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| CMS-HIN-18-007 ; CERN-EP-2019-247 | ||
| Studies of charm quark diffusion inside jets using PbPb and pp collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 4 November 2019 | ||
| Phy. Rev. Lett. 125 (2020) 102001 | ||
| Abstract: The first study of charm quark diffusion with respect to the jet axis in heavy ion collisions is presented. The measurement is performed using jets with ${p_{\mathrm{T}}^{\text{jet}}} > $ 60 GeV/$c$ and ${\mathrm{D^0}}$ mesons with ${p_{\mathrm{T}}^{\text{D}}} > $ 4 GeV/$c$ in lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV, recorded by the CMS detector at the LHC. The radial distribution of ${\mathrm{D^0}}$ mesons with respect to the jet axis is sensitive to the production mechanisms of the meson, as well as to the energy loss and diffusion processes undergone by its parent parton inside the strongly interacting medium produced in PbPb collisions. When compared to Monte Carlo event generators, the radial distribution in pp collisions is found to be well-described by PYTHIA, while the slope of the distribution predicted by SHERPA is steeper than that of the data. In PbPb collisions, compared to the pp results, the ${\mathrm{D^0}}$ meson distribution for 4 $ < {p_{\mathrm{T}}^{\text{D}}} < $ 20 GeV/$c$ hints at a larger distance on average with respect to the jet axis, reflecting a diffusion of charm quarks in the medium created in heavy ion collisions. At higher ${p_{\mathrm{T}}^{\text{D}}}$, the PbPb and pp radial distributions are found to be similar. | ||
| Links: e-print arXiv:1911.01461 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Examples of ${\mathrm{D^0}}$ candidate invariant mass distributions in pp (left) and PbPb (right) collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. |
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Figure 1-a:
Example of ${\mathrm{D^0}}$ candidate invariant mass distribution in pp collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. |
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Figure 1-b:
Example of ${\mathrm{D^0}}$ candidate invariant mass distribution in PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. |
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Figure 2:
Distributions of ${\mathrm{D^0}}$ mesons in jets, as a function of the distance from the jet axis ($r$) for jets of $ {{p_{\mathrm {T}}} ^\mathrm {jet}} > $ 60 GeV/$c$ and $ {| {\eta ^\mathrm {jet}} |} < $ 1.6 measured in pp and PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The measurement is performed in the ${{p_{\mathrm {T}}} ^\mathrm {D}}$ range 4-20 GeV/$c$ (left) and $ {{p_{\mathrm {T}}} ^\mathrm {D}} > $ 20 GeV/$c$ (right). Each spectrum is normalized to its integral in the region 0 $ < r < $ 0.3. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The PbPb spectra are compared to the CCNU energy loss model [23], while the pp spectra are compared with predictions from the PYTHIA and SHERPA pp MC event generators. The ratios of the ${\mathrm{D^0}}$ meson radial distributions in PbPb and pp data are shown in the middle panels. In the bottom panels the ratios of the ${\mathrm{D^0}}$ meson radial distributions of pp over the two MC event generators are presented. |
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Figure 2-a:
Distribution of ${\mathrm{D^0}}$ mesons in jets, as a function of the distance from the jet axis ($r$) for jets of $ {{p_{\mathrm {T}}} ^\mathrm {jet}} > $ 60 GeV/$c$ and $ {| {\eta ^\mathrm {jet}} |} < $ 1.6 measured in pp and PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The measurement is performed in the ${{p_{\mathrm {T}}} ^\mathrm {D}}$ range 4-20 GeV/$c$. Each spectrum is normalized to its integral in the region 0 $ < r < $ 0.3. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The PbPb spectra are compared to the CCNU energy loss model [23], while the pp spectra are compared with predictions from the PYTHIA and SHERPA pp MC event generators. The ratios of the ${\mathrm{D^0}}$ meson radial distributions in PbPb and pp data are shown in the middle panels. In the bottom panels the ratios of the ${\mathrm{D^0}}$ meson radial distributions of pp over the two MC event generators are presented. |
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Figure 2-b:
Distribution of ${\mathrm{D^0}}$ mesons in jets, as a function of the distance from the jet axis ($r$) for jets of $ {{p_{\mathrm {T}}} ^\mathrm {jet}} > $ 60 GeV/$c$ and $ {| {\eta ^\mathrm {jet}} |} < $ 1.6 measured in pp and PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The measurement is performed for $ {{p_{\mathrm {T}}} ^\mathrm {D}} > $ 20 GeV/$c$. Each spectrum is normalized to its integral in the region 0 $ < r < $ 0.3. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The PbPb spectra are compared to the CCNU energy loss model [23], while the pp spectra are compared with predictions from the PYTHIA and SHERPA pp MC event generators. The ratios of the ${\mathrm{D^0}}$ meson radial distributions in PbPb and pp data are shown in the middle panels. In the bottom panels the ratios of the ${\mathrm{D^0}}$ meson radial distributions of pp over the two MC event generators are presented. |
| Summary |
| In summary, this Letter presents the first measurement of the radial distributions of ${\mathrm{D^0}}$ mesons with respect to the jet axis in lead-lead (PbPb) and proton-proton (pp) collisions, performed with the CMS detector using jets with transverse momentum ${p_{\mathrm{T}}^{\text{jet}}}> $ 60 GeV/$c$ and ${\mathrm{D^0}}$ mesons with ${p_{\mathrm{T}}^{\text{D}}}> $ 4 GeV/$c$. When compared to Monte Carlo event generators, the radial distribution in pp collisions is found to be well-described by PYTHIA, while the slope of the distribution predicted by SHERPA is steeper than that of the data. The modification of the ${\mathrm{D^0}}$ meson radial distributions in PbPb collisions are studied by comparing them to those from pp collisions. The comparisons hint at a modification of the ${\mathrm{D^0}}$ meson radial profile in PbPb collisions at low ${p_{\mathrm{T}}^{\text{D}}}$ that vanishes at higher ${p_{\mathrm{T}}^{\text{D}}}$. The results show that this modification is different from that of the light flavor hadrons. The low ${p_{\mathrm{T}}^{\text{D}}}$ result is indicative of charm quark diffusion in the medium created in heavy ion collisions. This measurement provides new experimental constraints on the mechanisms of heavy-flavor production in pp collisions, as well as on the processes of parton energy loss and diffusion of heavy quarks inside the quark-gluon plasma. |
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