Mon·Mar 8, 2021Monday·March 8, 2021

1. 01*

   Measurement of the generalized spin polarizabilities of the neutron in the low $Q^2$ region

   V. Sulkosky · C. Peng · J.-P. Chen · A. Deur · S. Abrahamyan · K. A. Aniol · D. S. Armstrong · T. Averett · S. L. Bailey · A. Beck · P. Bertin · F. Butaru and 97 other authors

   Understanding the nucleon spin structure in the regime where the strong interaction becomes truly strong poses a challenge to both experiment and theory. At energy scales below the nucleon mass of about 1 GeV, the intense interaction among the quarks and gluons inside the nucleon makes them highly correlated. Their coherent behaviour causes the emergence of effective degrees of freedom, requiring the application of non-perturbative techniques, such as chiral effective field theory. Here, we present measurements of the neutron's generalized spin-polarizabilities that quantify the neutron's spin precession under electromagnetic fields at very low energy-momentum transfer squared down to 0.035 GeV${}^2$. In this regime, chiral effective field theory calculations are expected to be applicable. Our data, however, show a strong discrepancy with these predictions, presenting a challenge to the current description of the neutron's spin properties.

   nucl-exNat.Phys.(2021)·20 citations
2. 02*

   A measurement of two-photon exchange in Super-Rosenbluth separations with positron beams

   John R Arrington🇺🇸 · Mikhail Yurov🇺🇸

   The proton electric and magnetic form factors, $G_E$ and $G_M$, are intrinsically connected to the spatial distribution of charge and magnetization in the proton. For decades, Rosenbluth separation measurements of the angular dependence of elastic e${}^{-}$-p scattering were used to extract $G_E$ and $G_M$. More recently, polarized electron scattering measurements, aiming to improve the precision of $G_E$ extractions, showed significant disagreement with Rosenbluth measurements at large momentum transfers ($Q^2$). This discrepancy is generally attributed to neglected two-photon exchange (TPE) corrections. At larger $Q^2$ values, a new `Super-Rosenbluth' technique was used to improve the precision of the Rosenbluth extraction, allowing for a better quantification of the discrepancy, while comparisons of e${}^{+}$-p and e${}^{-}$-p scattering indicated the presence of TPE corrections, but at $Q^2$ values below where a clear discrepancy is observed. In this work, we demonstrate the significant benefits to combining the Super-Rosenbluth technique with positron beam measurements. This approach provides a greater kinematic reach and is insensitive to some of the key systematic uncertainties in previous positron measurements.

   nucl-exEPJA(2021)·8 citations
3. 03*

   ${}^{16}{\mathrm{O}}^{16}\mathrm{O}$ at RHIC and the LHC comparing $\alpha$ clustering vs substructure

   Nicholas Summerfield🇺🇸 · Bing-Nan Lu🇨🇳 · Christopher Plumberg🇺🇸 · Dean Lee🇺🇸 · Jacquelyn Noronha-Hostler🇺🇸 · Anthony Timmins🇺🇸

   Collisions of light and heavy nuclei in relativistic heavy-ion collisions have been shown to be sensitive to nuclear structure. With a proposed ${}^{16}{\mathrm{O}}^{16}\mathrm{O}$ run at the LHC and RHIC we study the potential for finding $\alpha$ clustering in ${}^{16}$O. Here we use the state-of-the-art iEBE-VISHNU package with ${}^{16}$O nucleonic configurations from {\rm ab initio} nuclear lattice simulations. This setup was tuned using a Bayesian analysis on pPb and PbPb systems. We find that the ${}^{16}{\mathrm{O}}^{16}\mathrm{O}$ system always begins far from equilibrium and that at LHC and RHIC it approaches the regime of hydrodynamic applicability only at very late times. Finally, by taking ratios of flow harmonics we are able to find measurable differences between $\alpha$-clustering, nucleonic, and subnucleonic degrees of freedom in the initial state.

   ↳ nucl-thnucl-exPRC(2021)·66 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.