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CMS-HIN-20-003 ; CERN-EP-2022-190
Search for medium effects using jets from bottom quarks in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV
Phys. Lett. B 844 (2023) 137849
Abstract: The first study of the shapes of jets arising from bottom (b) quarks in heavy ion collisions is presented. Jet shapes are studied using charged hadron constituents as a function of their radial distance from the jet axis. Lead-lead (PbPb) collision data at a nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV were recorded by the CMS detector at the LHC, with an integrated luminosity of 1.69 nb$^{-1}$. Compared to proton-proton collisions, a redistribution of the energy in b jets to larger distances from the jet axis is observed in PbPb collisions. This medium-induced redistribution is found to be substantially larger for b jets than for inclusive jets.
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Figure 1:
Upper row: jet shape distributions $ \rho(\Delta r) $ for inclusive (solid red squares) and b jets (solid blue circles) with $ p_{\mathrm{T}} > $ 120 GeV in three centrality bins (left to right) for PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV, as well as in pp collisions (open red and blue symbols) from Ref. [16], which are identical in all three panels. Second row: ratio of PbPb to pp jet shape results for inclusive (red) and b jets (blue). Third row: the difference between the charged-particle transverse momentum profile, defined by Eq. 1, between PbPb and pp collisions for 1 $ < p_{\mathrm{T}}^{\text{trk}} < $ 4 GeV for inclusive (red) and b jets (blue). Lower row: ratio of b to inclusive jet shapes for several PbPb centrality bins (green), as well as pp collisions from Ref. [16] (grey), which are identical in all three panels. In all panels, the vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively.

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Table 1:
Systematic uncertainties in percentage for jet shape measurements for the various sources and collision centralities. Columns correspond to different centrality selections as marked. Where an uncertainty range is given, the upper edge of the range corresponds to the bin with the smallest $ p_{\mathrm{T}}^{\text{trk}} $ values. The sources from the decontamination and tagging bias are exclusive for b jets.
In summary, the first jet shape measurement for bottom quark (b) jets in lead-lead (PbPb) collisions at a nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV are presented. A jet-track correlation technique was used with b jets of $ p_{\mathrm{T}} > $ 120 GeV and charged particles of transverse momenta $ p_{\mathrm{T}}^{\text{trk}} > $ 1 GeV to measure jet shapes as a function of radial distance $ \Delta r $ between the jet axes and particle tracks. The measured b jet shapes show a depletion of transverse momenta at small radial distances from the jet axis compared to inclusive jet shapes. This depletion is already present in proton-proton (pp) events at the same center-of-mass energy and does not notably change looking at PbPb collisions at different centralities. This observation may provide a quantitative measurement of the expected dead-cone effect for b jets. Comparisons of jet shapes from PbPb and pp data show that the presence of the quark-gluon plasma modifies the energy flow around b jets. These modifications include a depletion of the transverse momenta in the range of about $ \Delta r \lesssim $ 0.4 and an enhancement at larger radial distances from the jet axis. Furthermore, the large $ \Delta r $ enhancement is found to be greater than that previously reported for inclusive jets in PbPb collisions. This observation is consistent with an increased medium response to the propagation of a heavier quark. The enhancement is centrality dependent and is largest in the most central collisions. This measurement provides new constraints for theoretical calculations of parton flavor dependence of energy loss and jet-medium interactions in the quark-gluon plasma.
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