1. Abhilash Nair STAR Collaboration University of Illinois at Chicago 1 STAR

2. Beam Energy Scan at RHIC STAR has collected data from the following collision energies (in decreasing energy order) : 200, 130, 62.4, 39, 19.6, 11.5, 9, 7.7 GeV This presentation aims to show the work-in-progress analysis of the 39 GeV data sample Spectra shapes and particle ratios help connect collected data to observables that are expected to change in such a phase transition 2 Properties of matter created at 200 GeV are consistent with an expected 2 nd order phase transition Beam Energy Scan (BES) at RHIC looks to experimentally find the critical point of 1 st order phase transition predicted to exist at lower energies Simply a cartoon, does not represent actual experiential data

3. Time Projection Chamber (TPC) The TPC is a barrel shaped detector consisting of a gas-filled chamber, magnet providing a strong uniform magnetic field, and an electric field between the center and endplates We use measurements of the ionization energy loss (dE/dx) and momentum to identify particles 3

4. Event Selection From Min-Bias Trigger |Vz| < 30 cm |Vr| < 2 cm 10 M events pass cuts Track-Quality Cuts |Dca| < 3 cm NHitFits > 25 NHitFits/NHitPoss > 0.51 4 Vertex X [cm] Vertex Z Vertex Y [cm]

5. 2-D Particle-Centered Histograms π K p 5 Where superscript BB represents the expectation of dE/dx for a given particle i = ( π, K, p ) dE/dx (Arb Units) p (GeV/c) ZπZπ pT (GeV) Zk pT (GeV/c) Zp pT (GeV/c)

6. Gaussian Peak Fitting & Electron Contamination π+π+ e+ K+ p π-π- e- K- p bar e-/e+ peak overlaps with nearby peaks in certain pT ranges During crossover, e-/e+ peak is not reliable without further constraints Without constraints, e-/e+ peak positions will jump sharply over a relatively small pT range Zk

7. Estimate of E-/E+ Contamination For overlap areas we rely on extrapolated centroid positions and yields This is done independently in each particle-centered histogram, and across different rapidity bins 7 π K p e pT (GeV/c) dE/dx [-0.1 < y < 0.1) e+ Centroids e+ Yields dE/dx STAR Preliminary p (Gev/c)

8. Uncorrected Yields vs. mT-m0 8 STAR Preliminary Arbitrary Units [-0.1 < y < 0.1) mT-m0 (GeV) π+π+ π-π- K+ K- p p bar

9. Uncorrected Antiparticle to Particle Ratios Cannot make a physics conclusions without corrections, which include: Single Particle Efficiency Energy Loss Correction Acceptance Proton Background 9 STAR Preliminary mT-m0 (GeV) π- / π+K- / K+p bar / p [-0.1 < y < 0.1) Efficiency, absorption, & background uncorrected Arbitrary Units

10. Proton Background: DCA Plots 10 200 MeV < pT < 300 MeV Phys. Rev. C 79 (2009) 34909 DCA plot courtesy of Alex Brown (UIC) Dca [cm] dN/d(dca) [cm^-1] STAR Preliminary

11. Future Outlook Our work-in-progress min-bias π+/π- ratio at 39 GeV is similar to results from 19 GeV & 200 GeV Plans for the future include: Efficiency Corrections Proton Background Energy Loss Corrections Investigate Rapidity Dependence K/Pi, p/Pi Integrated Yields & Ratios 11 STAR Preliminary mT-m0 (GeV) π+ / π- [-0.1 < y < 0.1) Efficiency, absorption, & background uncorrected Arbitrary Units