Light and heavy flavor dijet production and dijet mass modification in heavy ion collisions

Back-to-back light and heavy flavor dijet measurements are promising experimental channels to accurately study the physics of jet production and propagation in a dense QCD medium. They can provide new insights into the path length, color charge, and mass dependence of quark and gluon energy loss in the quark-gluon plasma produced in reactions of ultrarelativistic nuclei. To this end, we perform a comprehensive study of both light and heavy flavor dijet production in heavy ion collisions. We propose the modification of dijet invariant mass distributions in such reactions as a novel observable that shows enhanced sensitivity to the quark-gluon plasma transport properties and heavy quark mass effects on in-medium parton showers. This is achieved through the combination of the jet quenching effects on the individual jets as opposed to their subtraction. The latter drives the subtle effects on more conventional observables, such as the dijet momentum imbalance shifts, which we also calculate here. Results are presented in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ for comparison to data at the Large Hadron Collider and in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ to guide the future sPHENIX program at the Relativistic Heavy Ion Collider.

Figure 1

Comparison of dijet mass distributions between Pythia 8 simulations and experimental measurements in $p+p$ collisions at the LHC at $\sqrt{s}=7\mathrm{TeV}$. The left is for inclusive dijets from the CMS Collaboration [32], while the right is for $b$-tagged dijets from the ATLAS Collaboration [33].

Figure 2

Double differential cross sections weighted by transverse momenta for $b$-tagged (left) and inclusive (right) dijet production in $p+p$ collisions at $\sqrt{s}=5.02\mathrm{TeV}$. Kinematic cuts are implemented in our simulations as in CMS measurements; see Ref. [25]. The roughness of the $b$-tagged dijet cross section relative to that for inclusive dijets is due to the inherently lower statistics.

Figure 3

Double differential cross sections weighted by transverse momenta for $b$-tagged (left) and inclusive (right) dijet production in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$. Kinematic cuts implemented in our simulations are the same as those from the sPHENIX Collaboration [34]. Here again, the slight bumpiness of the $b$-tagged dijet cross section is due to its lower statistics relative to the inclusive case.

Figure 4

The fractional contributions of different subprocesses to the $b$-dijet production cross sections vs leading jet $p_T^{\mathrm{L}}$ (left) and subleading jet $p_T^{\mathrm{S}}$ (right) in $p+p$ collisions at $\sqrt{s}=5.02\mathrm{TeV}$. Kinematic cuts are implemented in our simulations as in CMS measurements [25].

Figure 5

The fractional contributions of different subprocesses to the $b$-dijet production cross sections vs leading $p_T$ (left) and subleading $p_T$ (right) in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$. Kinematic cuts implemented in our simulations are the same as those from the sPHENIX Collaboration [34].

Figure 6

The fractional contributions of different subprocesses to the inclusive dijet production cross sections vs leading $p_T$ (left) and subleading $p_T$ (right) in $p+p$ collisions at $\sqrt{s}=5.02\mathrm{TeV}$. Kinematic cuts are implemented in our simulations as in CMS measurements [25].

Figure 7

The fractional contributions of different subprocesses to the inclusive dijet production cross sections vs leading $p_T$ (left) and subleading $p_T$ (right) in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$. Kinematic cuts implemented in our simulations are the same as those from the sPHENIX Collaboration [34].

Figure 8

Mass distributions (top) and their ratios (bottom) for $b$-tagged (left) and inclusive (right) dijet production in $p+p$ collisions at $\sqrt{s}=5.02\mathrm{TeV}$. Kinematic cuts are implemented in our simulations as in CMS measurements [25]. The upper panels display black histograms representing $d\sigma /d{m}_{12}$ simulated directly from Pythia 8, while the red histograms are $d\sigma /d{m}_{12}$ computed using Eq. (6) and Pythia 8-simulated $d\sigma /d{p}_{1T}d{p}_{2T}$. In the lower panels the Pythia calculation is given by the black lines and the red histograms represent the ratio of the approximate formula to the baseline.

Figure 9

Mass distributions (top) and their ratios (bottom) for $b$-tagged (left) and inclusive (right) dijet production in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$. Kinematic cuts implemented in our simulations are the same as those from the sPHENIX Collaboration [34]. The upper panels display black histograms representing $d\sigma /d{m}_{12}$ simulated directly from Pythia 8, while the red histograms are $d\sigma /d{m}_{12}$ computed using Eq. (6) and Pythia 8-simulated $d\sigma /d{p}_{1T}d{p}_{2T}$. In the lower panels the Pythia calculation is given by the black lines and the red histograms represent the ratio of the approximate formula to the baseline.

Figure 10

Nuclear modification factor for $b$-tagged (left) and inclusive (right) dijet production in $p+p$ collisions at $\sqrt{s}=5.02\mathrm{TeV}$. Kinematic cuts are implemented in our simulations as in CMS measurements [25].

Figure 11

Nuclear modification factor for $b$-tagged (left) and inclusive (right) dijet production in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$. Kinematic cuts implemented in our simulations are the same as those from the sPHENIX Collaboration [34].

Figure 12

Nuclear modification factor $R_{AA}$ is plotted as a function of dijet invariant mass $m_{12}$ for inclusive (left) and $b$-tagged (right) dijet production in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ at the LHC. Left: The band corresponds to a range of coupling strength between the jet and the medium: $g_{\mathrm{med}}=1.8–2.0$, respectively. Right: We fix $g_{\mathrm{med}}=1.8$, and the band corresponds to a range of masses of the propagating system between $m_b$ and $2m_b$.

Figure 13

Nuclear modification factor $R_{AA}$ plotted as a function of dijet invariant mass $m_{12}$ for inclusive (left) and $b$-tagged (right) dijet production in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ for sPHENIX at RHIC. Left: The band corresponds to a range of coupling strength between the jet and the medium: $g_{\mathrm{med}}=2.0–2.2$, respectively. Right: We fix $g_{\mathrm{med}}=2.0$, and the band corresponds to a range of masses of the propagating system between $m_b$ and $2m_b$.

Figure 14

Ratios of nuclear modification factors for $b$-tagged ($R_{AA}^{bb}$) vs inclusive ($R_{AA}^{jj}$) dijet production for CMS (left) and sPHENIX (right) are plotted as a function of dijet invariant mass $m_{12}$. For LHC (sPHENIX) energies, we choose $g_{\mathrm{med}}=1.8(2.0)$. For $b$-tagged dijets, the mass of the propagating system is held fixed at $m_b$.

Figure 15

The dijet imbalance $z_J$ distributions for inclusive (left) and $b$-tagged (right) dijet production at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ for CMS at the LHC. The black histogram is the result for $p+p$ collisions, while the colored curves are the results for central (0%–10%) $\mathrm{Pb}+\mathrm{Pb}$ collisions. Left: The band corresponds to a range of coupling strengths between the jet and the medium: $g_{\mathrm{med}}=1.8–2.0$, respectively. Right: We fix $g_{\mathrm{med}}=1.8$, and the band corresponds to a range of masses of the propagating system between $m_b$ and $2m_b$. The experimental data are from the CMS Collaboration [25].

Figure 16

The dijet imbalance $z_J$ distributions for inclusive (left) and $b$-tagged (right) dijet production at $\sqrt{s_{NN}}=200\mathrm{GeV}$ for sPHENIX at RHIC. The black histogram is the result for $p+p$ collisions, while the colored curves are the results for central (0%–10%) $\mathrm{Au}+\mathrm{Au}$ collisions. The blue “data” points are from preliminary simulations carried out by the sPHENIX Collaboration [34]. Left: The band corresponds to a range of coupling strengths between the jet and the medium: $g_{\mathrm{med}}=2.0–2.2$, respectively. Right: We fix $g_{\mathrm{med}}=2.0$, and the band corresponds to a range of masses of the propagating system between $m_b$ and $2m_b$.