# Presentation: 724916

## Status of STAR Local Polarimeter

-   A brief summary:  why forward pions?
-   What’s it like in the forward direction?
-   How things worked...
-   Plans and proposals

- L.C. Bland
- BNL

## Why Forward Pions?

**E704 effect**
- Non-zero values of AN have been observed in FNAL E704...
- [D.L. Adams, et al. Phys. Lett. B261 (1991) 201]

- p+ p + X, s = 20 GeV ,
-  0.5 < pT < 2.0 GeV/c

- ...and are predicted to persist at RHIC energies.

- STAR has substantial space in forward direction.

## The opportunity...

- T-438/452 setup at SLAC

- Tests of a prototype endcap calorimeter had been ongoing at SLAC...

- Pb-scintillator sampling calorimeter with scintillator-strip **shower maximum detector**

- ...a ‘perfect’ forward  detector for STAR?

## STAR Prototype FPD

- Prototype endcap EMC was pretty close to perfect as a  detector, but ***was not a polarimeter*** because it was a single arm detector.
-  Supplement with Pb-glass detectors from IHEP, Protvino (built for E-704).

- East

- West

## FPD Simulations forTransverse asymmetry measurement

- Use electromagnetic calorimeter (EMC) + shower maximum detector (SMD) to reconstruct  decays up to E~50 GeV

- SMD provides good  opening angle measurement and crude single g energy measurement  ********** reconstruction**.

- Expect single-spin asymmetry (AN) to be significant for  with large xF and moderate pT.
******Make first measurement of a polarization observable at RHIC**

- 1 year ago:
- ‘expected’ L=51030 cm-2 s-1 and Pbeam=50%

## What’s it like in the forward direction?

-   Counting rates of ~50(10) Hz for E>20 GeV at L=0.51030 cm-2s-1 and ~100 higher at 3 GeV threshold
-   Beam-gas background mostly from Yellow ring
-  Signal:background (without BBC coincidence) can be 10:1 and can also be much worse  use BBC.

## Forward Pi0 Detector:  mesons

- North arm of FPD = prototype Endcap EMC

- Resolution=20MeV
- for 20-60 GeV ’s

-  Etot from towers
-   energies,from SMD

- Reconstruction:

- M vs. Etot:

- Total number of ’s in standalone sample = 1.5M

## Mass vs. Energy Sharing CorrelationAcceptance and Background

-   energy sharing (|*z*|) related to opening angle.
-  distribution is uniform since  has J=0.
-  large energy asymmetry related to large diphoton opening angle.
-  for |*z*|<0.5, get uniform distribution.
-  most of the small *M* background comes from large |*z*|.

-  detected

- one lost

## Toward QM02:  cross section(G.Rakness for STAR)

- Abstract to QM02 for *pp*X cross sections
-                         at s = 200 GeV
- Two positions:  near  3.4 
-                           far     3.1 < 
-                            ~ 0.2 < *x*F 0.6

-  Input towards global analysis of E704 + HERMES spin   asymmetry data
-     Collins fragmentation  transversity
-  Towards understanding of  production from d+Au
-     possible first measurement of gluon shadowing

## Towards a cross section...

-       BBC E.W rate during **Van der Meer scan**

- Absolute normalization
-       from BBC E.W:

-             NEAR POSITION:
- MEAN± RMS = (12.0 ±1.3)

-                FAR POSITION
- MEAN± RMS = (12.0 ±0.6)

- Distribution of Fitted Yields:

- Yield stable w/in 10% for each pos’n

- 50% containment probability

- Still need to establish reconstruction efficiencies from existing simulations

-  in =0.4  0.4 box

## Physics Motivations for a Forward  Detector for STAR

-   Topic I:  Tuning STAR Spin Rotators
-   Topic II:  Spin Physics in the Forward Direction
-   Topic III:  Probing the color glass condensate in d-Au collision

## Proposed for Next Run

**Left/right symmetric calorimeters** ********
-  Pb-glass calorimeter:  two 7x7 matrices of 3.7cm  3.7cm  45 cm Pb-glass detectors
-   Scintillator strip shower maximum detector
-   7-element Pb-glass preshower detector
**Up/down symmetric calorimeters** ******total energy**
-   Pb-glass calorimeter:  two 5x5 matrices of 3.7cm  3.7cm  45 cm Pb-glass detectors
**Readout electronics**
-  employ existing design 8-bit ADC’s operating at RHIC clock frequency.
-  employ existing design of FPGA (data-storage and manipulation, DSM) boards for interface to STAR trigger.

## Probing gluon densities in heavy nuclei

-   Search for predicted modification of pT dependence of large rapidity  yield in   d-Au collisions from gluon saturation effects (Dumitru and Jalilian-Marian, hep-ph/0204028)
-   HIJING/GEANT simulations show that we can reconstruct  in d-Au collisions.  Will be able to reduce the background by tuning the reconstruction algorithm  utilize calorimeter segmentation to discriminate  from additional particles incident on calorimeter.

## Summary

-   Large rapidity  detection works.
-   Possibly large transverse spin effects, but further work is needed to establish consistency of left/right results.
-   A proposal will be submitted to build forward  polarimeters for STAR.
-   Forward  detection also of interest to probe gluon density in heavy nucleus in d-Au collisions.