Fri·Jan 20, 2017Friday·January 20, 2017

1. 01*

   The $2_1^{+}\to 3_1^{+}$ gamma width in ${}^{22}$Na and second class currents

   S. Triambak🇿🇦 · L. Phuthu · A. García🇺🇸 · G.C. Harper · J.N. Orce🇿🇦 · D.A. Short · S.P.R. Steininger · A. Diaz Varela🇨🇦 · R. Dunlop🇨🇦 · D.S. Jamieson · W.A. Richter🇩🇪 · G.C. Ball🇨🇦 and 3 other authors

   A previous measurement of the $\beta -\gamma$ directional coefficient in ${}^{22}$Na $\beta$ decay was used to extract recoil-order form factors. The data indicate the requirement of a significant induced-tensor matrix element for the decay.This conclusion largely relies on a Standard-Model-allowed weak magnetism form factor which was determined using an unpublished value of the analog $2_1^{+}\to 3_1^{+}$ $\gamma$ branch in ${}^{22}$Na, with the further assumption that the transition is dominated by its isovector $M1$ component. We obtain for the first time an unambiguous determination of the $2_1^{+}\to 3_1^{+}$ branch in ${}^{22}$Na to be $0.45(8)\%$. Using the Conserved Vector Current (CVC) hypothesis, our branch determines the weak magnetism form factor for ${}^{22}$Na $\beta$ decay to be $|b/A{c}_1|=8.7(1.1)$. Together with the $\beta -\gamma$ angular correlation coefficient, we obtain a large induced-tensor form factor for the decay that continues to disagree with theoretical predictions. Two plausible explanations are suggested.

   nucl-exPRC(2017)·9 citations
2. 02*

   PEN: a low energy test of lepton universality

   D. Pocanic🇺🇸 · L.P. Alonzi🇺🇸 · V.A. Baranov🇷🇺 · W. Bertl🇨🇭 · M. Bychkov🇺🇸 · Yu.M. Bystritsky🇷🇺 · E. Frlez🇺🇸 · C.J. Glaser🇺🇸 · V.A. Kalinnikov🇷🇺 · N.V. Khomutov🇷🇺 · A.S. Korenchenko🇷🇺 · S.M. Korenchenko🇷🇺 and 15 other authors

   Allowed charged $\pi$ meson decays are characterized by simple dynamics, few available decay channels, mainly into leptons, and extremely well controlled radiative and loop corrections. In that sense, pion decays represent a veritable triumph of the standard model (SM) of elementary particles and interactions. This relative theoretical simplicity makes charged pion decays a sensitive means for testing the underlying symmetries and the universality of weak fermion couplings, as well as for studying pion structure and chiral dynamics. Even after considerable recent improvements, experimental precision is lagging far behind that of the theoretical description for pion decays. We review the current state of experimental study of the pion electronic decay ${\pi }^{+}\to e^{+}{\nu }_e(\gamma )$, or ${\pi }_{e2(\gamma )}$, where the $(\gamma )$ indicates inclusion and explicit treatment of radiative decay events. We briefly review the limits on non-SM processes arising from the present level of experimental precision in ${\pi }_{e2(\gamma )}$ decays. Focusing on the PEN experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the prospects for further improvement in the near term.

   ↳ hep-exhep-phnucl-exPoS(2017)·2 citations
3. 03*

   Double Helicity Asymmetry in ${\pi }^0$ Production at Midrapidity in Polarized $p+p$ Collisions at $\sqrt{s}=510$ GeV

   Inseok Yoon (for the PHENIX Collaboration)🇰🇷

   PHENIX measurements are presented for the cross-section and double-helicity asymmetry ($A_{LL}$) of inclusive ${\pi }^0$ production ($A_{LL}^{{\pi }^0}$) at midrapidity from $p+p$ collisions at $\sqrt{s}=510$ GeV from data taken in 2012 and 2013 at the Relativistic Heavy Ion Collider (RHIC). The next-to-leading order (NLO) perturbative QCD (pQCD) calculation agrees excellently with the presented cross-section result. The $A_{LL}^{{\pi }^0}$ follows an increasingly positive asymmetry as functions of $p_T$ and $\sqrt{s}$ at the fixed $x_T$. The latest global analysis results, which support the positive spin contribution of gluon ($\Delta G$), agrees excellently with the presented asymmetry result. The asymmetry result extends the experimental sensitivity to the previously unexplored $x$ region down to $x\sim 0.01$ and provides additional constraints on $\Delta G$.

   ↳ hep-exnucl-ex0 citations
4. 04*

   Forward two-photon exchange in elastic lepton-proton scattering and hyperfine-splitting correction

   Oleksandr Tomalak🇩🇪

   We relate the forward two-photon exchange (TPE) amplitudes to integrals of the inclusive lepton-proton scattering cross sections. These relations yield an alternative way for the evaluation of the TPE correction to hyperfine-splitting (HFS) in the hydrogen-like atoms with an equivalent to the standard approach (Iddings, Drell and Sullivan) result implying the Burkhardt-Cottingham sum rule. For evaluation of the individual effects (e.g., elastic contribution) our approach yields a distinct result. We compare both methods numerically on examples of the elastic contribution and the full TPE correction to HFS in electronic and muonic hydrogen.

   ↳ hep-phnucl-exnucl-thphysics.atom-phEPJC(2017)·30 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.