Measurement of photons via conversion pairs with the PHENIX experiment at RHIC - Torsten Dahms - Master of Arts – Thesis Defense Stony Brook University January 20, 2005

01/20/2005Torsten Dahms - SUNY Stony Brook2 Outline Introduction –QGP and RHIC –Thermal Photons –Concept of new method The PHENIX experiment Analysis –Electron Identification –Extraction of beam pipe conversions –Monte Carlo Simulation –Corrections –Systematic errors –Cocktail comparison Result –Photon p T spectrum and ratio –Summary

01/20/2005Torsten Dahms - SUNY Stony Brook3 Introduction – Quark-Gluon Plasma New state of matter Deconfined quarks and gluons Phase shift above T c ≈170 MeV Increase in energy density State of matter in the early stage of the universe RHIC was built to produce and analyze QGP If QGP is produced what is its temperature? F. Karsch, Lect. Notes Phys. 583, 209 (2002)

01/20/2005Torsten Dahms - SUNY Stony Brook4 Thermal Photons Carry information about initial temperature Do not interact strongly  unaffected from final state effects Emitted from QGP like black body radiation Production mechanisms: –quark-gluon Compton scattering: –quark-antiquark annihilation: –Bremsstrahlung Other sources of direct photons are produced in initial hard scattering processes

01/20/2005Torsten Dahms - SUNY Stony Brook5 What do the Theorists say? Thermal photons dominate photon spectrum at 1-3 GeV/c S. Turbide, R. Rapp and C. Gale, Phys. Rev. C 29, (2004)

01/20/2005Torsten Dahms - SUNY Stony Brook6 Measurements of Direct Photons in heavy ion collisions CERN WA98 at SPSPHENIX New method: reconstruct photons from beam pipe conversions M. M. Aggarwal et al. [WA98 Collaboration], Phys. Rev. Lett. 93, (2004) J. Frantz [PHENIX Collaboration], J. Phys. G 30, (2004)

01/20/2005Torsten Dahms - SUNY Stony Brook7 Relativistic Heavy Ion Collider

01/20/2005Torsten Dahms - SUNY Stony Brook8 The PHENIX experiment

01/20/2005Torsten Dahms - SUNY Stony Brook9 Analyzed dataset: Au+Au Event criteria: –Minimum Bias event (BBC north and south fired both at least two phototubes) –|BBC z vtx | ≤ 25 cm 31.4 million events passed these criteria

01/20/2005Torsten Dahms - SUNY Stony Brook10 Electron identification –0.150 GeV/c ≤ p T ≤ 20 GeV/c –DC track quality: 63 || 51 || 31 –n0 ≥ 3 (# of RICH phototubes fired within associated area) –n3 ≥ 1 (same as n0, but additional timing requirement) –disp ≤5 (ring displacement) –χ 2 /npe0 ≤ 10 cm 2 –0.7 ≤ E/p ≤ 2.0 – Analyzed dataset: Au+Au

01/20/2005Torsten Dahms - SUNY Stony Brook11 Invariant mass of e + e - -pairs Dalitz decays beampipe conversions air conversions & combinatorial background

01/20/2005Torsten Dahms - SUNY Stony Brook12 Beam Pipe Conversions Track reconstruction relies on Pair obtains additional opening angle Pair gets mass > 0 Inv. Mass proportional to distance from collision vertex  conversion peak shifts w. r. t. to Dalitz decays

01/20/2005Torsten Dahms - SUNY Stony Brook13 Beam Pipe Conversions Beam Pipe West Arm East Arm γ e+e+ e-e- e+e+ e-e- γ Collision Vertex

01/20/2005Torsten Dahms - SUNY Stony Brook14 Pair-p T all pairs opposite arms same arm

01/20/2005Torsten Dahms - SUNY Stony Brook15 Pair properties I Conversion pairs have small intrinsic opening angle –magnetic field produces opening of the pair in azimuthal direction –orientation perpendicular to the magnetic field – Dalitz decays have larger intrinsic opening angles due to π 0 mass MC Simulation all pairs dalitz decay beam pipe conversions

01/20/2005Torsten Dahms - SUNY Stony Brook16 Pair properties II z y x ψ Pair e+e+ e-e- ξ Pair B z y x e+e+ e-e- B Conversion pair z y x e+e+ e-e- B Dalitz decay MC Simulation all pairs dalitz decay beam pipe conversions

01/20/2005Torsten Dahms - SUNY Stony Brook17 East and West arm East arm West arm Pair cuts: |ψ Pair | < 0.2 rad Δφ 0 < 0.0 rad

01/20/2005Torsten Dahms - SUNY Stony Brook18 Pairs (after pair cuts) Dalitz component (after pair cuts; MC) Extraction of conversion pairs 0.8 < pair p T ≤ 1.0 GeV/c

01/20/2005Torsten Dahms - SUNY Stony Brook19 Photon p T spectrum East arm West arm

01/20/2005Torsten Dahms - SUNY Stony Brook20 Monte Carlo Simulation Simulated 10 million π 0 decays with EXODUS flat in p T : 0-10GeV/c Rapidity: y ≤ 0.6 Azimuthal range: 0 < φ < 2 π Tracking in PHENIX detector model (PISA) Reconstruction done with same algorithms as used for real data

01/20/2005Torsten Dahms - SUNY Stony Brook21 Monte Carlo Simulation Use realistic p T distribution as a weight on simulation PHENIX Preliminary π 0 p T spectrum

01/20/2005Torsten Dahms - SUNY Stony Brook22 Monte Carlo Simulations Small conversion probability in beam pipe (0.2%) Changed average path length to the next pair production process x 1/20 Adjusted branching ratio: originalmodified %76.04 % %23.96 % East West

01/20/2005Torsten Dahms - SUNY Stony Brook23 Comparison MC and data MC data

01/20/2005Torsten Dahms - SUNY Stony Brook24 Acceptance and Reconstruction Efficiency Correction Detector is not perfect Does not cover 2π azimuth and |y| ≤ 0.5 Compare input and output of Monte Carlo Simulations Ratio returns correction function

01/20/2005Torsten Dahms - SUNY Stony Brook25 Acceptance and Reconstruction Efficiency Correction East arm West arm

01/20/2005Torsten Dahms - SUNY Stony Brook26 Corrected Photon Spectrum East arm West arm

01/20/2005Torsten Dahms - SUNY Stony Brook27 Systematic Errors Systematic uncertainties on electron identification and pair cuts Change one cut at a time Plot Raw photon spectrum MC correction function with this set of cuts Corrected photon spectrum (Unchanged – Changed)/Unchanged

01/20/2005Torsten Dahms - SUNY Stony Brook28 Systematic errors on cuts East ArmWest Arm n07%10% n32%1% chi2/npe05%9% displacement12%10% e/p3%4% pair cuts12%9% total19%20%

01/20/2005Torsten Dahms - SUNY Stony Brook29 The Cocktail Cocktail of light meson decays Use PHENIX measurements where available Otherwise: m T scaling of π 0 and π ± spectra to simulate shape of spectra Systematic error of 15% on cocktail

01/20/2005Torsten Dahms - SUNY Stony Brook30 Result: Cocktail Comparison East and west combined

01/20/2005Torsten Dahms - SUNY Stony Brook31 Result: Photon ratio

01/20/2005Torsten Dahms - SUNY Stony Brook32 Summary We are able to reconstruct photons from conversions The photon spectrum and cocktail agree within errors We do not see thermal photons Limited statistics has influence on systematic errors Better understanding of Monte Carlo Simulation  systematic errors ≈ 10% dataset has huge statistics Should be possible to study centrality dependence