 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

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 R. Bellwied, STAR meeting 6/01

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 6/01

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 R. Bellwied, STAR meeting 6/01

Key measurements in pp and pA p t -dependence x f -dependence R. Bellwied, STAR meeting 6/01

Summary of HEP results  is negatively polarized with respect to production plane for p t < 1 GeV/c the polarization is linearly increasing with p t with increasing slope as a function of x f for p t > 1 GeV/c the polarization is constant with p t but still increases linear as a function of x f effects due to incident energy and target size (in pA reactions) are small (P = -1.1 A ) other hyperons:  neg.P and decrease with x f,  pos.P and constant with x f, Anti-  and Anti-  : P=0, Anti-  shows polarization R. Bellwied, STAR meeting 6/01

E896-SDDA polarization results cos  in 5 x f bins x f = , =0.57 GeV/c x f = , =0.79 GeV/c x f = , =1.23 GeV/c x f = , =1.61 GeV/c x f > 0.5, =1.98 GeV/c R. Bellwied, STAR meeting 6/01

E896-SDDA polarization results R. Bellwied, STAR meeting 6/01

Comparison to pp/pA/AA data R. Bellwied, STAR meeting 6/01

Comparison to pp and pA data R. Bellwied, STAR meeting 6/01

Can we measure polarization in STAR ? R. Bellwied, STAR meeting 6/01 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  R. Bellwied, STAR meeting 6/01

STAR coverage R. Bellwied, STAR meeting 6/01

if x f = p z (  )/p z (beam) R. Bellwied, STAR meeting 6/01

STAR cos (  ) distribution (uncorrected) R. Bellwied, STAR meeting 6/01

Detector response function simulation R. Bellwied, STAR meeting 6/01

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 R. Bellwied, STAR meeting 6/01

STAR  polarization: P = -1.5% % All  in central events integrated over p t and x f and corrected for detector response function

STAR polarization results cos  in 5 p t bins x f = 0.01 – 0.04 p t = GeV/c p t = 0.5 – 1.0 GeV/c p t = 1.0 – 1.5 GeV/c p t = 1.5 – 2.0 GeV/c p t = 2.0 – 5.0 GeV/c

STAR p t dependent polarization apparent polarization effect at low momentum (statistically significant) (9.9%+-2.5% for pt = GeV/c) fake? systematic? due to low pt sweep? has to be wrong because of symmetry argument R. Bellwied, STAR meeting 6/01

How to measure polarization in STAR R. Bellwied, STAR meeting 6/01 do ‘asymmetric analysis’ by cutting on either rapidity or x f (if x f = p  / p b instead of x f = |p  | / |p b |) 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) R. Bellwied, STAR meeting 6/01