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Register a new userLifetime measurement of excited low-spin states via the $(p,p^{prime}gamma$) reaction
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In this article a method for lifetime measurements in the sub-picosecond regime via the Doppler-shift attenuation method (DSAM) following the inelastic proton scattering reaction is presented. In a pioneering experiment we extracted the lifetimes of 30 excited low-spin states of $^{96}$Ru, taking advantage of the coincident detection of scattered protons and de-exciting $gamma$-rays as well as the large number of particle and $gamma$-ray detectors provided by the SONIC@HORUS setup at the University of Cologne. The large amount of new experimental data shows that this technique is suited for the measurement of lifetimes of excited low-spin states, especially for isotopes with a low isotopic abundance, where $(n,n^{prime}gamma$) or - in case of investigating dipole excitations - ($gamma,gamma^{prime}$) experiments are not feasible due to the lack of sufficient isotopically enriched target material.
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New results are reported from a measurement of $pi^0$ electroproduction near threshold using the $p(e,e^{prime} p)pi^0$ reaction. The experiment was designed to determine precisely the energy dependence of $s-$ and $p-$wave electromagnetic multipoles as a stringent test of the predictions of Chiral Perturbation Theory (ChPT). The data were taken with an electron beam energy of 1192 MeV using a two-spectrometer setup in Hall A at Jefferson Lab. For the first time, complete coverage of the $phi^*_{pi}$ and $theta^*_{pi}$ angles in the $p pi^0$ center-of-mass was obtained for invariant energies above threshold from 0.5 MeV up to 15 MeV. The 4-momentum transfer $Q^2$ coverage ranges from 0.05 to 0.155 (GeV/c)$^2$ in fine steps. A simple phenomenological analysis of our data shows strong disagreement with $p-$wave predictions from ChPT for $Q^2>0.07$ (GeV/c)$^2$, while the $s-$wave predictions are in reasonable agreement.
High-statistics differential cross sections for the reactions gamma p -> p eta and gamma p -> p eta-prime have been measured using the CLAS at Jefferson Lab for center-of-mass energies from near threshold up to 2.84 GeV. The eta-prime results are the most precise to date and provide the largest energy and angular coverage. The eta measurements extend the energy range of the worlds large-angle results by approximately 300 MeV. These new data, in particular the eta-prime measurements, are likely to help constrain the analyses being performed to search for new baryon resonance states.
We measured both the differential cross section ($sigma_{p,p^prime}$ $=d^2sigma/dOmega dE_{x}$) and the $gamma$-ray emission probability ($R_gamma(E_x)$ $=sigma_{p,p^primegamma}$/$sigma_{p,p^prime}$) from the giant resonances excited by $rm^{12}C$(textit{p,p}$^prime$) reaction at 392 MeV and 0$^circ$, using a magnetic spectrometer and an array of NaI(Tl) counters. The absolute value of $R_gamma(E_x)$ was calibrated by using the well-known $gamma$-ray emission probability from $rm^{12}C^* ( 15.11$ MeV, $ 1^+$, $T=1$) and $rm^{16}O^* ( 6.9$ MeV, $2^+$, $T=0$) states within 5% uncertainty. We found that $R_gamma(E_x)$ starts from zero at $E_x=16$ MeV, increases to a maximum of 53.3$pm$0.4$pm$3.9% at $E_x=27$ MeV and then decreases. We also compared the measured values of $R_gamma(E_x)$ with statistical model calculation based on the Hauser-Feshbach formalism in the energy region $E_x=$ 16-32 MeV and discussed the features of $gamma$-ray emission probability quantitatively.
The reaction $gamma p to pi^circ gamma^prime p$ has been measured with the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI for energies between $sqrt{s}$ = 1221--1331 MeV. Cross sections differential in angle and energy have been determined for all particles in the final state in three bins of the excitation energy. This reaction channel provides access to the magnetic dipole moment of the $Delta^{+}(1232)$ resonance and, for the first time, a value of $mu_{Delta^+} = (2.7_{-1.3}^{+1.0}(stat.) pm 1.5 (syst.) pm 3(theo.)) mu_N$ has been extracted.
The circular photon asymmetry for pi0 eta photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Delta(1700)D33 resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D33 amplitude.
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