Improving the calibration of the STAR TPC for the high- 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 ( dE / 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 (anti-protons) and pions from Λ ( Λ ¯ ), and K S 0 decays are used to obtain the dE / dx value and its width at given β γ = p / m . We found that the deviation of the dE / dx from the Bichsel function can be up to 0.4 σ ( ∼ 3 % ) in p + p collisions at s NN = 200 GeV taken and subsequently calibrated in year 2005. The deviation is approximately a function of β γ independent of particle species and can be described with the function f ( x ) = A + B / ( C + x 2 ) . The deviations obtained with this method are used in 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 for the residual momentum distortion in the STAR TPC.