1. Finite Size Effects on Dilepton Properties in Relativistic Heavy Ion Collisions Trent Strong, Texas A&M University Advisors: Dr. Ralf Rapp, Dr. Hendrik van Hees Texas A&M University Cyclotron Institute Cyclotron Institute REU 2006

2. QCD (Quantum ChromoDynamics) -QCD describes the interactions between quarks and gluons. -There are six flavors of quarks, and eight gluons, all carrying color charge -The force between quarks is strong and is linear in distance! (coupling constant α s ≈1) -Force weakens at small distances (or high energies), so quarks essentially free within bounds (asymptotic freedom)

3. Relativistic Heavy-Ion Collisions At NA60: In-In @ 158 GeV/Nucleon b -Colliders accelerate nuclei to very relativistic speeds! (RHIC, γ ≈ 100, v=.9995c) -Nuclei collide, a hot and dense region is formed -In this region, the Quark-gluon plasma (QGP) and other forms of exotic matter like a hadron gas can form -They allow us to test further the theory of QCD and explore the early universe

4. -Quark-Gluon Plasma (QGP)- form of matter predicted by QCD at high temperature and density. -Predicted transition temperature is ~ 170 MeV, corresponding to a temperature on the order of 10 12 K. -As density and temperature become very large, hadrons formed by quarks overlap => quarks lose their affiliation with any particular hadron. -Quarks and gluons form a hot and dense soup! Quark-Gluon Plasma

5. Time Evolution of Relativistic Heavy-Ion Collision

6. Electromagnetic Probes: Dileptons and Photons Dileptons and photons good sources of information from a hot and dense medium since they: a.) are produced throughout the history of the collision. b.) do not interact strongly with the medium. The particles carry this information via their invariant mass and 4- momentum. In a hadronic medium expected from such a collision, the ρ meson is the dominant producer of dileptons.

7. NA60: Dilepton Data Invariant Mass Spectra Plots: S. Damjanovic, QM05

8. NA60: Dilepton Data Transverse Momentum Spectra -Data show signs of a two- component spectrum, one component dominates at low p T while the other dominates at high p T

9. Two-Component Model Idea: Attempt to model spectra using two contributions… -Cocktail: Component from hard- scattering processes; surface contribution -Thermal or In-Medium: Components from thermal medium, such as QGP or hadron gas; bulk contribution Collision Zone Total Spectra = a ∙ (Thermal) + b ∙ (Cocktail)

10. Results: Naive Two-Component Model in 4 Centrality Bins M[GeV] PeripheralSemiperipheral Semicentral Central

11. Naive Two-Component Model: Semicentral in two p T slices M[GeV] p T < 0.5 GeV p T > 1.0 GeV

12. Early Conclusions -Two Component Model seems to work well for inclusive p T bins, but shows deficiency in semicentral high-p T region. -Need to include smaller effects, other contributions to make model more complete

13. Backup Slides

14. Dilepton Spectra: Theory ρ Spectral Function: -Spectral function gives distribution of rho mesons being produced per unit four position and unit four momentum -To obtain observed spectra, convolute over the entire spacetime history of the fireball expansion.