# Presentation: 909594

## Forward Meson Spectrometer (FMS)status report

- Hank Crawford for FMS group
- L.Bland, F.S.Bieser, R.Brown, A.A.Derevschikov, J.Drachenberg, C.Eskew, C.Gagliardi, S.Heppelmann, J.Engelage, L.Eun, E.Judd, V.I.Kravtsov, Yu.A.Matulenko, A.P.Meschanin, D.A.Morozov, M.Ng, L.V.Nogach, S.B.Nurushev, A.Ogawa, J.Passaneau, C.Perkins, G.Rakness, J.Rothenberg, K.R.Shestermanov, M.Tatarowicz, A.V.Vasiliev, M.Zucker

## Overview

- STAR Forward  Detector (FPD) proved we can reconstruct forward  in pp, dAu, and CuCu environments

## Overview (2)Spin Physics Results

-   Discovered π0 asymmetry in pp
-   Proved NLO pQCD works in forward region at RHIC

- J. Adams et al. (STAR), Phys. Rev. Lett. **92** (2004) 171801

## Overview (3)Probing the gluon density in the Au nucleus

- Showed suppression of forward π0 in dAu

- G. Rakness, for STAR [hep-ex/0507093]

## Overview (4)FPD++ and Forward Meson Spectrometer (FMS)

- FMS proposed to enlarge acceptance for gluon distribution studies
- Stage FPD++ for Run6 as FMS engineering run
- Stage full FMS for Run7 dAu

## FMS Physics GoalsThese we know we can do from FPD analysis of π0

- 1. Measure gluon distributions xg(x) in protons and gold nuclei from 0.001<xb<0.1
- Check universality of xg(x) in region of overlap with DIS (0.02<xb<0.1)
- 2. Characterize correlated pion distributions as a function of Q2 to search for onset of saturation effects
- Is Au a Color Glass Condensate (CGC)?
- 3. Resolve the origin of large transverse spin asymmetries in p+p -> 0+x for forward 0 production

## Expanded FMS Physics GoalsWe intend to test these ideas using an FPD++ in Run6

- 4. Measure g(x) using direct photons
- Is known asymmetry in pp  π0 present in pp  ?
- Much simpler probe because no final-state effects
- 5. Measure J/ (with small acceptance)
- Simulations underway

## How do we measure g(x)?

- Quark from Blue beam scatters off gluon in Yellow beam to produce π0 or  in forward direction (2.8<<4.2)
- We measure E, , and  for π0 or  in coincidence with π0 ,   or leading hadron jet surrogate from BEMC, EEMC, or accompanying (s) within FMS to cover 0.001< x <0.1

## How do we tell if there is a CGC?

-  ln(1/x) and the scale (Q) is taken as pT
- Require two  (jets) in FMS  probes smallest x gluons in Au nucleus (largest )
- Look for broadening or disappearance of  peak as pT decreases

- pT decreasing

## FPD++ Physics for Run6

- We intend to stage a large version of the FPD to prove our ability to detect direct photons. These give a cleaner signal of the underlying *qg* interaction because they are free of final state interactions.

## How do we detect direct photons?

- Isolate photons by having sensitivity to partner in decay of known particles:
-  π0   M=0.135 GeV  BR=98.8%
-  K0  π0π0       0.497  31%
-                            0.547   39%
-   π0              0.782  8.9%
-  ’                         0.958  2.1%
- Other decay modes yield more photons with less Q
- Background simulations underway

## Where do decay partners go?

-   Gain sensitivity to direct photons by making sure we have high probability to catch decay partners
-   This means we need dynamic range, because photon energies get low (~0.25 GeV), and sufficient area (typical opening angles few degrees at our  ranges).

- m =  di-photon parameters
- z = |E1-E2|/(E1+E2)
-  = opening angle
- Mm = 0.135 GeV/c2 ()
- Mm=0.548 GeV/c2 ()

## Sample decays on FPD++

- With FPD++ module size and electronic dynamic range, have >95% probability of detecting second photon from  decay.

## On to the Full FMS

- Following Run6 we will have shown direct photon capability which will only improve with larger detector. We will have proven electronics and trigger schemes and may well have shown J/ capability, which will also only improve with acceptance of FMS

## STAR detector layout with FMS

**TPC: -1.0 < ******** < 1.0**
**FTPC: 2.8 < ******** < 3.8**
**BBC : 2.2 < ******** < 5.0**
**EEMC:1 < ******** < 2**
**BEMC:-1 < ******** < 1**
**FPD: |********| ~ 4.0 & ~3.7**

## Calorimeter cells - all for freeThanks to FNAL (U.Col) and Protvino

- 800 cells (5.8x5.8x60 cm3: 25 Lrad) of lead glass (PbGl) from E831 at FNAL including PMTs (XP2202) with active bases
- Arrived at BNL in June. Students from PSU, UCB, and TAMU working to refurbish, test, and characterize in bldg 510 rm 3-180
- 650 cells (3.8x3.8x45 cm3: 25 Lrad) of PbGl from IHEP Protvino including PMTs (FEU-84) needing bases
- 150 on site and used in FPD West.
- 500 more expected to arrive at BNL in September
- These are fully understood in terms of response

## FNAL E831Cells head to BNL

## Students prepare cells at test Lab at BNL

## FPD++ and FMS Timeline

## Electronics

- New digitizer boards - 32 channels in 9U VME
- 	12 bit ADC - 0.25 pC sensitivity
- 	5 bit TAC - 5 ns sensitivity
- 	10 MHz operation - fully pipelined
- 	bit selection for L0 triggering (sum, HT, ID, other?))
- Use existing FPDW DSM tree

## Conclusions

- We have shown we can do π0 reconstruction in the FPD for CuCu and pp and discovered large spin asymmetry at large  in polarized pp where the cross section is explained by NLO pQCD. We have observed suppression of forward π0 in dAu
- We intend to use the FPD++ arrays to show that we can measure direct  signal and to continue the study of asymmetry in pp
- Run6 will allow engineering test of “new” calorimeter cells and of the new 12 bit digitizer boards
- We will work with BNL management to produce the full electronics set for the FMS and stage it for Run7 measurements of the gluon distribution in protons and Au nuclei covering 0.001<x<0.1, checking the universality of the pdf’s determined through DIS

## Sivers Geometry

## FMS

- Inner cells are 3.8 cm edge, outer are 5.8 cm
- Showing FPD N,S,T,B for size scale
- Circles indicate  range of detector