Spectrometer Based Ratio Analysis Technique Discussion of Corrections Absorption Correction – As the collision products pass through the detector, some of them are absorbed. This results in a loss of anti-particles versus particles and a decrease in the anti-particle to particle ratio. Correction is obtained by studying the effect of hadronic interactions in the detector using Hijing. Feed-down Correction – Unstable primary collision product decays produces feed- down. Feed-down particles with sufficient hits in the spectrometer are reconstructed using tracking, affecting the ratios. Secondary Corrections – As the primary collision products pass through the beam pipe and detector materials, secondary particles are produced. Those which pass through the spectrometer may be reconstructed along with the primary particles. The effect of feed-down and secondaries is minimal in pions and kaons compared to protons. Vasundhara Chetluru University Of Illinois at Chicago Beam-Orbit Study Beam-orbit – Mean reconstructed vertex position of the collisions in the transverse plane for a given run. Steady beam-orbit ensures acceptance and efficiency cancellation for different polarities. Plan…. Extract anti-particle to particle primary ratios for pions, kaons and protons for low momentum (0.1<p T <0.8). Examine the ratios for Cu+Cu (64.2GeV, 200GeV) & Au+Au (64.2GeV, 200GeV) as a function of Centrality Reaction Plane Low p T ratios from spectrometer will be used to compliment results from a time-of-flight (TOF) based analysis at higher momentum (0.5<p T <3.0). The analysis techniques for determining antiparticle to particle ratios using the particle tracking and identification capabilities of the PHOBOS silicon spectrometer are presented. PHOBOS Spectrometer The PHOBOS detector has a two arm magnetic spectrometer at midrapidity, consisting of 16 planes of highly segemented silicon pads which makes the particle identification possible. Outer layers situated in 2T magnetic field. Oppositely charged particles have different bending directions for a given field polarity, hence different acceptance. PHOBOS magnet polarity is changed every couple of days. Independent measurements are taken for each polarity. near mid-rapidity Z Field Polarity: B -  - -  + +  - -  + + near mid-rapidity Z Field Polarity: B +  + +  + +  - -  - - Bending away from beam pipe: h - B +, h + B - Tracking in the Spectrometer Tracking within 10 cm of interaction point. Road-following algorithm finds straight tracks in the field-free region. Curved tracks in B-field found by clusters in (1/p,  ) space. Match pieces by , consistency in dE/dx and fit in yz-plane. Covariance matrix track fit for momentum reconstruction and ghost rejection. Energy Loss in the Silicon Ratios are obtained by dividing the normalized yields of oppositely charged particles in different magnetic polarities, so that the acceptance and efficiency cancel. For example, positive particles measured with negative magnet polarity are compared to negative particles measured with positive magnet polarity so that both bend in same direction and therefore have exactly the same trajectories. The probability of interaction is statistical and can be characterized by the average amount of energy lost per unit path length, dE/dx. Experimentally dE/dx is measured in units of minimum ionizing particles, MIPS. A MIP is defined as the minimum value of the dE/dx for a given material and is applicable to particles traveling at relativistic velocities, ≥ 0.9c. Particles studied have momenta below the “relativistic rise”. Data over the whole run range is classified depending on shifts in the beam-orbit. Ratios are calculated independently for each steady beam-orbit region. Preliminary results from PHOBOS, TOF based analysis. The hits that particles produce provide both momentum information (determined from the position of the hit) and energy loss information (determined from the ionization produced by the particle). The different energy loss characteristics of pions, kaons, and protons can be used conjointly with momentum to identify the particle type of a track. Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane,Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Constantin Loizides, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, Bolek Wysłouch ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER Bending toward beam pipe: h - B -, h + B + D±D± p±p± K±K± ±± e±e± PID Techniques PID Cuts Corrected Bethe-Bloch Function Technique Alternate Technique Taking the ratio of the PID distribution and Bethe-Bloch (m=const ) line. Gives a linear distribution of different PID bands centered around 1. Black - CuCu200Gev Data Color – Theoretical Bethe-Bloch Predictions Proton Band Sigma for each particle band is obtained. Plot PID bands from the corrected Bethe- Bloch function PID cuts are obtained by plotting sigma for given P around the local maxima for given PID band dE/dx slice for Momentum=0.5 Bin Local mean and sigma dE/dx dist for each momentum slice is obtained