Speaker
Description
The QCD phase structure at finite temperature and density remains largely unexplored because of the sign problem. At purely imaginary chemical potential, however, lattice QCD simulations can be performed without this difficulty. In particular, at $\mu_q/T=i\pi/3$, the Roberge--Weiss (RW) transition occurs as a consequence of the $Z_3$ symmetry. The nature of the RW endpoint depends on the quark mass: it is a triple point in the heavy-quark region and becomes a $Z_2$ critical point as the quark mass decreases. A tricritical point is therefore expected to separate these two regimes.
We propose a new method for locating this tricritical point using the temperature derivative of the Binder cumulant. Conventional analyses based on the Binder cumulant itself suffer from sizable finite-volume effects near tricriticality due to logarithmic corrections. Our method incorporates these corrections and uses the logarithm of the ratio of Binder-cumulant derivatives at two different volumes. The resulting peak allows us to determine the critical temperature and the thermal critical exponent $y_t$ simultaneously. Since $y_t$ takes distinct values for first-order, tricritical, and $Z_2$ critical behavior, it provides a useful criterion for identifying the phase structure.
We apply this method to the RW endpoint in heavy-quark QCD using an effective action derived from the hopping-parameter expansion, including contributions up to N$^3$LO. The reduced computational cost of this approach enables simulations at large volumes with high statistics. Using these data and the new finite-size scaling analysis, we aim to determine the location of the tricritical point with improved systematic control and precision.