Measurement Of Longitudinal Single-Spin Asymmtry, AL For W Boson Production In Polarized P+P Collisions At RHIC

Devika Gunarathne [Temple University, STAR collaboration]

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INTRODUCTION RESULTS

EXPERIMENT

ANALYSIS

~ 30%ΔΣ = (Δu+Δd +Δs+Δu +Δd +Δs )∫ dx

Quark / antiquark Contribution

spin sum rule

SPIN STRUCTURE OF THE PROTON

< Sp >=12=12ΔΣ+ΔG + L

polarizedbeam

proton proton

polarizedbeam

unpolarizedbeam

proton

probingd quark

protoncollision zone

polarizedbeam

unpolarizedbeam

proton

W-

e-probingd quark

protoncollision zone

polarizedbeam

unpolarizedbeam

proton

W-

e- e-probingu quark

probingd quark

protoncollision zone

polarizedbeam

unpolarizedbeam

proton

W-

e-

e-

e-probingu quark

probingquark sum (u+d)

protoncollision zone

polarizedbeam

unpolarizedbeam

proton

W-

W Boson Production in polarized p+p collisions

•Maximum parity violation•No fragmentation involve.

•Easy detection

DSSV global analysis of helicity PDF

•Direct access to Δq / Δq

Endcap Electro-Magnetic Calorimeter (EEMC)

Barrel Electro-Magnetic Calorimeter (BEMC)

Time Projection Chamber (TPC)

Beam - Beam Counter (BBC)

Forward Gem Tracker (FGT)

Magnet

STAR Detector

TPC : -1.3 < η < +1.3 , Tracking and Particle ID

BEMC : -1.0 < η < +1.0 EEMC : +1.1 < η < +2.0

Barrel and Endcap 2π calorimetry

BBC / ZDC : Relative Luminosity

Calorimeter response from a simulated QCD type di-jet background event

W candidate tracks as a function of transverse EnergyReconstruction of W bosons from decay electron and positrons

Calorimeter response from a simulated W event

• Isolated high energetic TPC tracks pointing to calorimeter tower. Energy from the maximum 2x2 cluster.

•Undetected neutrino leads to large opposite missing energy , large imbalance in the transverse momentum.

Transverse plane views

ptBalance4pi -

nearCone

electr

on

nearCone ΔR =0.7

ETΔR < 0.7

EeT / ET

ΔR < 0.7 > 88%

4X4

TPC track extrapolated to Barrel colorimeter tower grid

EeT

•Cuts designed to take the advantage of topological difference between W and QCD type BG event

!pTbalance =

!pTe +

!pTjets

ΔR>0.7∑

EeT / ET

4X4 > 95%

TPC Charge sign separation

Background Estimation

•QCD BG using data driven procedure

•Electroweak BG (W->tau, Z->e+/e-) using MC simulation

W-W+

STAR Run 2013 W AL Projections

STAR Run 2012 W AL Published Results

6r2

06d(x,Q2) dx1

0.05

DSSV+

DSSV++w/ proj. W data

DSSV++w/ STAR W data

Q2 = 10 GeV2

DSSV6r2=2%

0

5

10

15

-0.08 -0.06 -0.04 -0.02 0 0.02

AL =1P1N++ + N+− − N−− − N−+

N++ + N+− + N−− + N−+

AL =σ + −σ −

σ + +σ −

single spin asymmetry

=

TPC

BEMC

EEMC

FGT

Y

Z

η=-1.1

η=-0.5

η=0

η=+0.5

η=+1.112 3

4

DSSV++ Global Analysis includes published STAR W AL Results NNPDF analysis of anti U quark polarization.

6r2

0 6u(x,Q2) dx1

0.05

DSSV+

DSSV++w/ proj.W data

DSSV++w/ STAR W data

DSSV6r2=2%

0

5

10

15

-0.06 -0.04 -0.02 0 0.02 0.04 0.06

Impact of Recent STAR W AL Results on Recent Global Analyses and Predictions for future STAR Results

CONCLUSION

•Large uncertainty for sea quark polarization

•Large anti u quark polarization than theoretical prediction

•Large data set in run 2013 expect to reduce uncertainty further

• The production of W bosons in polarized P+P collisions provides an excellent way to study the spin and flavor asymmetries of the proton quark and antiquark distributions.

• STAR has measured the parity violating single spin asymmetry AL for pseudo-rapidity between -1.4 and +1.4 from STAR 2012 and 2011 data providing the first detailed look at the asymmetries η dependance.

• STAR 2012 W AL results provide significant constraints on anti u and anti d quark polarizations.

• Large data set of STAR 2013 is being analyzed currently in mid rapidity region (pseudo-rapidity between -1.0 and +1.0) and expects results reduce uncertainty further.

• High precision results from STAR 2013 data will improve he constraints on the anti u and anti d quark polarizations.

———— - Candidate track and BEMC cluster reconstructed. BLUE - EeT > 14 GeV and EeT / E4x4T > 0.95 Green - EeT / EΔR<0.7T > 0.88 Red - sign PT balance cut > 14 GeV W+

W-