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| CMS-PAS-HIN-16-006 | ||
| Splitting function in pp and PbPb collisions at $\sqrt{s_{\mathrm{NN}}}= $ 5.02 TeV | ||
| CMS Collaboration | ||
| July 2016 | ||
| Abstract: A measurement of the splitting function in pp and PbPb collisions at a center of mass energy of 5.02 TeV per nucleon pair is presented. The virtuality evolution of partons traversing the hot QCD medium created in a heavy ion collision is potentially modified due to interactions with the color charges of the medium. This measurement probes this phenomenon by measuring the generalized fragmentation function, or splitting function, over a wide range of jet transverse momentum and various collision centrality selections. The shared momentum fraction of two subjets resulting from a parton splitting is observed to be modified towards a more imbalanced fraction in central PbPb collisions compared to peripheral PbPb and pp collisions. In addition, the measurement in pp collisions is compared to the prediction of the splitting function from the PYTHIA and HERWIG event generators. | ||
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Links:
CDS record (PDF) ;
inSPIRE record ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, PRL 120 (2018) 142302. The superseded preliminary plots can be found here. |
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| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1-a:
Groomed jet energy fraction in pp (a) and 10% most central PbPb collisions (b) for jets with 140 $ < {p_{\mathrm {T,jet}}}< $ 160 GeV and $|\eta _{\mathrm {jet}}|< $ 1.3. The pp data is compared to the PYTHIA event generator and the PbPb data to PYTHIA embedded into the HYDJET event generator. Jets are reconstructed with the anti- ${k_{\mathrm {T}}}$ algorithm and groomed using Softdrop. The jets are selected based on the ungroomed transverse momentum. |
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Figure 1-b:
Groomed jet energy fraction in pp (a) and 10% most central PbPb collisions (b) for jets with 140 $ < {p_{\mathrm {T,jet}}}< $ 160 GeV and $|\eta _{\mathrm {jet}}|< $ 1.3. The pp data is compared to the PYTHIA event generator and the PbPb data to PYTHIA embedded into the HYDJET event generator. Jets are reconstructed with the anti- ${k_{\mathrm {T}}}$ algorithm and groomed using Softdrop. The jets are selected based on the ungroomed transverse momentum. |
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Figure 2:
Splitting function in pp collisions for several jet ${p_{\mathrm {T}}} $ intervals ranging between $ {p_{\mathrm {T,jet}}}= $ 120 GeV and $ {p_{\mathrm {T,jet}}}= $ 200 GeV compared to PYTHIA and HERWIG event generators. The shaded area indicates the systematic uncertainty on the measurement while the vertical lines represent the statistical uncertainty. |
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Figure 3:
Splitting function in PbPb for 160 $ |
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Figure 4-a:
Mean ${z_{\mathrm {g}}}$ in PbPb and smeared pp collisions as function of the average number of participants for two different jet ${p_{\mathrm {T}}} $ selections. The vertical error bars indicate the statistical uncertainty while the shaded area corresponds to the systematic uncertainty. |
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Figure 4-b:
Mean ${z_{\mathrm {g}}}$ in PbPb and smeared pp collisions as function of the average number of participants for two different jet ${p_{\mathrm {T}}} $ selections. The vertical error bars indicate the statistical uncertainty while the shaded area corresponds to the systematic uncertainty. |
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Figure 5:
Ratio of splitting function in PbPb and smeared pp collisions in 10% most central events for several ${p_{\mathrm {T,jet}}}$ ranges. The shaded area indicates the systematic uncertainty on the measurement while the vertical lines represent the statistical uncertainty. |
| Tables | |
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Table 1:
Centrality ranges with corresponding average number of participants (scaled by $N_{\mathrm {coll}}$) used in the analysis. The uncertainty on $< N_{\mathrm {part}}> $ originates from parameters used in the Glauber model. |
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Table 2:
Summary of individual contributions in percentiles to the systematic uncertainty of the ${z_{\mathrm {g}}}$ distributions. The maximum uncertainty along ${z_{\mathrm {g}}}$ is reported. The uncertainties on the smearing factors apply to the pp measurement smeared with the resolution difference between PbPb and pp for a given centrality selection. |
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Table 3:
Summary of individual contributions in percentiles to the systematic uncertainty of the mean ${z_{\mathrm {g}}}$. The uncertainties on the smearing factors apply to the pp measurement smeared with the resolution difference between PbPb and pp for a given centrality selection. |
| Summary |
| A measurement of the generalized fragmentation function, also referred to as 'Splitting Function', in pp and PbPb collisions at a center of mass energy of $5.02$ TeV per nucleon pair has been presented. Both the PYTHIA and HERWIG++ event generator agree with the measured splitting function in pp collisions at the level of 10%. This agreement is also observed in peripheral PbPb collisions. In central PbPb collisions, a significantly steeper slope for the splitting function is observed indicating that the splitting process of a parton traversing the hot medium created in heavy ion collisions is modified. |
| Additional Figures | |
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Additional Figure 1:
Splitting function in PbPb for 140 $ < p_{\mathrm{T,\,jet}}< $ 160 GeV in several centrality ranges compared to pp data. For this comparison the resolution of the pp data is deteriorated to the same resolution as the PbPb measurement for each centrality selection. The shaded area around the data points indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. |
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Additional Figure 2:
Splitting function in PbPb for 180 $ < p_{\mathrm{T,\,jet}}< $ 200 GeV in several centrality ranges compared to pp data. For this comparison the resolution of the pp data is deteriorated to the same resolution as the PbPb measurement for each centrality selection. The shaded area around the data points indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. |
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Additional Figure 3:
Splitting function in PbPb for 200 $ < p_{\mathrm{T,\,jet}}< $ 250 GeV in several centrality ranges compared to pp data. For this comparison the resolution of the pp data is deteriorated to the same resolution as the PbPb measurement for each centrality selection. The shaded area around the data points indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. |
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Additional Figure 4:
Mean $z_{\mathrm {g}}$ in PbPb and smeared pp collisions as function of the average number of participants for 140 $ < p_{\mathrm{T,\,jet}}< $ 160 GeV. The vertical error bars indicate the statistical uncertainty while the shaded area corresponds to the systematic uncertainty. |
png pdf |
Additional Figure 5:
Mean $z_{\mathrm {g}}$ in PbPb and smeared pp collisions as function of the average number of participants for 180 $ < p_{\mathrm{T,\,jet}}< $ 200 GeV. The vertical error bars indicate the statistical uncertainty while the shaded area corresponds to the systematic uncertainty. |
png pdf |
Additional Figure 6:
Ratio of splitting function in PbPb and smeared pp collisions in 10% most central events for several ${p_{\mathrm{T,\,jet}}}$ ranges. The shaded area indicates the systematic uncertainty on the measurement while the vertical lines represent the statistical uncertainty. The measurement is compared to predictions of the jet quenching event generator JEWEL. |
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Additional Figure 7:
Ratio of splitting function in PbPb and smeared pp collisions for various centrality selection and 160 $ < {p_{\mathrm{T,\,jet}}} < $ 180 GeV. The measurement is compared to the jet quenching event generator JEWEL. |
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