# Presentation: 501055

## Lambda production at high rapidity in

**        ****d+Au collisions at**

**200 GeV**

***NN***

***s***

- 

- Frank Simon, Max-Planck-Institut für Physik, Munich, Germany for the STAR collaboration

**The STAR Forward TPCs**

-  cover 2.5 < || < 4.0, extend STAR tracking into forward and backward rapidity region
-  2 FTPCs in STAR, 10 rows per detector, 960 pads per row  => 19200 channels
-  inner radius ~ 8 cm, outer radius ~ 30 cm, detector gas Ar:C02 50:50
-  radial electron drift for improved two-track resolution in high track density environment
-  tracking with momentum and charge determination
-  no particle ID from dE/dx (maximum 10 points on track, high average particle momentum)

**d+Au collisions @ STAR**

**Forward ********s: Analysis Technique**

- The s are reconstructed by combining possible decay daughters to V0 pairs and testing these pairs to see if they make up a . Since there is no particle identification to select the proton, all positive/negative pairs have to be analyzed. This leads to significant background contributions and the need for strict geometric cuts.

- Dominant decay mode: p BR 64%, c ~ 8 cm)

- dN/d

- 

- Deuteron

- Gold

- FTPC
- East

- TPC

- FTPC
- West

- Cuts on particle pairs:
-  at the primary vertex:
-  dca of proton candidate < 2 cm
-  dca of pion candidate > 2 cm
-  at decay vertex:
-  dca of the daughters <0.25 cm
-  pair cuts:
-  dca of back-extrapolated  candidate to primary vertex < 0.75 cm
-  decay length > 5 cm

- Background Studies

- Due to the lack of particle identification, Kaons are a sizeable contribution to the background. The figure on the right shows the invariant mass distribution of Kaons from a HIJING simulation assuming the proton mass for one of the two daughters.

**Analysis Results**

- From a sample of 10.6 million d+Au minimum bias events Lambda and Anti-Lambda signals around || ~ 3.2 have been extracted:

- Raw  signal from FTPC West (d side)
- [half of total available statistics]
- The background estimation based on the assumption of misidentified Kaons,  normalized on the integral from 1.3 GeV to 2 GeV is shown in red.

- The signal after background subtraction for both  and anti- in both FTPCs. The yield is extracted by summing up all bins from 1.08 GeV/c2 to 1.2 GeV/c2

- Kinematic parameters of  candidates

- rapidity and pseudorapidity of s on the d side (half of total stats)
- the shaded area shows the candidates accepted by the rapidity cuts
- mean rapidity: 2.6
- mean pseudorapidity: 3.3

- decay time for  candidates
- FTPC W,  0.4 GeV/c < pt < 0.7 GeV/c
- selected to avoid acceptance constraints
- The exponential fit gives c ~ 7.9 cm

- Anti-particle to particle ratios

- So far, no efficiency corrections for  reconstruction at high rapidity exist, but it is expected and indicated by simulation studies that the reconstruction efficiency for particle and anti-particle is equal. This permits the extraction of  anti-particle to particle ratios from the uncorrected yields. Note that there are no corrections for absorption or feeddown. Simulations show that due to the high total momentum of particles in the forward region, the effects of absorption are less than 2%, and thus negligible compared to the other error contributions.
- All errors are statistical only, the systematic errors on the ratios in this preliminary analysis are estimated to be of the order of 0.1 by using different methods of background subtraction and varying the cuts.

- 0.4 GeV/c < pt < 1.5 GeV/c,  y cuts on <y> ± 0.25

- <y> = 2.69 (deuteron side)  anti- /  = 0.58 ± 0.02

- <y> = -2.72 (gold side)         anti- /  = 0.71 ± 0.04

- Comparison of the measured anti- /  with HIJING. The HIJING plot is made without analysis-specific cuts. The mid-rapidity value from STAR is corrected for absorption, the values at high rapidity are the uncorrected FTPC values. It is known from other collision systems (Au+Au and p+p) that HIJING tends to over-predict  ratios. The disagreement on the Au side is expected since HIJING does not model rescattering processes.

- Transverse momentum resolution for  in FTPC W for 0.4 GeV/c < pt < 0.7 GeV/c from full HIJING simulations.
- The Gaussian fit yields ~21%, which is due to the limited momentum resolution for tracks not originating from the primary event vertex.

- The forward TPCs in STAR are well placed to study the asymmetric collision system d+Au. In the STAR experiment, deuterons enter from the east, gold from the west. This leads to the higher multiplicity in FTPC E, as seen on the left. The asymmetry increases with collision centrality. The dashed lines are saturation model calculations, the solid blue line is a HIJING model. HIJING gives a good description of the multiplicity density in the collisions. It will be used later for comparison with strange particle ratios, although it is known that HIJING over-predicts these ratios. For more details on forward spectra, see the poster by J.Putschke.

**Motivation: anti-particle to particle ratios**

- In relativistic heavy ion collisions, the ratio of yields of anti-particles and particles reach values
- near unity at mid-rapidity, indicating an almost net baryon free environment.
- At forward rapidity, the baryon content of the beam nuclei plays a significant role, and in addition to particle-antiparticle pair production, other processes such as associated production contribute significantly to the particle production. In d+Au collisions, an asymmetry between the gold and deuteron side is expected due to different contributions of production processes.
- For results on anti-proton/proton in Au+Au collisions see PRL 90 102301 (2003) (BRAHMS)

- Acceptance for  candidates in FTPC W as a function of pt and y. Since the FTPC covers a constant  region, the mean pt of the s rises with increasing y.

**Conclusions**

- The first study of anti- and  at forward rapidity in d+Au collisions at RHIC shows a slight asymmetry of the anti-/ ratio. This indicates some recattering effects in the cold nuclear matter of the Au nucleus and possibly differences in baryon transport. These can be explained by the fact that nucleons from the deuteron suffer several collisions while passing through the Au nucleus, while nucleons from the Au typically only participate in one collision.
- More detailed comparisons with different models are needed in future studies.

**STAR preliminary**