Incident-energy and system-size dependence of directed flow Gang Wang (UCLA) for STAR Collaboration Introduction to directed flow Detectors: ZDC-SMD,

Incident-energy and system-size dependence of directed

flowGang Wang (UCLA)

for STAR Collaboration

Introduction to directed flow

Detectors: ZDC-SMD, (F)TPC

Cross check with PHOBOS

Comparison between 4 systems

Summary

Gang Wang (UCLA) Nov 19, 2006 2

r

pt

Higher harmonics

rapidity

<px> or directed flow

)](cos[

)](cos[212

1

1

2

3

3

rn

nrn

tt

nv

nvdydpp

Nd

pd

NdE

Anisotropic flow

Directed flow (v1)

Elliptic flow (v2)


J. -Y. Ollitrault, Nucl. Phys. A638, 195c (1998).

RHIC

Easier to measure

Harder!

Non-flow effects

Flow fluctuations

Low statistics

STAR Collaboration, Phys. Rev. Lett. 92, 062301 (2004)


STAR Main Detector


STAR ZDC-SMD

SMD is 8 horizontal slats & 7 vertical slats located at 1/3 of the depth of the ZDC

• New knowledge of the direction of the impact parameter vector• Minimal, if any, non-flow effects• Worse resolution than from TPC, but that disadvantage is minor

ZDC side view

Scintillator slats of Shower Max Detector

Transverse plane of

ZDC


Cross check with PHOBOS

Within errors, STAR v1 of charged hadrons agrees with PHOBOS’ results.

PHOBOS Collaboration, Phys. Rev. Lett. 97, 012301 (2006)

5% - 40%


Comparisons of charged hadron v1

between 4 collision systems:

200 GeV Au +Au200 GeV Cu +Cu

62.4 GeV Au +Au 62.4 GeV Cu +Cu


Resolution of 1st-order event plane

from STAR ZDC-SMDFull EP resolution depends on spectator v1 and the number of hits falling on ZDC-SMDs.

)cos(2 ba


Best case: 200 GeV Au + Au

Charged hadron v1 is in the direction opposite to that of spectators.


Best case: 200 GeV Au + Au

In mid-rapidity, v1(pt) crosses zero at pt above 1 GeV/c in the central and mid-central collisions.

Not seen in the forward region.

Hydro model predicts the zero-crossing in v1(pt), but the prediction also applies to the forward region.

If we assume that pions and protons flow oppositely, then the change in their relative abundances leads to the

zero-crossing.

With the yields of (anti)proton and pion, we can fit the v1(pt) points assuming v1(pt) for pions and (anti)protons are straight lines.


AMPT

v1(η): system-size dependence

System size doesn’t seem to influence v1(η).


v1(η): incident-energy dependence

v1 magnitude is bigger at 62.4 GeV than at 200 GeV.The hypothesis of limiting fragmentation is supported.


v1(pt)

At 200 GeV, v1(pt)shows similarity in Au +Au and Cu +Cu collision systems.

At 62.4 GeV, this is true for low pt. More statistics is needed in higher pt for Cu +Cu.

For all 4 cases, v1 (pt) goes to zero when pt is zero, and increases in magnitude with pt until reaching the maximum and turning back.


Integrated v1

At 200 GeV, integrated v1 shows similarity in Au +Au and Cu +Cu collision systems.

At 62.4 GeV, this is seemingly true. More statistics is needed for Cu +Cu.

For all 4 cases, integrated v1

goes to zero in central collisions, and increases in magnitude from central to peripheral collisions.


Summary• Best case: 200 GeV Au + Au

– Charged particle v1(η) tends to turn back towards zero at forward pseudo-rapidities in central collisions.

– v1(pt) crosses zero at pt above 1 GeV/c in central and mid-central collisions in mid-rapidities.

• v1 shows similiarities between AuAu and CuCu as functions of η, pt and centrality at both 200 GeV and seemingly at 62.4 GeV.

• AMPT model predicts this effect at 200 GeV.

• Limiting fragmentation hypothesis is supported by AuAu data, and seemingly by CuCu data.

• Results of v1{ZDC-SMD} will be cross-checked by other methods.


Backup Slides


Flow study with ZDC-SMD: 4 terms

In analysis:

For example:

To systematically study the method with ZDC-SMD, we can use the sub event plane from only east or west ZDC-SMD, instead of the full event plane, and even break down the correlation into X and Y direction.

Definitions of 4 terms:


Identified particles : v1 in 62 GeV Au +Au

STAR Collaboration, Phys. Rev. C 73, 034903 (2006)

Documents

Incident-energy and system-size dependence of directed flow Gang Wang (UCLA) for STAR Collaboration Introduction to directed flow Detectors: ZDC-SMD,