Exposing the parton-hadron transition within jets with energy-energy correlators in pp collisions at s = 5.02     TeV

This paper presents a fully corrected measurement of the energy-energy correlator (EEC) within jets in pp collisions. The EEC traces the energy flow as a highly energetic parton undergoes a QCD shower followed by the confinement of partons into hadrons, probing the correlation function of the energy flow inside jets. The EEC observable is measured as a function of the charged particle pair angular distance, <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:msub> <a:mi>R</a:mi> <a:mi mathvariant="normal">L</a:mi> </a:msub> </a:math> , for <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:mn>20</d:mn> <d:mo><</d:mo> <d:msubsup> <d:mrow> <d:mi>p</d:mi> </d:mrow> <d:mrow> <d:mi mathvariant="normal">T</d:mi> </d:mrow> <d:mrow> <d:mi>ch</d:mi> <d:mtext> </d:mtext> <d:mi>jet</d:mi> </d:mrow> </d:msubsup> <d:mo><</d:mo> <d:mrow> <d:mn>80</d:mn> <d:mtext> </d:mtext> <d:mtext> </d:mtext> <d:mi>GeV</d:mi> <d:mo>/</d:mo> <d:mi>c</d:mi> </d:mrow> </d:mrow> </d:math> . In the perturbative region (large <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:msub> <g:mi>R</g:mi> <g:mi mathvariant="normal">L</g:mi> </g:msub> </g:math> ), a good agreement between the data and a next-to-leading-log perturbative QCD calculation is observed. In the nonperturbative region (small <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:msub> <j:mi>R</j:mi> <j:mi mathvariant="normal">L</j:mi> </j:msub> </j:math> ), the data exhibit a linear <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:msub> <m:mi>R</m:mi> <m:mi mathvariant="normal">L</m:mi> </m:msub> </m:math> dependence. There is a transition region in between, characterized by a turnover in the EEC distribution, indicating where hadronization effects begin to dominate. The peak of this transition region is located at <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:mn>2.43</p:mn> <p:mo>±</p:mo> <p:mn>0.14</p:mn> <p:mtext> </p:mtext> <p:mtext> </p:mtext> <p:mrow> <p:mi>GeV</p:mi> <p:mo>/</p:mo> <p:mi>c</p:mi> </p:mrow> <p:mo>/</p:mo> <p:mo stretchy="false">⟨</p:mo> <p:msubsup> <p:mi>p</p:mi> <p:mi mathvariant="normal">T</p:mi> <p:mrow> <p:mi>ch</p:mi> <p:mtext> </p:mtext> <p:mi>jet</p:mi> </p:mrow> </p:msubsup> <p:mo stretchy="false">⟩</p:mo> </p:math> for jets of various energies, indicating a common energy scale for the hadronization process. State-of-the-art Monte Carlo event generators are compared with the measurements, and can be used to constrain the parton shower and hadronization mechanisms.