# Presentation: 821580

## Qinghua Xu,  LBNL & Shandong University 
for the STAR Collaboration
PANIC05,  Santa Fe, October 27, 2005

*for the STAR Collaboration*
- PANIC05,  Santa Fe, October 27, 2005

-  Introduction
-  Lambda reconstruction in TPC at STAR
-  Preliminary results for Lambda and Lambda-bar polarization
-  Summary and outlook

- Measurement of Lambda polarization in longitudinally polarized proton-proton collisions at √s=200 GeV

## Introduction

-     polarized fragmentation and parton distribution functions!

-  Polarized parton distribution function (spin structure of nucleon):
-    g(x),flavor separation                              , q(x)...
-  Polarized fragmentation function D(z):  limited knowledge...
-  Spin transfer: the relation between the spins of initial and final hadrons
-   --- a connection of q(x) and D(z) in polarized pp collisions!

##  Theoretical predictions for Λ polarization:

**SU(6), set II**

**DIS,  set I**

**SU(6), set I(GRSV00)**

**DIS, set II(GRSV96)**

**D.de Florian, M.Stratmann, W.Vogelsang, **
**PRL81,(1998)530**

**Xu Q.H., Liu C.X, Liang Z.T., **
**PRD65(2002)114008**

**QM**

**DIS’**

**equal contr.**

-  Theoretical predictions for Λ polarization:

-  polarized frag. function
-      from available data

- (b) model for pol. frag. function
-      according to the origin

-  Main difference comes from D:

##  Lambda-bar polarization is sensitive to 

**DIS, set II**

**DIS,  set I**

**SU(6), set I(GRSV00_val)**

**SU(6), set II(GRSV00_std)**

-  A prediction for anti-lambda polarization using method (b)

**Xu Q.H., E. Sichtermann, Liang Z.T., in preparation.**

-  Lambda-bar polarization is sensitive to
** ******S from inclusive DIS and semi-inclusive DIS (Hermes) are both the sum of them

## RHIC - First Polarized pp Collider

| pp Run  | 2002  | 2003 | 2004 |   2005                                                      |  > 2006 LongTermGoals |  |
| --- | --- | --- | --- | --- | --- | --- |
| CM Energy |                                  200 GeV |  |  |  | 200 GeV | 500 GeV |
| <Pb> and direction at STAR | 0.15  T       | 0.30 T/L | 0.40  L | 0.45  L | 0.7 L/T | 0.7  L/T |
| Lmax [ 1030 s-1cm-2 ] | 2 |  6 | 6 | 16 | 80 | 200 |
| Lint    [pb-1 ] at STAR | 0.3  | 0.5 / 0.4 | 0.4 | 9 | 320  | 800  |

- AGS Heclical Partial Snake

- 4 spin orientations:
- ++,+-,-+,--

- Stable polarization direction - transverse
- Longitudinal polarization at STAR/Phenix

## STAR Detector

-   **primary vertex**

- p

**V0_vertex**

**V0_DCA**

- Λ

-   Λ is reconstructed by combining TPC
-    tracks with opposite charges and
-    applying topological cuts
-   Data sample(2003+2004):
-    2M* minbias+*3M* high tower EMC trigger*

- TPC

##  data of 2003 and 2004

-  Invariant mass distribution and Armenteros plot :

-  Kinematics of Λ candidates (Minbias+Barrel high tower trigger):

** ****<pT>~1.5GeV**

**|<*****xF*****>|~0.008**

-  data of 2003 and 2004

- Note: only a part of whole sample

**M****Λ****=1.1157 GeV(PDG)**

##   Standard method to extract Lambda polarization:

-    momentum distribution of decay proton in Λ’s rest frame(Λ->p-)

**A(cos****θ****): detector acceptance **
**from MC simulation**

**decay parameter**
**0.642(PDG)**

-  **unit vector along proton  momentum in ****Λ**** ****rest frame**

**Λ**** polarization vector, is its moving direction in longi. polarized case**

-  Consider a narrow bin in : [1,2], and define:

- (acceptance is cancelled!)

-  Symmetry relations:

##   data in bins of cosθ:

- ~30K of Λ candidates (~27K for anti-lambda) used in our following analysis!

- Ks0  bkg

- only the first 4 bins were used for analysis! (~60%)

## Preliminary results of Λand      polarization from 03+04

**  ****Kinematics ****:  **
** ****|<xF>|~0.008**
** ****<pT>~1.5GeV**

- (limited by statistics)

- (statistic error only)

- (statistic error only)

**STAR Preliminary**

**STAR Preliminary**

**  ****No. of events:**
** **~30 K for Lambda
-  ~27 K for anti-lambda

## Spin transfer D_LL

-   D_LL:  spin transfer from beam to Λ in

**( |<xF>|~0.008,<pT>~1.5GeV**)

-  Relation :

-  Discussion:

-   Since |xF|~0.01, pT~1.5GeV, small value not surprising.
-   Gluon frag. at such low pT should be important !

## Summary & outlook

-  Preliminary results are: ( |<xF>|~0.008, <pT>~1.5GeV)

-   2005 prospects:

-  The integrated luminosity was ~4 times to 03+04;
-  Beam polarization was ~45%, ~2 times to 2003;
-  xF and pT (~3GeV) still low, theoretical predictions?!

|    |  |  |
| --- | --- | --- |
| <η> =0.5  | 0.01±0.05(stat) | -0.06±0.05(stat) |
| <η>=-0.5 | 0.06±0.05 | 0.06±0.05 |

- Statistics limited, systematic error considered from relative luminosity ratio(±0.01), beam polarization(~20%); trigger bias study underway....