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Register a new userMeasurement of double-polarization asymmetries in the quasi-elastic $^3vec{mathrm{He}}(vec{mathrm{e}},mathrm{e}mathrm{p})$ process
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We report on a precise measurement of double-polarization asymmetries in electron-induced breakup of $^3mathrm{He}$ proceeding to $mathrm{pd}$ and $mathrm{ppn}$ final states, performed in quasi-elastic kinematics at $Q^2 = 0.25,(mathrm{GeV}/c)^2$ for missing momenta up to $250,mathrm{MeV}/c$. These observables represent highly sensitive tools to investigate the electromagnetic and spin structure of $^3mathrm{He}$ and the relative importance of two- and three-body effects involved in the breakup reaction dynamics. The measured asymmetries cannot be satisfactorily reproduced by state-of-the-art calculations of $^3mathrm{He}$ unless their three-body segment is adjusted, indicating that the spin-dependent part of the nuclear interaction governing the three-body breakup process is much smaller than previously thought.
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We present a precise measurement of double-polarization asymmetries in the $^3vec{mathrm{He}}(vec{mathrm{e}},mathrm{e}mathrm{d})$ reaction. This particular process is a uniquely sensitive probe of hadron dynamics in $^3mathrm{He}$ and the structure of the underlying electromagnetic currents. The measurements have been performed in and around quasi-elastic kinematics at $Q^2 = 0.25,(mathrm{GeV}/c)^2$ for missing momenta up to $270,mathrm{MeV}/c$. The asymmetries are in fair agreement with the state-of-the-art calculations in terms of their functional dependencies on $p_mathrm{m}$ and $omega$, but are systematically offset. Beyond the region of the quasi-elastic peak, the discrepancies become even more pronounced. Thus, our measurements have been able to reveal deficiencies in the most sophisticated calculations of the three-body nuclear system, and indicate that further refinement in the treatment of their two- and/or three-body dynamics is required.
The existence of a new force beyond the Standard Model is compelling because it could explain several striking astrophysical observations which fail standard interpretations. We searched for the light vector mediator of this dark force, the $mathrm{U}$ boson, with the KLOE detector at the DA$Phi$NE $mathrm{e}^{+}mathrm{e}^{-}$ collider. Using an integrated luminosity of 1.54 fb$^{-1}$, we studied the process $mathrm{e}^{+}mathrm{e}^{-} to mathrm{U}gamma$, with $mathrm{U} to mathrm{e}^{+}mathrm{e}^{-}$, using radiative-return to search for a resonant peak in the dielectron invariant-mass distribution. We did not find evidence for a signal, and set a 90%~CL upper limit on the mixing strength between the Standard Model photon and the dark photon, $varepsilon^2$, at $10^{-6}$--$10^{-4}$ in the 5--520~MeV/c$^2$ mass range.
High statistics measurements of the photon asymmetry $mathrm{Sigma}$ for the $overrightarrow{gamma}$p$rightarrowpi^{0}$p reaction have been made in the center of mass energy range W=1214-1450 MeV. The data were measured with the MAMI A2 real photon beam and Crystal Ball/TAPS detector systems in Mainz, Germany. The results significantly improve the existing world data and are shown to be in good agreement with previous measurements, and with the MAID, SAID, and Bonn-Gatchina predictions. We have also combined the photon asymmetry results with recent cross-section measurements from Mainz to calculate the profile functions, $check{mathrm{Sigma}}$ (= $sigma_{0}mathrm{Sigma}$), and perform a moment analysis. Comparison with calculations from the Bonn-Gatchina model shows that the precision of the data is good enough to further constrain the higher partial waves, and there is an indication of interference between the very small $F$-waves and the $N(1520) 3/2^{-}$ and $N(1535) 1/2^{-}$ resonances.
According to sensitivity studies, the $^{38}mathrm{K}left( p, gamma right){}^{39}mathrm{Ca}$ reaction has a significant influence on $mathrm{Ar}$, $mathrm{K}$, and $mathrm{Ca}$ production in classical novae. In order to constrain the rate of this reaction, we have performed a direct measurement of the strengths of three candidate $ell = 0$ resonances within the Gamow window, at $386 pm 10~mathrm{keV}$, $515 pm 10~mathrm{keV}$, and $689 pm 10~mathrm{keV}$. The experiment was performed in inverse kinematics using a beam of unstable $^{38}mathrm{K}$ impinged on a windowless $mathrm{H}_2$ target. The $^{39}mathrm{Ca}$ recoils and prompt $gamma$ rays from $^{38}mathrm{K}left( p, gamma right){}^{39}mathrm{Ca}$ reactions were detected in coincidence using a recoil mass separator and a BGO array, respectively. For the $689$ keV resonance, we observed a clear recoil-$gamma$ coincidence signal and extracted resonance strength and energy values of $120^{+50}_{-30}~mathrm{(stat.)}^{+20}_{-60}~mathrm{(sys.)}~mathrm{meV}$ and $679^{+2}_{-1}~mathrm{(stat.)} pm 1~mathrm{(sys.)}~mathrm{keV}$, respectively. We also performed a singles analysis, extracting a resonance strength of $120 pm 20~mathrm{(stat.)} pm 15~mathrm{(sys.)}~mathrm{meV}$, consistent with the coincidence result. For the $386$ keV and $515$ keV resonances, we extract $90%$ confidence level upper limits of $2.54$ meV and $18.4$ meV, respectively. We have established a new recommended $^{38}mathrm{K}(p, gamma){}^{39}mathrm{Ca}$ rate based on experimental information, which reduces overall uncertainties near the peak temperatures of nova burning by a factor of ${sim} 250$. Using the rate obtained in this work in model calculations of the hottest oxygen-neon novae reduces overall uncertainties on $mathrm{Ar}$, $mathrm{K}$, and $mathrm{Ca}$ synthesis to factors of $15$ or less in all cases.
We extract the $e^+e^-rightarrow pi^+pi^-$ cross section in the energy range between 600 and 900 MeV, exploiting the method of initial state radiation. A data set with an integrated luminosity of 2.93 fb$^{-1}$ taken at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider is used. The cross section is measured with a systematic uncertainty of 0.9%. We extract the pion form factor $|F_pi|^2$ as well as the contribution of the measured cross section to the leading order hadronic vacuum polarization contribution to $(g-2)_mu$. We find this value to be $a_mu^{pipi,rm LO}(600-900;rm MeV) = (368.2 pm 2.5_{rm stat} pm 3.3_{rm sys})cdot 10^{-10}$.
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