Study QGP in different Centrality

Most Central events (highest multiplicity), e.g. top 5% central, i.e. 5% of the events with largest multiplicity

Mid Central events

Most Peripheral events

From most central to most peripheral event, the collision is more like a p+p collisions.

One can also collision smaller size of nuclear, e.g. Cu+Cu, Si+Si, instead of Au+Au to gain more luminosity.

N_coll: 8 N_part: 6

Centrality can be quantified by the number of collisions (N_coll) and number of participants (N_part) through the glauber model calculation with

Fermi-Dirac and Bose-Einstein distribution

( ) /( )1 E kT

gn E

e

Number of particles with energy E

... for fermions (half-integer spin):

(Fermi-Dirac distribution)

... for bosons (integer spin):

( ) /( )1E kT

gn E

e

(Bose-Einstein distribution)

:

:

:

g

T

# degrees of freedom (degeneracy)

Chemical potential

Temperature

Ways to Reveal the QGP properties---flow

V1: directed flow

Higher order

V2: elliptic flow

x

yz

The System Thermalized very Early

– “Pressure” converts spatial anisotropy to momentum anisotropy.

– Hydodynamic describe the results well

– Requires early thermalization ~ 1 fm

dN

/d

x

yz

• Same phenomena observed in gases of strongly interacting atoms– M. Gehm, S. Granade, S. Hemmer, K, O’Hara, J. Thomas

Science 298 2179 (2002)

weakly coupled

strongly coupled

The RHIC fluid behaves like this,

that is, a strongly coupled fluid

22

22

2yx

yx

pp

ppV

Ideal Hydrodynamics

• Why the interest in viscosity?

A.) Its vanishing is associated with the applicability of ideal hydrodynamics (Landau, 1955):

B.) Successes of ideal hydrodynamics applied to RHIC data suggest that the fluid is “as perfect as it can be”, that is, it approaches the (conjectured) quantum mechanical limit

11 so )(

1Forces DragForces Inertial

Number Reynolds Hydro Ideal

mfpL

mfpvLV

mfpv

LV

thermal

BULKthermal

BULK

s

4

)densityentropy (4

ViscosityQuote from Wikipedia: Viscosity is a measure of the resistance of a fluid to being deformed by either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.[1] The study of viscosity is known as rheology.

Viscosity

Viscosity is a measure of the resistance of a fluid to being deformed by either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.[1] The study of viscosity is known as rheology.