Speaker
Description
The spin alignment of mesons has been a popular subject in relativistic heavy-ion collisions . It is only until last year that non-vanishing spin alignment of $D^{*+}$ meson was measured. We propose that through the QCD analogue of the Sokolov-Ternov effect, magnetic synchrotron radiation of gluons can polarize the $c$ and $\bar d$ quarks and result in spin alignment of $D^{*+}$ mesons. The spectator magnetic field is intense but short-lived. For single-gluon emission, the polarization time is much longer than the magnetic-field lifespan. Only a small fraction of quarks are therefore polarized, and the resulting spin-alignment magnitude remains small. Nevertheless, the quark polarization from single-gluon synchrotron radiation gives the experimentally observed sign of $D^{*+}$ spin alignment at high $p_T$.
We then go on to find that that the polarization can be enhanced when partonic cascade is included. Emitted gluons can split into $q\bar q$ pairs, whose constituents undergo further synchrotron radiation and emit additional gluons. This cascade multiplies the radiative and spin-flip channels, shortens the polarization time, and allows more quarks to become polarized within the magnetic-field lifespan. The quark polarization is then transferred to $D^{*+}$ during hadronization, suggesting that magnetic field can have nontrivial contribution to $D^{*+}$ spin alignment.
