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| CMS-HIN-21-002 ; CERN-EP-2022-199 | ||
| Azimuthal anisotropy of dijet events in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 15 October 2022 | ||
| JHEP 07 (2023) 139 | ||
| Abstract: The path-length dependent parton energy loss within the dense partonic medium created in lead-lead collisions at a nucleon-nucleon center-of-mass energy of $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV is studied by determining the azimuthal anisotropies for dijets with high transverse momentum. The data were collected by the CMS experiment in 2018 and correspond to an integrated luminosity of 1.69 fb$^{-1}$. For events containing back-to-back jets, correlations in relative azimuthal angle and pseudorapidity ($ \eta $) between jets and hadrons, and between two hadrons, are constructed. The anisotropies are expressed as the Fourier expansion coefficients $v_n$, $ n = $ 2-4 of these azimuthal distributions. The dijet $v_n$ values are extracted from long-range (1.5 $ < |\Delta\eta| < $ 2.5) components of these correlations, which suppresses the background contributions from jet fragmentation processes. Positive dijet $v_2$ values are observed which increase from central to more peripheral events, while the $v_3$ and $v_4$ values are consistent with zero within experimental uncertainties. | ||
| Links: e-print arXiv:2210.08325 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; Physics Briefing ; CADI line (restricted) ; | ||
| Figures & Tables | Summary | Additional Figures | References | CMS Publications |
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| Figures | |
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Figure 1:
Illustration on how the long-range correlation distribution is constructed. The shape of the $ \Delta\varphi $ projection corresponding to the range 1.5 $ < |\Delta\eta| < $ 2.5 is determined from both leading and subleading jet-hadron correlation distributions for $ |\Delta\varphi| < \pi/ $ 2. The whole 2 $ \pi $ range for the $ \Delta\varphi $ distribution is obtained by combining these two components. |
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Figure 2:
The dijet $v_n$ data points factorized using different associated hadron $ p_{\mathrm{T}} $ bins for 0-10% (left), 10-30% (middle), and 30-50% (right) centrality bins. The data points are corrected for the jet reconstruction bias effects. The vertical bars represent statistical uncertainties, while the $ p_{\mathrm{T}} $-independent systematic uncertainties are plotted as shaded areas on the left side of the panels. |
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Figure 3:
Final dijet $v_2$ (left), $v_3$ (middle), and $v_4$ (right) results presented as functions of centrality. The dijet $v_2$ results are compared to CMS high-$ p_{\mathrm{T}} $ hadron $v_2$ results from Ref. [28]. The shaded areas represent systematic uncertainties and the vertical bars are the statistical uncertainties. |
| Tables | |
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Table 1:
The breakdown of different sources of systematic uncertainty for dijet $v_n$, separately for the three centrality bins considered in the analysis. |
| Summary |
| The Fourier coefficients $v_2$, $v_3$, and $v_4$ are determined for jets from events containing back-to-back jets (``dijet $v_n$'') in lead-lead collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV. The jet-hadron correlation technique used for this measurement has been developed to unambiguously separate jet fragmentation-related contributions from the long-range correlations due to the in-medium path length and medium density dependencies of parton energy loss. The dijet $v_2$ values are found to be positive, meaning that more jets are observed coplanar with the event plane than perpendicular to this plane. The dijet $v_2$ values increase with increasing eccentricity of the initial collision region, from about 2.0% in the 0-10% centrality bin to about 4.4% in the 30-50% centrality bin. These results are qualitatively consistent with expectations from a path-length dependence of in-medium energy loss. For all measured centrality bins, the dijet $v_3$ and $v_4$ values are consistent with zero within experimental uncertainties. Within the accuracy of this analysis, this shows that no significant modifications due to the initial state geometry and medium density fluctuations are seen in the jet azimuthal distributions. The measured dijet $v_n$ values provide valuable input to a more precise and quantitative description of the partonic energy loss in the quark-gluon plasma. |
| Additional Figures | |
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Additional Figure 1:
Dijet $ v_{2} $ results presented as a function of centrality. The shaded areas represent systematic uncertainties and the vertical bars are the statistical uncertainties. |
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Additional Figure 2:
Dijet $ v_{2} $, $ v_{3} $, and $ v_{4} $ results presented as functions of centrality. The shaded areas represent systematic uncertainties and the vertical bars are the statistical uncertainties. |
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