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, $R_{\rm L}$, for $20 <~ p_{\rm T}^{\rm ch \, jet} <~ 80$ GeV/$c$. In the perturbative region (large $R_{\rm L}$), a good agreement between the data and a next-to-leading-log perturbative QCD calculation is observed. In the non-perturbative region (small $R_{\rm L}$), the data exhibits a linear $R_{\rm L}$ dependence. There is a transition region in between, characterized by a turnover in the EEC distribution, corresponding to the confinement process. The peak of this transition region is located at $2.39 \pm 0.17$ GeV/$c/\langle p_{\rm T}^{\rm ch \, jet}\rangle$ 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.
Submitted to: PRL
e-Print: arXiv:2409.12687 | PDF | inSPIRE
CERN-EP-2024-245
Figure group
Figure 3
Normalized $\Sigma_{\text{EEC}}$ as a function of $\langle p_{\rm T}^{\textrm{ch jet}} \rangle R_L$. ln$\langle p_{\rm T}^{\textrm{ch jet}} \rangle$ in the y-axis represents ln$(\langle p_{\rm T}^{\textrm{ch jet}} \rangle/({\rm GeV}/c))$ as explained in footnote 2. The gray line corresponds to the maximum location of the distribution and the gray band corresponds to a $\pm$0.17 GeV/$c$ uncertainty along the x-axis. The orange curves show pQCD calculations, which are normalized to data such that the integral inside $R_L$ range of [12 GeV/$c$/$\langle p_{\rm T}^{\textrm{ch jet}} \rangle$, 0.4] are the same. The purple curve represents a linear functional form that is fit to data in the $R_L$ range of [0.01, 0.7 GeV/$c$/$\langle p_{\rm T}^{\textrm{ch jet}} \rangle$]. Bottom: Ratios of the pQCD calculation and linear fit to data. As the fitting range for the linear curve is mostly accessible by the data in 20$-$40 GeV/$c$, the ratio of linear fit to data is only shown for 20$-$40 GeV/$c$. As the normalization range for the orange pQCD curve is mostly accessible by the data in 60$-$80 GeV/$c$, the ratio of pQCD to data is only shown for 60$-$80 GeV/$c$. |  |
Figure A.2
Left: the jet $\pT$ dependence of the transition peak position $R_L^{\text{peak}}$. Right: the jet $\pT$ dependence of the transition peak height $\Sigma_{\text{EEC}}^{\text{peak}}$. ln$\langle p_{\rm T}^{\textrm{ch jet}} \rangle$ in the y-axis represents $\ln(\langle p_{\rm T}^{\textrm{ch jet}} \rangle/({\rm GeV}/c))$ as explained in footnote 2. |   |
Figure A.3
Comparison $\Sigma_{\text{EEC}}$ as a function of $R_L$ with 0.15 GeV/$c$, 0.5 GeV/$c$, and 1 GeV/$c$ track $\pT$ threshold cut in the $p_{\rm T}^{\textrm{ch jet}}$ intervals 20$-$40, 40$-$60, and 60$-$80 GeV/$c$. Systematic uncertainties are not shown here. |    |
Figure A.4
Comparison of $\Sigma_{\text{EEC}}$ as a function of $R_L$ with different values for TimeShower:pTmin using generated Pythia events. The default setting is 0.5 (blue markers). The parton level distributions are made from parton jets that are matched to the charged jets with $\pT$ range indicated in each figure. |    |
Figure A.5
Comparison of $\Sigma_{\text{EEC}}$ section as a function of $R_L$ with different values for StringPT:sigma using generated PYTHIA 8 events. The default setting is 0.335 (blue markers). The parton level distributions are made from parton jets that are matched to the charged jets with $\pT$ range indicated in each figure. |    |