Contemporary presence of dynamical and statistical production of intermediate mass fragments in midperipheral 58^{58}Ni+58^{58}Ni collisions at 30 MeV/nucleon

The $^{58}Ni+^{58}Ni$ reaction at 30 MeV/nucleon has been experimentally investigated at the Superconducting Cyclotron of the INFN Laboratori Nazionali del Sud. In midperipheral collisions the production of massive fragments (4$\le$Z$\le$12), consistent with the statistical fragmentation of the projectile-like residue and the dynamical formation of a neck, joining projectile-like and target-like residues, has been observed. The fragments coming from these different processes differ both in charge distribution and isotopic composition. In particular it is shown that these mechanisms leading to fragment production act contemporarily inside the same event.Comment: 9 pages, minor correction

Strong enhancement of extremely energetic proton production in central heavy ion collisions at intermediate energy

The energetic proton emission has been investigated as a function of the reaction centrality for the system 58Ni + 58Ni at 30A MeV. Extremely energetic protons (EpNN > 130 MeV) were measured and their multiplicity is found to increase almost quadratically with the number of participant nucleons thus indicating the onset of a mechanism beyond one and two-body dynamics.Comment: 5 pages, 2 figures, submitted to Physical Review Letter

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 ($Q^2$ = 0.27 -- 1.3 (GeV/c)$^2$) and final hadronic state mass in the nucleon resonance region ($W$ = 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 ($^{15}$ND$_3$) and detected the scattered electrons with the CEBAF Large Acceptance Spectrometer (CLAS). From our data, we extract the longitudinal double spin asymmetry $A_{||}$ and the spin structure function $g_1^d$. 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 $g_1^d$ and study its approach to both the deep inelastic limit at large $Q^2$ and to the Gerasimov-Drell-Hearn sum rule at the real photon limit ($Q^2 \to 0$). We find that the first moment varies rapidly in the $Q^2$ range of our experiment and crosses zero at $Q^2$ between 0.5 and 0.8 (GeV/c)$^2$, indicating the importance of the $\Delta$ resonance at these momentum transfers.Comment: 13 pages, 8 figures, ReVTeX 4, final version as accepted by Phys. Rev.

Photoproduction of phi(1020) mesons on the proton at large momentum transfer

The cross section for $\phi$ 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 $\phi$.Comment: 5 pages; 7 figure

Q2Q^2 Dependence of Quadrupole Strength in the γ∗p→Δ+(1232)→pπ0\gamma^*p\to\Delta^+(1232)\to p \pi^0 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 $Q^2$ dependence of the magnetic ($M_{1+}$), electric ($E_{1+}$), and scalar ($S_{1+}$) multipoles in the $\gamma^* p \to \Delta^+ \to p \pi^0$ transition. We report new experimental values for the ratios $E_{1+}/M_{1+}$ and $S_{1+}/M_{1+}$ over the range $Q^2$= 0.4-1.8 GeV$^2$, extracted from precision $p(e,e 'p)\pi^{\circ}$ 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.

The e p -> e' p eta reaction at and above the S11(1535) baryon resonance

New cross sections for the reaction e p -> ep eta are reported for total center of mass energy W = 1.5--1.86 GeV and invariant momentum transfer Q^2 = 0.25--1.5 GeV^2. This large kinematic range allows extraction of important new information about response functions, photocouplings, and eta N coupling strengths of baryon resonances. Expanded W coverage shows sharp structure at W \~ 1.7 GeV; this is shown to come from interference between S and P waves and can be interpreted in terms of known resonances. Improved values are derived for the photon coupling amplitude for the S11(1535) resonance.Comment: 11 pages, RevTeX, 5 figures, submitted to Phys. Rev. Let

Study of polarized-e polarized-p --> e-prime p pi0 in the Delta(1232) mass region using polarization asymmetries

Measurements of the angular distributions of target and double spin asymmetries for the Delta(1232) in the exclusive channel (polarized-p polarized-e,e-prime p)pi0 obtained at Jefferson Lab in the Q2 range from 0.5 to 1.5 gev2 are presented. Results of the asymmetries are compared with the unitary isobar model, dynamical models, and the effective Lagrangian theory. Sensitivity to the different models was observed, particularly in relation to the description of background terms on which the target asymmetry depends significantly.Comment: 14 pages, 13 figure

Tensor Polarization of the phi meson Photoproduced at High t

As part of a measurement of the cross section of $\phi$ meson photoproduction to high momentum transfer, we measured the polar angular decay distribution of the outgoing $K^+$ in the channel $\phi \to K^+K^-$ in the $\phi$ center-of-mass frame (the helicity frame). We find that s-channel helicity conservation (SCHC) holds in the kinematical range where $t$-channel exchange dominates (up to $-t \sim 2.5$ GeV$^2$ for $E_{\gamma}$=3.6 GeV). Above this momentum, $u$-channel production of a $\phi$ meson dominates and induces a violation of SCHC. The deduced value of the $\phi NN$ coupling constant lies in the upper range of previously reported values.Comment: 6 pages; 5 figure