How Does STAR Identify Charged Hadrons at High Transverse Momentum?
By Yichun Xu and Zhangbu Xu
Study of identifying charged hadrons is important for almost every aspect of physics in the field of relativistic heavy ion collisions[1]. In the last few years the STAR collaboration has developed methods to identify charged hadrons at high transverse momentum (pT) at middle rapidity. Recently the identification of charged pions and protons in STAR has been pushed to pT~=15 GeV/c. In this article we provide the method developed for p+p collisions with data from the 2005 RHIC run. The central piece of the work is to understand the ionization energy loss (dE/dx) and its relativistic rise for a charged particle when traversing the Time Projection Chamber (TPC). Identified hadron spectra at high pT in p+p collisions provide a good test of perturbative quantum chromodynamics (pQCD) [2] and a baseline for studying the color charge effects of parton energy loss in heavy ion collisions [3].
Figure 1 dE/dx (keV/cm) distribution vs momentum (GeV/c). Both
axes are in log scale. A 1-σ band is drawn around the center of
the dE/dx for a given particle.
Fig.1 shows a typical dE/dx distribution including electron, pion, kaon and proton bands as a function of momentum. The dE/dx for a given particle at low momentum decreases with increasing momentum to reach a minimum ionization, then increases due to the relativistic rise. For a minimum ionizing particle (MIP) the dE/dx resolution in the STAR TPC is 6-8% for a track with the maximum of 45 sampled dE/dx points. The pions are well separated from the rest of the particles (e,K,p) at pT of 0.3 to 0.6 GeV/c. This clear separation has been used to calibrate the TPC dE/dx without other means of identification. It provides the fixed points for extrapolating the dE/dx function to higher momentum. In the thin material (TPC gas) the Bichsel function has proved to be a very good approximation for the dE/dx curves and has been adopted by STAR as a standard method of predicting the dE/dx value for charged hadrons in all momentum ranges [4]. We refer to dE/dx in the relativistic rise region as rdE/dx to distinguish it from that at the low momentum as it is clear that many features are different at low and high momentum.
It is observed that effects such as the saturation effect [4], the gains and noise of the TPC electronics, and event pileup in high luminosity environments may make the rdE/dx deviate from the Bichsel function predictions. In the relativistic rise region the rdE/dx separations among Π, K and p at a given momentum are about 1-3σ with the dE/dx amplitude of pions the highest and that of protons the lowest. Pions are the dominant particle species for inclusive and jet hadrons, and they shadow the kaons and protons in the dE/dx distribution. In a given momentum slice, clear peak separations of these three hadrons are not possible. This results in large systematic errors due to the uncertainty of the dE/dx positions. Knowledge of the precise dE/dx positions for those hadrons is important to understand the efficiencies of PID selection and to reduce the systematic uncertainty in the identified hadron yields. In order to improve the particle identification at high pT we developed a method to locate the rdE/dx positions for the different hadrons with high precision[5].
Figure 2 nSigma of dE/dx for e, π, k, p and their anti-particles
from EMC trigger electron enhanced events (the peak position is
plotted
offset by +-6σ for positively and negatively charged hadrons).
Although the electron dE/dx curve is relatively far away from those for the other charged particles, the electron yields are orders of magnitudes lower than the yields for pions. In order to identify the electron and obtain its rdE/dx position a dataset with a special trigger based on the energy deposited in the Electromagnetic Calorimeter (EMC) tower is used to enhance the yield of electrons relative to the other particles. Fig.2 shows the dE/dx distribution from this dataset with clear electron dE/dx peak well separated from the rest. In order to obtain the rdE/dx position of protons at the relativistic rise range, protons (anti-protons) are selected from Λ( Λ)through the Λ→p + π- (Λ→p+π+) decay mode. Similarly, decay pions from K0S through the K0S→π+ + π- mode can be used to get dE/dx and rdE/dx positions of pions for 0.2<pT<3 GeV/c. These results are shown in Fig.3.
Figure 3 dE/dx positions of pions and protons inferred from selected daughters of Ks and Lamda decays.
With identified pions, protons and electrons mentioned above, the experimental results on the deviation of the normalized dE/dx relative to the Bichsel theoretical values, as a function of Βγ are shown on Fig. 4. Since there is almost no particle species dependence of dE/dx, we can describe it with a function f(x) = A + B/(C+x2) . With this we can determine the dE/dx positions and width of any given charged particles to better than < 0.1σ or < 1%. The corrected separations of dE/dx among particles relative to pions are plotted as a function of pT in Fig.5.
Figure 4 dE/dx deviation from Bischel function as function of Βγ.
Figure 5 dE/dx separation in the standard deviation
among charged hadrons (relative to pions) as a function of pT.
In summary, with enhanced electrons using the BEMC triggered events, pure pions decayed from K0S and protons decayed from Λ, rdE/dx positions and widths of different charged particles are determined precisely. Their deviations relative to theoretical values versus Βγ and pT /mass are described by the function f(x) = A + B/(C+x2) very well. With this method, rdE/dx positions of charged particles are re-calibrated to be better than 0.1σ. The particle identification of charged hadrons is thus improved, and the uncertainty is reduced significantly. These are important steps toward fulfilling the physics goals of the STAR experiment at RHIC in the future.
References:
[1] J. Adams et al., Nucl. Phys. A 757 (2005) 102
[2] J. Adams et al., Phys. Lett. B 637 (2006) 161
[3] B.I. Abelev et al., Phys.Rev.Lett. 97 (2006) 152301;
Q. Wang and X.N. Wang, Phys.Rev.C 71 (2005) 014903
[4] H. Bichsel, Nucl.Instrum.Meth.A 562 (2006) 154-197
[5] Y.C. Xu et al., arXiv:0807.4303
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