Qinghua Xu, LBNL 1

Longitudinal Spin Transfer of in Polarized pp Collisions at 200 GeV

Qinghua Xu, LBNL

for the STAR Collaboration

SPIN 2006, Oct 6, 2006

)(ΛΛ

• Motivation

• Proof-of-principle analysis at low pT with STAR

• Preliminary results for DLL in inclusive production

• Summary and outlook

)(ΛΛ

Qinghua Xu, LBNL 2

Why study ?

• Due to its self-analyzing weak decay, Λ polarization has been widely studied in different processes with many interesting results (following talks).

)(ΛΛ

decay parameter 0.642 0.013

)ˆ( 1d

dNppP ⋅+∝

Ω Λ

rα Unit vector along proton mom

entum in Λ rest frame.

Λ polarization vector

• What can we learn from polarization with polarized pp collider at RHIC?

)(ΛΛ

Qinghua Xu, LBNL 3

How sensitive is anti-Lambda DLL

to

• Longitudinal spin transfer in polarized proton-proton collisions:

• contains a strange quark, can it provide sensitivity to

?ss ΔΔ or

What can we learn with in polarized pp collisions ?

€

DLL ≡σ

p + p →Λ+X−σ

p + p →Λ−X

σp + p →Λ+X

+ σp + p →Λ−X

,

)(ΛΛ

Λ

?sΔ

ΛΛ &

measures the transfer of beam polarization to hyperon.

fraction production

fΔ DΔ

Δ

Λ production is expected to be dominated by u quark, while s_bar plays a dominate role for production at large pT!

Λ

[GeV]

)(ΛΛ

PYTHIA

€

s = 200 GeV

Qinghua Xu, LBNL 4

• Anti-Lambda DLL may give new insights into at high pT!sΔ

1)(PRD63,200 GRSV2000

standard

valence

Q2=5 GeV

-0.090.02 (Inclusive DIS)

0.010.03 (Semi-inclusive

DIS-HERMES)

GeV 8

GeV 200

>=

Tps

Q. X, E. Sichtermann, Z. Liang, PRD 73,2006

Pol. frag. func. model

x

models sΔ

:ss Δ+Δ

• Spin transfer of in pp collisionΛ

€

xΔs (x)

Qinghua Xu, LBNL 5

XppXpp

XppXppLLD

−+++

−+++

Λ→Λ→

Λ→Λ→

+

−≡

σσσσ

XppXpp

XppXppLLA

Λ→Λ→

Λ→Λ→

−+++

−+++

+

−≡

σσσσ

• Spin transfer versus double spin asymmetry

• DLL is ~ 4X more sensitive to than ALL.

• ~0.01 measurement will distinguish parameterizations.

sΔsΔ

models s with ΔLLA

models s with ΔLLD

standard GRSV00 valenceGRSV00

0<<1 for DLL

-1<<1 for ALL

[GeV]

Qinghua Xu, LBNL 6

RHIC- the first polarized pp collider

AGS Heclical Partial Snake

Stable polarization direction - transverseLongitudinal polarization at STAR/Phenix

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

Qinghua Xu, LBNL 7

STAR Detector

• primary vertex

p

V0_vertex

V0_DCA

Λ

rr

• is reconstructed by combining TPC tracks with opposite charges after particle identification from energy loss and applying topological cuts.

TPC

−π

)(ΛΛ

(-1.6<<1.6)

Qinghua Xu, LBNL 8

• 2005 data: ~3 pb-1, ~50% beam polarization ~3X106 minimum bias events, ~30X103 (24X103) analyzed.)(ΛΛ• Invariant mass & kinematics

M=1.1157 GeV(PDG)

Λ

Λ

nucl-ex/0607033, submitted to PRC

• Cross section of reasonably described by pQCD.

Λ+Λ

<pT>~1.3 GeV<|xF|>~0.0075

Qinghua Xu, LBNL 9

Method to extract DLL

• Λ polarization is usually extracted from the momentum distribution of its weak decay ( ):

: decay parameter: 0.642A(cos): detector acceptance after integrating over momentum space

12 )cos)((cos Λ+= PAdN totN

−→Λ πp: angle between the momentum of decay proton in Λ’s rest frame and Λ’s momentum at the lab frame

• PΛ() in inclusive pp reaction change its sign when flipping beam helicity, due to parity conservation: )()( +

Λ−Λ −= PP

• Asymmetry between Λ counts with opposite polarization within : [1,2]:

2

coscos

)()(

)()( 21 +⋅≈

−+−−

ΛΛΛ

ΛΛ PPNPNPNPN

€

N(P) = N0A(cosθ)(1+ αPΛ cosθ)d cosθθ1

θ 2

∫ ⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

−+

−+

−−

⋅><⋅

=NN

NN

PD

beamLL θα cos

1−−+−−

−++++

+=

+=

+−

−−

−+

−−

++

−−

NNN

NNN

LL

LL

LL

• Equation to extract DLL:

Qinghua Xu, LBNL 10

Signal and Background

• Subtracting bg. contribution to DLL

• Background from K0s: --- a cut of cos<-0.2 applied.

sK0

%60

rawLLDbkg

LLD

r: fraction of background under the peak

r

rDDD

bkgLL

rawLLsig

LL −−

=1

1)

r

DrDD

bkgLL

rawLLsig

LL −−

=1

222 δδδ

)() _bkgsiderawLL NND − 2

Qinghua Xu, LBNL 11

€

DLL (N raw − N side _ bkg ) : 0.092 ± 0.081

€

DLLsig = (DLL

raw − rDLLbkg ) /(1− r) : 0.094 ± 0.077

• Comparison of DLL with two methods:

• Time stability of DLL:

# fill

Qinghua Xu, LBNL 12

Results of DLL :

• Positive is along the polarized beam direction. • Statistical uncertainty is ~0.05, systematic < 0.01.

<pT>~1.3 GeV<|xF|>~0.0075

Systematic error

€

---scale uncertainty from beam polarization measurement not included.

Qinghua Xu, LBNL 13

Systematics:• 4X10-3 from relative luminosity measurement.• 2% from decay-parameter (0.6420.013).• 2% from transverse beam polarization components at STAR.• +17% scale uncertainty from RHIC beam polarization measurement.

Cross check with K0s:

• K0s are spin-0 meson -> null  measurement.

• reconstruction/analysis similar to (anti)Lambda.

• Statistical error is ~1/5 of (anti)Lambda’s DLL.

# fill

K0s: 0.010.01

δL

L/Pb

eam

Qinghua Xu, LBNL 14

Summary

• Spin transfer DLL for anti-Lambda in proton collisions is sensitive to at large pT (pT > ~6 GeV/c)

• Proof of principle analysis from 2005 minimum bias data

sΔ

<pT>~1.3 GeV<|xF|>~0.0075

Systematic error

---scale uncertainty from beam polarization measurement not included.

Qinghua Xu, LBNL 15

• Triggering is needed to efficiently reach high pT.

• In 2005, we collected most of the data with a jet-patch trigger and the projected precision is:

• Considering dedicated trigger development, and

looking forward to high-luminosity running periods.

Outlook

1.6 pb-1 of BJP20<<1

Λ

Qinghua Xu, LBNL 16

Backup slides

Possible dedicated trigger for high pT anti-Lambda:

• Different pre-shower response to anti-proton in electromagnetic calorimeter than photon

• Triggering on high pT pion with muon trigger

• Higher level trigger on distribution of invariant mass versus momentum ratio of proton and pion

Qinghua Xu, LBNL 17

Invariant mass distribution of Λ in different pT range (BJP)

Qinghua Xu, LBNL 18

(anti)Lambda counts versus pT with BJP2 trigger of 2005

Qinghua Xu, LBNL 19

Decay contribution to anti_Lambda production in pp

Qinghua Xu, LBNL 20

Λ