Spin Recent Results, Future Directions

Carl Gagliardi – STAR Spin: Recent Results, Future Directions 1

Spin

Recent Results, Future Directions

Carl A. GagliardiTexas A&M University

for the CollaborationSTARSTAR

STARSTAR


What contributes to the proton spin?Consider a proton moving

toward the right

Δq(x)Δg(x)

δq(x)

Proton spin

Proton spin

Spin sum rule:

Polarized DIS: ~ 0.3 Both are poorly constrained

Transversity – very little data

STAR spin program:

Exploring poorly determined components of the proton


RHIC: the world’s first polarized hadron collider

• Spin varies from rf bucket to rf bucket (9.4 MHz)• Spin pattern changes from fill to fill• Spin rotators provide choice of spin orientation• “Billions” of spin reversals during a fill with little if any depolarization

BRAHMS

PHENIX

AGS

BOOSTER

Spin Rotators(longitudinal polarization)

Solenoid Partial Siberian Snake

Siberian Snakes

200 MeV PolarimeterAGS Internal Polarimeter

Rf Dipole

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC Polarimeters

Strong Helical AGS Snake

Helical Partial Siberian Snake

Spin Rotators(longitudinal polarization)

Spin flipper

Siberian Snakes

STAR

PHOBOS

Pol. H- SourceLINAC


STAR detector in cross section

BEMC

EEMC FMSFPD TPC

BBC


Essential benchmark – unpolarized cross sections

• Mid-rapidity jet cross section is consistent with NLO pQCD over 7 orders of magnitude

• Forward rapidity π0 cross section also consistent with NLO pQCD• Many other examples• pQCD works over a very broad kinematic range at RHIC energies

STARSTAR PRL 97, 252001 STARSTAR PRL 97, 152302


STAR longitudinal spin program

• What is the polarization of the gluons in the proton?

Three recent fits of equal quality:– ΔG = 0.13 ± 0.16– ΔG ~ 0.006– ΔG = -0.20 ± 0.41all at Q2 = 1 GeV2

Leader et al, PRD 75, 074027

• Additional question for future 500 GeV runs:

What is the polarization of the anti-quarks?


LLba

baLL a

ff

ffA ˆ

f: polarized parton distribution functions

cos

For most RHIC kinematics, gg and qg dominate, making ALL for inclusive jets and hadrons sensitive to gluon polarization. 10 20 30 pT(GeV)

Partonic fractions in jet production at 200 GeV

0

Longitudinally polarized pp collisions at RHIC

0

0.25

0.5

0.75

1

0 0.2 0.4 0.6 0.8 1

|cos (theta*)|

qg

->

qg

FO

M


STAR inclusive π0 ALL at various rapidities

• During Run 6, STAR measured ALL for inclusive π0 for three different rapidity regions

• Mid-rapidity result excludes large gluon polarization scenarios• Larger rapidity correlates to stronger dominance of qg scattering

with larger x quarks and smaller x gluons

• Expect ALL to decrease as increases

• Forward rapidity: baseline for future γ and γ-jet measurements

|| < 0.95 1 < < 2 = 3.2, 3.7


STAR inclusive charged pions

• STAR measured ALL for inclusive charged pions during Run 5

• ALL(π+) – ALL(π-) is sensitive to the sign of G

• Difficult to trigger on charged pions• Used the EMCal jet patch trigger as a surrogate, which introduces

significant trigger bias (dominates syst. error band)

STARSTAR

π+ π-


Lambda longitudinal polarization transfer

• STAR can measure the longitudinal polarization transfer from one of the proton beams to outgoing Λ and anti-Λ

• Outgoing anti-Λ polarization is particularly sensitive to s• Has some of the same trigger difficulties as charged pions

STARSTAR


STAR inclusive jet ALL from Run 6

200 GeV-0.7 < < 0.9

STARSTAR

• Confidence level calculations from comparison to the GRSV polarized parton distributions

• Also compare to other polarized parton fits

• Large gluon polarization scenarios excluded, except for GS-C


DSSV – first global analysis with polarized jets

• The first global NLO analysis to include inclusive DIS, SIDIS, and RHIC pp data on an equal footing

• Finds a node in the gluon distribution near x ~ 0.1, but with the opposite phase from GS-C

de Florian et al., PRL 101, 072001

STAR


Future inclusive jet ALL sensitivity

• Goal for the current 200 GeV run:– 50 pb-1 @ 60% pol – reduce ALL uncertainties a factor of ~4

– Will provide much stronger constraints on gluon polarization

• Goal for future 500 GeV running:– 300 pb-1 @ 70% pol

– Extend precision determination to lower xg

Projected improvement in xg from Run 9

Projected sensitivities:Run 9 & future 500 GeV running


Looking beyond inclusive ALL measurements

• Inclusive ALL measurements at fixed pT average over a broad x range.

• Can hide considerable structure if Δg(x) has a node• Correlation measurements can constrain the shape of

Δg(x)

STARSTAR PRL 100, 232003


Beyond inclusives: charged pions opposite jets

STARSTAR

• Making lemons into lemonade

• Beat the trigger bias by using it

• Trigger and reconstruct a jet, then look for a charged pion on the opposite side

• Correlation measurement significantly increases the sensitivity of ALL(π+)

ALL ALL


Jet+hadron correlations at NLOfrom de Florian, arXiv:0904.4402

• NLO calculations show strong correlation between the real x and z values and LO estimates

• NLO calculations verify the increased sensitivity of ALL(π+)

• Typical scale dependence ±20% of calculated ALL value


Remaining trigger bias in charged pions + jets

• Trigger bias will be reduced with the Run 9 data:– Larger acceptance and lower thresholds increase efficiency, reduce bias

– Higher statistics will permit finer binning in jet pT


Di-jets and Δg(x)

• Di-jets provide direct access to parton kinematics at LO

Mass Rapidity |cos(*)|

Ratio

2005 preliminary di-jet distributions

2tanhcos

2ln

2

1

|ηη||θ|

ηη

x

xy

sxxM

epeps

1x

epeps

1x

43*

43

2

1

21

η4

η32

η4

η31

43

43

STARSTAR


Di-jets and g(x): projected sensitivity for Run 9

2exp 43

21

ηη

s

Mx,x

• Di-jets provide direct access to initial parton kinematics at LO

• Di-jets in different regions of the STAR detector– Sample different

mixtures of qq, qg, gg– Sample different

ranges of xg

• Goal for the current run: 50 pb-1 @ 60% pol


Projected di-jet sensitivity for √s = 500 GeV

• Higher energy accesses lower xg

• Expect smaller ALL

• Assumes 300 pb-1 at 70% polarization

2exp 43

21

ηη

s

Mx,x


Gamma + jet

• 90% of gamma+jet yield from quark-gluon Compton scattering• Forward photons + mid-rapidity jets provide clean access to g(x)

via scattering off highly polarized valence quarks

STAR sensitivity for: pT,γ > 10 GeV/c 1.09 < ηγ < 2 (EEMC) √s = 200 GeV 50 pb-1 @ 60% pol.

NOT INCLUDING: Efficiency and background effects


Gamma+jet experimental issues

• Signal yield is very small compared to di-jet backgrounds• Background suppression is VERY CHALLENGING• Efforts underway for photons in the STAR EEMC

– Current status from Run 6: 25-40% purity at 70% efficiency– Have less material in STAR to produce backgrounds in Run 9– FGT will improve photon isolation beginning in Run 12

• Gamma+jet with photons in the STAR FMS investigates even lower xg

– To date, only PYTHIA+GEANT background studies have been performed


500 GeV pp: quark and anti-quark polarizations

• Projected sensitivity for parity-violating W asymmetries

• With two polarized beams and W+ and W-, can separate u, d, u, d polarizations

• Explores the origin of the sea quarks

Will constrain u


Mid-rapidity W asymmetries

• W cross section is much larger at mid-rapidity than at forward and backward rapidity

• Mid-rapidity W asymmetries mix quark and anti-quark sensitivities• Provide high precision information about anti-quarks when quark

polarizations are well known

Will constrain d


Extending the Q2 reach

• W asymmetry measurements will extend the reach of polarized data to Q2 ~ 6500 GeV2

• Significant test of the evolution when combined with our g(x) measurements

World DIS

database with

DGLAP fits

All fixed-target data

World Data on g1p as of 2005


STAR transverse spin program

• Large rapidity transverse single-spin asymmetries at 200 GeV are BIG

• May arise from the Sivers effect, Collins effect, or a combination

PRL 92, 171801STARSTAR

Parton orbital motion

Transversity


Run 6 inclusive π0 AN at forward rapidity

• Large transverse single-spin asymmetries at large xF

• xF dependence matches Sivers effect expectations qualitatively (but not quantitatively)

• pT dependence at fixed xF does not follow ~1/pT expectation of pQCD-based calculations

PRL 101, 222001STARSTAR


• To date, the η meson has looked like a “high-mass, low-yield π0” in all measurements at RHIC

• AN for the η mass region is much larger at high xF

STAR 2006 PRELIMINARY

η ~ 3.66

STARSTARRun 6 inclusive AN at large xF


Separating Sivers and Collins effects in pp collisions

Collins mechanism: asymmetry in the forward jet fragmentation

Sivers mechanism: asymmetry in the forward jet or γ production

SPkT,q

p

p

SP

p

p

Sq kT,πSensitive to proton spin – parton transverse motion correlations

Sensitive to transversity

• Need to go beyond inclusive hadrons to measurements of jets or direct γ


Sivers di-jet measurementPRL 99, 142003

• Observed asymmetries are an order of magnitude smaller than seen in semi-inclusive DIS by HERMES

• Detailed cancellations of initial vs. final state effects and u vs. d quark effects?

STARSTAR


FMS: expanding STAR’s forward acceptance

• Expanded pT range for inclusive π0 AN during Run 8

STAR Forward Meson Spectrometer2.5 < η < 4.0 STARSTAR


More than just π0 out there

• FMS acceptance is large enough to provide good efficiency for higher mass hadrons over a broad kinematic range

Pythia+GEANT(3γ background only)

STAR Preliminary

η γγω π0γ γγγ


Run 8 forward π0 + hadron correlation measurements

• Trigger on a π0 in the FMS with pT > 2.5 GeV/c• Correlate with hadrons with pT > 1.5 GeV/c• FMS-TPC and FMS-FMS back-to-back correlations enable di-hadron / di-jet Sivers effect measurements• FMS-FMS near-side correlations sensitive to transversity• FMS-TPC: key step toward future transverse spin γ + jet study

Un

corr

ect

ed C

oin

cid

ence

P

rob

abil

ity

(rad

ian

-1)

pp π0 (FMS) + h± (TPC) + XSTARSTAR pp π0 (FMS) + π0 (FMS) + X


First look at jet-like events with the STAR FMS

• Comparisons of the jet profile and effective mass in data vs. PYTHIA + GEANT simulations

• If we find:– AN for jets must arise from Sivers effect

– Azimuthal variation in jet shape must arise from Collins effect


Proposed addition: Forward Hadron Calorimeter

• Install hadronic calorimetry behind the FMS left and right of the beam– Forward Lambdas

– Improved FMS direct photon isolation

– Forward jets

Estimated sensitivity for FMS+FHC


Future: transverse spin forward + mid-rapidity jet

Bacchetta et al., PRL 99, 212002

• Conventional calculations predict the asymmetry to have the same sign in SIDIS and +jet

• Calculations that account for the repulsive interactions between like color charges predict opposite sign

• Critical test of our basic theoretical understanding


Transverse spin possibilities with mid-rapidity jets

• Run 6 transverse spin measurements– Measured AN, AΣ while investigating systematics in ALL

– Study of Collins effect in jet fragmentation underway (please calculate)

• Will measure ATT for inclusive jets during next extended transverse spin run (please calculate)

STARPreliminary

STAR Preliminary

PT (GeV/c)

PT (GeV/c) PT (GeV/c)

PT (GeV/c)

-0.9 < < -0.5

STAR Preliminary

0 < < 0.5STAR Preliminary

0.5 < < 0.9

STAR Preliminary

-0.5 < < 0

AΣ

AN

STARSTAR


Conclusions

• STAR is making significant contributions to three poorly constrained pieces of the spin puzzle– Gluon polarization– Flavor-separated quark and anti-quark polarizations– Parton orbital motion and transversity

• STAR in embarking on precision explorations of these phenomena through correlation measurements

• STAR is generating a wealth of new data regarding the spin structure of the proton


E-M Calorimeter

Time of Flight

Projection Chamber

STAR detector


Other global analyses

• There have been many global analyses of the polarized DIS data

xΔg(x) atQ2 = 10 GeV2


Full set of DSSV polarized distributionsde Florian et al, PRL 101, 072001 and arXiv:0904.3821


Current status on transversity

• Global analysis combining Collins effect measurements in SIDIS from HERMES and COMPASS with measurements of the Collins fragmentation function by BELLE

Anselmino et al, arXiv:0812.4366

Spin Recent Results, Future Directions

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