Investigating the broadening phenomenon in two-particle correlations induced by gluon saturation

It has been found that the gluon density inside the proton grows rapidly at small momentum fractions. Quantum chromodynamics (QCD) predicts that this growth can be regulated by nonlinear effects, ultimately leading to gluon saturation. Within the color glass condensate framework, nonlinear QCD effects are predicted to suppress and broaden back-to-back angular correlations in collisions involving heavy nuclei. While suppression has been observed in various experiments in <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>d</a:mi><a:mo>/</a:mo><a:mi>p</a:mi><a:mo>+</a:mo><a:mi mathvariant="normal">A</a:mi></a:mrow></a:math> collisions compared to <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:mi>p</d:mi><d:mo>+</d:mo><d:mi>p</d:mi></d:math> collisions, the predicted broadening remains unobserved. This study investigates the contributions of intrinsic transverse momentum (<f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:msub><f:mi>k</f:mi><f:mi>T</f:mi></f:msub></f:math>), which is associated with saturation physics, as well as parton showers and transverse motion from fragmentation (<h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:msubsup><h:mi>p</h:mi><h:mi>T</h:mi><h:mrow><h:mi>frag</h:mi></h:mrow></h:msubsup></h:math>), which are not saturation dependent, to the width of the correlation function. Our findings show that the non-saturation dependent effects, especially the initial-state parton shower and <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:msubsup><j:mi>p</j:mi><j:mi>T</j:mi><j:mrow><j:mi>frag</j:mi></j:mrow></j:msubsup></j:math>, which occur independently of the collision system, smear the back-to-back correlation more than gluon saturation does, making the broadening phenomenon difficult to observe.