Particle physicists detect unexpected asymmetry in rare meson decays
Researchers at Brookhaven's particle accelerator have measured a subtle asymmetry in how eta mesons behave when produced by spinning protons—a finding that challenges existing physics models. The discovery could reshape understanding of fundamental forces and may have implications for materials science and quantum computing applications.
Originaltitel: Transverse single-spin asymmetry of forward <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>η</mml:mi> </mml:math> mesons in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msup> <mml:mrow> <mml:mi>p</mml:mi> </mml:mrow> <mml:mrow> <mml:mo stretchy="false">↑</mml:mo> </mml:mrow> </mml:msup> <mml:mo>+</mml:mo> <mml:mi>p</mml:mi> </mml:mrow> </mml:math> collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> <mml:mo>=</mml:mo> <mml:mn>200</mml:mn> <mml:mtext> </mml:mtext> <mml:mtext> </mml:mtext> <mml:mi>GeV</mml:mi> </mml:math>
Utilizing the 2012 transversely polarized proton data from the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, the forward <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>η</a:mi> </a:math> -meson transverse single-spin asymmetry ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msub> <c:mi>A</c:mi> <c:mi>N</c:mi> </c:msub> </c:math> ) was measured for <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:msup> <e:mrow> <e:mi>p</e:mi> </e:mrow> <e:mrow> <e:mo stretchy="false">↑</e:mo> </e:mrow> </e:msup> <e:mo>+</e:mo> <e:mi>p</e:mi> </e:mrow> </e:math> collisions at <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:msqrt> <h:mi>s</h:mi> </h:msqrt> <h:mo>=</h:mo> <h:mn>200</h:mn> <h:mtext> </h:mtext> <h:mtext> </h:mtext> <h:mi>GeV</h:mi> </h:math> as a function of Feynman-x ( <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:msub> <j:mi>x</j:mi> <j:mi>F</j:mi> </j:msub> </j:math> ) for <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mn>0.2</l:mn> <l:mo><</l:mo> <l:mo stretchy="false">|</l:mo> <l:msub> <l:mi>x</l:mi> <l:mi>F</l:mi> </l:msub> <l:mo stretchy="false">|</l:mo> <l:mo><</l:mo> <l:mn>0.8</l:mn> </l:math> and transverse momentum ( <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:msub> <p:mi>p</p:mi> <p:mi>T</p:mi> </p:msub> </p:math> ) for <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"> <r:mn>1.0</r:mn> <r:mo><</r:mo> <r:msub> <r:mi>p</r:mi> <r:mi>T</r:mi> </r:msub> <r:mo><</r:mo> <r:mn>5.0</r:mn> <r:mtext> </r:mtext> <r:mtext> </r:mtext> <r:mi>GeV</r:mi> <r:mo>/</r:mo> <r:mi>c</r:mi> </r:math> . Large asymmetries at positive <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"> <t:msub> <t:mi>x</t:mi> <t:mi>F</t:mi> </t:msub> </t:math> are observed ( <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"> <v:mrow> <v:mrow> <v:mo stretchy="false">⟨</v:mo> <v:msub> <v:mrow> <v:mi>A</v:mi> </v:mrow> <v:mrow> <v:mi>N</v:mi> </v:mrow> </v:msub> <v:mo stretchy="false">⟩</v:mo> </v:mrow> <v:mo>=</v:mo> <v:mn>0.086</v:mn> <v:mo>±</v:mo> <v:mn>0.019</v:mn> </v:mrow> </v:math> ), agreeing well with previous measurements of <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"> <z:msup> <z:mi>π</z:mi> <z:mn>0</z:mn> </z:msup> </z:math> and <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"> <bb:mrow> <bb:mi>η</bb:mi> <bb:mtext> </bb:mtext> <bb:msub> <bb:mrow> <bb:mi>A</bb:mi> </bb:mrow> <bb:mrow> <bb:mi>N</bb:mi> </bb:mrow> </bb:msub> </bb:mrow> </bb:math> , but with reach to higher <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"> <db:msub> <db:mi>x</db:mi> <db:mi>F</db:mi> </db:msub> </db:math> and <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"> <fb:msub> <fb:mi>p</fb:mi> <fb:mi>T</fb:mi> </fb:msub> </fb:math> . The contribution of initial-state spin-momentum correlations to the asymmetry, as calculated in the collinear twist-3 framework, appears insufficient to describe the data.