1

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 ΔS

• 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, π0)

RHIC 500 GeV (jet, π0…)

?STAR

6

ΔS from polarized DIS

• Inclusive DIS:

⇒ ΔΣ= 0.33 ±0.03±0.03

-- with neutron, hyperon β decay data using SU(3)_f symmetry,

ΔU~ 0.84, ΔD~ −0.43, ΔS~ −0.08

!

s

KHERMES, PLB666,446(2008)

!

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

(HERMES,Q2=5 GeV2)

!

"S'= 0.037 ± 0.019 ± 0.0270.02<x<0.6, Q2=2.5 GeV2

• Semi-inclusive DIS:

Measurements in other channels/reactions are needed!

7

Study ΔS at RHIC with hyperons?

decay parameter 0.642 ±0.013

!

dN

d"#1+$ (

r P % &

r p p*) Unit vector along proton

momentum in Λ’s rest frame.

Λ polarization vector

• Λ’s contain a strange quark, whose spin is expected to carry most of the Λ spin,

• Λ polarization can be measured in experiment via weak decay

• Can polarization measurements provide sensitivity to ΔS at RHIC?

)(!!

8

• The factorized framework enables perturbative description,

- STAR data on described by pQCD with suitable choice of D.!+!

)(ˆ)()( 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!

- Λ DLL is less sensitive to Δs, 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

11

BBCEast

BarrelEMC

BBCWest

TPC

STAR - Solenoid Tracker At RHIC

p

V0_vertex

V0_DCA

Λ

rr

!"

Time Projection Chamber enables PID

Plus topological reconstruction:

4.2m

!

(|" |< 1.3)

for |η|<~1.3

12

M=1.1157 GeV(PDG)

!

!

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

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

STAR data - 2005

~ 30X103 Λ candidates~ 25X103 !

!

xF = 2p/// s

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Method to extract DLL

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

α: decay parameter: 0.642A(cosθ*): detector acceptance

!

dN =Ntot

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

!"# $p

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

!

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: 0.01±0.01

δ LL/

P bea

m

# fill

!

take "K0s

= 1

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

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Results I

• First DLL results from RHIC:

systematic error

<pT>~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 (0.642±0.013).• 2% from non-longitudinal beam polarization components at STAR.• <0.01 from relative luminosity measurement.• <5% background fraction.• <4% pile-up effects in TPC.• <15% trigger bias estimated from MC simulation.

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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 (π0),

--> 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 δq(x)

!

PTH

=d"

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

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

( p#p$H&X )

!

"µµµ#

!

"

d

dzDxfxfdzdxdxdp

d

ddcba

TH

cTbba

abcd

aba

p

T

XHpp

T

T

)(222

)( ˆ),(),(),(

min

$$$$ %% &&=

&'((

rrrr

• Transverse spin transfer of hyperons to study transverse spin structure of nucleon:

transversity distribution :δf(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:) θ

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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

producedΛ

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 Λ counts vs. cosθ*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 Δz 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 Λ→nπ0 (xF~0.4)

Forward Lambda physics at STAR

- Les Bland, Mar. 09, Collaboration mtg

FHC

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Summary & Outlook

• Expectations for spin transfer DLL measurements at RHIC, show sensitivity to at high pT .

• We have performed the first proof-of-principle measurement at RHIC; reached ~10% precision with transverse momentum up to 4 GeV. - not yet to discriminate between quark distribution scenarios, - need to extend the pT coverage with specific trigger

• Transverse spin transfer of hyperons can shed light on transversely polarized parton distributions.

s!

!

"

• Promising forward Lambda spin physics with FHC at STAR. - longitudinal & transverse spin transfer at large xF.

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Thank you !

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Paper proposal: Lambda/Anti-Lambda DLL

P.A.: Q.H. Xu, E. Sichtermann, W. Zhou

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Λ→ n+π0 with FMS+FHC not so different from ω→γ+π0 with FMS alone (A.Gordon, Moriond, 2009).

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SN438

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Signal and Background

• Subtracting bg. contribution to DLL

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

sK0

%60

raw

LLDbkg

LLD

r: fraction of background under the peak!

DLL

sig=DLL

raw" rDLL

bkg

1" r

r

DrDD

bkg

LL

raw

LLsig

LL!

!=

1

222 """

Reject

Accept

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• spin transfer versus double spin asymmetry:

models s with !LLA

models s with !LLD

standard GRSV00

valenceGRSV00

0<η<1 for DLL

-1<η<1 for ALL

[GeV]

XppXpp

XppXpp

LLD!+++

!+++

"#"#

"#"#

+

!$

%%

%%

XppXpp

XppXpp

LLA

!"!"

!"!"

#+++

#+++

+

#$

%%

%%

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

- net advantage owing to anti-Lambda spin being carried mostly by anti-s quark and the dominance of anti-s quark fragmentation

s!

!