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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

2

RHIC- also a polarized pp collider

2008

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 ]

4560

2006

45-5040-453015< Polarization> %

2005200420032002pp Run Year

++---+-+

3

!

"S

!

1

2=1

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

1

# ]

!

"q(x) = q+(x) # q

#(x)

!

q(x) = q+(x) + q

"(x)

!

"

!

+

Polarized parton densities:

4

World data on pol. and unpol. deep-inelastic scattering

!

g1 (x) =1

2ei2

i

" #qi (x)

!

F2(x) = ei

2

i

" xqi(x)

5

From the newest global analysis --fit all the available data in DIS, SDIS and pp

D. de Florian et al, PRL101(2008)

W@ RHIC 500 GeV

RHIC 200 GeV (jet, 0)

RHIC 500 GeV (jet, 0)

?STAR

6

S from polarized DIS

Inclusive 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

!

s

KHERMES, PLB666,446(2008)

!

x["s(x) + "s (x)]

(HERMES,Q2=5 GeV2)

!

"S'= 0.037 0.019 0.0270.02

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Study S at RHIC with hyperons?

decay parameter 0.642 0.013

!

dN

d"#1+$ (

r P % &

r p p*) Unit vector along proton

momentum in s rest frame.

polarization vector

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?

)(!!

8

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

9

GeV 8

GeV 200

>

=

Tp

s

Q. X, E. Sichtermann, Z. Liang, PRD 73(2006)077503

models s!

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.

s!

- DLL is less sensitive to s, due to large u,d quark fragmentation.

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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

+,

,

!!"

#!$

%

+0

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BBCEast

BarrelEMC

BBCWest

TPC

STAR - Solenoid Tracker At RHIC

p

V0_vertex

V0_DCA

rr

!"

Time Projection Chamber enables PID

Plus topological reconstruction:

4.2m

!

(|" |< 1.3)

for ||

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M=1.1157 GeV(PDG)

!

!

~3X106 events collected with a beam-collision trigger (minimum bias, bandwidth limited),

~1.3 GeV~0.0075

STAR data - 2005

~ 30X103 candidates~ 25X103 !

!

xF = 2p// / s

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Method to extract DLL

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

: decay parameter: 0.642A(cos*): detector acceptance

!

dN =Ntot

2A(cos"*)(1+# P$ cos"*)

!"# $p

DLL can thus be extracted from counts with opposite beam polarization within a small interval of cos*:

!

DLL

=1

" # Pbeam

< cos$* >#N

+% N

%

N+% N

% , where the acceptance cancels.

!

N"+

= N+ + L##

L+ ++ N

+# L##

L+#

N"#

= N# + L##

L#++ N

##

Relative luminosity ratio measured with BBC, and Pb in RHIC.

!

cos"*#r P $ %

r p p*

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Cross-check of DLL

)(!LLD

The extracted DLL exhibits the expected statistical variation with time:

# fill

K0s: 0.010.01

LL/

P bea

m

# fill

!

take "K0s

= 1

Null measurements with spin-0 K0s ( larger statistics than )

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Results I

First DLL results from RHIC:

systematic error

~1.3 GeV

- Statistics and pT limited,- Need better precision and higher pT.

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STAR triggered data - 2005

STAR was triggered on energy deposits in jet-patches of the Barrel E.M. Calorimeter,

(MB~25K, JP ~45K)

Jet-patch trigger

Minimum bias

!

"

raw

sign

al c

ount

sTrigger on high pT jets --> higher pT hyperons in jets

Recorded a (biased) sample of candidates with considerably higherpT, although not directly triggered.

!

" and "

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!

DLL ypreliminarSTAR

systematic error

Results II

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.

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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)

3% mis-match

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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)

BHT2 (3.6GeV)

-uncorrected

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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

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Transverse polarization of hyperons in pp and q(x)

!

PTH

=d"

( p#p$H#X ) % d"( p#p$H&X )

d"( p#p$H#X ) + d"

( p#p$H&X )

!

"#

!

"

d

dzDxfxfdzdxdxdp

d

ddcba

TH

cTbba

abcd

aba

p

T

XHpp

T

T

)(222

)( ),(),(),(

min

$$$$ %% &&=

&'((

rrrr

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

rrr+=

qin

qout

Transverse polarization direction of the hard scattering parton:

A rotation in scattering plane!

!

" =1

2

1 PTe#i$

PTei$

1

%

& '

(

) * Helicity density

matrix:)

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Measurements at STAR:

DNN ,

DTT (?): real transfer require the correlation with jet reconstruction

DNN measurement by E704 (fixed target) - w.r.t. production plane

target

produced

The production plane

Normal of the productionplane

!

r N :

!

r N =

r p b "

r p # /(|

r p b "

r p # |)

bpr

!

(= 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)

PRL78,4003(1997)

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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

FHC

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Summary & Outlook

Expectations for