# Presentation: 123922

## Directed Flow at RHIC Based on Transverse Deflection of Spectator Neutrons

- Gang Wang  (UCLA)
- for STAR Collaboration

## pt

*p**t*

- Higher harmonics

- Anisotropic flow

- Directed flow (*v*1)

- Elliptic flow (*v*2)

- Gang Wang (UCLA) Jun 7, 2006

## v1(y) structure

***v*****1****(*****y*****) structure**

- Models predict that** anti-flow/3****rd**** flow component, **with** **QGP** ******** ***v*1(*y*)** flat **or a** *****wiggle***** **structure**.**

***L.P. Csernai, D. Rohrich***
***PLB 458, 454 *****(1999)**

***J. Brachmann et al.***
***PRC 61, 024909  *****(2000)**

- Gang Wang (UCLA) Jun 7, 2006

## v1(y) structure

- Models without QGP also predict a** *****wiggle***** **structure in** **peripheral or mid-peripheral events.

***M. Bleicher and H. Stöcker, PLB 526, 309 *****(2002)***** ***

**RQMD**

**UrQMD**

***R. Snellings, H. Sorge, S. Voloshin, F. Wang, N. Xu, PRL 84, 2803 *****(2000);  *****also H. Liu et al., PRC 59, 348 (1999).***

- Baryon stopping and positive space-momentum correlation

- Gang Wang (UCLA) Jun 7, 2006

## Easier to measure

- RHIC

- Harder!

**Non-flow effects**

**Flow fluctuations**

**Low statistics**

- Gang Wang (UCLA) Jun 7, 2006

## STAR Main Detector

- Gang Wang (UCLA) Jun 7, 2006

## STAR ZDC

**Each of the RHIC experiments has a pair of Zero Degree Calorimeters for beam monitoring, triggering, and locating interaction vertices.**** **
**ZDCs detect neutrons emitted along beam directions and measure their total energy (multiplicity) .**
***Baseline ZDCs have no ***_***transverse segmentation***_***, *****which motivates upgrade.**

- Gang Wang (UCLA) Jun 7, 2006

## 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**
** ****Minimal, if any, effects from flow fluctuations**
** ****Worse resolution than from TPC, but that disadvantage is minor**

**ZDC side view**

**Scintillator slats of**** ****Shower Max Detector**

**Transverse plane of ZDC**

- Gang Wang (UCLA) Jun 7, 2006

## Flow study with ZDC-SMD: 4 terms

**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:**

- Gang Wang (UCLA) Jun 7, 2006

## First result: cross check

**First result: cross check**

**Au +Au 62 GeV**

**Models do well at larger ****|**************|**** ****but their *****v*****1****(**********) is too flat near ********** = 0**

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

- Gang Wang (UCLA) Jun 7, 2006

## Comparisons between 3 collision systems:  62 GeV Au +Au200 GeV Au +Au200 GeV Cu +Cu

- Gang Wang (UCLA) Jun 7, 2006

## Mid-rapidity

**Mid-rapidity**

- Charged particle v1 is a monotonic function of *η***.**

**AMPT model gives a**

- Gang Wang (UCLA) Jun 7, 2006

## Forward rapidity

**AMPT model gives a**

**AMPT model shows a similar tendency, with smaller magnitude.**** **

**AMPT model gives a**

- Gang Wang (UCLA) Jun 7, 2006

## More Hydro?

**Assume (anti-)proton and pion *****v*****1****(*****p******t*****) are straight lines starting from zero, and neglect other particles. Charged particle *****v*****1****(*****p******t*****) can be fit simply.**

- Gang Wang (UCLA) Jun 7, 2006

## Scaling behavior

**Scaling behavior**

**The hypothesis holds here.**

**It has been observed that particle emission(both spectra and flow) as a function of rapidity in the vicinity of beam rapidity appears unchanged over a wide range of beam energies, a pattern known as ****limiting fragmentation****.**

- Gang Wang (UCLA) Jun 7, 2006

## More Scaling

**More Scaling**

***STAR preliminary***

- Gang Wang (UCLA) Jun 7, 2006

## Summary

- For all 3 collision systems, charged particle *v*1(η) decreases with η in mid-rapitidies, flowing opposite to spectators**.**
- Charged particle *v*1(η) approaches zero at forward η sooner or later. The more central collision, the sooner.
- AMPT model predicts this effect, with much smaller magnitude. (cross-section?)
- Hydro-dynamics model also predicts the zero-approaching.
- Charged particle v1(*p**t*) crosses zero at *p**t* > 1 GeV/*c *in the TPC region at 200 GeV Au +Au**.** The more central collision, the sooner.
- Hydro-dynamics model predicts the sign-change.
- With an assumption of the straight-line shape in v1(*p**t*) for identified particles, the slopes can be fit with particle yields.
- Identified particle *v*1 still need more investigation.
- Limiting fragmentation is observed when comparing 62 GeV and 200 GeV Au +Au.
- System size doesn’t matter in *v*1?
- 200 GeV Au +Au and Cu +Cu
- Would look at 62 GeV Cu +Cu, and compare it with 62 GeV Au +Au.
*v*1 results call for more input from model calculations.

- Gang Wang (UCLA) Jun 7, 2006

## Backup Slides

- Gang Wang (UCLA) Jun 7, 2006

## Identified particles : v1 in 62 GeV Au +Au

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

- Gang Wang (UCLA) Jun 7, 2006