High-energy nuclear collisions create a quark-gluon plasma, whose initial condition and subsequent expansion vary from event to event, impacting the distribution of the eventwise average transverse momentum [<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>P</a:mi><a:mo stretchy="false">(</a:mo><a:mo stretchy="false">[</a:mo><a:msub><a:mi>p</a:mi><a:mi mathvariant="normal">T</a:mi></a:msub><a:mo stretchy="false">]</a:mo><a:mo stretchy="false">)</a:mo></a:math>]. Disentangling the contributions from fluctuations in the nuclear overlap size (geometrical component) and other sources at a fixed size (intrinsic component) remains a challenge. This problem is addressed by measuring the mean, variance, and skewness of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mi>P</h:mi><h:mo stretchy="false">(</h:mo><h:mo stretchy="false">[</h:mo><h:msub><h:mi>p</h:mi><h:mi mathvariant="normal">T</h:mi></h:msub><h:mo stretchy="false">]</h:mo><h:mo stretchy="false">)</h:mo></h:math> in <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mrow><o:mrow><o:mmultiscripts><o:mrow><o:mi>Pb</o:mi></o:mrow><o:mprescripts/><o:none/><o:mrow><o:mn>208</o:mn></o:mrow></o:mmultiscripts></o:mrow><o:mo>+</o:mo><o:mrow><o:mmultiscripts><o:mrow><o:mi>Pb</o:mi></o:mrow><o:mprescripts/><o:none/><o:mrow><o:mn>208</o:mn></o:mrow></o:mmultiscripts></o:mrow></o:mrow></o:math> and <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mrow><q:mrow><q:mmultiscripts><q:mrow><q:mi>Xe</q:mi></q:mrow><q:mprescripts/><q:none/><q:mrow><q:mn>129</q:mn></q:mrow></q:mmultiscripts></q:mrow><q:mo>+</q:mo><q:mrow><q:mmultiscripts><q:mrow><q:mi>Xe</q:mi></q:mrow><q:mprescripts/><q:none/><q:mrow><q:mn>129</q:mn></q:mrow></q:mmultiscripts></q:mrow></q:mrow></q:math> collisions at <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mrow><s:msqrt><s:mrow><s:msub><s:mrow><s:mi>s</s:mi></s:mrow><s:mrow><s:mi>NN</s:mi></s:mrow></s:msub></s:mrow></s:msqrt><s:mo>=</s:mo><s:mn>5.02</s:mn></s:mrow></s:math> and 5.44 TeV, respectively, using the ATLAS detector at the LHC. All observables show distinct features in ultracentral collisions, which are explained by a suppression of the geometrical component as the overlap area reaches its maximum. These results demonstrate a new technique to separate geometrical and intrinsic fluctuations, providing constraints on initial conditions and properties of the quark-gluon plasma, such as the speed of sound. © 2024 CERN, for the ATLAS Collaboration 2024 CERN