Thu·Jun 15, 2017Thursday·June 15, 2017

1. 01*

   Photon beam asymmetry $\Sigma$ in the reaction $\vec{\gamma }p\to p\omega$ for $E_{\gamma }$ = 1.152 to 1.876 GeV

   CLAS Collaboration: P. Collins🇺🇸 · B.G. Ritchie🇺🇸 · M. Dugger🇺🇸 · F.J. Klein🇺🇸 · A.V. Anisovich🇩🇪 · E. Klempt🇩🇪 · V.A. Nikonov🇩🇪 · A. Sarantsev🇩🇪 · K.P. Adhikari🇺🇸 · S. Adhikari🇺🇸 · D. Adikaram🇺🇸 · Z. Akbar🇺🇸 and 137 other authors

   Photon beam asymmetry $\Sigma$ measurements for $\omega$ photoproduction in the reaction $\vec{\gamma }p\to \omega p$ are reported for photon energies from 1.152 to 1.876 GeV. Data were taken using a linearly-polarized tagged photon beam, a cryogenic hydrogen target, and the CLAS spectrometer in Hall B at Jefferson Lab. The measurements obtained markedly increase the size of the database for this observable, extend coverage to higher energies, and resolve discrepancies in previously published data. Comparisons of these new results with predictions from a chiral-quark-based model and from a dynamical coupled-channels model indicate the importance of interferences between $t$-channel meson exchange and $s$- and $u$-channel contributions, underscoring sensitivity to the nucleon resonances included in those descriptions. Comparisons with the Bonn-Gatchina partial-wave analysis indicate the $\Sigma$ data reported here help to fix the magnitudes of the interference terms between the leading amplitudes in that calculation (Pomeron exchange and the resonant portion of the $J^P=3/2^{+}$ partial wave), as well as the resonant portions of the smaller partial waves with $J^P$= $1/2^{-}$, $3/2^{-}$, and $5/2^{+}$.

   nucl-exPLB(2017)·14 citations
2. 02*

   A revised $B(E2;2_1^{+}\to 0_1^{+})$ value in the semi-magic nucleus ${}^{210}$Po

   D. Kocheva · G. Rainovski · J. Jolie · N. Pietralla · A. Blazhev · A. Astier · Th. Braunroth · M.L. Cortés · A. Dewald · M. Djongolov · C. Fransen · K. Gladnishki and 20 other authors

   The lifetimes of the $2_1^{+}$, the $2_2^{+}$ and the $3_1^{-}$ states of ${}^{210}$Po have been measured in the ${}^{208}$Pb(${}^{12}$C,${}^{10}$Be)${}^{210}$Po transfer reaction by the Doppler-shift attenuation method. The results for the lifetime of the $2_1^{+}$ state is about three times shorter than the adopted value. However, the new value still does not allow for consistent description of the properties of the yrast $2_1^{+}$, $4_1^{+}$, $6_1^{+}$, and $8_1^{+}$ states of ${}^{210}$Po in the framework of nuclear shell models. The Quasi-particle Phonon Model (QPM) calculations also cannot overcome this problem thus indicating the existence of a peculiarity which is neglected in both theoretical approaches.

   nucl-exnucl-thEPJA(2017)·23 citations
3. 03*

   Small-$x$ Asymptotics of the Gluon Helicity Distribution

   Yuri V. Kovchegov🇺🇸 · Daniel Pitonyak🇺🇸 · Matthew D. Sievert🇺🇸

   We determine the small-$x$ asymptotics of the gluon helicity distribution in a proton at leading order in perturbative QCD at large $N_c$. To achieve this, we begin by evaluating the dipole gluon helicity TMD at small $x$. In the process we obtain an interesting new result: in contrast to the unpolarized dipole gluon TMD case, the operator governing the small-$x$ behavior of the dipole gluon helicity TMD is different from the operator corresponding to the polarized dipole scattering amplitude (used in our previous work to determine the small-$x$ asymptotics of the quark helicity distribution). We then construct and solve novel small-$x$ large-$N_c$ evolution equations for the operator related to the dipole gluon helicity TMD. Our main result is the small-$x$ asymptotics for the gluon helicity distribution: $\Delta G\sim {\left(\frac{1}{x}\right)}^{{\alpha }_h^G}$ with ${\alpha }_h^G=\frac{13}{4\sqrt{3}}\sqrt{\frac{{\alpha }_s{N}_c}{2\pi }}\approx 1.88\sqrt{\frac{{\alpha }_s{N}_c}{2\pi }}$. We note that the power ${\alpha }_h^G$ is approximately 20$\%$ lower than the corresponding power ${\alpha }_h^q$ for the small-$x$ asymptotics of the quark helicity distribution defined by $\Delta q\sim {\left(\frac{1}{x}\right)}^{{\alpha }_h^q}$ with ${\alpha }_h^q=\frac{4}{\sqrt{3}}\sqrt{\frac{{\alpha }_s{N}_c}{2\pi }}\approx 2.31\sqrt{\frac{{\alpha }_s{N}_c}{2\pi }}$ found in our earlier work.

   ↳ nucl-thhep-exhep-phnucl-exJHEP(2017)·113 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.