longitudinal spin transfer of in polarized pp collisions at 200 gev
DESCRIPTION
Longitudinal Spin Transfer of in Polarized pp Collisions at 200 GeV. Qinghua Xu, LBNL for the STAR Collaboration SPIN 2006, Oct 6, 2006. Motivation Proof-of-principle analysis at low p T with STAR Preliminary results for D LL in inclusive production - PowerPoint PPT PresentationTRANSCRIPT
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