Observation of the Electromagnetic Field Effect via Charge-Dependent Directed Flow in Heavy-Ion Collisions at the Relativistic Heavy Ion Collider

Abstract

The deconfined quark-gluon plasma (QGP) created in relativistic heavy-ion collisions enables the exploration of the fundamental properties of matter under extreme conditions. Noncentral collisions can produce strong magnetic fields on the order of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msup><a:mn>10</a:mn><a:mn>18</a:mn></a:msup><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi mathvariant="normal">G</a:mi></a:math>, which offers a probe into the electrical conductivity of the QGP. In particular, quarks and antiquarks carry opposite charges and receive contrary electromagnetic forces that alter their momenta. This phenomenon can be manifested in the collective motion of final-state particles, specifically in the rapidity-odd directed flow, denoted as <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mrow><d:msub><d:mrow><d:mi>v</d:mi></d:mrow><d:mrow><d:mn>1</d:mn></d:mrow></d:msub><d:mo stretchy="false">(</d:mo><d:mi mathvariant="sans-serif">y</d:mi><d:mo stretchy="false">)</d:mo></d:mrow></d:math>. Here, we present the charge-dependent measurements of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>d</i:mi><i:msub><i:mi>v</i:mi><i:mn>1</i:mn></i:msub><i:mo>/</i:mo><i:mi>d</i:mi><i:mi mathvariant="sans-serif">y</i:mi></i:math> near midrapidities for <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:msup><l:mi>π</l:mi><l:mo>±</l:mo></l:msup></l:math>, <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"><n:msup><n:mi>K</n:mi><n:mo>±</n:mo></n:msup></n:math>, and <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"><p:mi>p</p:mi><p:mo stretchy="false">(</p:mo><p:mover accent="true"><p:mi>p</p:mi><p:mo stretchy="false">¯</p:mo></p:mover><p:mo stretchy="false">)</p:mo></p:math> in <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:mrow><v:mi>Au</v:mi><v:mo>+</v:mo><v:mi>Au</v:mi></v:mrow></v:math> and isobar (<x:math xmlns:x="http://www.w3.org/1998/Math/MathML" display="inline"><x:mrow><x:mmultiscripts><x:mrow><x:mi>Ru</x:mi></x:mrow><x:mprescripts/><x:mrow><x:mn>44</x:mn></x:mrow><x:mrow><x:mn>96</x:mn></x:mrow></x:mmultiscripts></x:mrow><x:mo>+</x:mo><x:mrow><x:mmultiscripts><x:mrow><x:mi>Ru</x:mi></x:mrow><x:mprescripts/><x:mrow><x:mn>44</x:mn></x:mrow><x:mrow><x:mn>96</x:mn></x:mrow></x:mmultiscripts></x:mrow></x:math> and <z:math xmlns:z="http://www.w3.org/1998/Math/MathML" display="inline"><z:mrow><z:mmultiscripts><z:mrow><z:mi>Zr</z:mi></z:mrow><z:mprescripts/><z:mrow><z:mn>40</z:mn></z:mrow><z:mrow><z:mn>96</z:mn></z:mrow></z:mmultiscripts></z:mrow><z:mo>+</z:mo><z:mrow><z:mmultiscripts><z:mrow><z:mi>Zr</z:mi></z:mrow><z:mprescripts/><z:mrow><z:mn>40</z:mn></z:mrow><z:mrow><z:mn>96</z:mn></z:mrow></z:mmultiscripts></z:mrow></z:math>) collisions at <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML" display="inline"><bb:msqrt><bb:msub><bb:mi>s</bb:mi><bb:mrow><bb:mi>NN</bb:mi></bb:mrow></bb:msub></bb:msqrt><bb:mo>=</bb:mo><bb:mn>200</bb:mn><bb:mtext> </bb:mtext><bb:mtext> </bb:mtext><bb:mi>GeV</bb:mi></bb:math>, and in <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mrow><db:mi>Au</db:mi><db:mo>+</db:mo><db:mi>Au</db:mi></db:mrow></db:math> collisions at 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:msub><fb:mi>v</fb:mi><fb:mn>1</fb:mn></fb:msub></fb:math> signal on collision system, particle species, and collision centrality can be qualitatively and semiquantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the <hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline"><hb:mi>u</hb:mi></hb:math> and <jb:math xmlns:jb="http://www.w3.org/1998/Math/MathML" display="inline"><jb:mi>d</jb:mi></jb:math> quarks transported from initial-state nuclei, those in peripheral events reveal the impacts of the electromagnetic field on the QGP. Our data put valuable constraints on the electrical conductivity of the QGP in theoretical calculations. Published by the American Physical Society 2024