Abhilash NairSTAR Collaboration

University of Illinois at Chicago

1STARSTAR

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 expected2nd order phase transition

Beam Energy Scan (BES) at RHIC looks to experimentally find the critical point of 1st order phase transition predicted to exist at lower energies

Simply a cartoon, does not represent actual experiential data

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

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Event SelectionFrom Min-Bias Trigger |Vz| < 30 cm |Vr| < 2 cm10 M events pass cuts

Track-Quality Cuts |Dca| < 3 cmNHitFits > 25NHitFits/NHitPoss > 0.51

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GeV39NNAu Au s

Vertex X [cm]

Vertex Z

2-D Particle-Centered Histograms

π K p

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Where superscript BB represents the expectation of dE/dx for a given particle i = ( π, K, p )

GeV39NNAu Au s

p (GeV/c)

Zπ Zk Zp

Gaussian Peak Fitting & Electron Contamination

π+

e+K+p

π-

e-K-

pbar

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

GeV39NNAu Au s

Zk Zk

Zk

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

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GeV39NNAu Au s

π

K p

e

pT (GeV/c) pT (GeV/c)

[-0.1 < y < 0.1)[-0.1 < y < 0.1)

e+ Centroids e+ Yields

STAR Preliminary STAR Preliminary

p (Gev/c)

Uncorrected Yields vs. mT-m0

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STAR Preliminary STAR Preliminary

STAR Preliminary

STAR Preliminary STAR Preliminary

STAR Preliminary

GeV39NNAu Au s

[-0.1 < y < 0.1)

[-0.1 < y < 0.1)

[-0.1 < y < 0.1)

[-0.1 < y < 0.1)[-0.1 < y < 0.1)

[-0.1 < y < 0.1)

mT-m0 (GeV) mT-m0 (GeV) mT-m0 (GeV)

mT-m0 (GeV) mT-m0 (GeV) mT-m0 (GeV)

π+

π-

K+

K-

p

pbar

Uncorrected Antiparticle to Particle Ratios Cannot make a physics conclusions without corrections, which

include: Single Particle Efficiency Energy Loss Correction Acceptance Proton Background

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STAR PreliminarySTAR Preliminary STAR Preliminary

GeV39NNAu Au s

mT-m0 (GeV) mT-m0 (GeV) mT-m0 (GeV)

π- / π+ K- / K+ pbar / p

[-0.1 < y < 0.1) [-0.1 < y < 0.1) [-0.1 < y < 0.1)

Efficiency, absorption, &background uncorrected

Efficiency, absorption, &background uncorrected

Efficiency, absorption, &background uncorrected

Proton Background: DCA Plots

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200 MeV < pT < 300 MeV

Phys. Rev. C 79 (2009) 34909

Phys. Rev. C 79 (2009) 34909

DCA plot courtesy of Alex Brown (UIC)

Dca [cm]

STAR Preliminary

Future OutlookOur 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

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STAR Preliminary

mT-m0 (GeV)

π+ / π-

[-0.1 < y < 0.1)

Efficiency, absorption, &background uncorrected