electroproduction from Hydrogen at CLAS
The longitudinal and transverse components of the cross section for the ep â eâČ pÏ0 reaction were measured in Hall B at Jefferson Laboratory using the CLAS detector. The data were taken with a 4.247 GeV electron beam and were analyzed in a range of xB from 0.2 to 0.6 and of Q2 from 1.5 to 3.0 GeV2. The data are compared to a Regge model based on effective hadronic degrees of freedom and to a calculation based on Generalized Parton Distributions. It is found that, at our lowest xB values, the transverse part of the cross section is well described by the former approach while the longitudinal part can be reproduced by the latter
Test of phi renormalization in nuclei through phi photoproduction
We propose an experimental procedure to find out the medium modifications of
the meson. The reaction is inclusive photoproduction in nuclei,
looking for pairs from the decay with total momentum smaller
than 100-150 , which are made possible at energies of present
laboratories from center of mass backward production and the help of
Fermi motion. We have conducted a many body calculation of the mass
distribution of the adapted to the experimental set up of a recent JLAB
experiment where the backwards photoproduction has been measured. Using
recent results for the in medium properties of the , we find that the
width of the invariant mass distribution for photoproduction on medium to heavy
nuclei is larger than the free width by a factor of two or more.Comment: 10 pages, 3 figure
Photoproduction of phi(1020) mesons on the proton at large momentum transfer
The cross section for meson photoproduction on the proton has been
measured for the first time up to a four-momentum transfer -t = 4 GeV^2, using
the CLAS detector at the Thomas Jefferson National Accelerator Facility. At low
four-momentum transfer, the differential cross section is well described by
Pomeron exchange. At large four-momentum transfer, above -t = 1.8 GeV^2, the
data support a model where the Pomeron is resolved into its simplest component,
two gluons, which may couple to any quark in the proton and in the .Comment: 5 pages; 7 figure
Dependence of Quadrupole Strength in the Transition
Models of baryon structure predict a small quadrupole deformation of the
nucleon due to residual tensor forces between quarks or distortions from the
pion cloud. Sensitivity to quark versus pion degrees of freedom occurs through
the dependence of the magnetic (), electric (), and
scalar () multipoles in the
transition. We report new experimental values for the ratios
and over the range = 0.4-1.8 GeV, extracted from
precision data using a truncated multipole expansion.
Results are best described by recent unitary models in which the pion cloud
plays a dominant role.Comment: 5 pages, 5 figures, 1 table. To be published in Phys. Rev. Lett.
(References, figures and table updated, minor changes.
Measurement of the Polarized Structure Function for in the Resonance Region
The polarized longitudinal-transverse structure function
has been measured using the reaction in the
resonance region at and 0.65 GeV. No previous
data exist for this reaction channel. The kinematically
complete experiment was performed at Jefferson Lab with the CEBAF Large
Acceptance Spectrometer (CLAS) using longitudinally polarized electrons at an
energy of 1.515 GeV. A partial wave analysis of the data shows generally better
agreement with recent phenomenological models of pion electroproduction
compared to the previously measured channel. A fit to both
and channels using a unitary isobar model suggests the unitarized
Born terms provide a consistent description of the non-resonant background. The
-channel pion pole term is important in the channel through a
rescattering correction, which could be model-dependent.Comment: 6 pages, LaTex, 5 eps figures: Submitted to PRC/Brief Reports v2:
Updated referenc
Deeply virtual and exclusive electroproduction of omega mesons
The exclusive omega electroproduction off the proton was studied in a large
kinematical domain above the nucleon resonance region and for the highest
possible photon virtuality (Q2) with the 5.75 GeV beam at CEBAF and the CLAS
spectrometer. Cross sections were measured up to large values of the
four-momentum transfer (-t < 2.7 GeV2) to the proton. The contributions of the
interference terms sigma_TT and sigma_TL to the cross sections, as well as an
analysis of the omega spin density matrix, indicate that helicity is not
conserved in this process. The t-channel pi0 exchange, or more generally the
exchange of the associated Regge trajectory, seems to dominate the reaction
gamma* p -> omega p, even for Q2 as large as 5 GeV2. Contributions of handbag
diagrams, related to Generalized Parton Distributions in the nucleon, are
therefore difficult to extract for this process. Remarkably, the high-t
behaviour of the cross sections is nearly Q2-independent, which may be
interpreted as a coupling of the photon to a point-like object in this
kinematical limit.Comment: 15 pages,19 figure
Measurement of Inclusive Spin Structure Functions of the Deuteron
We report the results of a new measurement of spin structure functions of the
deuteron in the region of moderate momentum transfer ( = 0.27 -- 1.3
(GeV/c)) and final hadronic state mass in the nucleon resonance region (
= 1.08 -- 2.0 GeV). We scattered a 2.5 GeV polarized continuous electron beam
at Jefferson Lab off a dynamically polarized cryogenic solid state target
(ND) and detected the scattered electrons with the CEBAF Large
Acceptance Spectrometer (CLAS). From our data, we extract the longitudinal
double spin asymmetry and the spin structure function . Our
data are generally in reasonable agreement with existing data from SLAC where
they overlap, and they represent a substantial improvement in statistical
precision. We compare our results with expectations for resonance asymmetries
and extrapolated deep inelastic scaling results. Finally, we evaluate the first
moment of the structure function and study its approach to both the
deep inelastic limit at large and to the Gerasimov-Drell-Hearn sum rule
at the real photon limit (). We find that the first moment varies
rapidly in the range of our experiment and crosses zero at between
0.5 and 0.8 (GeV/c), indicating the importance of the resonance at
these momentum transfers.Comment: 13 pages, 8 figures, ReVTeX 4, final version as accepted by Phys.
Rev.
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