J. Seele - WWND 1

The STAR Longitudinal Spin Program

Joe Seele (MIT) for the Collaboration

WWND 2009

J. Seele - WWND 2

Outline

• The Spin Puzzle• The STAR experiment• STAR Longitudinal Spin Results

• Future DirectionsSee C. Gagliardi’s Talk Tomorrow For Transverse Spin Results

J. Seele - WWND 3

The Spin Puzzle

€

1

2=

1

2ΔΣ + Lq

z + ΔG + Lgz

Fairly well measuredonly ~30% of spin

A future challenge

The proton is viewed as being a “bag” of bound quarks and gluons interacting via QCD

Spins + orbital angular momentum needto give the observed spin 1/2 of proton

Being measuredat RHIC

€

ΔΣ= (Δu + Δd + Δs + Δu + Δd + Δs +L )dx∫

€

ΔG = Δgdx∫

J. Seele - WWND 4

Helicity Asymmetries

€

ALL ≈ aggΔgΔg + aqgΔqΔg + aqqΔqΔq'€

ALL =σ ++ −σ +−

σ ++ + σ +−=

Δfa ⊗Δfb

a,b,c

∑ ⊗ d ˆ σ fa fb → fc X ⋅ ˆ a LLfa fb → fc X ⊗D fc

h

fa ⊗ fb

a,b,c

∑ ⊗ d ˆ σ fa fb → fc X ⊗D fc

h

and translating…

Take the asymmetry of proton helicity configurations

Needed for hadrons but not jets

J. Seele - WWND 5

RHIC

The world’s first polarized proton collider

J. Seele - WWND 6

STAR

=0

Forward Pion Detector

Endcap EM Calorimeter

Beam-Beam Counters

Time Projection Chamber

1 2-4.1 -3.3

3 5

Solenoidal MagneticField 5kG

=2= -1

200320042005

= - ln(tan2)

=2.5

=4

FMS EM Calorimeter

2008

Barrel EM Calorimeter

-1 1

-2<< 2

J. Seele - WWND 7

STAR Measurements

J. Seele - WWND 8

Inclusive Measurements

A Pro A Con

Reconstruct a piece of the final state without considering the structure of the event

€

rp +

r p → Jet + X

€

rp +

r p → h + Xe.g.

Larger cross section in our acceptance than exclusive or correlation measurements

Does not constrain initial partonkinematics well (e.g. a jet of a given pT could have come from an initial parton with a range of x values) -> does not measure shape

Currently all STAR longitudinal spin results are inclusive measurements

or

J. Seele - WWND 9

Inclusive Jets

Phys. Rev. Lett. 97 (2006) 252001

€

rp +

r p → Jet + X

Smeared x-range for jets at a few pTs

Jets at each pT are a different mix ofsubprocesses

J. Seele - WWND 10

Inclusive Jets ALL (2005)

0.2 < < 0.8

STAR Collaboration, PRL 100, 232003 (2008).

Maximum gluon polarization scenario (GRSV-MAX) ruled out

J. Seele - WWND 11

Inclusive Jets ALL (2006)

ALL systematics (x 10 -3)

Reconstruction + Trigger Bias

[-1,+3] (pT dep)

Non-longitudinal Polarization

~ 0.03 (pT dep)

Relative Luminosity

0.94

Backgrounds1st bin ~ 0.5else ~ 0.1

pT systematic ± 6.7%With the increased statistics in 2006 (~x10)

the uncertainties are greatly reduced

J. Seele - WWND 12

Inclusive Jets - Constraint on ΔG

Using the availablefits the confidence levelis calculated using the2005+2006 data sets

Many of the sets withlarge gluon polarization(neg or pos) are ruledout

Integral constrained to be over the x range probed by STAR

J. Seele - WWND 13

Before RHIC…

J. Seele - WWND 14

Inclusive Jets - Impact on Global Fits

RHIC range

The STAR data provides a strong constraint on ΔG over 0.05<x<0.2 The data favor a small gluon polarization in this range of x

There is a big need for a true mapping of the x dependenceof the gluon polarization and an increased range in x

Phys. Rev. Lett. 101 (2008) 072001

J. Seele - WWND 15

Inclusive Jets vs. Inclusive Hadrons

€

rp +

r p → Jet + X

No fragmentation functions

HadronsJets

€

rp +

r p → h + X

π0

Suffers from JES uncertainty

Needs a convolution with FFs but this gives a selectivity to different flavors

Less uncertainty in pT measurement

Information from inclusive hadron measurements is complementary to

inclusive jet measurements

Jet definition provides a complication when relating theory and measurement

J. Seele - WWND 16

Inclusive Hadrons

π0

Good agreement between theory and data over largerange in pT

MinBias Data - Phys. Lett. B 637 (2006) 161

J. Seele - WWND 17

Neutral Pion ALL (2005+2006)

Reconstructed using the di-photon decay channel

Maximum gluon polarizationScenario (GRSV-Max) is ruled out

2006 preliminary result uncertainties are comparable to PHENIX at pT~8 GeV/c and data extend to a high pT than PHENIX

J. Seele - WWND 18

Charged Pion ALL (2005)

Charged pions are useful for

constraining ΔG1. Can give the sign of

Δg

€

Δg > 0 → ALLπ +

> ALLπ −

€

Δg < 0 → ALLπ +

< ALLπ −

2. π+ is a strong “lever-arm” for measuring Δg especially since Δg is small

€

ALLπ +

∝ΔgΔg + ΔgΔu → ΔgΔu

J. Seele - WWND 19

Charged Pion ALL (2006)A new way of measuring the charged pion. Triggering on a jet and measuring away side pion gives less trigger bias. Allows less biased measurement of something akin to z.

Trigger Jet

Measured Pions

J. Seele - WWND 20

Future Measurements

J. Seele - WWND 21

Di-jets at STAR

Correlation measurementsprovide information about x1 and x2 which can be used to get the shapeof Δg

The plots are expectations from STAR run9 BUR with 50pb-1 and 60% polarization

€

M

s= x1x2

€

3 + η 4 = lnx1

x2

At LO

J. Seele - WWND 22

Ws at STAR (mid-rapidity)

€

u(x1)d (x2) + d (x1)u(x2) → W +

€

u (x1)d(x2) + d(x1)u (x2) → W −

At sqrt(s)=500 GeV, W’s will be produced in p+p collisions

They can be tagged through their lepton+neutrino decay channel

€

ALW +

=Δu(x1)d (x2) − Δd (x1)u(x2)

u(x1)d (x2) + d (x1)u(x2)

€

ALW −

=Δd(x1)u (x2) − Δu (x1)d(x2)

d(x1)u (x2) + u (x1)d(x2)

Single spin asymmetries can be measured to give information about polarized quark pdfs.

Run9 projections

500 GeV Program Projections

€

AL (W +)

€

AL (W +)€

AL (W −)

€

AL (W −)

J. Seele - WWND 23

Ws at STAR (mid-rapidity)

The simulations use full detector response and realistic QCD background.

The main source of background is hadrons so good e/h separation is necessary.

In preparation for the upcoming 500 GeV run, STAR has been studying the reconstruction of the W.

The current analysis uses a combination of tracking, shower shape, isolation style, missing energy style, and event shape cuts.

Run9 W Algorithm Simulation Results

J. Seele - WWND 24

Ws at STAR (forward rapidity)

€

ALW +

(yW >> 0) ≈Δu(x)

u(x)

€

ALW +

(yW << 0) ≈ −Δd (x)

d (x)€

ALW −

(yW >> 0) ≈Δd(x)

d(x)

€

ALW −

(yW << 0) ≈ −Δu (x)

u (x)

At forward or backward rapidity (defined by single polarized proton), the formulas for the single spin asymmetries simplify to

At forward and backward rapidity the rapidity of the lepton, the rapidity of the W and the partonic x are all strongly correlated.

500 GeV Program Projections

€

ALW +(yW >> 0)

€

ALW +(yW << 0) €

ALW −(yW >> 0)

€

ALW −(yW << 0)

J. Seele - WWND 25

Summary

• STAR inclusive measurements at sqrt(s)=200 GeV have made a strong contribution to our knowledge of Δg.

• STAR will continue to impact our knowledge Δg as higher statistics sqrt(s)=200 GeV and sqrt(s)=500 GeV inclusive measurements are included in the global fits.

• Future di-jet (and other correlation) measurements will constrain the shape of Δg.

• The STAR W measurements will probe the polarizations of the anti-quarks in the proton.

J. Seele - WWND 26

Backup Slides

J. Seele - WWND 27

What did we know about ΔG ?

Polarized DIS data does not constrain the gluon distribution very well at

present

Global fits that include DIS data do not constrain ΔG very

well.

x

J. Seele - WWND 28

Inclusive Jets

€

rp +

r p → Jet + X

Require jet pT be large to be in hard scattering regionNo fragmentation functions but relation of measurement to theory is complicated

Suffers from JES uncertainty

Jets at each pT are a mix of subprocesses from a

range in x

J. Seele - WWND 29

STAR

BEMC (Barrel Electromagnetic

Calorimeter: -1 < η < 1)

EEMC (Electromagnetic

Endcap Calorimeter: 1.09

< η < 2)

FMS (Forward Meson

Spectrometer: 2.5 < η < 4)

TPC (Time Projection

Chamber)

FTPC (Forward Time

Projection Chamber)

SSD (Silicon-Strip Detector) BBC (Beam-Beam

Counter)

ZDC (Zero-Degree

Calorimeter)

J. Seele - WWND 30

Helicity Asymmetries

€

ALL =σ ++ −σ +−

σ ++ + σ +−=

Δfa ⊗Δfb

a,b,c

∑ ⊗ d ˆ σ fa fb → fc X ⋅ ˆ a LLfa fb → fc X ⊗D fc

h

fa ⊗ fb

a,b,c

∑ ⊗ d ˆ σ fa fb → fc X ⊗D fc

h

The LO result for aLL is nonzerofor all subprocesses

J. Seele - WWND 31

Inclusive Hadrons

More statistics than jets, but not as clean because of FFsBut also have flavor information to use

€

rp +

r p → h + X

A type of final state particle is measured regardless of rest of event

J. Seele - WWND 32

ΔG in Polarized p+p Collisions

Three Things Needed to Measure ΔG1) Gluons in colliding protons2) Detectable final state sensitive to gluon scattering3) Helicity dependent cross section

STARmid-rapidity

In hard interactions at RHIC energies the

proton is roughly 40% glue

J. Seele - WWND 33

ΔG in Polarized p+p Collisions

Three Things Needed to Measure ΔG1) Gluons in colliding protons2) Detectable final state sensitive to gluon scattering3) Helicity dependent cross section

Gluons enter p+p scattering at leading order and are dominant in the collisions

at low pT

e.g. π0 subprocess fractions at mid-radpidity

J. Seele - WWND 34

ΔG in Polarized p+p Collisions

Three Things Needed to Measure ΔG1) Gluons in colliding protons2) Detectable final state sensitive to gluon scattering3) Helicity dependent cross section

€

ALL =σ ++ −σ +−

σ ++ + σ +−=

Δfa ⊗Δfb

a,b,c

∑ ⊗ d ˆ σ fa fb → fc X ⋅ ˆ a LLfa fb → fc X ⊗D fc

h

fa ⊗ fb

a,b,c

∑ ⊗ d ˆ σ fa fb → fc X ⊗D fc

h

The LO result for aLL is nonzerofor all subprocesses

J. Seele - WWND 35

Spin Transfer