u d

dud̄

n+

…or how the sea came to be …

W.W. Jacobs for the STAR Collaboration

Indiana University Dept. of Physics/IUCF

CIPANP09 26-31 May, 2009, San Diego

• what we know

• why W’s?

• what we need

• status, data, etc.

• future & plan

The W Spin Physics Program with s = 500 GeV Polarized pp

Collisions at RHIC

STAR STAR

PRL 80 (1998) 3715

Flavor Asymmetry of the Sea Flavor Asymmetry of the Sea

SU(2): symmetric light quarks

Gottfried sum rule in DIS:

Quantitative calc’s of Pauli blocking insufficient to explain ratio/x-dependence (but see Bourelly and Soffer, Nucl Phys B423, 329 (1994))

Purely perturbative mechanism => equal numbers of light anti-quarks

Non-perturbative processes seem to be needed in generating the sea

=> indication of flavor symmetry breaking in light sea!

DIS on nuclei

SIDIS

Drell-Yan

2

B. Dressler et al., Chiral Quark Soliton Model Predictions (hep-ph/9910464)

m2 = (600 MeV)2

m2 = (5 GeV)2

x(d-u)̄¯

x(u-d)¯ ¯

E866 Results

∫0

1

∫0

1

Polarized q Flavor Asymmetry _

Look at differences d(x)-u(x) and Δu(x)-Δd(x) (also useful quantities for the Q2 evolution)

E866 are qualitatively consistent with pion cloud models, instanton models, chiral quark soliton models, etc.

pQCD models predict ∫(Δu(x)-Δd(x))dx [≤ ∫(d(x)-u(x))dx; chiral models generally disagree

¯ ¯ ¯ ¯

3

¯ ¯

Spin Puzzle & Current Constraints on Polarized SeaSpin Puzzle & Current Constraints on Polarized Sea

D. De Florian et al. PRL 101, 072001 (2008)

“DSSV” one example: except for Δg (RHIC), the main constraints come from (SI)DIS

Valence u and d distr’s are ~well determined; strange distr “Δs=Δs” w/ small εSU(2,3) breaking

Δu and Δd fit distr’s are slightly asymmetric

gz

qz

pz LLGS

2

1

2

1

polarized DIS: 0.2~0.3 poorly constrained

measure at RHIC

Distributions from recent global fitting:

4

Fit difference distr x(Δu-Δd) ~ positive re: analysis update arXiv:0904.382 [hep-ph]

x(Δu-Δd) fit smaller than χQSM, but ~ similar to x(d-u)

V-A coupling gives perfect spin separation LH u and RH d couple to W+

LH d and RH u couple to W-

only LH W’s produced

Reconstruct W’s through High pT lepton (e+/e-) decay channels [chrg sign discr!]

Parity violating single spin asymmetry (helicity flip in one beam; average over the other)

ALW ֿ ~ u(x1)d(x2)-d(x1)u(x2)

_ _

5

ALW ⁺ ~ d(x1)u(x2)-u(x1)d(x2)

__

W+(-) Production in p-p at s = 500 GeV/c2

Neutrino helicity gives preferred direction in the decay kinematics

W¯ preserves initial state kinematics; decay electron emitted along W¯ trajectory (and conversely for W+ where decay positron is anti-parallel to trajectory)

¯¯

(ang dep suppressed)

(similarly W+: -Δd/d, Δu/u)

W Decay Kinematics W Decay Kinematics L

epto

n p T

20

30

40

W+ kinematics W- kinematics

x1 MW

seyW

x2 MW

se yW

y leplab y lep

* yW

y lep*

1

2ln

1 cos*

1 cos*

p'T ,lMW

2sin*

WqT

small|rapidity|

>>0

Lepton proxy works best at ~ large |rapidity|; x1 and x2 also calc/assignable

At mid- rapidity the assignment of x1, x2 more ambiguous & correlation of lepton and W momenta not tight; but information still there (e.g., Ayl, ApT)

6

assoc. low x w/ sea

NLO Theory Calculations (RHICBOS)NLO Theory Calculations (RHICBOS)

RHICBOS, Nucl. Phys. B666, 31 (2003)

NLO theoretical framework exists -> allowing global analysis of data sets! (framework is general for all rapidity W’s)

7

The shape of RHICBOS and MC-Pythia agree (there is a calc normalization (“K”) factor ~ 1.3 applied to Pythia)

formalism in RICHBOS includes possibility of NNLL resummation; but for inclusive AL(ye), NLO is likely sufficient as discussed by (DeFlorian et al. arXiv:0904.382 [hep-ph])

RHIC … World’s 1RHIC … World’s 1stst pp Collider pp Collider

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

Extensive Spin Analysis Infrastructure Spin dependent Lumi (ZDC+BBC+VpD) Control/estimate of spin dependent bkgrds Effects of trigger on spin asymmetries, etc. [see other STAR talks this conference]

100 & 250 GeV proton beams: s = 200, 500 GeV

Spin Rotators at IR’s: transverse and longitudinal spin orientation

CNI polarimeters + Hydrogen Jet target: run & abs. polarization

8

Inclusive jet results

eta=2.0

eta=1.0

TPC

FGT

Endcap EMC

Barrel EMCThe STAR Detector for W Physics IThe STAR Detector for W Physics I

9

FGT (under construction) needed for high pseudorapidity tracking; require charge sign discrimination for High PT leptons (e+/e-)

10

The STAR Detector for W Physics IIThe STAR Detector for W Physics II STAR: large solid angle detector w/ full TPC, EMC coverage

Endcap EMC: 720 projective towers. Design features to aid in lepton/hadron discrimination

SMD: triangle shape scint strips; fine grained shower shape analysis and forward tracking point

Forward GEM Tracker (FGT) Upgrade at STARForward GEM Tracker (FGT) Upgrade at STAR

11

Charge reconstruction efficiency

TPC tracks only

Add FGT tracks

TPC

FGT

Endcap acceptance

Charge sep. in forward region essential!

TPC tracking degrades in forward direction

Adding FGT allows >80% c.s. separation out to limit of acceptance of the Endcap EMC

Isolation cut will be based on tracks for η>2

12

Forward GEM Tracker Implementation Forward GEM Tracker Implementation

6 light weight triple-GEM disks mount on common fiber; “flats” accommodate TPC inner field cage, support structures/utilities

Disks divided into quarter sections with 2D readout (Radius, phi)

GEM foil (TechEtch)

Detector strip readout w/ APV25S1 electronics mounted on GEM ass’bly; 2 APV boards/quadrant each with 5 chips (128 chns/)

Custom built readout (APV readout controller and modules – ARC, ARM) will be housed in remote crate

expect resolution~ 60 μm in x and y

Square prototype GEMS w/ APV tested in FNAL beam

Forward e/h Discrimination (simulation)Forward e/h Discrimination (simulation)

Shower “shape” in Calorimeter

Isolation about electron

Veto on energy opposite in phi

Example - 3 of several - cuts: (spans list of cut categories)

13

Forward Rapidity: Suppression of QCD BackgroundForward Rapidity: Suppression of QCD Background

QCD events(background)

W events(signal)

sequ

entia

l cu

ts

All simulations scaled to LT=300/pb

Full Pythia/GEANT simulation of QCD background and W signal sample

1010 QCD background events generated w/ full detector response; similar statistics as expected in data

Initial studies involve both global and detector specific sequential cuts electron/positron Isolation away side “veto” (missing ET) detailed EMC shower shapes and PID

The full cuts strongly (~ 103) suppress hadrons while preserving ~ 80% of the electrons/positrons

Present cut scheme (14 conditions) achieves a signal to background ratio > 1:1 for detected ET greater than ~ 30 GeV 14

Mid-Rapidity: Suppression of QCD BackgroundMid-Rapidity: Suppression of QCD Background

MC simulations The W cross section is ~ 3x larger

at mid- vs forward-rapidity

Develop mid-rapidity W reconstruct algo w/ using full coverage by the STAR TPC and Barrel EMC

RHICBOS W simulation @ 500 GeV

QCD (background) and W (signal) events before cuts

mid-rapidity QCD and W after sequential cut sequence

these simulations suggest good S/B should be possible; there may be additional effects in the real data environment

15

500 GeV longitudinal pp running in 2009

Longitudinal Polarization = 50%

Luminosity: 10 pb-1

FOM = 2.5 pb-1

Effective signal ~ 250 (W+) and ~ 60 (W-)

MC simulations: assumes W reconstr algo at mid-rapidity yields S/B >1 for ET > 30 GeV

Goal for run 9: test new data taking environment; measure and extract W signal from first physics run at 500 GeV

Projected sensitivity to the parity violating longitudinal single spin asymmetry:

Status April 12, 2009 upon switch to 200 GeV recorded ~4 weeks of data w/ detectors working well average (online) beam polarizations ~ 35% luminosity sampled ~10pb-1 analysis underway through fast production route

16

W(+/-) prod w/ forward rapidity decay leptons and kinematics with Δu, Δd (Δu, Δd) isolation

Assume LT ~ 300 pb¯¹ from ~ 5 years running

Realistic Background subtraction via cuts

PDF’s w/ current allowed Δu/Δd range shown

17

W’s @ Forward Rapidity: Projected STAR SensitivityW’s @ Forward Rapidity: Projected STAR Sensitivity

¯ ¯

¯

u d

ud

MC simulations for 1 < η < 2; LT = 300 pb-1

Phase I: consistency check with existing DIS quark polarizations (LT ~ 100 pb-1)

Phase II: strong impact constraining unknown antiquark pol w/ full LT and 70% beam polarization

AL

AL

17

Flavor dependence of spin dependent Δq and Δq PDF’s relate to fundamental questions concerning origin of the sea and QCD physics

Parity Violating single spin asymmetries in W boson production can provide constraints on the flavor (a)symmetry of the sea

An exciting program of W production in polarized proton-proton collisions is beginning with the STAR detector at RHIC

Upgrade of the STAR forward tracking is required at forward rapidity where sensitivity is most clear

Triple-GEM is cost effective technology for the forward tracking upgrade (a.k.a. Forward GEM Tracker)

Goal: full installation of FGT in summer 2011 for anticipated long 500 GeV longitudinally polarized pp RHIC run in FY12

Realistic simulations suggest S/B >1 obtainable in relevant ET range for both Barrel and Endcap EMC regions; expect mid-rapidity x-sec from 2009 run

Integrate ~ 300 pb-1 of pol pp collisions w/ full operation over next ~ 5 years; expect significant results from both singular (esp. forward rapidity) and global (including mid-rapidity) analyses. 18

Summary Summary and OutlookSummary and Outlook

BACKUP SLIDES

Dressler et al. predict large sensitivity

W+(-) Production in p-p at s = 500 GeV/c2

V-A coupling gives perfect spin separation LH u and RH d couple to W+

LH d and RH u couple to W-

only LH W’s produced

Reconstruct W’s through High pT lepton (e+/e-) decay channels [chrg sign discr!]

Neutrino helicity gives preferred direction in the decay kinematics

W¯ preserves initial state kinematics; decay electron emitted along W¯ trajectory (and conversely for W+ where decay positron is anti-parallel to trajectory)

collisions are highly asymmetric. W production favors initial quark momentum

W Kinematics at STARSTAR

Valence + sea quark collisions favored at √s=500 GeV

Xvalence X sea

d2dywdcos *

W

~ u x1 d x2 1 cos * 2 d x1 u x2 1 cos * 2

q q

AL

d d AL

uu

Projections vs. Pseudorapidity

• d sensitivity spread over

Jan Balewski, MIT

Exploring W/lepton phase space @ STAR

eForw’dd-pol

Aftubar-pol

backward

unpolarizedbeam

polarizedbeam

W-

forward

polarizedbeam

unpolarizedbeam

13

Example Cuts: isolation & veto away side ET

•Barrel tower isolation cut•Look at ratio of Eт in 3x3 tower patch to Eт in 30x40 tower patch

•Away-side Eт cut•Require opposite Eт < 10 GeV

Mid rapidity MC simulations

f

E(3

x3)

GeV

E(3

x3)

GeVf

QCD events W events

away-side ET (GeV) away-side ET (GeV)

QCD events W eventselectron

neutrino

y

x

W event

Jan Balewski, MIT

Principle of e+ vs. e- differentiation

1 of reco track

2mmS

agit

ta (

cm)

100cmY/cm

40cm

20cm

X/mm

1.0Vertex=200μm

Endcap SMDhit=1.5mm

reco track

Limit f

or ∞ p T

trac

k

3 FGT hits=70μm

0 2.0 mm

Sagitta=2mm

Wrong Q-signGood Q-sign

Include vertex & Esmd

30 GeV PT