Improving the dE/dx calibration of the STAR TPC for the high-pT hadron identification
We derive a method to improve particle identification (PID) at high transverse momentum ($p_T$) using the relativistic rise of the ionization energy loss ($rdE/dx$) when charged particles traverse the Time Projection Chamber (TPC) at STAR. Electrons triggered and identified by the Barrel Electro-Magnetic Calorimeter (BEMC), pure protons and pions from $Λ\to p+π^{-}$ ($\barΛ\to \bar{p}+π^{+}$), and $K^{0}_{S}\toπ^{+}+π^{-}$ decays are used to obtain the $dE/dx$ value and its width at given $βγ=p/m$. We found that the deviation of the $dE/dx$ from the Bichsel function can be up to $0.4σ$ ($\sim3%$) in p+p collisions at $\sqrt{s_{NN}}=200$ GeV taken and subsequently calibrated in year 2005. The deviation is approximately a function of $βγ$ independent of particle species and can be described with a function of $f(x) = A+\frac{B}{C+x^{2}}$. The deviations obtained with this method are used to re-calibrate the data sample from p+p collision for physics analysis of identified hadron spectra and their correlations up to transverse momentum of 15 GeV/$c$. The ratio of $e^{-}/e^{+}$ (dominantly from $γ$-conversion) is also used to correct the residual asymmetry in the negative and positive charged hadrons due to momentun distortion in the STAR TPC.