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CMS-PAS-HIN-16-024
Measurement of groomed jet mass in PbPb and pp collisions at $\sqrt{s_{_\mathrm{NN}}} = $ 5.02 TeV
Abstract: A measurement of the groomed jet mass ($M_g$) normalized by the jet transverse momentum ($p_\text{T}^\text{jet}$) for anti-$k_\text{T}$ jets with the radius parameter 0.4 in PbPb and pp collisions at a center of mass energy of 5.02 TeV per nucleon pair is presented. The jet grooming algorithm is a recursive procedure which sequentially removes soft constituents of a jet until a pair of hard subjets is found. The results of this grooming can be used to study modifications to partons and their evolution while traversing the hot and dense medium created in heavy ion collisions, via small angle splitting of quarks and gluons inside the jet cores. The CMS detector at the LHC is used to perform this analysis for $p_\text{T}^\text{jet}$ between 140 and 300 GeV and pseudo-rapidity less than 1.3 in both PbPb and pp collisions, and for a range of PbPb collision centralities. The measurements in pp collisions are compared to predictions from the PYTHIA and HERWIG++ event generators and agreement at the 20% level is found. When compared to pp data, an increase of jets with large jet mass is observed for the PbPb results. However, the core of the jet is observed to be unmodified for all event centrality classes.
Figures Summary Additional Figures References CMS Publications
Figures

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Figure 1:
Groomed jet energy fraction in pp (left) and the 10% most central PbPb collisions (right) for jets with 140 $ < {p_\mathrm {T}^\mathrm {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. The parameters used for the SD algorithm are $z_\text {cut} = $ 0.5, $\beta = $ 1.5. The jets are selected based on the ungroomed jet transverse momentum.

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Figure 1-a:
Groomed jet energy fraction in pp collisions for jets with 140 $ < {p_\mathrm {T}^\mathrm {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. The parameters used for the SD algorithm are $z_\text {cut} = $ 0.5, $\beta = $ 1.5. The jets are selected based on the ungroomed jet transverse momentum.

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Figure 1-b:
Groomed jet energy fraction in the 10% most central PbPb collisions for jets with 140 $ < {p_\mathrm {T}^\mathrm {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. The parameters used for the SD algorithm are $z_\text {cut} = $ 0.5, $\beta = $ 1.5. The jets are selected based on the ungroomed jet transverse momentum.

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Figure 2:
The $ {p_\mathrm {T}^\mathrm {jet}} $ dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for pp events for two SD settings: $(0.1,0.0)$ (upper panels) and $(0.5,1.5)$ (lower panels). Results are compared to PYTHIA and HERWIG++ event generators. The ratio of data to MC is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Figure 2-a:
The $ {p_\mathrm {T}^\mathrm {jet}} $ dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for pp events for the $(0.1,0.0)$ SD setting. Results are compared to PYTHIA and HERWIG++ event generators. The ratio of data to MC is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Figure 2-b:
The $ {p_\mathrm {T}^\mathrm {jet}} $ dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for pp events for the $(0.5,1.5)$ SD setting. Results are compared to PYTHIA and HERWIG++ event generators. The ratio of data to MC is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Figure 3:
The centrality dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for events with 160 $ < {p_\mathrm {T}^\mathrm {jet}} < $ 180 GeV for two SD settings: $(0.1,0.0)$ (upper panels) and $(0.5,1.5)$ (lower panels). Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Figure 3-a:
The centrality dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for events with 160 $ < {p_\mathrm {T}^\mathrm {jet}} < $ 180 GeV for the $(0.1,0.0)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Figure 3-b:
The centrality dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for events with 160 $ < {p_\mathrm {T}^\mathrm {jet}} < $ 180 GeV for the $(0.5,1.5)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Figure 4:
The ${p_\mathrm {T}^\mathrm {jet}} $ dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for events in the centrality class 0-10% for two SD settings: $(0.1,0.0)$ (upper panels) and $(0.5,1.5)$ (lower panels). Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Figure 4-a:
The ${p_\mathrm {T}^\mathrm {jet}} $ dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for events in the centrality class 0-10% for the $(0.1,0.0)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Figure 4-b:
The ${p_\mathrm {T}^\mathrm {jet}} $ dependence of $ {M_{\mathrm {g}}} / {p_\mathrm {T}^\mathrm {jet}} $, for events in the centrality class 0-10% for the $(0.5,1.5)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.
Summary
The first measurement of the groomed jet mass normalized by the transverse momentum of the jet, $M_{g}/p_{\mathrm{T}}^{\text{jet}}$, 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 HERWIGpp event generators do not reproduce the measurement in pp collisions.

When the grooming of jets is performed with a profile that removes more radiation at distances far away from the jet axis, the $M_{g}/p_{\mathrm{T}}^{\text{jet}}$ distribution in PbPb collisions is within uncertainties equivalent to that measured in pp collisions for all studied centrality and$ p_{\mathrm{T}}^{\text{jet}} $ regions. Using a grooming setting which is independent of the angular separation of the subjets, no modification of the $M_{g}/p_{\mathrm{T}}^{\text{jet}}$ spectra in 10-80% peripheral collisions with respect to the measurement in pp collisions is observed. In 0-10% central collisions, however, an increased probability to produce a jet with large $M_{g}/p_{\mathrm{T}}^{\text{jet}}$ is observed when compared to pp collisions for jets with 140 $ < p_{\mathrm{T}}^{\text{jet}} < $ 180 GeV. This measurement shows that the core of the jet is not altered in central PbPb collisions within the uncertainties of this measurement, but the periphery of the jet is sensitive to interactions of the partons with the dense colored medium during the parton shower evolution.
Additional Figures

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Additional Figure 1:
The $p_\text {T}^\text {jet}$ dependence of $M_\text {g}/p_\text {T}^\text {jet}$, for events in the centrality class 10-30% for the $(0.1,0.0)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Additional Figure 2:
The $p_\text {T}^\text {jet}$ dependence of $M_\text {g}/p_\text {T}^\text {jet}$, for events in the centrality class 30-50% for the $(0.1,0.0)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Additional Figure 3:
The $p_\text {T}^\text {jet}$ dependence of $M_\text {g}/p_\text {T}^\text {jet}$, for events in the centrality class 50-80% for the $(0.1,0.0)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Additional Figure 4:
The $p_\text {T}^\text {jet}$ dependence of $M_\text {g}/p_\text {T}^\text {jet}$, for events in the centrality class 10-30% for the $(0.5,1.5)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Additional Figure 5:
The $p_\text {T}^\text {jet}$ dependence of $M_\text {g}/p_\text {T}^\text {jet}$, for events in the centrality class 30-50% for the $(0.5,1.5)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Additional Figure 6:
The $p_\text {T}^\text {jet}$ dependence of $M_\text {g}/p_\text {T}^\text {jet}$, for events in the centrality class 50-80% for the $(0.5,1.5)$ SD setting. Results are compared to the smeared pp spectra. The ratio of PbPb data over smeared pp is also shown. The height of the vertical lines (colored boxes) indicate statistical (systematic) uncertainties.

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Additional Figure 7:
The $M_\text {g}/p_\text {T}^\text {jet}$ dependence of systematic uncertainties for the $(0.1,0.0)$ SD setting for jets with 140 $ < p_\text {T}^\text {jet} < $ 160 GeV in an event in the centrality class 0-10%, expressed as percentages relative to the size of the mass spectrum at each $M_\text {g}/p_\text {T}^\text {jet}$ value.

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Additional Figure 8:
The $M_\text {g}/p_\text {T}^\text {jet}$ dependence of systematic uncertainties for the $(0.5,1.5)$ SD setting for jets with 140 $ < p_\text {T}^\text {jet} < $ 160 GeV in an event in the centrality class 0-10%, expressed as percentages relative to the size of the mass spectrum at each $M_\text {g}/p_\text {T}^\text {jet}$ value.

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Additional Figure 9:
The $M_\text {g}/p_\text {T}^\text {jet}$ dependence of systematic uncertainties for the $(0.1,0.0)$ SD setting for jets with 200 $ < p_\text {T}^\text {jet} < $ 300 GeV in an event in the centrality class 50-80%, expressed as percentages relative to the size of the mass spectrum at each $M_\text {g}/p_\text {T}^\text {jet}$ value.

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Additional Figure 10:
The $M_\text {g}/p_\text {T}^\text {jet}$ dependence of systematic uncertainties for the $(0.5,1.5)$ SD setting for jets with 200 $ < p_\text {T}^\text {jet} < $ 300 GeV in an event in the centrality class 50-80%, expressed as percentages relative to the size of the mass spectrum at each $M_\text {g}/p_\text {T}^\text {jet}$ value.
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