Single-spin asymmetry of charm di-jet in longitudinally polarized pp collisions at STAR

Introduction and motivation

Reconstruction of W and Z signals

Single-spin asymmetry of di-jet

Summary and outlook

Nucleons’ spin structure

• Spin sum rule for nucleon:

• In naive Quark Model, the nucleon is made up of three quarks, its spin (½) is carried by the three quarks’ spin.

“Spin Crisis” : In 1988, EMC experiment found proton spin carried by quarks’ spin is 20%-30%.

1 1

2 2 q gG L L

Quark spin

u u d d s sGluon spin

Orbital Angular Momentum

Introduction and motivation

Introduction and motivation

Longitudinal spin program at RHIC

( , )

LLjets and leading hadrons A etc

G prompt photons

heavy flavor production

, ( , )LLs s hyperons D etc

, ( , , )

,L LL

u uweak bosons A A etc

d d

Introduction and motivation

SSA of W bosons

p p W X p p W XWL

p p W X p p W X

A

- ( ) 0 -:

p p W Xthe cross section for W bosons production when

helicity of the polarized beam is - (+) .

: W ud W Leading order diagrams for

Introduction and motivation

SSA of W bosons

: ++ - +

In weak interaction, parity is vialated.

W is only left - hand u d W

1 2 1 2( ) ( ) ( ) ( )p p W u d W u d W

d u x d x d d x u x d

1 2 1 2( ) ( ) ( ) ( )p p W u d W u d W

d u x d x d d x u x d

1 2 1 2

1 2 1 2

( ) ( ) ( ) ( )

( ) ( ) ( ) ( )W

L

u x d x d x u xA

u x d x d x u x

Introduction and motivation

SSA of W bosons

1 2 1 2

1 2 1 2

( ) ( ) ( ) ( )

( ) ( ) ( ) ( )W

L

u x d x d x u xA

u x d x d x u x

To abtain asymmetry of , one interchanges and .-W u d

0 :

0 : -

WLW

WLW

uy A approaches

u

dy A approaches

d

1 2, W Wy yW WM Mx e x e

s s

Motivation-----1

Introduction and motivation

Reconstruction of W

Leptonic channel :

eW e

t almost all high P electrons

come from this process,

the background is small

neutrinos can not be detected,

there is missing energy,

the kinematics

Advantages :

Disadvantages:

of W can not be claimed clear

Hadronic channel :

W di jet

/ 0

the branching ratio is 6 times

larger than leptonic channel,

the kinematics of W can be claimed

the background is big,

QCD 2 2 process

Z

di - jet

Advantages :

Disadvantages:

Introduction and motivation

Leptonic channel

: t

P distribution of electron in W e+

: (10.72 0.16)%Branching ratio

* lab *e e W e

Relation between electron rapidity and W rapidity : y = y + y where y is lepton rapidity in W rest frame

Introduction and motivation

Hadronic channel

2 2 - Di jets from QCD process di jets from W bosons

(67.96 0.35)Branching ratio: %

0

supress the QCD di- jets very much

supress Z di- jet more than W di- jet

Charm di- jet ( at least one charm jet )

Motivation-----2

& the kinematics of W can be claimed directly

Introduction and motivation

0

0( ) ( ) ( )

( ) ( ) ( )

di jet W W Z

L L L L

W W ZA A A A

tot tot tot

0( ) ( ) ( ) ( */ ) ( )tot W W Z QCD

( ) ( )

( ) ( )di jetL

p p di jet p p di jetA

p p di jet p p di jet

+ -

0

W ,W

production of di - jet Z

QCD 2 2 process

LQCD 2 2 process : A = 0 Parity conservation

SSA of di-jet

Reconstruction of W and Z signals

UA2 experiment

-1

630GeV p p collisions

Luminosity : 0.73 pb

mass resolution : 10%

630 190 0

signals are observed in

the mass region of W and Z

R.Ansari et al [UA2 Collaboration], Phys. Lett. B 186, 452 (1987)

Reconstruction of W and Z signals

Fast Simulation of the invariant mass distribution of di-jet in pp collision at STAR.

Mass resolution : 5%

Fast simulation of the invariant mass distribution of charm di-jet in pp collision at STAR.

Reconstruction of W and Z signals

Mass resolution : 5%

From W+ From W- From Z0 QCD 2->2 W+/total

common di-jet 59256 19764 4200 7.04E+6 0:0083

charm di-jet 29236 9924 704 1.92E+5 0:126

cross section:-6

0 -7

-3

:1.133 10

:3.278 10

2 2 : 2.381 10

pp W mb

pp Z mb

QCD process mb

comparison:

1

s = 500GeV; luminosity = 400pb ; E > 20GeV; < 1t

mass resolution = 5%; mass window= [75GeV - 85GeV]

Reconstruction of W and Z signals

Reconstruction of W and Z signals

di-jet charm di-jet

6

1+(-)W 1 210Z 1

1QCD 1 30

Single-spin asymmetry of di-jet

SSA of pure W bosons

PDF : GRV98polarized PDF : GRSV2000

Single-spin asymmetry of di-jet

SSA of di-jet

PDF : GRV98polarized PDF : GRSV2000

Single-spin asymmetry of di-jet

SSA of charm di-jet

PDF : GRV98polarized PDF : GRSV2000

Single-spin asymmetry of di-jet

SSA of charm di-jet events & the charge of charm can be determined

PDF : GRV98polarized PDF : GRSV2000

Summary and outlook

Summary

L L

L

• q, q of light flavor A of W bosons A of di - jet

• W from di- jet significance is not good W from charm di- jet improve significance

• A of pure W signal is large with

L

L

GRV98 & GRSV2000

• A of di- jet is too small, not very hopeful large background

• A of charm di- jet improve significance 4 times hopeful, need further work

Summary and outlook

Outlook

0L

L

• Contribution of Z bosons ( production and A )

• Detail study of QCD background ( different sub process )

• Behavior in different resolution and acceptance

• A with other polarized PDF

• Kin tematic and topological cuts ( E , , etc. )

• Comparison between leptonic channel and hadronic channel

Why DLL of (anti-)lambda in STAR?

P is more sentive to s model

0 0

0 0

p p H X p p H X

LL

p p H X p p H X

D

f D

Longitudinal spin transfer in polarized proton-proton collisions:

Pol. frag. func. model

models s

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

GeV 8

GeV 200

Tp

s

Theory prediction on polarization of anti-lambda in RHIC energy:

into LL

D may give new insights s at high pT

P is more sentive to s model

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

Data sample:

Minbias data from run 2005 About 3M events

Kinetics range:

Qinghua Xu for STAR

Jet define algorithms:

Kt algorithms and cone algorithms

See Salam’s talk

RHIC : Relativistic Heavy Ion Collider

STAR : The Solenoidal Tracker at RHIC

TPC : time projection chamber

EMC : E-M calorimeter