Curvature of the chiral phase transition line from the magnetic equation of state of (2+1)-flavor QCD
We analyze the dependence of the chiral phase transition temperature on baryon number and strangeness chemical potentials by calculating the leading order curvature coefficients in the light and strange quark flavor basis as well as in the conserved charge ($B, S$) basis. Making use of scaling properties of the magnetic equation of state (MEoS) and including diagonal as well as off-diagonal contributions in the expansion of the energy-like scaling variable that enters the parametrization of the MEoS, allows to explore the variation of $T_c(μ_B,μ_S) = T_c ( 1 - (κ_2^B \hatμ_B^2 + κ_2^S \hatμ_S^2 + 2κ_{11}^{BS} \hatμ_B \hatμ_S))$ along different lines in the $(μ_B,μ_S)$ plane. On lattices with fixed cut-off in units of temperature, $aT=1/8$, we find $κ_2^B=0.015(1)$, $κ_2^S=0.0124(5)$ and $κ_{11}^{BS}=-0.0050(7)$. We show that the chemical potential dependence along the line of vanishing strangeness chemical potential is about 10\% larger than along the strangeness neutral line. The latter differs only by about $3\%$ from the curvature on a line of vanishing strange quark chemical potential, $μ_s=0$. We also show that close to the chiral limit the strange quark mass contributes like an energy-like variable in scaling relations for pseudo-critical temperatures. The chiral phase transition temperature decreases with decreasing strange quark mass, $T_c(m_s)= T_c(m_s^{\rm phy}) (1 - 0.097(2) (m_s-m_s^{\rm phys})/m_s^{\rm phy}+{\cal O}((Δm_s)^2)$.