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

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)

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

-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 1012 K.

-As density and temperature become very large, hadrons formed by quarks overlap =>quarks lose their affiliation with any particularhadron.

-Quarks and gluons form a hot and dense soup!

Quark-Gluon Plasma

Time Evolution of Relativistic Heavy-Ion Collision

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.

NA60: Dilepton DataInvariant Mass Spectra

Plots: S. Damjanovic, QM05

NA60: Dilepton Data Transverse Momentum Spectra

-Data show signs of a two-component spectrum, one component dominates at low pT while the other dominates at high pT

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)

Results: Naive Two-Component Model in 4 Centrality Bins

M[GeV]M[GeV]

Peripheral Semiperipheral

Semicentral Central

Naive Two-Component Model:Semicentral in two pT slices

M[GeV]

M[GeV]

pT < 0.5 GeV

pT > 1.0 GeV

Early Conclusions

-Two Component Model seems to work well for inclusive pT bins, but shows deficiency in semicentral high-pT region.

-Need to include smaller effects, other contributions to make model more complete

Backup Slides

Dilepton Spectra: Theory

D

g

mTqf

Mqxdd

Nd B Im),(1

~2

4

044

8

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.