Future Investigations of the Flavor Dependence of Sea Quark Helicities at STAR

STARSTAR

Future Investigations of the Flavor Dependence of Sea Quark

Helicities at STAR

J. Sowinski

for the

Collaboration STARSTAR

STARSTAR 10/4/2008 J. Sowinski RHIC Spin Collab. Mtg. 2

u u

d

p u d

d

u

d̄

n+

Virtual ’s imply an excess of d over u

_ _

Perturbative picture implies that u(x)~d(x)

_ _

How is the Sea Generated?

“Middle Ground” chiral symmetry +

large Nc models

’s = Goldstone bosons nucleons = solitons


NMC and

• Gottfried Sum Rule in DIS• DIS on nuclei• SIDIS• Drell-Yan• Quantitative calculations

of Pauli blocking not sufficient

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

Phys.Rev.Lett. 80 (1998) 3715

Flavor Asymmetry of the Sea


B. Dressler et al., Chiral Quark Soliton Model Predictions

m2 = (600 MeV)2

m2 = (5 GeV)2

x(d-u)̄¯

x(u-d)¯ ¯

• d(x)-u(x) and u(x)-d(x) better for Q2 evolution

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

¯ ¯ ¯¯

E866 Results Polarized q Flavor Asymmetry_

• pQCD motivated models predict

[u(x)-d(x)]dx [d(x)-u(x)]dx

• Chiral motivated models tend to disagree

¯ ¯¯ ¯1

01

0


Current Constraints on the

Polarized Sea

D. De Florian et al. Phys. Rev. Lett. 101, 072001 (2008)

• One example of global fit: DSSV• Except for G main constraints

from (SI)DIS• Flavor and valence distributions

well determined• u and d best fit distributions

asymmetric• But error bands allow flavor

symmetric sea


• V-A coupling – only LH u and RH d couple to W+

– Likewise LH d and RH u to W -

– Only LH W’s produced

• Neutrino decay gives preferential directionality in decay

_

_

Parity violating single spin asymmetry AL

(Helicity flip in one beam while averaging over other)

ALW - ~ u(x1)d(x2)+d(x1)u(x2)

_ _

Allows kinematic separation especially for W- in EEMC

Dressler et al. predict large sensitivity

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


NLO Theory Calculations

NLO theoretical framework with NNLL resummation accuracy allows interpretation of AL(ye) and Monte Carlo calculations. Nucl. Phys. B666, 31 (2003) RHICBOS


STAR•Need to detect e+ e- and measure pT

•Need to find in huge hadronic background•Need to separate e+ and e- charge

Large solid angle of STAR

Forward GEM Tracker

EM Calorimetry

h

e


Tracking Upgrade at STAR

TPC Only

Add FGT

• Charge sep. in forward region essential• TPC tracking degrades in forward dir.• Add FGT allows >80% c.s. separation• Isolation cut based on tracks for >2


Endcap ElectroMagnetic Calorimeter• Pb Scint sampling calorimeter• 21 radiation lengths• 720 projective towers• Depth Segmentation

– 2 preshower layers, e/h 0/ disc.– High position resol. SMD 0/disc.– Postshower layer e/h discc.

• L0 trigger- high tower, jet patches


Examples (3 of many) cuts

• Shower shape in calo.• Isolation about electron• Veto on energy opposite in

e/h Discrimination


e/h discrimination• Full cuts strongly suppress (~1000) hadrons

while preserving ~80% of electron/positrons

• Full Pythia/GEANT simulation

• Isolation cone about electron

• Awayside veto• Calorimeter

shower shapes


Projected Signal to Background Ratios• Succesfully suppress hadronic

background to give S/B >1 for pT> 28 GeV

• Allows for 30% conversion of neutrals before tracking

30% neutrals convert to tracks

Full neutral rejection


Projections vs pT

• 1 < < 2• 300 pb-1

• Realistic BG subtraction

• Recent PDFs ~representing current allowed u/d range

• d and u isolated


Projections vs. Pseudorapidity

• u sensitivity spread over


Summary

• First 500 GeV run planned for February

• Over next 5 yrs. of full operation could expect 300 pb-1 of pol. pp collsiions

• W program at STAR and RHIC should provide new constraints on the flavor (a)symmetry of the sea quarks

Documents

Future Investigations of the Flavor Dependence of Sea Quark Helicities at STAR