Beam-Normal Single-Spin Asymmetry in Pb 208 at Low Energy: Discrepancy Resolved or New Kinematic Puzzle?

A long-standing discrepancy between measured and predicted beam-normal single-spin asymmetries <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mrow> <a:mi>A</a:mi> </a:mrow> <a:mrow> <a:mi>n</a:mi> </a:mrow> </a:msub> </a:mrow> </a:math> in elastic electron scattering off <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mmultiscripts> <c:mrow> <c:mi>Pb</c:mi> </c:mrow> <c:mprescripts/> <c:none/> <c:mrow> <c:mn>208</c:mn> </c:mrow> </c:mmultiscripts> </c:mrow> </c:math> has challenged our understanding of two-photon exchange (TPE) in heavy nuclei. We report a new measurement at 570 MeV and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:msup> <e:mrow> <e:mi>Q</e:mi> </e:mrow> <e:mrow> <e:mn>2</e:mn> </e:mrow> </e:msup> <e:mo>=</e:mo> <e:mn>0.04</e:mn> <e:mtext> </e:mtext> <e:mtext> </e:mtext> <e:msup> <e:mrow> <e:mi>GeV</e:mi> </e:mrow> <e:mrow> <e:mn>2</e:mn> </e:mrow> </e:msup> <e:mo>/</e:mo> <e:msup> <e:mrow> <e:mi>c</e:mi> </e:mrow> <e:mrow> <e:mn>2</e:mn> </e:mrow> </e:msup> </e:mrow> </e:math> , yielding <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mrow> <g:msub> <g:mrow> <g:mi>A</g:mi> </g:mrow> <g:mrow> <g:mi>n</g:mi> </g:mrow> </g:msub> <g:mo>=</g:mo> <g:mo stretchy="false">[</g:mo> <g:mo>−</g:mo> <g:mn>9.1</g:mn> <g:mo>±</g:mo> <g:mn>2.1</g:mn> <g:mrow> <g:mo stretchy="false">(</g:mo> <g:mi>stat</g:mi> <g:mo stretchy="false">)</g:mo> </g:mrow> <g:mo>±</g:mo> <g:mn>0.7</g:mn> <g:mrow> <g:mo stretchy="false">(</g:mo> <g:mi>syst</g:mi> <g:mo stretchy="false">)</g:mo> </g:mrow> <g:mo stretchy="false">]</g:mo> <g:mtext> </g:mtext> <g:mtext> </g:mtext> <g:mi>ppm</g:mi> </g:mrow> </g:math> . This nonzero value contrasts with previous results at higher energies and suggests a kinematic dependence of TPE effects not captured by current theory, prompting a reevaluation of earlier interpretations.