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Longitudinal spin transfer of Lambda and anti-Lambda in pp collisions at STAR
Qinghua Xu, Shandong UniversitySTAR-TOF workshop, Hangzhou, April 2009
Spin structure of nucleon and its strangeness part S
Longitudinal spin transfer of hyperons in pp at STAR
Transverse spin transfer and forward hyperon physics
Summary & Outlook
RHIC- also a polarized pp collider
0/3.18.5 / 3.43.1 / 0.10.4 / 00.3 / 0.250 / 0.3Lint [pb-1 ] at STAR(Longitudinal/Transverse)
3530166 62Lmax [ 1030 s-1cm-2 ]
45-5040-453015< Polarization> %
2005200420032002pp Run Year
2"# + "G+ < Lq, g >
Quark spin,Best known(~30%) -DIS
Gluon spin,Poorly known,@RHIC
Orbital Angular MomentaLittle known -DVCS
Spin sum rule (longitudinal case):
Spin structure of nucleon
["q = "q(x)dx0
"q(x) = q+(x) # q
q(x) = q+(x) + q
Polarized parton densities:
World data on pol. and unpol. deep-inelastic scattering
g1 (x) =1
" #qi (x)
F2(x) = ei
From the newest global analysis --fit all the available data in DIS, SDIS and pp
D. de Florian et al, PRL101(2008)
[email protected] RHIC 500 GeV
RHIC 200 GeV (jet, 0)
RHIC 500 GeV (jet, 0)
S from polarized DIS
= 0.33 0.030.03
-- with neutron, hyperon decay data using SU(3)_f symmetry,
U~ 0.84, D~ 0.43, S~ 0.08
x["s(x) + "s (x)]
"S'= 0.037 0.019 0.0270.02
Study S at RHIC with hyperons?
decay parameter 0.642 0.013
r P % &
r p p*) Unit vector along proton
momentum in s rest frame.
s contain a strange quark, whose spin is expected to carry most of the spin,
polarization can be measured in experiment via weak decay
Can polarization measurements provide sensitivity to S at RHIC?
The factorized framework enables perturbative description,
- STAR data on described by pQCD with suitable choice of D.!+!
)()()( 21 zDdxfxfd ba!""#$ % &&
Hyperon production in pp
| y |< 0.5
s = 200 GeV
STAR: B. Abelev et al, PRC 75 (2007) 064901 DSV: D. de Florian et al, PRD 57 (1998) 581AKK: B.A. Kniehl et al, NPB 597 (2001) 337
Q. X, E. Sichtermann, Z. Liang, PRD 73(2006)077503
Expectations at LO show sensitivity of DLL for anti-Lambda to :
GRSV00-M.Gluck et al, Phys.Rev.D63(2001)094005
Typ. range at RHIC
DLL-Longitudinal spin transfer at RHIC
Pol. frag. func. models
- Promising measurements---effects potentially large enough to be observed.
- DLL is less sensitive to s, due to large u,d quark fragmentation.
How about other (anti-)hyperons?
- good opportunity when large amount of data available.
Y. Chen et al, PRD78, 054007(2008)
(u u s )
(u s s )
(d d s )
(d s s )
(u d s )
.d and ufor infomation providecan
, as ,s y tosensitivit provideon polarizati
STAR - Solenoid Tracker At RHIC
Time Projection Chamber enables PID
Plus topological reconstruction:
(|" |< 1.3)
~3X106 events collected with a beam-collision trigger (minimum bias, bandwidth limited),
STAR data - 2005
~ 30X103 candidates~ 25X103 !
xF = 2p// / s
Method to extract DLL
polarization is usually extracted from the momentum distribution of its weak decay ( ):
: decay parameter: 0.642A(cos*): detector acceptance
2A(cos"*)(1+# P$ cos"*)
DLL can thus be extracted from counts with opposite beam polarization within a small interval of cos*:
" # Pbeam
< cos$* >#N
% , where the acceptance cancels.
= N+ + L##
L+ ++ N
= N# + L##
Relative luminosity ratio measured with BBC, and Pb in RHIC.
cos"*#r P $ %
r p p*
Cross-check of DLL
The extracted DLL exhibits the expected statistical variation with time:
Null measurements with spin-0 K0s ( larger statistics than )
First DLL results from RHIC:
- Statistics and pT limited,- Need better precision and higher pT.
STAR triggered data - 2005
STAR was triggered on energy deposits in jet-patches of the Barrel E.M. Calorimeter,
(MB~25K, JP ~45K)
sTrigger on high pT jets --> higher pT hyperons in jets
Recorded a (biased) sample of candidates with considerably higherpT, although not directly triggered.
" and "
Systematic uncertainties: 5% scale uncertainty from RHIC beam polarization measurement. 2% from decay-parameter (0.6420.013). 2% from non-longitudinal beam polarization components at STAR.
Anti-proton trigger with BEMC
TPC track (anti-proton from anti-Lambda) is projected to BEMC tower, check whether its DSM adc # passes BHT1 trigger threshold (13, ~2.6GeV):
adc # of p (" ) matched tower
adc # of p (") matched tower
Large fraction (43%) of anti-proton matched tower passed threshold, and only tiny fraction in the case of proton from Lambda.
DLL analysis with this special sample is almost trigger bias free, with pT extended to 4 GeV.
43% passed HT1 threshold (13)
Anti-proton trigger with BEMC
The ratio of anti-Lambda to Lambda is enhanced ~4 times higher compared with MB results.
- This anti-proton trigger (BEMC only) is not efficient for high pT due to limited energy deposit through annihilation effect.
" /" BHT1 (2.6 GeV)
Ideas on anti-Lambda trigger
Higher level trigger (L2) with TOF (Time of Flight) and BEMC: require online a hit in the EM calorimeter (the annihilation effect) and a hit in TOF to veto signals from photon (0),
--> trigger on (high pT) anti-proton and thus anti-Lambda
High level trigger (L3)
- online reconstruction of (anti-)Lambda with TPC L3 tracks
Transverse polarization of hyperons in pp and q(x)
( p#p$H#X ) % d"( p#p$H&X )
d"( p#p$H#X ) + d"
( p#p$H&X )
$$$$ %% &&=
Transverse spin transfer of hyperons to study transverse spin structure of nucleon:
transversity distribution :f(x) = f (x) - f(x) pQCD
J.Collins et al, NPB420 (1994)565
)( )( qqqqV ssSrrr
+=)( )( qqqqV ssS
Transverse polarization direction of the hard scattering parton:
A rotation in scattering plane!
) * Helicity density
Measurements at STAR:
DTT (?): real transfer require the correlation with jet reconstruction
DNN measurement by E704 (fixed target) - w.r.t. production plane
The production plane
Normal of the productionplane
r N :
r N =
r p b "
r p # /(|
r p b "
r p # |)
(= 2p// / s)
DNN: production plane close to hard scattering plane; but precision reduced ~ one half (beam pol. projected to N.)
Example of counts vs. cos*w.r.t. production plane at STAR
Sizable effects may exist at large xF region (in forward direction)
At a minimal z from the FMS, positioned symmetrically left(South) and right (North) of west DX magnet
A proposal of FHC at STAR may enable the study of forward Lambda physics together with FMS through n0 (xF~0.4)
Forward Lambda physics at STAR
- Les Bland, Mar. 09, Collaboration mtg
Summary & Outlook