Measurement of Compton Scattering from the Deuteron and an Improved Extraction of the Neutron Electromagnetic Polarizabilities

The electromagnetic polarizabilities of the nucleon are fundamental properties that describe its response to external electric and magnetic fields. They can be extracted from Compton-scattering data—and have been, with good accuracy, in the case of the proton. In contradistinction, information for the neutron requires the use of Compton scattering from nuclear targets. Here, we report a new measurement of elastic photon scattering from deuterium using quasimonoenergetic tagged photons at the MAX IV Laboratory in Lund, Sweden. These first new data in more than a decade effectively double the world data set. Their energy range overlaps with previous experiments and extends it by 20 MeV to higher energies. An analysis using chiral effective field theory with dynamical $\mathrm{\Delta }(1232)$ degrees of freedom shows the data are consistent with and within the world data set. After demonstrating that the fit is consistent with the Baldin sum rule, extracting values for the isoscalar nucleon polarizabilities, and combining them with a recent result for the proton, we obtain the neutron polarizabilities as ${\alpha }_n=[11.55\pm 1.25(\text{stat})\pm 0.2(\mathrm{BSR})\pm 0.8(\mathrm{th})]\times {10}^{-4}{\mathrm{fm}}^3$ and ${\beta }_n=[3.65\mp 1.25(\text{stat})\pm 0.2(\mathrm{BSR})\mp 0.8(\mathrm{th})]\times {10}^{-4}{\mathrm{fm}}^3$, with ${\chi }^2=45.2$ for 44 degrees of freedom.

Figure 1

A typical accidental-corrected scattered-photon spectrum together with a simulation (red) of the response function of the detector. The vertical, dashed lines indicate the $-2.0<\mathrm{ME}<2.0\mathrm{MeV}$ yield-integration region.

Figure 2

The $1\sigma$ (${\chi }^2+2.3$, solid ellipse) and ${\chi }^2+1$ (dashed ellipse) regions of the free fit compared to the Baldin-constrained fit (line); gray band: BSR.

Figure 3

Deuteron Compton scattering cross sections $(\mathrm{in}\mathrm{nb}/\mathrm{sr})$ in the lab frame as a function of lab angle (in degrees). Data are within 2 MeV of the nominal energy. Data and statistical uncertainties: this work $\times$, Ref. [8] •, Ref. [9] $⧫$, Ref. [10] ▴. Curves: results of the one (two) parameter fits [solid (dashed)]. Band: statistical uncertainty of the one-parameter fit. Note that the two points with $\mathrm{\Delta }{\chi }^2>5$ are not included in the fit, and the normalization of each data set is floated within quoted uncertainties. See text for details.