Probing nuclear matter with jet conversions

We discuss the flavor of leading jet partons as a valuable probe of nuclear matter. We point out that the coupling of jets to nuclear matter naturally leads to an alteration of jet chemistry even at high transverse momentum $p_T$. In particular, quantum chromodynamics (QCD) jets coupling to a chemically equilibrated quark gluon plasma in nuclear collisions will lead to hadron ratios at high transverse momentum $p_T$ that can differ significantly from their counterparts in $p+p$ collisions. Flavor measurements could complement energy loss as a way to study interactions of hard QCD jets with nuclear matter. Roughly speaking they probe the inverse mean free path $1/\lambda$ while energy loss probes the average squared momentum transfer ${\mu }^2/\lambda$. We present some estimates for the rate of jet conversions in a consistent Fokker-Planck framework and their impact on future high-$p_T$ identified hadron measurements at RHIC and LHC. We also suggest some novel observables to test flavor effects.

Figure 1

Photon spectrum in $p+p$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV as function of transverse momentum. Data are taken from the PHENIX Collaboration [40].

Figure 2

Photon spectra in central Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV as functions of transverse momentum with (left panel) and without conversions (right panel). Direct photons are from initial hard collisions (solid line), final state jet fragmentation (dotted line), and jet-photon conversions (dashed line). Data are taken from PHENIX [42].

Figure 3

Nuclear modification factor $R_{\mathit{AA}}^{\gamma }$ of direct photons (left panel) and the double ratios of $r_{\gamma /{\pi }^{+}}=R_{\mathit{AA}}^{\gamma }/R_{\mathit{AA}}^{{\pi }^{+}}$ (middle panel) and $r_{p/{\pi }^{+}}=R_{\mathit{AA}}^p/R_{\mathit{AA}}^{{\pi }^{+}}$ in central Au+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV as functions of transverse momentum $p_T$. Results are with (solid line) or without (dotted line) conversions.

Figure 4

Elliptic flow $v_2^{\gamma }$ of direct photons in peripheral Au+Au collisions with centrality of 20–30% at $\sqrt{s_{\mathit{NN}}}=200$ GeV as functions of transverse momentum for two different scenarios of with (left panel) or without conversions (right panel), respectively. Data are taken from the PHENIX Collaboration [37].

Figure 5

Elliptic flow $v_2^{\gamma }$ of direct photons in peripheral Au+Au collisions with centrality of 30–40% (left) and 40–50% (right) at $\sqrt{s_{\mathit{NN}}}=200$ GeV as a function of transverse momentum.

Figure 6

$K_S^0$ (left panel) spectra from quark and gluon jet fragmentation via AKK fragmentation function [22] in $p+p$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. Data are from the STAR Collaboration [45]. Nuclear modification factor $R_{\mathit{AA}}$ (right panel) for $K_S^0$ in Au+Au collisions at same center-of-mass energy as a function of transverse momentum.

Figure 7

Photon spectra in central Pb+Pb collisions at $\sqrt{s_{\mathit{NN}}}=5.5$ TeV as functions of transverse momentum $p_T$ with (left panel) or without conversions (right panel).

Figure 8

Same quantities as those in Fig. 3 in central Pb+Pb collisions at $\sqrt{s_{\mathit{NN}}}=5.5$ TeV as functions of transverse momentum with or without conversions.

Figure 9

Elliptic flow $v_2^{\gamma }$ of direct photons in peripheral Pb+Pb collisions with centrality of 20–30% at $\sqrt{s_{\mathit{NN}}}=5.5$ TeV as functions of transverse momentum with (left panel) or without conversions (right panel).

Figure 10

Elliptic flow $v_2^{\gamma }$ of direct photons in peripheral Pb+Pb collisions with centrality of 30–40% (left) and 40–50% (right) at $\sqrt{s_{\mathit{NN}}}=5.5$ TeV as functions of transverse momentum.

Figure 11

Nuclear modification factor $R_{\mathit{AA}}$ for $K_S^0$ in Pb+Pb collisions at $\sqrt{s_{\mathit{NN}}}=5.5$ TeV as function of transverse momentum.