189 research outputs found
Photodisintegration of He4 into p+t
The two-body photodisintegration of 4He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid 4He target. This is the first measurement of the photodisintegration of 4He above 0.4 GeV. The differential cross sections for the Ī³4Heāpt reaction were measured as a function of photon-beam energy and proton-scattering angle and are compared with the latest model calculations by J.-M. Laget. At 0.6-1.2 GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. These results reinforce the conclusion of our previous study of the three-body breakup of 3He that demonstrated the great importance of three-body mechanisms in the energy region 0.5-0.8 GeV
Coherent photoproduction of Ļ+ from 3He
We have measured the differential cross section for the Ī³He3āĻ+t reaction. This reaction was studied using the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.50 to 1.55 GeV were incident on a cryogenic liquid 3He target. The differential cross sections for the Ī³3HeāĻ+t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model
Photodisintegration of \u3csup\u3e4\u3c/sup\u3eHe into \u3ci\u3ep\u3c/i\u3e + \u3ci\u3et\u3c/i\u3e
The two-body photodisintegration of 4He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid 4He target. This is the first measurement of the photodisintegration of 4He above 0.4 GeV. The differential cross sections for the Ī³ 4He ā pt reaction were measured as a function of photon-beam energy and proton-scattering angle and are compared with the latest model calculations by J.M. Laget. At 0.6ā1.2 GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. These results reinforce the conclusion of our previous study of the three-body breakup of 3He that demonstrated the great importance of three-body mechanisms in the energy region 0.5ā0.8 GeV
Polarized structure function ĻLTā² for 1H(eā,eā²K+)Ī in the nucleon resonance region
The first measurements of the polarized structure function ĻLTā² for the reaction 1H(eā,eā²K+)Ī in the nucleon resonance region are reported. Measurements are included from threshold up to W=2.05 GeV for central values of Q2 of 0.65 and 1.00 GeV2, and nearly the entire kaon center-of-mass angular range. ĻLTā² is the imaginary part of the longitudinal-transverse response and is expected to be sensitive to interferences between competing intermediate s-channel resonances, as well as resonant and nonresonant processes. The results for ĻLTā² are comparable in magnitude to previously reported results from CLAS for ĻLT, the real part of the same response. An intriguing sign change in ĻLTā² is observed in the high Q2 data at Wā1.9 GeV. Comparisons to several existing model predictions are shown
Search for Medium Modifications of the Ļ Meson
The photoproduction of vector mesons on various nuclei has been studied using the CLAS detector at Jefferson Laboratory. The vector mesons, Ļ, Ļ, and Ļ, are observed via their decay to e+eā, in order to reduce the effects of final-state interactions in the nucleus. Of particular interest are possible in-medium effects on the properties of the Ļ meson. The Ļ mass spectrum is extracted from the data on various nuclei,2 H, C, Fe, and Ti. We observe no significant mass shift and some broadening consistent with expected collisional broadening for the Ļ meson
Chiral Symmetry in Nuclei -- Theoretical Expectations and Hard Facts
It is widely believed that chiral symmetry is restored not only at high
temperatures, but also at high nuclear densities. The drop of the order
parameter of the chiral phase transition, the chiral condensate, with density
has indeed been calculated in various models and is as such a rather robust
result. In this talk I point out that the connection of this property with
actual observables is far less clear. For this task a good hadronic description
of the primary production of hadrons, their propagation inside the nuclear
medium, their decay and the propagation of the decay products through the
medium to the detector all have to be treated with equal accuracy and weight.
In this talk I illustrate with the examples of production and
production how important in particular final state interactions
are.Comment: Invited Talk at Chiral07, Osaka, Japan, Nov. 13 - 16, 200
Coherent Photoproduction of \u3ci\u3eĻ\u3c/i\u3e\u3csup\u3e+\u3c/sup\u3e from 3He
We have measured the differential cross section for the Ī³3Heā Ļ+t reaction. This reaction was studied using the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.50 to 1.55 GeV were incident on a cryogenic liquid 3He target. The differential cross sections for the Ī³3Heā Ļ+t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model
Coherent photoproduction of pi(+) from He-3
We have measured the differential cross section for the gamma He-3 -\u3e pi(+)t reaction. This reaction was studied using the Continuous Electron Beam Accelerator Facility (CEBAF) Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.50 to 1.55 GeV were incident on a cryogenic liquid He-3 target. The differential cross sections for the gamma He-3 -\u3e pi(+)t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model
Medium Effects in rho-Meson Photoproduction
We compute dilepton invariant mass spectra from the decays of rho mesons
produced by photon reactions off nuclei. Our calculations employ a realistic
model for the rho photoproduction amplitude on the nucleon which provides fair
agreement with measured cross sections. Medium effects are implemented via an
earlier constructed rho propagator based on hadronic many-body theory. At
incoming photon energies of 1.5 -3 GeV as used by the CLAS experiment at JLAB,
the average density probed for iron targets is estimated at about half
saturation density. At the pertinent rho-meson 3-momenta the predicted medium
effects on the rho propagator are rather moderate. The resulting dilepton
spectra approximately agree with recent CLAS data.Comment: One numerical error corrected, conclusions unchange
Exclusive electroproduction of K+ Lambda and K+ Sigma^0 final states at Q^2 = 0.030-0.055 (GeV/c)^2
Cross section measurements of the exclusive p(e,e'K+)Lambda,Sigma^0
electroproduction reactions have been performed at the Mainz Microtron MAMI in
the A1 spectrometer facility using for the first time the Kaos spectrometer for
kaon detection. These processes were studied in a kinematical region not
covered by any previous experiment. The nucleon was probed in its third
resonance region with virtual photons of low four-momenta, Q^2= 0.030-0.055
(GeV/c)^2. The MAMI data indicate a smooth transition in Q^2 from
photoproduction to electroproduction cross sections. Comparison with
predictions of effective Lagrangian models based on the isobar approach reveal
that strong longitudinal couplings of the virtual photon to the N* resonances
can be excluded from these models.Comment: 16 pages, 7 figure
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