First Measurement Using Elliptically Polarized Photons of the Double-Polarization Observable E for γp→pπ0 and…

We report the measurement of the helicity asymmetry <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>E</a:mi></a:math> for the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>p</c:mi><c:msup><c:mi>π</c:mi><c:mn>0</c:mn></c:msup></c:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>n</e:mi><e:msup><e:mi>π</e:mi><e:mo>+</e:mo></e:msup></e:math> final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g., <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>G</g:mi></g:math>) and circularly polarized photons (e.g., <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>E</i:mi></i:math>) and a longitudinally polarized target. The new data cover a photon energy range 270–1400 MeV for the <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mi>p</k:mi><k:msup><k:mi>π</k:mi><k:mn>0</k:mn></k:msup></k:math> final state (230–842 MeV for the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>n</m:mi><m:msup><m:mi>π</m:mi><m:mo>+</m:mo></m:msup></m:math> final state) and the full range of pion polar angles, <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>θ</o:mi></o:math>, providing the most precise measurement of the observable <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mi>E</q:mi></q:math>. A moment analysis gives a clear observation of the <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>p</s:mi><s:mi>η</s:mi></s:math> cusp in the <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi>p</u:mi><u:msup><u:mi>π</u:mi><u:mn>0</u:mn></u:msup></u:math> final state. Published by the American Physical Society 2024