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| CMS-HIN-16-006 ; CERN-EP-2017-205 | ||
| Measurement of the splitting function in pp and PbPb collisions at ${\sqrt{\smash [b]{s_{_{\mathrm{NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 30 August 2017 | ||
| Phys. Rev. Lett. 120 (2018) 142302 | ||
| Abstract: Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions. However, the stage in the shower evolution at which the modifications occur is not known. By removing the soft wide-angle radiation inside a jet, the two leading partons can be resolved as subjets. The momentum ratio of these subjets provides information about the corresponding partons at an early stage of the shower. This substructure observable, known as the splitting function, relates to the process of a parton splitting into two other partons. The measurement is performed for jets with transverse momentum larger than 140 GeV and smaller than 500 GeV, using pp and PbPb collision samples collected by the CMS detector at a center-of-mass energy of 5.02 GeV per nucleon pair. In central PbPb collisions, the splitting function is observed to be modified toward a more unbalanced momentum ratio compared to peripheral PbPb and pp collisions. The measurements are compared to the predictions from event generators and analytical calculations using various models for the medium modification of the jet evolution. | ||
| Links: e-print arXiv:1708.09429 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Groomed jet energy fraction in pp and the 10% most central PbPb collisions for jets with 160 $ < {p_{\mathrm {T,jet}}} < $ 180 GeV and $ | \eta _{\text {jet}} | < $ 1.3. The pp data is compared to the PYTHIA 6 event generator and the PbPb data to PYTHIA 6 embedded into the HYDJET event generator. |
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Figure 2:
Splitting function distribution in pp collisions for 160 $ < p_{\mathrm {T,jet}} < $ 180 GeV compared to predictions from event generators. The shaded area indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. |
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Figure 3:
Splitting function distribution in PbPb collisions for 160 $ < p_{\mathrm {T,jet}} < $ 180 GeV in several centrality ranges compared to pp data smeared to account for the difference in resolution between pp and PbPb collisions for each centrality selection. The shaded area indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. |
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Figure 4:
Ratio of splitting function distributions in PbPb and smeared pp collisions in the 10% most central events for several ${p_{\mathrm {T,jet}}} $ ranges compared to various jet quenching theoretical calculations. The shaded area indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. The diagonally hatched band denotes the uncertainty from the treatment of the medium response utilizing the JEWEL event generator. |
| Summary |
| In summary, the first measurement of the splitting function in pp and PbPb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair has been presented. This represents the first application of a grooming technique to PbPb data, allowing for the removal of soft wide-angle radiation from the jet and thereby isolating the two leading subjets. The momentum sharing between these subjets is used to obtain information about hard parton splitting in the early stages of the shower in pp and PbPb collisions. Both the PYTHIA and HERWIG++ event generators agree with the measured splitting function in pp collisions at the level of 15%. This agreement is also observed in peripheral PbPb collisions. In central PbPb collisions, a steeper distribution of $ z_{\mathrm{g}} $ is observed, indicating that the splitting process of a parton is modified when traversing the hot medium created in heavy ion collisions. These results provide new insight into the role of color coherence and other attributes of the early interactions of partons in the quark-gluon plasma. |
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