# Presentation: 123921

## Directed flow in Au+Au collisions at 62GeV 

**Introduction**
**     ****** **Directed Flow**
**     ******** ****Wiggle**
**     ******** ****Previous result**
**     ******** ****3 Methods**
**     ****** **ZDC upgrade**
**     ****** **ZDC-SMD method**
**Physics results**
**     ******** ****10% - 70%**
**     ******** ****Centrality dependence**
**     ******** ****Pt dependence**
**     ******** ****Integrated v1**
**     ******** ****Limiting fragmentation**
-  **Summary**

***Gang Wang  ******for******  ******STAR Collaboration***

## Flow

**Among different kinds of flow, such as radial flow, directed flow, elliptic flow and higher harmonics, directed flow is the first discovered type of flow (20 years ago).**

**Flow**

**Radial Flow (isotropic)**

**Anisotropic Flow**

**Directed Flow**

**Elliptic Flow**

**Higher Harmonics**

- Gang Wang, KSU

## Directed Flow

** *****Directed flow*****:**** the first harmonic (****v****1****) in the Fourier expansion of the particle azimuthal anisotropy with respect to the reaction  plane**
** ****It’s likely influenced by the the compression in the initial stages of the collision, and makes a probe for the early stage of the collision. **

**v****1**** changes with the collision energy.**

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

- Gang Wang, KSU

**Notes:**
Harder now

## Wiggle

**Wiggle**

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

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

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

- Gang Wang, KSU

## Wiggle

**Models without QGP also predicted 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)***

**Baryon stopping and positive space-momentum correlation**

- Gang Wang, KSU

## Previous result

**Previous result**

***STAR Collaboration, PRL 92, 062301 (2004)***

- Gang Wang, KSU

## Experimental complications

**We try to reduce sensitivity to non-flow effects:**
**v****1****{3} **
** ****the three-particle cumulant method**
** ****by *****Aihong Tang***
**v****1****{EP****1****, EP****2****} **
** ****the event plane method with mixed harmonics**
** ****by *****Markus Oldenburg, Ilya Selyuzhenkov***
**v****1****{ZDC-SMD} **
** ****the standard method with the 1st-order event plane reconstructed from neutral fragments of the incident beams**
** ****the event plane is determined from the sideward deflection of spectator neutrons (“bounce-off”) measured in ZDC-SMD**
** ****by *****Gang Wang***

**At RHIC, most detectors cover the central rapidity region where the directed signal is small and the analysis procedures are easily confused by ****non-flow effects**** (azimuthal correlations not related to the reaction plane orientation).**

- Gang Wang, KSU

## STAR ZDC

**Each of the RHIC experiments has a pair of Zero Degree Calorimeter**

**Each of the RHIC experiments has a pair of Zero Degree Calorimeter detectors 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, KSU

## STAR ZDC-SMD

**SMD is an 8 channel by 7 channel hodoscope that sits directly on the face of the 2nd ZDC module**

- Gang Wang, KSU

## Raw event plane from ZDC-SMD

**Raw event plane from ZDC-SMD**

**STAR Preliminary**

**The raw event plane distributions are not flat. We can use the inverse of the bin content as psi weight in the analysis.**

- Gang Wang, KSU

## Flatten event plane distribution

**Flatten event plane distribution**

**STAR Preliminary**

***E877 Collaboration, Phys. Rev. C 56, 3254 (1997)***

- Gang Wang, KSU

## Comparison between 2 analysis

**Comparison between 2 analysis**

**STAR Preliminary**

**50% -- 60%**

**The analysis with psi weight gives the same result as the one with flat psi.**

- Gang Wang, KSU

## Systematic study: 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, KSU

## Systematic study : Corrections

**East and west correction:**

**where       denotes another 1****st****-order event plane, e.g. from FTPC**

**When east and west ZDC-SMD have different RP resolutions, we have to use a 3rd 1****st****-order event plane (      ) as an independent reference to correct this effect. **

***E877 Collaboration, Phys. Rev. C 55, 1420 (1997)***

**They are inverse of each other**

- Gang Wang, KSU

## Systematic study : Corrections

**X - Y correction: (from Sergei V. and Ilya S.)**

**When detectors like TPC and FTPC don’t have the same sensitivity in X and Y direction,  we  have  to  replace  the  “2”  in  the numerator with                    **
**                   ****for X direction or                    for Y direction. For example:**

- Gang Wang, KSU

## Systematic study : Comparison

**Systematic study : Comparison**

**STAR Preliminary**

**50% -- 60%**

**After corrections applied, the 4 separate terms get much closer to each other.**

- Gang Wang, KSU

## Systematic study : Comparison

**STAR Preliminary**

**50% -- 60%**

**The average of the 4 separate terms almost coincides with the result of  the full-event-plane analysis.**

**Systematic study : Comparison**

- Gang Wang, KSU

## Charged hadrons: Eta dependence

**Charged hadrons: Eta dependence**

**STAR Preliminary**

- Gang Wang, KSU

## Charged hadrons : Eta dependence

**Charged hadrons : Eta dependence**

**STAR Preliminary**

- Gang Wang, KSU

## Charged hadrons: Pt dependence

**Charged hadrons: Pt dependence**

**STAR Preliminary**

**The magnitude of ****v****1 ****is**
**observed to increase with particle transverse momentum and then appears to saturate.**

- Gang Wang, KSU

## Charged hadrons: Integrated v1

**0.15 GeV < pt < 2 GeV.**
** ****The magnitude of ****v****1 ****decreases with centrality.**
** ****v****1 ****in the more forward pseudorapidity region varies more strongly with centrality than in the region closer to midrapidity.**

**Charged hadrons: Integrated v1**

- Gang Wang, KSU

## Limiting Fragmentation

**Limiting Fragmentation**

**STAR Preliminary**

**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, KSU

## Summary

**We present the first measurements of charged particle directed flow in Au+Au collisions at 62GeV.**
**The analysis has been performed using three different methods and the results agree very well with each other.**
**STAR ZDC-SMD now allows to determine the reaction plane from the bounce-off of fragmentation neutrons, the first measurement of this type at RHIC.**
**Charged particles in the pseudorapidity region covered by the STAR TPC and FTPCs (up to eta = 4.0) flow in the opposite direction to the fragmentation nucleons.**
**Over the pseudorapidity range studied, the v1 for charged particles is found to vary monotonically with pseudorapidity at all centralities.**
**There’s no evidence so far for pion wiggle. We are working on v1 of  identified particles.**
**Our data provide further support for the limiting fragmentation picture.**

- Gang Wang, KSU

## 

Backup Slides

- Backup Slides

- Gang Wang, KSU

## Event Plane Resolution for ZDC-SMD

**STAR Preliminary**

- Gang Wang, KSU

## EOS data: pion flow

- Gang Wang, KSU