Improving the dE/dx calibration of the STAR TPC for the high-pT hadron identification

Abstract

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 $\Lambda \to p+{\pi }^{-}$ ($\bar{\Lambda }\to \bar{p}+{\pi }^{+}$), and $K_S^0\to {\pi }^{+}+{\pi }^{-}$ decays are used to obtain the $dE/dx$ value and its width at given $\beta \gamma =p/m$. We found that the deviation of the $dE/dx$ from the Bichsel function can be up to $0.4\sigma$ ($\sim 3$) in p+p collisions at $\sqrt{s_{NN}}=200$ GeV taken and subsequently calibrated in year 2005. The deviation is approximately a function of $\beta \gamma$ 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 $\gamma$-conversion) is also used to correct the residual asymmetry in the negative and positive charged hadrons due to momentun distortion in the STAR TPC.

18pages, 10 figures

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