Searching for missing direct photons in heavy-ion collisions with P and CP violation

TL;DR

This work develops a first-principles framework to compute synchrotron photon emission in a magnetized plasma with chiral imbalances, by solving the modified Dirac equation with $b_0$ and $b_3$ and constructing exact Landau-level spinors. Using these spinors, the authors compute single-particle and plasma photon emission, finding that nonzero $b_0$ or $b_3$ modestly increases the total photon yield while significantly reducing the elliptic flow coefficient $v_2$, potentially addressing the missing direct photons puzzle in heavy-ion collisions. They also derive the chiral magnetic current from the same spinors and confirm that the current depends on $b_0$ but is independent of $b_3$ in the explored regime. The results, anchored to a massive quark scenario with $m\approx 300$ MeV and $|eB|\approx m_\pi^2$, suggest that chirality-imbalanced QGP could reconcile photon yields and anisotropies, with rotation and further generalizations offering avenues for future work.

Abstract

We compute synchrotron radiation from a plasma in which $P$- and $CP$-violating parameters, a chiral chemical potential and a chiral gradient, couple to fermions. To do this, we compute exact wavefunctions for the fermions in the presence of these parameters and an external constant magnetic field. We find that these parameters increase the synchrotron radiation emitted by the fermions while also decreasing the traditionally large synchrotron radiation elliptic flow coefficient $v_2$. We apply these results to the quark-gluon plasma, where just such a contribution could provide a solution to the missing direct photons puzzle. We also use our wavefunctions to give a derivation of the chiral magnetic effect.

Figures (4)

• Figure 1: The single-particle radiation intensity due to a magnetic field $qB = 0.01 m^2 = 900$ MeV$^2$. Left: The chiral chemical potential $\mu_5 = b_0$ is varied while $b_3 = 0$ is fixed. Right: $\mu_5 = b_0 = 3$ MeV is fixed while $b_3$ varies. The blue curve in the left figure is the $b_0 = b_3 = 0$ result, well-approximated by standard semiclassical computations Buzzegoli:2023vne. Differences in $s-$ and $u-$helicity have been made apparent.
• Figure 2: The same as Figure \ref{['fig:SP']}, but for various values of $b_3$ with $\mu_5 = b_0 = 3$ MeV. The difference due to the sign of $b_3$ is evident.
• Figure 3: $v_0$ and $v_2$ for the photon spectrum of a plasma of up and down quarks with temperature $T = m = 300$ MeV. $b_0 = \mu_5$ is a chiral chemical potential. The magnetic field is $|e|B = m_\pi^2 = 18000$ MeV$^2$. We have estimated $L = c \Delta t = 10$ fm. The red points are 20-40% centrality results from the PHENIX collaboration PHENIX:2014nkkPHENIX:2022rsxPHENIX:2015iglPHENIX:2025ejr.
• Figure 4: The same as Fig. \ref{['fig:PlasmaUpDown']}, but with both $b_0 = \mu_5$ and $b_3$ turned on. Left: $b_0 = 3$ MeV is fixed. Right: $b_3 = 3$ MeV is fixed. We have estimated $L = c \Delta t = 10$ fm. The red points are 20-40% centrality results from the PHENIX collaboration PHENIX:2014nkkPHENIX:2022rsxPHENIX:2015iglPHENIX:2025ejr.