transverse polarization in star au-au collisions polarization definition and models e896 results...

Transverse Polarization in STAR Au-Au Collisions

Polarization definition and models E896 results STAR analysis STAR results Significance of results Future analysis

R. Bellwied for M. Castro R. Bellwied for M. Castro

What is transverse polarization ?

Preferred spin direction of perpendicular to reaction plane Measure angular distribution of decay proton in rest frame of

dN/dcos = A(cos(1+Pcos) = 0.65 (-decay asymmetry

parameter = s- and p-wave interference term)

A(cos) = detector acceptance P = polarization + or - cos = spin up or down

R. Bellwied, STAR meeting 6/01


Models on the Quark Level

Parton Recombination Model polarization due to Thomas precession in quark recombination process valence di-quark recombines with sea s-quark slow sea s-quark has transverse momentum component strong push in longitudinal direction from recombination spin of s-quark interacts with Thomas precession vector

Lund Model color field between di-quark and collision region color field materializes into s-sbar pair generate pair has angular momentum perpendicular to string which

has to be compensated by spin of pair



Models on the Quark Level

Quark Fragmentation Model polarization due fragmentation of a color field s-quark (D. Boer et al.)

Regge Model polarization due to intermediate baryon dissociation or

electromagnetic decay of intermediate SU(3) baryon one pion exchange model polarization can be introduced in final state interaction

R. Bellwied, STAR meeting 6/01R. Bellwied, STAR meeting 6/01

Heavy Ion Models

Disappearance of polarization due to QGP color field in QGP possesses momentum color field provides transverse momentum to s-sbar pair no correlation between inherent transverse momentum and spin

of s-quark

Simple rescattering any final state interaction of the Lambda itself or the decay proton

can potentially destroy the preferred spin direction measurements at high transverse momentum and high

Feynman-x should reduce rescattering probability



Key measurements in pp and pA

pt-dependence xf-dependence



Summary of HEP results

is negatively polarized with respect to production plane for pt < 1 GeV/c the polarization is linearly increasing with p t

with increasing slope as a function of xf

for pt > 1 GeV/c the polarization is constant with pt but still increases linear as a function of xf

effects due to incident energy and target size (in pA reactions) are small (P = -1.1 A -0.15)

other hyperons: neg.P and decrease with xf, pos.P and constant with xf, Anti- and Anti-: P=0, Anti- shows polarization



E896-SDDA polarization results

cos in 5 xf bins

xf = 0.1-0.2 , <pt>=0.57 GeV/c

xf = 0.2-0.3, <pt>=0.79 GeV/c

xf = 0.3-0.4, <pt>=1.23 GeV/c

xf = 0.4-0.5, <pt>=1.61 GeV/c

xf > 0.5, <pt>=1.98 GeV/c



E896-SDDA polarization results



R a n g e P in %

xf = 0.1 – 0.2 -0.3 +- 6.0 %

xf = 0.2 – 0.3 -2.1 +- 3.2 %

xf = 0.3 – 0.4 -4.5 +- 3.9 %

xf = 0.4 – 0.5 -14.1 +- 5.9 %

xf > 0.5 -25.7 +- 7.6 %

Comparison to pp/pA/AA data



Comparison to pp and pA data

R. Bellwied, STAR meeting 6/01R. Bellwied, STAR meeting 6/01

Can we measure polarization in STAR ?



detector is perfectly symmetric

beam direction for generated Lambda is unknown, choose beam always in +z direction

if Lambda is polarized the polarization would cancel out

solution: do an ‘asymmetric’ analysis

STAR data analysis

540,000 central Au-Au events

apply Matt’s cuts

around 70,000



STAR coverage



if xf = pz()/pz(beam)



STAR cos () distribution (uncorrected)



Detector response function simulation



Cause for detector response function

simulation input (y=+-0.5) 1 hit on positive + negative

1 hit on positive only MC – RC associated, no cuts

Detector response function summary

symmetric shape of response function due to loss of one of the two daughters even if the is within y=+-0.5

additional asymmetric component due to magnetic field and cuts in V0 reconstruction



STAR polarization: P = -1.5% +- 1.8%

All in central events integrated over pt and xf

and corrected for detector response function

STAR polarization results

cos in 5 pt bins xf = 0.01 – 0.04

pt = 0 - 0.5 GeV/c

pt = 0.5 – 1.0 GeV/c

pt = 1.0 – 1.5 GeV/c

pt = 1.5 – 2.0 GeV/c

pt = 2.0 – 5.0 GeV/c

STAR pt dependent polarization

apparent polarization effect at low momentum (statistically significant) (9.9%+-2.5% for pt =0.5-1.0 GeV/c)

fake? systematic? due to low pt sweep?

has to be wrong because of symmetry argument



How to measure polarization in

STAR



do ‘asymmetric analysis’ by cutting on either rapidity or xf (if xf = p / pb instead of xf = |p| / |pb|)

we cut on rapidity ( from 0.2 to 0.5 and –0.2 to –0.5)

if polarization, then it should change sign (but not magnitude) for the two rapidity bins

Preliminary results of two hemisphere analysis

P for y = -0.2 to –0.5 P for y = 0.2 to 0.5

there seems to be a systematic asymmetry when comparing the two rapidity bins (still under investigation)



transverse polarization in star au-au collisions polarization definition and models e896 results...

Documents

star meeting

transverse polarization

gevc x f

x f bins x f

color field squark

regge model polarization

castro slide

function of x f effects