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| CMS-HIN-18-018 ; CERN-EP-2020-019 | ||
| Measurement of quark- and gluon-like jet fractions using jet charge in PbPb and pp collisions at 5.02 TeV | ||
| CMS Collaboration | ||
| 1 April 2020 | ||
| JHEP 07 (2020) 115 | ||
| Abstract: The momentum-weighted sum of the electric charges of particles inside a jet, known as jet charge, is sensitive to the electric charge of the particle initiating the parton shower. This paper presents jet charge distributions in ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5.02 TeV lead-lead (PbPb) and proton-proton (pp) collisions recorded with the CMS detector at the LHC. These data correspond to integrated luminosities of 404 $\mu$b$^{-1}$ and 27.4 pb$^{-1}$ for PbPb and pp collisions, respectively. Leveraging the sensitivity of the jet charge to fundamental differences in the electric charges of quarks and gluons, the jet charge distributions from simulated events are used as templates to extract the quark- and gluon-like jet fractions from data. The modification of these jet fractions is examined by comparing pp and PbPb data as a function of the overlap of the colliding Pb nuclei (centrality). This measurement tests the color charge dependence of jet energy loss due to interactions with the quark-gluon plasma. No significant modification between different centrality classes and with respect to pp results is observed in the extracted fractions of quark- and gluon-like jet fractions. | ||
| Links: e-print arXiv:2004.00602 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
(Upper) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for the $ {p_{\mathrm {T}}}$-weighting factor $\kappa = $ 0.5 and a minimum track ${p_{\mathrm {T}}}$ of 1 GeV. (Lower) Ratio of the jet charge measurements to the results of template fits. |
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Figure 2:
The standard deviation of the jet charge distributions with different track ${p_{\mathrm {T}}}$ thresholds and $\kappa $ values for pp collisions and in the various event centrality bins for PbPb collisions compared with the PYTHIA 6 prediction. The systematic and statistical uncertainties in the standard deviation measurements are shown by the shaded regions and vertical bars, respectively. |
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Figure 3:
Fitting results for the extraction of gluon-like jet fractions in pp and PbPb data shown for different track ${p_{\mathrm {T}}}$ threshold values and event centrality bins in PbPb collisions. The systematic and statistical uncertainties are represented by the shaded regions and vertical bars, respectively. The predictions for the gluon jet fractions from PYTHIA 6 are shown in dashed red lines. |
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Figure 4:
Fitting results for the extraction of gluon-like jet fractions in pp and PbPb data shown for ${p_{\mathrm {T}}}$-weighting factor $\kappa $ values of 0.3, 0.5, and 0.7 in different event centrality bins in PbPb. The markers for track $ {p_{\mathrm {T}}} > $ 1 and $ > $2 GeV have been separated horizontally for clarity. The systematic and statistical uncertainties are represented by the shaded regions and vertical bars, respectively. The predictions for the gluon jet fractions from PYTHIA 6 are shown in dashed red lines. |
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Figure 5:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for $\kappa = $ 0.5 and a minimum track ${p_{\mathrm {T}}}$ of 2, 4, and 5 GeV (top, middle, and bottom, respectively). (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
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Figure 5-a:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for $\kappa = $ 0.5 and a minimum track ${p_{\mathrm {T}}}$ of 2 GeV. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
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Figure 5-b:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for $\kappa = $ 0.5 and a minimum track ${p_{\mathrm {T}}}$ of 4 GeV. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
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Figure 5-c:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for $\kappa = $ 0.5 and a minimum track ${p_{\mathrm {T}}}$ of 5 GeV. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
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Figure 6:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for a minimum track ${p_{\mathrm {T}}}$ of 1 GeV and a $\kappa $ value of 0.3, 0.5, and 0.7 (top, middle, and bottom, respectively). (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
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Figure 6-a:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for a minimum track ${p_{\mathrm {T}}}$ of 1 GeV and a $\kappa $ value of 0.3. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
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Figure 6-b:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for a minimum track ${p_{\mathrm {T}}}$ of 1 GeV and a $\kappa $ value of 0.5. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
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Figure 6-c:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for a minimum track ${p_{\mathrm {T}}}$ of 1 GeV and a $\kappa $ value of 0.7. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
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Figure 7:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for a minimum track ${p_{\mathrm {T}}}$ of 2 GeV and a $\kappa $ value of 0.3, 0.5, and 0.7 (top, middle, and bottom, respectively). (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
png pdf |
Figure 7-a:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for a minimum track ${p_{\mathrm {T}}}$ of 2 GeV and a $\kappa $ value of 0.3. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
png pdf |
Figure 7-b:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for a minimum track ${p_{\mathrm {T}}}$ of 2 GeV and a $\kappa $ value of 0.5. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
png pdf |
Figure 7-c:
(Upper row of each figure) Unfolded jet charge measurements shown for inclusive jets in data along with the extracted fractions of up, and down quark jets, gluon jets, and the "other flavor" jets. The systematic and statistical uncertainties in the distributions are shown by the shaded regions and vertical bars, respectively. The jet charge measurements shown here are for a minimum track ${p_{\mathrm {T}}}$ of 2 GeV and a $\kappa $ value of 0.7. (Lower row of each figure) Ratio of the jet charge measurements to the results of template fits. |
| Tables | |
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Table 1:
Relative systematic uncertainties in percentage for the measurements of gluon-like jet fractions in pp and PbPb events. The PbPb results are given in intervals of centrality. When an uncertainty range is given, the range of the values are the maximum variation in the fractions for different selections on $\kappa $ and track ${p_{\mathrm {T}}}$ threshold values. |
| Summary |
| Jet charge, defined as the momentum-weighted sum of the electric charges of particles inside a jet, is measured for the first time in heavy ion collisions and is presented along with pp results at the same energy. The analysis uses lead-lead (PbPb) and proton-proton (pp) collision data collected with the CMS detector at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The unfolded jet charge distributions, measured using the jet constituents with transverse momentum ${p_{\mathrm{T}}} > $ 1 GeV for jets having ${p_{\mathrm{T}}} > $ 120 GeV and pseudorapidity $|{\eta}| < $ 1.5, are presented. The widths of the jet charge distributions for pp collisions are in good agreement with predictions from the event generator PYTHIA{6} and are shown to be independent of PbPb collision centrality. The jet charge distributions for quark- and gluon-initiated jets from PYTHIA{6} events are used as fitting templates to estimate the respective contributions in the measured jet samples. The gluon-like jet fractions extracted from these template fits are found to be similar between pp data and all studied PbPb centrality ranges. These are the first measurements in heavy ion collisions which exploit the electric charge of the initiating parton to discriminate between quark and gluon jets. No evidence is seen for a significant decrease (increase) in gluon-like (quark-like) prevalence in a sample of jets with ${p_{\mathrm{T}}} > $ 120 GeV in PbPb collisions. These observations do not support recent interpretations of other heavy ion results [11,12], which are based on a decreased (increased) gluon (quark) fraction caused by color-charge dependent jet quenching. |
| Additional Figures | |
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Additional Figure 1:
The standard deviation of the jet charge distributions with different track $p_{\mathrm{T}}$ cuts in the 0-10% most central bin for PbPb collisions compared to predictions from PYTHIA6 and PYQUEN (Collisional and Radiational energy loss models) with $p_{\mathrm{T}}$ weighting factor $\kappa = $ 0.5. |
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Additional Figure 2:
Response matrices derived from PYTHIA used in unfolding the PbPb data jet charge measurements for background fluctuations shown for different event centrality bins with $p_{\mathrm{T}}$ weighting factor $\kappa = $ 0.5 and track $p_{\mathrm{T}} > $ 2 GeV. |
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Additional Figure 3:
Response matrices derived from PYTHIA used in unfolding the data jet charge measurements for reconstruction effects shown for different event centrality bins with $p_{\mathrm{T}}$ weighting factor $\kappa = $ 0.5 and track $p_{\mathrm{T}} > $ 2 GeV. |
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