# Presentation: 342167

## Measurement of Transverse Single Spin Asymmetry AN in Eta Mass Region

- Len Eun
- For STAR Collaboration

- Measurement of Transverse Single Spin Asymmetry AN in Eta Mass Region
- at Large Feynman XF
- with the STAR Forward Pion Detector

- APS DNP OCT 08

##   STAR Forward Pion Detector (FPD)

-   STAR Forward Pion Detector (FPD)

-  STAR forward calorimeters have gone through significant upgrades since run3.
-  In run6, the original FPD remained in the east, while the west FPD was expanded to FPD++.
-  The east FPD is consisted of two 7X7 Pb-glass modules, EN and ES. During run6, it was placed at the “far” position. (x-offset~30cm, <>~3.7)

**1**

## Forward 0 Single Spin Asymmetry

- Forward 0 Single Spin Asymmetry

- From Spin2008 talk by J.Drachenberg

- At √s=200GeV, 0 cross-section measured by STAR FPD is consistent with the NLO pQCD calculation. Results at <>=3.3 and <>=3.8 have been included in the DSS global pion fragmentation function analysis. *(Phys.Rev.D75(2007) 114010)*

- We find that AN increases with xF, roughly consistent with theoretical predictions. Contrary to predictions, however, AN does not fall as a function of pT at fixed xF in this kinematic region.

**2**

*Accepted by PRL*

## Acceptance and Reconstruction

-  7x7 FPD has limited acceptance for Eta mesons. At 40GeV, a symmetrically decaying Eta needs to point to the center of the FPD to fit in. Acceptance improves greatly at higher energy.
-  0 starts to push the reconstruction efficiency over 60GeV, where the separation between two photons for symmetric decay becomes ~1 cell width.

- _Fast Simulator_
- The ratio of N(reconstructed particles) to N(generated particles with CoM within FPD)

**Eta**

**3**

## Event Selection and Mass Reconstruction

- Event Cuts
-  2 photon events
-  Z< 0.85, Etotal>25GeV
-  Hardware threshold nominally at 25GeV
-  “Center Cut” for 2 CoM defined as

-  Etot: Detector summed energy
-  Z and photon separation: Fitted photon energy/locations
-  Reconstructs on the entire FPD
-  Vertex set at zero for all events

**4**

##  and 0 Energy Sharing (Z) Distribution

-  and 0 Energy Sharing (Z) Distribution

**5**

## Observation of Eta Signal

-  Center Cut
-  Z < 0.85
-  3 colums for 3 energy bins
-  Each column shows a single plot  in log and linear scale.

- Di-Photon Invariant Mass Spectra in 3 Energy Bins

**6**

## Mass Dependence of AN

- Mass Dependence of AN

-  Yellow beam asymmetry clearly reveals the shape of two mass resonances.
-  There is an “asymmetry valley” in between 0 and Eta mass regions.

- Nphoton = 2
- Etotal > 40GeV
- No Center Cut
- Z< 0.85
- Average Yellow Beam Polarization = 56%

**STAR 2006 PRELIMINARY**

**7**

## Single Spin Asymmetry for 0 and 

- Single Spin Asymmetry for 0 and 

-  Nphoton = 2
-  Center Cut ( and )
-  Pi0 or Eta mass cuts
-  Z< 0.85
-  Average Yellow Beam Polarization = 56%

**8**

**Notes:**
Mass and Energy Distribution for evts > 40 GeV
(N and S) energy Distribution

## Summary

- Previously, the STAR Forward Pion Detectors at RHIC (Brookhaven National Laboratory) have been used to successfully measure the forward single spin asymmetry, AN, for 0 meson in <>=3.3~4.0 region.
- In RHIC run 6, during √s=200 GeV p+p collisions, 0 and Eta mesons were observed in the east FPD. We measured and compared the single spin asymmetry in the 0 and the Eta mass regions, at <>~3.65 and xF above 0.4.
- AN as a function of the invariant mass reveals 0 and Eta resonance peaks.
- From 55GeV to 75GeV, (xF=0.55~0.75) the average transverse single spin asymmetry in the Eta mass region was measured to be AN  = 0.361 ±  0.064,  about 4 standard deviations greater than the average AN in the 0 mass region.
- The dominant errors in the asymmetry measurements in the Eta mass region are statistical errors.

- Len Eun